Download Developing an ellipsoidal reflector for measuring otoacoustic

Vesa Heiskanen Developing an ellipsoidal reflector for measuring otoacoustic emissions School of Electrical Engineering Thesis submitted for examination for the degree of Master of Science in Technology. Espoo 15.5.2015 Thesis supervisor: Prof. Ville Pulkki Thesis instructor: Prof. Bastian Epp A! Aalto University School of Electrical Engineering aalto university school of electrical engineering abstract of the master’s thesis Author: Vesa Heiskanen Title: Developing an ellipsoidal reflector for measuring otoacoustic emissions Date: 15.5.2015 Language: English Number of pages:10+44 Department of Signal Processing and Acoustics Professorship: Acoustics and Audio Signal Processing Code: S-89 Supervisor: Prof. Ville Pulkki Instructor: Prof. Bastian Epp Otoacoustic emissions (OAEs) are faint sounds emanated by the eardrum. They can be recorded and are used as an objective measure to monitor the health of the inner ear. Conventional measurement methods use a miniature microphone housed in a probe that is fitted to the ear canal. In this thesis a novel method for measuring OAEs was experimented by using a large one-inch-diameter microphone which has the benefit of very low inherent noise. A modular ellipsoidal reflector was developed and used to collect OAEs and focus them to the microphone. The applicability of a such reflector for recording OAEs was studied. Response measurements were made to examine the acoustical properties of the different reflector setups comprised of different part-combinations. The results show that the reflector setups providing the most gain could be employed for OAE measurements where the temporal response of the measurement system is not overridingly critical such as distortion product otoacoustic emissions (DPOAEs), stimulus-frequency otoacoustic emissions (SFOAEs) or spontaneous otoacoustic emissions (SOAEs). This limitation is due to reflections in the response that take over 10 ms to decay. DPOAE measurements were made to two subjects to test the reflector measurement system in practice. Succesful measurements were accomplished at the two used stimulus frequencies which is a proof of concept that this kind of measurement system can be used to record OAEs. Further investigations are needed to be able to compare the properties of the reflector system to conventional measurement probes. Keywords: acoustics, ellipsoidal reflector, otoacoustic emission (OAE), DPOAE, SFOAE, SOAE aalto-yliopisto sähkötekniikan korkeakoulu diplomityön tiivistelmä Tekijä: Vesa Heiskanen Työn nimi: Ellipsoidin heijastimen kehittäminen otoakustisten emissioiden mittaamiseen Päivämäärä: 15.5.2015 Kieli: Englanti Sivumäärä:10+44 Signaalinkäsittelyn ja akustiikan laitos Professuuri: Akustiikka ja äänenkäsittelytekniikka Koodi: S-89 Valvoja: Prof. Ville Pulkki Ohjaaja: Prof. Bastian Epp Otoakustiset emissiot (OAE) ovat kuulojärjestelmän tuottamia heikkoja ääniä, jotka voidaan mitata. Mittausta voidaan käyttää sisäkorvan terveydentilan tarkasteluun. OAE:t mitataan tavallisesti käyttämällä korvakäytävään sovitettavaa anturia, joka sisältää pienikokoisen mikrofonin. Tässä diplomityössä kokeiltiin uutta tapaa otoakustisten emissioiden mittaamiseen käyttämällä suurta yhden tuuman halkaisijan mikrofonia, jonka etuna on todella pieni sisäinen kohina. Osista koostuva ellipsoidin muotoinen heijastin kehitettiin kokoamaan OAE:t ja kohdistamaan ne mikrofoniin. Työn tarkoitus oli tutkia tällaisen heijastinjärjestelmän soveltuvuutta OAE:iden mittaamiseen. Vastemittauksilla tarkasteltiin eri heijastinkokoonpanojen akustisia ominaisuuksia. Eniten vahvistusta tarjoavia kokoonpanoja voisi käyttää sellaisten OAE-tyyppien mittaamiseen, joissa mittausjärjestelmän aikavasteen lyhyys ei ole ensisijaisen ratkaiseva ja jotka voidaan tehdä esimerkiksi käyttämällä jatkuvaa ja pitkäkestoista herätettä. Tämä rajoitus johtuu kokoonpanojen aikavasteissa olevista heijastuksista, joiden vaimeneminen kestää yli 10 ms. Järjestelmän testaamiseksi käytännössä, DPOAE-mittaus tehtiin kahdelle koehenkilölle. Mittaukset onnistuivat kahdella käytetyllä herätetaajuudella, joka osoittaa, että kehitettyä heijastinjärjestelmää voi käyttää OAE:iden mittaamiseen. Heijastinjärjestelmän vertaaminen normaalissa käytössä oleviin OAE-antureihin edellyttää lisätutkimuksia. Avainsanat: akustiikka, ellipsoidi heijastin, otoakustinen emissio (OAE) Acknowledgements This work is carried out in Department of Signal Processing and Acoustics, Aalto University School of Electrical Engineering, during the year 2014. I would like to thank my instructor professor Bastian Epp from the Technical University of Denmark for providing knowledge, guidance and practical advice along the course of this work. I express appreciation towards my supervisor Ville Pulkki for the opportunity of doing this work as well as his ideas, guidance and comments. I am thankful to Ilkka Huhtakallio for teaching me how to use laboratory equipment and using his time and vision on helping me to build measurement setups. From the process of building the reflector I am grateful to Hannu Paajanen from the Department of Design for lending his know-how of manufacturing techniques. Also thanks to Kari Kääriäinen from Design Factory and Tapani Honkavaara. Thanks to Juha Backman and Benedict Slotte from Nokia for providing their knowledge and a miniature speaker for use in the measurements. And thanks to Javier Gómez and the rest of the staff in the Department of Signal Processing and Acoustics for the assistance. Finally, I would like to thank my family and friends for all the love and support. Otaniemi, 24.2.2015 Vesa Heiskanen iv Contents Abstract . . . . . . . Abstract (in Finnish) Acknowledgements . Contents . . . . . . . Abbreviations . . . . List of Figures . . . . List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii . iii . iv . v . vii . viii . x 1 Introduction 1 2 Otoacoustic emissions 2.1 Cochlear amplifier and otoacoustic emissions . . 2.2 Measurement of otoacoustic emissions . . . . . . 2.2.1 Transient-evoked otoacoustic emissions . 2.2.2 Distortion product otoacoustic emissions 2.2.3 Stimulus-frequency otoacoustic emissions 2.2.4 Spontaneous otoacoustic emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3 6 7 8 10 10 3 Microphones and reflectors 12 3.1 Inherent noise in condenser microphone systems . . . . . . . . . . . . 12 3.2 Ellipsoidal reflectors . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 4 Methodology and experiments 4.1 Design of a reflector used to measure otoacoustic emissions . . . . . . . . . . 4.1.1 Design of the reflector . . . . . 4.1.2 Reflector gain measurements . . 4.2 Otoacoustic emission measurements . . 4.2.1 Organization of the experiments 4.2.2 Stimulus parameters . . . . . . 4.2.3 Data collection and analysis . . 4.2.4 System distortion measurement 18 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 18 20 22 22 23 24 25 5 Results 26 5.1 Reflector gain measurement results . . . . . . . . . . . . . . . . . . . 26 5.1.1 Focus to focus gain . . . . . . . . . . . . . . . . . . . . . . . . 26 v vi 5.2 5.1.2 Effect of offset from the focus . . . . . . . . . . . . . . . . . . 5.1.3 Discussion of the acoustical characteristics of the reflector . . . Otoacoustic emission measurement results . . . . . . . . . . . . . . . 5.2.1 Comparison of open and sealed ear in DPOAE measurements 5.2.2 DPOAE growth . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.3 System distortion . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.4 Discussion of the DPOAE measurement results . . . . . . . . 27 30 31 31 35 36 38 6 Conclusions 40 Bibliography 44 Abbreviations CF DPOAE EOAE IHC OHC SFOAE SNR SOAE SPL SSOAE TEOAE Charasteristic frequency Distortion product otoacoustic emission Evoked otoacoustic emission Inner hair cell Outer hair cell Stimulus-frequency otoacoustic emission Signal-to-noise ratio Spontaneous otoacoustic emission Sound pressure level Synchronized spontaneous otoacoustic emission Transient-evoked otoacoustic emission vii List of Figures 2.1 2.2 2.3 2.4 2.5 Structure of the ear. The outer ear is the part external to the tympanic membrane featuring the pinna and the external auditory canal. The middle ear is the portion internal to the tympanic membrane and external to the cochlea. The inner ear is comprised of the cochlea and the semicircular canals of the vestibular system. Picture adapted from (46). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Illustration of the traveling wave motion along the basilar membrane. The spiral appearance of the cochlea has been stretched out and the displacement of the basilar membrane is greatly exaggerated. Figure adapted from (49). . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cross section of the cochlea featuring a schematic view of the organ of Corti. Figure adopted from (42). . . . . . . . . . . . . . . . . . . Example response of a clinical TEOAE measurement. On the left waveform of two time-locked averages are shown. There is a few millisecond delay preceding the response. The right side of the figure shows the levels of the emission divided into frequency bands (blue) and noise levels in these given bands (red). . . . . . . . . . . . . . . . A spectrum of an DPOAE measurement from the ear of a 16-monthold child. Stimulus tones (f1 = 1.807 kHz; f2 = 2.002 kHz; L1 = 65 dB SPL and L2 = 60 dB SPL) and resulting DPOAEs are labeled to the figure. Several DPOAEs occur at frequencies below the primaries (4f1 – 3f2 = 1.222 kHz; 3f1 – 2f2 = 1.417 kHz and 2f1 – f2 = 1.612 kHz) and one above the primaries ( 2f2 – f1 = 2.197 kHz). Figure adapted from (35) [frequency component labels are redrawn due to low-quality copy of the figure]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 3 4 5 7 9 Equivalent levels of inherent noise (1/3 octave bandwidth) produced by different elements of a microphone system equipped with a 1” free-field microphone (sensitivity 50mV/Pa). The microphone, the preamplifier and the measurement amplifier all contribute significantly to the noise floor of the system in certain parts of the frequency range. Figure adopted from (6). . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.2 Comparison of 1/3 octave noise spectra of microphone systems equipped with different size microphones. Figure adopted from (6). . . . . . . . 14 viii ix Inherent noise comparison between a conventional OAE probe microphone (ER-10B+) and a 1” low-noise instrumentation microphone (G.R.A.S. 40HF) measured in an anechoic chamber. SPL is a 10s average. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Principle of the focusing effect of an elliptical shape. A ray originating from one focal point (f1) reflects to the other focal point (f2) independent of the part on the elliptical shape where it reflects. . . . 3.5 Principle of the focusing effect of a truncated elliptical shape. If a ray originating from the other focus (f1) collides surface of this truncated elliptical shape it reflects to the other focus (f2). . . . . . . . . . . . . 3.6 Ellipse dimensions and ray travel distance. . . . . . . . . . . . . . . . 3.3 4.1 4.2 4.3 4.4 4.5 5.1 5.2 5.3 5.4 General idea of using a truncated ellipsoidal reflector in recording of otoacoustic emissions. The ear canal and the microphone capsule are at the focal points of the reflector. The distance between the foci is 404 mm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Different reflector assemblies with test subject and microphone positioning: (a) half reflector, (b) two-sided reflector, (c) two-sided reflector with detachable end. . . . . . . . . . . . . . . . . . . . . . . Manufactured reflector parts: (a) all parts separately, (b) two halves together and the detachable end alongside, (c) detachable end with the microphone mounted in place . . . . . . . . . . . . . . . . . . . . Picture of the setup used to measure the responses of the reflector. The mobile device speaker element mounted in an enclosure made out of a plastic tube, play dough and foam is on the left side of the reflector. The measurement microphone is on the right side. . . . . . The setup from preliminary measurements shows the equipment used in the final measurements with the difference of speaker model and placement. An artificial head is placed under the reflector to demonstrate the position of the test subject. The measurement microphone is inserted inside the reflector from the other end. The reflector is mounted to a three-legged stand featuring vibration isolating foam. . Magnitude responses of the gain measurements between the two foci. Gain responses of the different reflector setups plotted as a difference to the free field response at the same distance. . . . . . . . . . . . . . Energy decay curves for different reflector setups. . . . . . . . . . . . The effect of offsetting the sound source away from the axis going through the focal points of the reflector. 1/12 octave smoothing was employed in the curves. . . . . . . . . . . . . . . . . . . . . . . . . . . The effect of offsetting the sound source along the axis going through the focal points. 1/12 octave smoothing was employed in the curves. . 15 16 17 17 19 20 21 22 23 27 28 29 29 x 5.5 5.6 5.7 5.8 5.9 5.10 Sound spectra measured with the reflector with two loudspeakers producing sinusoids at 3000 Hz and 3660 Hz and the subject lying under the reflector with his ear canal approximately at the focal point. Left: response with sealed ear canal. Right: response with open ear canal, where most prominent otoacoustic emissions are marked with red circles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sound spectra measured with reflector from all the subjects at f2 = 2745 and f2 /f1 = 1.22. The stimulus level was approximately L1 = L2 = 65 dB. On the left the measurement with a sealed ear and on the right the measurement with an open ear. (a) test subject S, (b) test subject A, (c) test subject A with a different positioning of the stimulus producing speakers. . . . . . . . . . . . . . . . . . . . . . . . Sound spectra measured with reflector from all the subjects at f2 = 3660 and f2 /f1 = 1.22. The stimulus level was approximately L1 = L2 = 65 dB. On the left the measurement with sealed ear and on the right the measurement with open ear. (a) test subject S, (b) test subject A, (c) test subject A with a different positioning of the stimulus producing speakers. . . . . . . . . . . . . . . . . . . . . . . . Measured DPOAE levels from all the test subjects. Level of 2f1 – f2 distortion component, f2 /f1 = 1.22. Orange line: DPOAE levels, dark line: linear growth reference, grey area height: noise levels. Red circles denote that the emission level satisfies the 12 dB SNR criterion (sect. 4.2.3). (a) test subject S, (b) test subject A, (c) test subject A with a different positioning of the stimulus producing speakers. . . . . . . . . Comparison of speaker intermodulation distortion with two different speaker positionings without a test subject. Left figure: speakers positioned next to each other on the same stand. Right figure: speakers positioned on a 2 m distance from each other. Red circles denote second order intermodulation distortion componets, blue circles third order components, a yellow circle a one fourth order component and pink circles fifth order components. . . . . . . . . . . . . . . . . . . . Comparison of speaker intermodulation distortion with two different speaker positionings without a test subject. Red circles denote second order intermodulation distortion components. . . . . . . . . . . . . . 31 32 34 36 37 38 Chapter 1 Introduction Otoacoustic emissions (OAEs) are faint sounds emanating from the ear. The level of these sounds is usually too low to be heard, but they can be measured with a sensitive microphone and techniques designed for the purpose. OAEs can be evoked with a sound stimulus but they can be present also without a stimulus. They are an objective measure to assess the function of the inner ear (23). The most familiar test of hearing is pure-tone audiometry, where different-level pure-tone sounds are reproduced to the test subject’s ears and the subject responds by pressing a button when he hears the sound. Hearing thresholds are determined at different frequencies according to the minimum levels of the pure-tones that the test subject is able to detect. In a way this test offers the most accurate information about a persons’ ability to hear since it involves the whole process of hearing. In addition to behavioural audiometric tests, objective physiological examinations such as measurement of OAEs have been developed. OAEs have various applications. OAE tests can be used to evaluate the functionality of the test subject’s inner ear when he cannot be expected to respond in an anticipated manner. As an example of this, OAE testing is widely used in newborn hearing screenings. OAEs are useful in audiological diagnostics along with other physiological measures on monitoring different stages of the auditory pathway (2). As information of the function of the inner ear is retrieved with this measure, it is also of interest in science. OAEs are commonly measured with a probe fitted to the ear canal. This probe features a microphone and possibly loudspeakers for reproducing a stimulus sound. Clearly the microphone has to be in miniature-size to fit in a such probe. Small microphones have higher inherent noise compared to microphones larger in diameter. This inherent noise limits the lower end of the microphone’s dynamic range. When recording low level sounds such as OAEs, low inherent noise would be a truly desireable quality for a microphone to achieve a good signal-to-noise ratio (SNR). The motivation of this study is to investigate if a higher SNR compared to the common method can be obtained employing a sensitive low-noise one-inch microphone at a considerable distance from the ear canal opening and an acoustic reflector to collect and focus the sound emerging from the ear canal. 1 An ellipsoidal shape for the acoustic reflector has the correct properties for this study. To our knowledge a such reflector has not been used for recording OAEs. If the intented technique is succesful it could offer better quality recordings compared to the common method, e.g., higher SNR, and it could provide new information about OAEs since the ear canal is not sealed with the microphone probe. The objectives of this thesis is to answer the following questions: • Is recording of OAEs possible by using an acoustic ellipsoidal reflector? • What is the gain achieved with the reflector to be designed i.e. what is the frequency response? What about temporal characteristics? • What is the overall SNR achievable with this system including the lower noise floor of the measurement microphone compared to a common OAE probe? Comparing the levels of OAEs measured with the reflector setup and conventional methods requires quantifying the relation between sound pressure level at the entrance of the ear canal to the level which is measured with the reflector setup farther away. Conventional OAE measurement probes are fitted to the ear canal and thus also pick up body noises such as breathing or swallowing, which increase the background noise of the recording. The reflector measurement system can be mounted without touching the test subject’s body at all, and therefore benefit from lower body noise. Due to the noise originating from the human body (accompanied by the inherent noise from the miniature microphone) the low frequency limit of OAE recordings is around 500 Hz with the conventional methods. An interesting question is if the frequency range of OAE recordings could be extended towards low frequencies with the reflector measurement setup. OAEs have already been recorded without sealing the ear canal with a miniature microphone that is not housed in a probe that seals the ear canal (4). Sealing the ear canal alters it’s acoustic load and thus OAEs recorded from an open ear reflect more natural characteristics of the emission. The reflector technique would also have this advantage of an open ear. The structure of this work is described as follows. Chapter 2 focuses on OAEs: the mechanisms behind them is discussed briefly as well as methods how to measure them. In chapter 3 the relevant attributes of key measurement tools in this study, condenser microphones and ellipsoidal reflectors are explained in general terms. Chapter 4 presents the methodology used to answer the research questions and the organization of the experiments. Details of the reflector designed for the study can be found in this chapter. Results of the study are presented and discussed in chapter 5 and finally the thesis is concluded in chapter 6. Chapter 2 Otoacoustic emissions 2.1 Cochlear amplifier and otoacoustic emissions Ossicles Figure 2.1: Structure of the ear. The outer ear is the part external to the tympanic membrane featuring the pinna and the external auditory canal. The middle ear is the portion internal to the tympanic membrane and external to the cochlea. The inner ear is comprised of the cochlea and the semicircular canals of the vestibular system. Picture adapted from (46). The structure of the ear is shown in figure 2.1. The sound entering the ear canal causes the tympanic membrane (eardrum) to vibrate. Three small bones known as the ossicles transfer the energy from the eardrum to the cochlea in the inner ear. This results in pressure variations in the fluids of the cochlea that displace the flexible cochlear partition composed of basilar membrane, organ of Corti and 3 CHAPTER 2. OTOACOUSTIC EMISSIONS 4 tectorial membrane. The displacement takes the form of a wave pattern, that moves from the base of the cochlea to the apex (7). A.k.a. the traveling wave. This means that the flexible cochlear partition carries a transversal wave that is similar to the waves formed in water. Figure 2.2 illustrates the traveling wave motion along the basilar membrane. The amplitude increases as the traveling wave moves from the base of the cochlea towards the apex and at a certain point the amplitude reaches a maximum followed by a rapid decrease. The location of the maximum amplitude peak along the cochlear partition is frequency dependent. High frequencies peak close to the base of the cochlea and as the frequency is decreased the peaking occurs closer to the apex in systematic order depending on the frequency. A frequency corresponding to one certain place along the cochlear partition is referred to as the charasteristic frequency (CF). (34) Base Apex Figure 2.2: Illustration of the traveling wave motion along the basilar membrane. The spiral appearance of the cochlea has been stretched out and the displacement of the basilar membrane is greatly exaggerated. Figure adapted from (49). A schematic slice through the cochlea is shown in figure 2.3 displaying the flexible cochlear partition in the center. The organ of Corti runs along the entire length of the basilar membrane and contains inner hair cells (IHCs) and outer hair cells (OHCs) (42). In the human cochlea IHCs form a single row of cells while OHCs are arranged in three to five rows (31). The IHCs convert the vibration at a certain place along the basilar membrane into a neural code in auditory nerve (31). The OHCs seem to involve in an active process in hearing. The basic features of this CHAPTER 2. OTOACOUSTIC EMISSIONS 5 Figure 2.3: Cross section of the cochlea featuring a schematic view of the organ of Corti. Figure adopted from (42). phenomenon are provided next in this chapter. Active biologic mechanisms have been measured in mammalian ears. Basilar membrane responses measured at the charasteristic place (peak of the traveling wave) grow linearly at low stimulus levels but demonstrate compressive growth at high stimulus levels in a healthy living cochlea. Death of the cochlea abolishes this nonlinear behaviour (39). A healthy living cochlea also exhibits nonlinear behaviour in the form of refined tuning at the CF (36). The influence of active mechanisms to cochlear function is shown also in other measurements (33, 37). These active biological mechanisms are often referred to as the ”active process” or more specifically as the ”cochlear amplifier”. The cochlear amplifier is believed to be responsible of the nonlinear behaviour and the exceptional sensitivity and frequency selectivity in a healthy cochlea in comparison to a damaged or a dead cochlea (17, 8, 9). Current evidence suggests that OHCs contribute to this amplification process. The consequence to the loss of OHCs appers to be loss of the cochlear amplifier (16, 35). Otoacoustic emissions (OAEs) are intimately associated to the cochlear amplifier (23). Somehow related to this amplification process hearing produces these faint but measurable sounds which are utilized in clinical applications and research. OAEs are vulnerable to acoustic trauma, hypoxia and ototoxic medications, all of which cause hearing loss and damage to OHCs (35). There is a link between OHCs and the production of OAEs similarly as OHCs are linked to the cochlear amplifier. However, OAEs have been measured also from species without OHCs (29). Therefore OHCs are unlikely to be the sole producer of OAEs. For a long time all types of OAEs were thought to be caused by the same mechanism. More recent knowledge suggests that mammalian OAEs arise from two fundamentally CHAPTER 2. OTOACOUSTIC EMISSIONS 6 different mechanisms: nonlinear distortion and linear coherent reflection (43). Emissions from nonlinear distortion are attributed directly to the action of OHCs, and the source of this type of emissions is believed to follow the traveling wave envelope of the stimulus that is used to evoke the emission. That means that the actual physical place where the emission originates in the cochlea is dependent on the frequency of the stimulus. The linear coherent reflection emissions or ”place-fixed” emissions are proposed to originate from impedance perturbations in the mechanics of the cochlea or impedance mismatches present at or near the largest displacement of the traveling wave. These impedance perturbations could be for example variations in hair cell number and geometry (20). Slightly changing the frequency of the stimulus does not change the physical source of the emission – hence the name ”place-fixed”. Changing the frequency of the stimulus alters the peaking location of the traveling wave, but as the source point of the emission stays fixed this results in phase changes due to the altered traveling time along the basilar membrane. OAEs measured in the ear canal are likely a combination of energy from the two mechanisms. The association between different types of OAEs and the two generation mechanisms will be explained later in this chapter in the sections dedicated for different emission types. (43, 35) 2.2 Measurement of otoacoustic emissions A usual measurement setup for detecting otoacoustic emissios (OAEs) consists of a minituare microphone housed in a small probe that fits the ear canal. The probe is coupled to the ear by a foam or rubber tip. The probe can also contain one or two speakers for providing sound stimuli. The microphone signal is amplified and sampled via an AD-converter. The output is then appropriately analyzed depending on the type of OAE that is being measured. (35) OAEs have been traditionally divided to different types based on the type of stimulus used or lack of it (35). Spontaneous otoacoustic emissions (SOAEs) are recorded without any external stimulus. Evoked otoacoustic emissions (EOAEs) are measured during or after presenting an acoustic stimulus to the ear. EOAEs are further categorized by the type of stimulus used. Transient evoked otoacoustic emissions (TEOAEs) are elicited by using transient stimuli such as clicks. Distortion product otoacoustic emissions (DPOAEs) are present when a stimulus containing two pure tones is reproduced to the ear. Stimulus-frequency otoacoustic emissions (SFOAEs) are elicited by one pure tone. Synchronized spontaneous otoacoustic emissions (SSOAEs) use a transient stimulus as an averaging point, but the emission itself is assumed to be spontaneous (38). CHAPTER 2. OTOACOUSTIC EMISSIONS 7 Figure 2.4: Example response of a clinical TEOAE measurement. On the left waveform of two time-locked averages are shown. There is a few millisecond delay preceding the response. The right side of the figure shows the levels of the emission divided into frequency bands (blue) and noise levels in these given bands (red). 2.2.1 Transient-evoked otoacoustic emissions Transient-evoked otoacoustic emissions (TEOAEs) are evoked by transient or brief stimulus such as a click or a tone burst (35). A emission response is present after a short time delay after stimulus onset. A single response is very low in level and hence multiple measurements are required to accomplish a sufficient signal-to-noise ratio (SNR) by using time-synchronous averaging. Commonly responses resulting from about 200 stimulus repetitions need to be averaged to get a valid TEOAE response (19). A TEOAE from a healthy ear has a broad spectrum resulting from the transient stimulus used to evoke the emission. Thus a TEOAE has a broad spectrum similarly as transient signals do. The level of the stimulus in most clinical applications is around 80 dB peak equivalent SPL or about 45 dB greater than perceptual threshold. (38) Figure 2.4 shows an example of the outcome from a clinical TEOAE measurement. The left side of the figure shows two overlapping waveforms (green and blue) of the averaged response. The two waveforms are from two distinct averaging buffers to which multiple click responses are divided for extracting a single TEOAE response. Similarity of the two averaged buffers (as in the figure) communicates reliability of the measurement. It can be seen that the response starts around 3 ms after the stimulus onset. The first few milliseconds are usually eliminated from the final averaged waveform because energy from the stimulus may persist in the ear canal long enoungh to obscure the onset of the TEOAE response (35). On the right side of the figure 2.4 are shown the TEOAE response levels divided to half octave frequency bands. CHAPTER 2. OTOACOUSTIC EMISSIONS 8 In general, TEOAE levels for humans are largest around 1–1.5 kHz, and decrease at lower and higher frequencies. TEOAE levels are typically very small or immeasurable below 0.5 kHz and above approximately 4 kHz particularly in adult ears. Middle-ear effects and contamination by subject-generated noises such as breathing and body movements are a likely cause for the decline below 1 kHz. The decline at high frequencies likely reflects many factors: middle ear, frequency dependend latency of the response and restrictions in the measurement equipment (e.g. the frequency response of the loudspeakers built into measurement probes). (28) Different frequency components of a TEOAE response have a different latency between ∼3 and 20 ms and on average the latency decreases with increasing frequency (21). As usually the first few milliseconds are eliminated from a TEOAE response in order to remove the stimulus artifact, some information at high frequencies is lost. TEOAEs are present basically in every normally hearing subject (19). TEOAEs are used in newborn hearing screening, which is a procedure for deciding whether further examinations are advised. When it comes to the two different OAE generation mechanisms in the cochlea, low-level TEOAEs arise predominantly through the linear coherent reflection mechanism (43). 2.2.2 Distortion product otoacoustic emissions Distortion product otoacoustic emissions (DPOAEs) arise by stimulation with two continuous pure tones simultaneously. DPOAEs are attractive as clinical tools because the emission frequencies can be predicted exactly by the stimulus frequencies (38). The two pure tones of the stimulus are designated by f1 (lower frequency) and f2 (higher frequency) and the emissions are elicited at frequencies which correspond to two-tone intermodulation distortion components (35). This means that the emissions are at different frequencies compared to the stimulus which allows to reproduce the stimulus and to record the emission at the same time. On average the distortion component 2f1 – f2 yields the highest amplitude in humans and other mammalian ears (13). Therefore 2f1 – f2 is the component that is most extensively investigated and well-suited for clinical purposes (15). The two tones f1 and f2 also known as primaries, interact in the cochlea close to the characteristic place of f2 (19). Thus, it has been suggested that DPOAEs could be used as a frequency-specific assessment of cochlear dysfunction at the f2 place. DPOAE responses are influenced by stimulus amplitude, relative amplitude between the two primaries and frequency ratio between them (38). That is, the primaries need to be reasonably close in frequency and at a reasonable level to elicit a notable response – in addition these variables can be optimized to yield high response levels (13). When considering the two emission generation mechanisms in the cochlea (sec 2.1) DPOAEs are a mixture of both, which have also succesfully been unmixed to separate components (20). The two stimulus frequencies are usually reproduced by separate transducers to avoid CHAPTER 2. OTOACOUSTIC EMISSIONS f1 3f1–2f2 9 f2 2f1–f2 2f2–f1 4f1–3f2 Figure 2.5: A spectrum of an DPOAE measurement from the ear of a 16-month-old child. Stimulus tones (f1 = 1.807 kHz; f2 = 2.002 kHz; L1 = 65 dB SPL and L2 = 60 dB SPL) and resulting DPOAEs are labeled to the figure. Several DPOAEs occur at frequencies below the primaries (4f1 – 3f2 = 1.222 kHz; 3f1 – 2f2 = 1.417 kHz and 2f1 – f2 = 1.612 kHz) and one above the primaries ( 2f2 – f1 = 2.197 kHz). Figure adapted from (35) [frequency component labels are redrawn due to low-quality copy of the figure]. generating distortion and especially intermodulation distortion in the measurement system itself. Intermodulation components would be at the same frequencies with the DPOAE components. Time-synchronous averaging is used to reduce noise in the measurement result. Figure 2.5 shows an FFT analysis of a DPOAE response where both, stimulus and emission frequency components can be seen. The two stimulus components clearly have the highest amplitude while there are four DPOAE components at lower levels. Detecting OHC impairment requires DPOAEs to be measured with stimulus levels as close as possible to the hearing threshold where OHC function is maximal (25). In practice this means DPOAE measurements to be done at various stimulus levels from low to moderate or high, the range being about 20–65 dB. DPOAE level input/output functions plot the levels of a DPOAE component measured at a fixed CHAPTER 2. OTOACOUSTIC EMISSIONS 10 stimulus frequency f2 as a function of stimulus level L2 . In an ear without hearing impairment the slopes of DPOAE I/O functions are steep at low stimulus levels while at higher stimulus levels the slopes decrease (19). This mirrors the high amplification of the cochlear amplifier at low stimulus levels and decreasing amplification at higher stimulus levels. This kind of a compressive behaviour is observed when using an appropriate stimulus setting where the level difference L1 – L2 increases with decreasing stimulus level (25). A similar stimulus setting has been suggested later (26). 2.2.3 Stimulus-frequency otoacoustic emissions A stimulus-frequency otoacoustic emission (SFOAE) is a constant low-level pure-tone that occurs at the same frequency and at the same time as the applied constant pure-tone stimulus. Because the stimulus and the emission superimpose in time and frequency, specialized measurement techniques must be used to extract the SFOAE from the evoking stimulus. For example using a second stimulus slightly higher or lower in frequency can be used to suppress the SFOAE (35). Then a measurement can be made with and without the presence of the suppressor tone. The difference of the two cases can be attributed to the SFOAE, or depending on the specifics of the second tone, the portion that remains unsuppressed by the second tone. Other measurement methods have been used that produce similar results (35, 5, 40, 41). Out of the two OAE generation mechanisms in the cochlea, like TEOAEs low-level SFOAEs arise predominantly through linear coherent reflection (43). The properties of SFOAEs are also similar to those of TEOAEs. SFOAEs are present in nearly all normally hearing human subjects, are strong at same frequencies where TEOAEs are and demonstrate latencies and compressive nonlinearity similar to those of TEOAEs (24). Charasteristics of SFOAEs in ears with normal hearing and ears with hearing loss have not been extensively studied and therefore SFOAEs are not used clinically (35). A laboratory experiment in 85 ears showed that SFOAEs identified hearing loss at 0.5 kHz better than TEOAEs and DPOAEs while performing similarly to other evoked emissions at higher frequencies except a slight performance reduction at 4 kHz (11). 2.2.4 Spontaneous otoacoustic emissions Spontaneus otoacoustic emissions (SOAEs) are narrow-band, often almost tonal, acoustic signals present in the ear canal withouth the presence of an external stimulus (28). They are not present in all the human ears with normal hearing. Estimates of the prevalence of SOAEs go from 40% (3) to about 70% (45, 3). They are more commonly found in females compared to males and more common in the right ear compared to the left ear (3). SOAEs are presumed to be caused by the linear reflection mechanism (sec 2.1) via CHAPTER 2. OTOACOUSTIC EMISSIONS 11 multiple reflections within the cochlea (43). SOAEs have been hypotesized to result from feedback to the generator of linear reflection emissions: the output of this generator is reflected from stapes (a bone at the ’input’ of the cochlea) back to the input of the generator (22). In (28) this process is described followingly: ”At frequencies where this feedback is positive, self-sustaining oscillation results if the loop gain is sufficient. The resulting self-sustaining oscillation is observed in the ear canal as an SOAE. Thus, SOAEs can be thought of as continuously self-eliciting evoked OAEs.”. While no stimulus is used to record SOAEs there exists a variant, synchronized spontaneous otoacoustic emission (SSOAE). They are recorded using a transient stimulus as an averaging point with the goal to reduce background noise. Timelocked averaging is done over a ∼80 ms window after a click stimulus. As TEOAEs persist roughly 20 ms after the click stimulus and are thus present at the start of the averaging window, spectral analysis is done on the latter part of the ∼80 ms window to reveal possible spontaneous emission(s). (14) Chapter 3 Microphones and reflectors 3.1 Inherent noise in condenser microphone systems The lower end of the dynamic range of a microphone system is limited by it’s noise floor. In an electrical system such as a microphone system, noise is generated as a result of physics of the devices and materials that make up the system (30). Thermal noise and low-frequency 1/f noise are examples of noise types present in electronic devices. In microphone systems parameters such as the size of the microphone’s diaphragm have an effect on the noise floor. Condenser microphones are used in acoustical instrumentation because of their stability, flat frequency response, wide dynamic range, reasonable sensitivity and remarkably low self-noise (27). These factors make condenser microphones a suitable tool also for measuring OAEs. A condenser which is more commonly in electronics know as a capacitor, is an electronic component which is used to store energy electrostatically in an electric field. This electric field is usually created between two electrodes such as metal plates insulated from each other so that electrons are not able to move directly between them. The ability to store electrical energy, a charge, is called capacitance. A condenser microphone is a capacitor where the microphone diaphragm is one of the electrodes and a backplate is provided to function as the other. These electrodes are insulated from each other by air. The movements of the diaphragm cause changes in the capacitance which converts to voltage fluctuations (27). This allows transducing acoustic signals to electric ones. The capacitance of a capacitor is proportional to the surface area of the electrodes. Therefore among condenser microphones a microphone with large diaphragm has larger capacitance compared to a microphone with a small diaphragm. Capacitance of condenser measurement microphones varies from about 3 pF for 1/8” microphones to 70 pF for 1” microphones (6). A condenser microphone is used together at least with a preamplifier and the 12 CHAPTER 3. MICROPHONES AND REFLECTORS 13 5 dB re. 20 µPa 0 a) Complete system –5 – 10 b) Preamplifier +Measurement Amplifier – 15 – 20 c) Measurement Amplifier – 25 – 30 8 16 31.5 63 125 250 500 1k 2k 4k 31.5 k 8k 16 k Frequency (Hz) Figure 3.1: Equivalent levels of inherent noise (1/3 octave bandwidth) produced by different elements of a microphone system equipped with a 1” free-field microphone (sensitivity 50mV/Pa). The microphone, the preamplifier and the measurement amplifier all contribute significantly to the noise floor of the system in certain parts of the frequency range. Figure adopted from (6). noise produced by the amplifier adds it’s part to the noise floor. Commonly also a measurement amplifier is used which also increases the noise floor. Figure 3.1 illustrates the noise produced by different components in a microphone system. The noise voltages of the preamplifier and the measurement amplifier are converted to equivalent SPL by dividing the noise voltage by the sensitivity of the microphone. Preamlifier produces most noise at low frequecies while the measurement amplifier noise is strongest at high frequencies. Both are the same order of magnitude. The noise produced by the microphone itself is dominating from about 200 Hz to 10 kHz. (6) The noise generated by the preamplifier is a significant noise source. It may be regarded that the preamplifier noise is composed of two main parts. One source is a CHAPTER 3. MICROPHONES AND REFLECTORS 14 40 dB re. 20 µPa 35 30 1/4" (4 mV/Pa) 25 20 15 1/2" (12.5 mV/Pa) 10 5 1/2" (50 mV/Pa) 0 1/1" (50 mV/Pa) –5 – 10 8 16 31.5 63 125 250 500 1k 2k 4k 8k 16 k 31.5 k 63 k Frequency (Hz) Figure 3.2: Comparison of 1/3 octave noise spectra of microphone systems equipped with different size microphones. Figure adopted from (6). low frequency noise originating from the input circuit of the preamplifier. The voltage of this noise is inversely proportional to both the frequency and the capacitance of the microphone. The other noise source is related to the amplifier and has a flat voltage spectrum in the entire operational range. (6) Because the low frequency noise generated in the preamplifier is a function of microphone capacitance, the bigger the microphone diaphragm the lower the noise that is generated in the preamplifier. It can be noted that increasing capacitance leads to higher sensitivity, and thus, the aforementioned relation can also be thought in terms of sensitivity. For small microphones in sizes such as 1/4” and 1/8”, only the preamplifier noise needs to be taken into account because it dominates over the microphone noise. In large and most sensitive microphones the microphone noise is most significant. A comparison of noise spectra for microphone systems equipped with different size microphones is shown in figure 3.2. (6) CHAPTER 3. MICROPHONES AND REFLECTORS 15 The miniature microphones used in common OAE measurement probes can be electret consenser microphones, which unlike normal air condenser microphones, do not require a polarizing voltage supplied from the measurement amplifier. Instead, a permanently charged material provides the electric field between the diaphragm and the backplate. The material can be used in the backplate or in the diaphragm (10). In a study comparing different microphone types it was concluded that the electret microphone type is slightly quieter at low frequencies than a comparable air condenser microphone due to absence of a polarization voltage resistor (48). In this work it was assumed that a remarkably bigger sized air condenser microphone outperforms a miniature electret condenser microphone in terms of lower inherent noise. Our experimentation with the noise levels between a 1” low-noise air condenser microphone and one conventional OAE measurement probe microphone support this claim. A plot of the noise levels from this experimentation made in an anechoic chamber is shown in figure 3.3. The 1” air condenser has clearly lower noise at most frequencies. The background noise of the anechoic chamber is included in the plots and the inherent noise of the microphones can be lower at some frequecies. Nevertheless the measurement conditions were similar for the both microphones. 25 ER-10B+ G.R.A.S. 40HF 20 15 SPL, avg (dB) 10 5 0 -5 -10 -15 -20 10 2 10 3 10 4 Frequency (Hz) Figure 3.3: Inherent noise comparison between a conventional OAE probe microphone (ER-10B+) and a 1” low-noise instrumentation microphone (G.R.A.S. 40HF) measured in an anechoic chamber. SPL is a 10s average. CHAPTER 3. MICROPHONES AND REFLECTORS 3.2 16 Ellipsoidal reflectors Ellipsoidal reflectors can be used to collect and focus acoustical waves in a similar manner as a reflector of a spotlight focuses light. Ellipsoidal reflectors have been used in acoustics for example in medical applications to focus pressure pulses to break up, e.g., kidney stones (1, 47) and underwater acoustics (32). f1 f2 Figure 3.4: Principle of the focusing effect of an elliptical shape. A ray originating from one focal point (f1) reflects to the other focal point (f2) independent of the part on the elliptical shape where it reflects. The principle of the reflecting properties of an elliptical shape can be seen in figure 3.4. A ray originating from the other focus (f1 in picture) refclects to the other focus (f2) independent of the part on the elliptical shape it collides and reflects from. An elliptical object can naturally be truncated. Then all the rays originating from one focus reflect to the other focus if they collide to the objects surface as seen in figure 3.5. Basically it is possible to truncate the elliptical shape where desired and the remaining surface has the reflection properties of an elliptical shape. In practical applications reflectors are naturally three-dimensional and most commonly axially symmetric. Rotating the truncated ellipse in figure 3.5 around the axis going through the two focal points would result in an axially symmetric ellipsoidal reflector. In figure 3.6 an ellipse and it’s two axes are presented. The distance f from the center of the ellipse to one of the focal points can be calculated by √ f = a2 − b 2 , where a is the length of the semi-major axis and b the length of the semi-minor axis. The distance a reflected ray originating from focus travels to the other focus is constant. That is, in an ellipsoidal reflector every reflected ray from one focal point to the other travel the same distance between the foci. Travel distance of a ray is equal to the length of the major axis, i.e., semi-major axis times two n + v = 2a. This also means that rays with equal propagation velocity arrive to the other focus at the same time. This is a good quality for a reflector because there will be no CHAPTER 3. MICROPHONES AND REFLECTORS 17 f1 f2 Figure 3.5: Principle of the focusing effect of a truncated elliptical shape. If a ray originating from the other focus (f1) collides surface of this truncated elliptical shape it reflects to the other focus (f2). b f1 a f2 f n v Figure 3.6: Ellipse dimensions and ray travel distance. cancellation of frequencies due to phase differences of the arriving waves. Therefore there is no coloration in this sense. Only the small part of the waves that go directly from one focus to another without reflecting come there at a different time. How much sooner the direct part of waves arrive depends on the reflector’s dimensions. Chapter 4 Methodology and experiments 4.1 4.1.1 Design of a reflector used to measure otoacoustic emissions Design of the reflector A reflector was to be designed to collect and focus sound coming from the ear for measuring otoacoustic emissions. Sound emanating from the ear canal will naturally spread to all directions available limited by the head and the pinna. A reflector should gather as much of this sound as possible. Frequency response of the reflector should be flat and the impulse response as short as is conceivable. In an ellipsoidal reflector all reflecting waves originating from one focal point arrive to the other focal point at the same time if they have the same propagation velocity (see sect. 3.2). This is a good feature for a reflector since there are no phase differences among the reflected waves to cause coloration. Only the small amount of direct sound arriving before the reflected sound can cause coloration. An ellipsoidal reflector was decided to be made for measuring otoacoustic emissions. The ear canal opening would be placed to the other focus and microphone to the other. The reflector would have the shape of a prolate ellipsoid. A prolate ellipsoid is an ellipsoid of revolution respect to the axis running trough the focal points, i.e. an axially symmetric ellipsoid. An ellipsoidal reflector can be truncated to fit specific needs. Naturally this results in less reflecting surface. If a minimally truncated ellipsoidal reflector is used the sound will at least partly be reflected back to the ear canal and then back to the microphone due to reflections from the subject and diffraction from the edge of the reflector, as shown with dashed line in figure 4.1. This would result in longer impulse response and create modes to the frequency response. If the reflector is truncated more, less reflections going back and forth will result and therefore shorter impulse response and milder modes in the frequency response can be obtained. To test the effect of truncating the reflector in practice a modular structure was 18 CHAPTER 4. METHODOLOGY AND EXPERIMENTS 19 ellipsoidal reflector 220 mm ear canal microphone 460 mm Figure 4.1: General idea of using a truncated ellipsoidal reflector in recording of otoacoustic emissions. The ear canal and the microphone capsule are at the focal points of the reflector. The distance between the foci is 404 mm. designed. The reflector was designed to break into three parts: two halves and a detachable end. The two halves are boat-like shapes that would result from cutting a prolate reflector in half lengthwise along the axis of symmetry. The detachable end was a end cap to the microphone-side of the reflector to test how much the area behind the microphone causes reflections. This kind of reflector functions also as a resonator due to the sound reflecting back and forth as was shown in figure 4.1. The amount of these reflections can be decreased by using the reflector without the detachable end thus reducing the resonance. Three reasonable reflector setups could be put together from the three parts. The three setups are shown with microphone and test subject placement in figure 4.2. The setup (a) was used to determine the gain achieved with the most truncated half reflector, which would not reflect the sound back to the ear canal. The two other setups were used to measure the gain with less truncation and to test the amount of reflections that result from the detachable end of setup (c). Many manufacturing techniques and materials were considered when designing a reflector. These included vacuum forming of plastic, using a computer-controlled cutting machine (CNC router) and 3D printing. Vacuum forming would have been most likely done over a CNC router cut mold. At first a fairly small reflector was decided to be made and possibly try with a different size later. Also the reflector was decided to be small enough to fit above a test subjects shoulder while one ear would be at the focal point of the reflector. Reflector’s inner surface length was decided to be 460 mm and width 220 mm as seen in figure √ 4.1. Distance between the two foci now resulted to be approximately 404 mm (= 4602 − 2202 ). The most reasonable manufacturing technique for this size model was to directly cut the ellipsoid form to a material by a CNC router. 400 kg/m3 density polyurethane foam model board was decided as the material which was believed to be hard and dense enough for the purpose. It is also sufficiently stable material to avoid any deformations. The surface of the milled parts were smoothed by sanding them. The reflection coefficient of the surface was further increased by painting it with a glossy black paint containing a hardening agent (the result is similar to a car paint surface). The paint also gave a smooth and uniform coating to the porous material. The CHAPTER 4. METHODOLOGY AND EXPERIMENTS (a) 20 (b) (c) Figure 4.2: Different reflector assemblies with test subject and microphone positioning: (a) half reflector, (b) two-sided reflector, (c) two-sided reflector with detachable end. manufactured reflector parts are shown in figure 4.3. 4.1.2 Reflector gain measurements Measurements were made to study how much gain the manufactured reflector gives at different frequencies. A swept sine method (12) was used to measure the impulse and frequency response of the reflector. A small mobile device loudspeaker (diaphragm 9 x 14 mm) was placed to the other focus of the reflector and a 1/2” measurement microphone (G.R.A.S 40 HT system) was place to the other focus. A photograph of the measurement setup is seen in figure 4.4. Measurements were made in the big anechoic chamber of the Department of Signal Processing and Acoustics in Aalto University. Sweep measurements were done with Fuzzmeasure software. Impulse responses of three different reflector assemblies were measured. The three assemblies were as in figure 4.2: a half reflector, a two-sided reflector and the twosided reflector with a detachable end. The measurement was done with both the sweep reproducing speaker and the microphone at focal points for all the three reflector assemblies. In addition the effect of displacing the speaker from the focus was measured for all the reflector assemblies. The displacement was in the range CHAPTER 4. METHODOLOGY AND EXPERIMENTS 21 (a) (b) (c) Figure 4.3: Manufactured reflector parts: (a) all parts separately, (b) two halves together and the detachable end alongside, (c) detachable end with the microphone mounted in place between 1 to 3 cm and was done in two directions: along the axis going through the focal points and perpendicularly to that axis. As a comparison a free field response was measured at the distance of the focal points (404 mm) and at 3 cm distance. A first order RC high-pass filter was employed in the speaker wiring to protect the speaker due to limitations of specifying the frequency range of the measurement sweep in the software. The cutoff frequency of the high-pass filter was approximately at 200 Hz and the roll-off was 6 dB per octave. CHAPTER 4. METHODOLOGY AND EXPERIMENTS 22 Figure 4.4: Picture of the setup used to measure the responses of the reflector. The mobile device speaker element mounted in an enclosure made out of a plastic tube, play dough and foam is on the left side of the reflector. The measurement microphone is on the right side. 4.2 4.2.1 Otoacoustic emission measurements Organization of the experiments The experiments were carried out by the author at the Department of Signal Processing and Acoustics in Aalto University School of Electrical Engineering. A measurement setup was built to an anechoic chamber to achieve a minimal background noise. The measurement was done in a such way that a test subject could lay down his other ear pointing upwards. The developed reflector was mounted above the ear and the ear canal opening was positioned to one of the reflector’s focal points. A low-noise large diaphragm microphone (G.R.A.S. 40HF system) was positioned to the reflector’s other focal point. Stimuli were reproduced using two active Genelec 8020B active speakers. The measurement equipment is shown in picture 4.5. The stand holding the reflector featured vibration isolating foam to mitigate noise originating from test subject’s body and movements from entering the microphone. A comparison measurement was made by sealing the ear canal of the test subject with an earplug and repeating the measurement. This was done to examine if the two tone distortion products originate from the ear or possibly from somewhere else in the measurement setup. Also an additional measurement was done with CHAPTER 4. METHODOLOGY AND EXPERIMENTS 23 Figure 4.5: The setup from preliminary measurements shows the equipment used in the final measurements with the difference of speaker model and placement. An artificial head is placed under the reflector to demonstrate the position of the test subject. The measurement microphone is inserted inside the reflector from the other end. The reflector is mounted to a three-legged stand featuring vibration isolating foam. an alternate stimulus producing speaker positioning related to measurement of distortion originating from the measurement system itself (see sec. 4.2.4). This was done with the author as a test subject to observe the possible changes in the results. The measurement procedure was identical to the principal measurement. The given measurement was done without confirming the stimuli levels at the destination position and thus the results are not directly comparable to the principal measurement. Two male test subjects aged 28–30 participated the measurement which includes the author. Letters A and S are used as identifiers for the author and the other test subject respectively. 4.2.2 Stimulus parameters Distortion product otoacoustic emissions (DPOAEs) were evoked by two continuous pure tones. f1 was the lower tone and f2 the higher. Selected frequencies were [f1 = 2250 Hz, f2 = 2745 Hz] and [f1 = 3000 Hz, f2 = 3660 Hz]. In this work a stimulus, CHAPTER 4. METHODOLOGY AND EXPERIMENTS 1. 2. 3. 4. 24 L1 [dB] L2 [dB] 65 65 60 52.5 55 40 50 27.5 Table 4.1: Levels of the primary tones f1 and f2 . a tone pair, is repeatedly referred to by naming only the frequency of the higher frequency f2 . The ratio of the frequencies f1 and f2 was 1.22 for the both stimuli. Both primary tones were reproduced at four different levels. Levels of the tones f1 and f2 are referred to as L1 and L2 respectively. Levels were calibrated by placing a G.R.A.S 46AF measurement microphone directly under the reflectors focal point and measuring the sound pressure level of the stimulus tone. Speaker gains were adjusted to meet the desired levels. The levels of the primary tones are shown in table 4.1. These levels follow a paradigm L1 = 0.4 L2 + 39 dB used in other experiments in the field (25, 18). Stimulus tones were generated using a LabView software controlled National Instruments hardware. Length of the stimulus was 65 seconds of which approximately a 60 second long window starting 2.5 s after stimulus onset was analyzed. 4.2.3 Data collection and analysis The microphone signal was fed from the microphone’s preamplifier to a National Instruments PXI-4461 card controlled by LabView software and recorded with a script programmed for the purpose. Further data analysis was done in Matlab software environment. Recordings were averaged in the time domain and then analyzed with an FFT algorithm. The same size window was used for both the averaging and the FFT analysis. The window size was selected so that an integer number of periods of the both tones in the stimulus fit exactly to the window. This allowed the use of rectangular window which enabled studying accurate levels and frequencies in the FFT analysis. At stimulus frequency [f1 = 2250 Hz, f2 = 2745 Hz] the length of the window was 6860 samples which is about 156 ms at the 44.1 kHz sample rate that was used. At the other stimulus frequency [f1 = 3000 Hz, f2 = 3660 Hz] the length of the window was 7350 samples which is about 166 ms. Approximately a 60 second part of the 65 second long stimulus was analyzed starting 2.5 s after stimulus onset. 386 windows were averaged in the time domain at the stimulus frequency f2 = 2745 Hz and 360 windows at the stimulus frequency f2 = 3660 Hz. A signal-to-noise ratio (SNR) criterion was decided for accepting only otoacoustic emissions at sufficiently high level above the background noise as valid emissions. CHAPTER 4. METHODOLOGY AND EXPERIMENTS 25 Noise level was defined as the mean value of the FFT bins surrounding the emission bin at a 1/12–octave band. That is, the frequency of the emission was the center frequency of the 1/12–octave band. Emission was accepted to be valid if the level of the frequency bin containing the emission was 12 dB above the noise level. 4.2.4 System distortion measurement Measurements conducted with exactly the same stimulus parameters as in the DPOAE measurements, but without a test subject, were done to study the amount of distortion in the measurement setup itself. Also analysis was done with the same parameters as in the DPOAE measurements and are described in section 4.2.3. In addition, the effect of alternate loudspeaker positioning to the amount of distortion was experimented. In the principal test setup the two stimulus producing loudspeakers were placed side by side on the same stand. In the alternate test setup one of the two loudspeakers was taken from the stand and placed 2 meters away from the other speaker on a block of foam, while keeping the distance from the reflector’s focus (’stimulus destination’) the same 2.5 m. Also the input gains of the stimuli were kept the same. Chapter 5 Results 5.1 5.1.1 Reflector gain measurement results Focus to focus gain Measured and unsmoothed magnitude responses of different reflector setups are presented in figure 5.1. All of the responses are divided piecewise with the free field response measured at the distance of the focal points of the reflector, i.e. the difference has been plotted to the graph. This subtracts the response of the loudspeaker (and the microphone) from the plot. Hence the free field response is a straight line at 0 dB. Also a free field response at 3 cm distance is added as a comparison. Full reflector with all parts has strong modes in the response at frequencies below 2 kHz, a strong notch with a depth of about 10 dB between 3.5 – 4.5 kHz and a rippled response at frequencies above 7 kHz. The amplification between 100 – 2500 Hz and above 4.5 kHz is 20 dB at minimun compared to free field at the same distance. The strong modes below 2 kHz peak at frequencies of 172 Hz, 612 Hz, 1016 Hz, 1397 Hz and 1778 Hz. The two-sided reflector comprised of two halves has slightly less pronounced modes than the full reflector and less amplification at frequencies below 2 kHz. The response shows less ripples in general. There are some notches around 2.5 kHz and 6.5 kHz. There is a double peak at [280 Hz, 360 Hz] and other mode peaks below 2 kHz are at 744 Hz and 1188 Hz. The single-sided (i.e. half) reflector has overall lower gain and less ripples in the magnitude response compared to the other reflector setups. The full reflector gives higher gain compared to the 3 cm distance free field response at nearly all frequencies above 5 kHz. The response of the two-sided reflector has also higher gain than the 3 cm free field response at 5 kHz, but then has a notch between about 6–7 kHz. From 7.3 kHz and above the two-sided reflector has higher gain compared to the 3 cm free field response. The single-sided reflector response reaches the amplification of the 3 cm free field response from about 11 kHz on. 26 CHAPTER 5. RESULTS 27 70 Full Two-sided Single-sided Free field 3 cm Free field focal distance 60 Relative level [dB] 50 40 30 20 10 0 -10 128 256 512 1024 2048 4096 8192 16384 Frequency [Hz] Figure 5.1: Magnitude responses of the gain measurements between the two foci. Gain responses of the different reflector setups plotted as a difference to the free field response at the same distance. The energy decay curves measured for different reflector setups can be seen in figure 5.2. In the case of the full reflector the curve shows strong reflections spaced about 2.7 ms in time decaying only 6 dB between the first reflections. It takes 28.6 ms for a reflection decayed by 40 dB. The reflections decay faster for the two-sided reflector the first reflection being over 10 dB lower in level and decaying at slightly lower pace from there on. In the case of the single-sided reflector the decay of the reflections happen very fast within few milliseconds. The first reflection is already about 27 dB lower in level and the second 50 dB lower compared to the direct sound. 5.1.2 Effect of offset from the focus The effect of displacing the sound source perpendicularly away from the reflector’s axis of symmetry (line going through the focal points) was measured. The measurement microphone was always at the other focal point of the reflector. Figure 5.3 shows gain measurements responses from the full reflector with the loudspeaker at focus and with an sideways offset of 1, 2 and 3 cm (the diaphragm size of the loudspeaker CHAPTER 5. RESULTS 28 0 Full Two-sided Single-sided Energy [dB] -10 -20 -30 -40 -50 0 5 10 15 Time [ms] Figure 5.2: Energy decay curves for different reflector setups. is 9x14 mm). The curves are smoothed with 1/12 octave smoothing. The four different responses are identical from 128 Hz to 2 kHz. The curve of 3 cm offset starts to deviate from the others after 2 kHz, showing lower or similar gain compared to the others from there on. The given response has a remarkably stronger notch than other responses at 4 kHz and the gain decreases clearly from 10 kHz on with an eminent difference compared to the other responses. Response from the 2 cm offset shows similar behaviour to the 3 cm offset response, but with larger gain on most frequencies. The 1 cm offset response starts to deviate slightly from the focal response around 4 kHz and deviates more when reaching 10 kHz and frequencies above. An offset of the sound source along the line going through the reflector’s focal points was also measured. Responses of these measurements are shown in figure 5.4 with focal response, an offset of 2.5 cm from the focus towards the microphone and an offset of 3 cm to the other direction away from the microphone. At frequencies below 6.5 kHz the smallest distance between the sound source and the microphone results in greatest gain although being offset from the focus. From 6.5 kHz on the in focus response has the highest gain at most frequencies. The response measured with offset away from the microphone features a larger gain between 7–9 kHz than the other offset response, but has lower gain at all other frequencies compared to the other two responses. This response also has a deep notch at about 4 kHz. CHAPTER 5. RESULTS 29 70 Focal 1 cm off sideways 2 cm off sideways 3 cm off sideways Free field focal distance 60 Relative level [dB] 50 40 30 20 10 0 -10 128 256 512 1024 2048 4096 8192 16384 Frequency [Hz] Figure 5.3: The effect of offsetting the sound source away from the axis going through the focal points of the reflector. 1/12 octave smoothing was employed in the curves. 70 2,5 cm offset towards Focal 2 cm offset away Free field focal distance 60 Relative level [dB] 50 40 30 20 10 0 -10 128 256 512 1024 2048 4096 8192 16384 Frequency [Hz] Figure 5.4: The effect of offsetting the sound source along the axis going through the focal points. 1/12 octave smoothing was employed in the curves. CHAPTER 5. RESULTS 5.1.3 30 Discussion of the acoustical characteristics of the reflector The results show that the full reflector expectedly yields the highest amplification. The promiment reflections at about 2.7 ms intervals clearly limit the use of a such reflector in time-domain measurements such as measuring TEOAEs. The travel time of a sound wave from one focus to the other focus and back is 2.68 ms which matches the reflection interval. Therefore the reflecting likely occurs between the sound source and the microphone. To increase the interval between reflections the reflector’s dimensions should be made larger. If 10 ms time window between reflections would be desired, the reflector’s dimensions should be made four times larger. A 10 ms time window already could be applicable for TEOAE measurements. The two-sided reflector, which is the more truncated version, has faster decaying temporal response, but still strong enough first reflections to make it unlikely applicable in time-domain OAE measurements in this size. The response of the single-sided reflector decays fast and would be in this respect perfect for time-domain measurements, but unfortunately it has low gain at low frequecies, which limits it’s applicability. 1–4 kHz being the common band in TEOAE measurements, at 1 kHz the difference to the 3 cm free field response is about 12 dB and at 4 kHz 6 dB. The full and two-sided reflector setups could be used in measurements where the temporal response is not that critical such as DPOAE measurements, where two continuous sinusoids are used as a stimulus and the stimulus and OAE are present at the same time. The response of the full reflector has higher gain compared to the 3 cm distance free field response also at low frequencies due to resonance peaks, but above frequencies about 5 kHz the gain is nearly always higher. DPOAEs seem to be generally measured at frequencies up to little over 6 kHz, which means the frequency of the higher stimulus tone f2 . DPOAE data exists also from measurements at f2 = 8 kHz in a research application (44). The optimum frequency ratio f2 /f1 in DPOAE generation from humans being about 1.2 the 2f1 – f2 frequency component would be at frequency 3.84 kHz with f2 = 6 kHz and at 5.33 kHz with f2 = 8 kHz. At frequency 5.3 kHz the full reflector’s response has already about 3 dB higher gain compared to the free field response at 3 cm distance. Other DPOAE components at frequencies higher than the stimulus are evidently at the optimum frequencies to be recorded with this kind of reflector. For example the third order component 2f2 – f1 is above 5 kHz for stimulus frequecies f2 = 4.3 kHz and above. One could also utilize the gain provided by the strong resonance peaks at frequencies below 2 kHz when measuring OAEs of which frequencies are accurately predicted such as DPOAEs or SFOAEs. It has to be noted that in the given reflector design placing the test subjects ear canal to the reflector’s focus brings the test subject’s head close to the reflector’s opening and thus can alter the acoustic load and the resonant frequecies. This may bring unwanted variability to the test conditions if employing such a technique. The offset of sound source away from the reflector’s axis of symmetry starts to affect CHAPTER 5. RESULTS 31 the frequency response above 2 kHz frequencies as seen in the results. Offset along the reflector’s axis of symmetry affects the shape of the response above about 2 kHz frequecies, while below this frequency the offset only causes a shift in gain for this part of the response related to the distance from the microphone. These factors have to be taken into account in the margin of error in measurements when absolute values of emission level are of interest. However these measurements do not have the information of the transfer function from the ear to the microphone to deduce absolute SPLs emanating from the ear canal. 5.2 Otoacoustic emission measurement results 5.2.1 Comparison of open and sealed ear in DPOAE measurements SPL [dB] Sealed ear Open ear 60 60 40 40 20 20 0 0 -20 -20 -40 -40 -60 -60 -80 1 2 4 6 Frequency [kHz] 8 10 -80 1 2 4 6 Frequency [kHz] 8 10 Figure 5.5: Sound spectra measured with the reflector with two loudspeakers producing sinusoids at 3000 Hz and 3660 Hz and the subject lying under the reflector with his ear canal approximately at the focal point. Left: response with sealed ear canal. Right: response with open ear canal, where most prominent otoacoustic emissions are marked with red circles. An FFT analysis from the DPOAE measurements for subject S is shown in figure 5.5. The case of an ear sealed with an earplug is shown on the left side and the case of an open ear is shown on the right side. The frequency components of the stimulus and distortion components from the speakers are shown in both cases. In the case of an open ear, some otoacoustic emissions are clearly shown. Most prominent ones of them corresponding to frequencies 2f1 – f2 and 2f2 – f1 . These are third order two-tone intermodulation distortion components. CHAPTER 5. RESULTS 32 In figures 5.6 and 5.7 similar measured data as in the previous figure (5.5) is shown from the two test test subjects at two different stimulus frequencies. Subfigures (b) and (c) both show the data from subject A with different speaker positioning which affected the distortion originating from the measurement system (see sec 5.2.3). This was an additional experiment and the cases (b) and (c) are not strictly comparable because it was not verified that in case (c) the speakers produce the same level stimuli as in case (b). Yet, the speakers gains were kept intact and the distance of the speaker is approximately the same. DPOAE, f2 = 2745 Hz (c) SPL [dB] (b) SPL [dB] (a) SPL [dB] Sealed ear Open ear 50 50 0 0 -50 -50 50 50 0 0 -50 -50 50 50 0 0 -50 -50 1 2 4 6 8 10 Frequency [kHz] 1 2 4 6 8 10 Frequency [kHz] Figure 5.6: Sound spectra measured with reflector from all the subjects at f2 = 2745 and f2 /f1 = 1.22. The stimulus level was approximately L1 = L2 = 65 dB. On the left the measurement with a sealed ear and on the right the measurement with an open ear. (a) test subject S, (b) test subject A, (c) test subject A with a different positioning of the stimulus producing speakers. The FFT analysis of the measurements at a stimulus frequency of f2 = 2745 Hz in figure 5.6 show frequency components of the stimulus and distortion components CHAPTER 5. RESULTS 33 from the speakers (or elsewhere in the measurement system) in the case of a sealed ear on the left side. In addition to these frequency components DPOAEs are shown in the case of the open ear on the right side. The stimulus level was the highest that was used L1 = L2 = 65 dB. 2f1 – f2 frequency components are marked with a red circle. The SNR of this component for subject S was 9.4 dB and did not satisfy the set 12 dB SNR criterion. For subject A the SNR criterion for component 2f1 – f2 was met in both cases shown in the figure. Interestingly the frequency component 2f2 – f1 from subject S is shown also in the case of sealed ear. This indicates that the component arose from the measurement system itself and is shown in section 5.2.3. The system distortion measurements showed also that the 2f1 – f2 distortion can be produced by the measurement system itself when there is no test subject. In figure 5.7 the FFT analysis from measurements at the highest stimulus level at frequency f2 = 3660 Hz is shown. Frequency components on the right side marked with red circles are third order intermodulation components corresponding to frequencies 2f1 – f2 and 2f2 – f1 . They have emerged when there is no earplug and are thus clearly DPOAEs. SNR values for these DPOAEs are 29 dB or greater fullfilling the 12 dB criterion. For subject A the levels of the 2f2 – f1 components in open ear case are similar to the levels of the noise around the given component in the sealed ear case, and are thus neglected. Also other third order components and one fifth order intermodulation component marked with blue circles were present when the ear canal was open. The SNRs of these components were 17 dB or higher. CHAPTER 5. RESULTS 34 DPOAE, f2 = 3660 Hz (c) SPL [dB] (b) SPL [dB] (a) SPL [dB] Sealed ear Open ear 50 50 0 0 -50 -50 50 50 0 0 -50 -50 50 50 0 0 -50 -50 1 2 4 6 8 10 Frequency [kHz] 1 2 4 6 8 10 Frequency [kHz] Figure 5.7: Sound spectra measured with reflector from all the subjects at f2 = 3660 and f2 /f1 = 1.22. The stimulus level was approximately L1 = L2 = 65 dB. On the left the measurement with sealed ear and on the right the measurement with open ear. (a) test subject S, (b) test subject A, (c) test subject A with a different positioning of the stimulus producing speakers. CHAPTER 5. RESULTS 5.2.2 35 DPOAE growth DPOAE measurements were done at two different stimulus frequencies and both frequencies were measured at four different stimulus levels which can be found in table 5.1. Figure 5.8 shows the levels of 2f1 – f2 DPOAE component resulting from the four different stimulus levels used at two different stimulus frequencies. These input/output functions present the level of the given DPOAE component as a function of growing stimulus level L2 . DPOAE levels are depicted with an orange line while the height of the grey area presents the noise level. Red circles denote that the DPOAE level satisfies the 12 dB SNR criterion. A dark line with a slope of 0.39 has been added as a reference for linear growth in every plot. The slope of the given line is determined by calculating the slopes between the first two points of the DPOAE growth functions obtained with the f2 = 3660 Hz stimulus and taking the mean of these three slopes. 1. 2. 3. 4. L1 [dB] L2 [dB] 65 65 60 52.5 55 40 50 27.5 Table 5.1: Levels of the primary tones f1 and f2 . In the DPOAE measurements at frequency f2 = 2745 Hz only 1/3 of the measurement points fullfill the 12 dB SNR criterion and are considered valid. The level functions obtained are nonmonotonic for subject S and A in the normal test setup (figures a and b), but both only contain one valid data point.The I/O function at frequency f2 = 2745 Hz increases monotonically for subject S with the alternate measurement setup. Out of the DPOAE measurements done at a frequency f2 = 3660 Hz there is one nonvalid measurement at the lowest stimuli level for subject A with the alternate measurement setup. All the other points fullfilled the 12 dB SNR criterion. At the given stimulus frequency all the obtained I/O functions increase monotonically. For subject S the slope of the I/O function decreases slightly after the second highest stimulus level L2 = 52.5 dB, thus exhibiting compressive behaviour. For subject A the slope decreases after the third highest stimulus level, but increases again on from the second highest stimulus level. For subject A with the alternative test setup the I/O function is roughly linear. CHAPTER 5. RESULTS 36 (a) SPL [dB] f 2 = 2745 Hz 0 -10 -10 -20 -20 -30 -30 -40 -40 30 SPL [dB] 0 (b) SPL [dB] 40 50 60 L 0 -10 -10 -20 -20 -30 -30 -40 -40 30 0 (c) f 2 = 3660 Hz 0 40 50 60 0 -10 -10 -20 -20 -30 -30 -40 -40 30 40 50 60 Stimulus level L2 [dB] dp ref 30 40 50 60 30 40 50 60 30 40 50 60 Stimulus level L2 [dB] Figure 5.8: Measured DPOAE levels from all the test subjects. Level of 2f1 – f2 distortion component, f2 /f1 = 1.22. Orange line: DPOAE levels, dark line: linear growth reference, grey area height: noise levels. Red circles denote that the emission level satisfies the 12 dB SNR criterion (sect. 4.2.3). (a) test subject S, (b) test subject A, (c) test subject A with a different positioning of the stimulus producing speakers. 5.2.3 System distortion System distortion measurements were done with the exactly same measurement setup and stimulus levels as in the DPOAE measurements but without a test subject. Figure 5.9 shows an FFT analysis of the measurement at stimulus frequency f2 = 2745 Hz at the highest level L1 = L2 = 65 dB. The plot on the left shows the result from the measurement with the loudspekers next to each other and the plot on the right side shows the result after moving one of the speakers 2 meters away from the other. Frequency components that arise from intermodulation distortion are marked to the plot. Two second order intermodulation components at frequencies f2 – f1 and CHAPTER 5. RESULTS 37 Intermodulation distortion, f2 = 2745 Hz SPL [dB] Speakers next to each other Speakers 2 m apart 60 60 40 40 20 20 0 0 -20 -20 -40 -40 -60 -60 -80 1 2 4 6 8 10 Frequency [kHz] -80 1 2 4 6 8 10 Frequency [kHz] Figure 5.9: Comparison of speaker intermodulation distortion with two different speaker positionings without a test subject. Left figure: speakers positioned next to each other on the same stand. Right figure: speakers positioned on a 2 m distance from each other. Red circles denote second order intermodulation distortion componets, blue circles third order components, a yellow circle a one fourth order component and pink circles fifth order components. f2 + f1 are marked with red circles; third order components at 2f1 – f2 , 2f2 – f1 , 2f1 + f2 and f1 + 2f2 with blue circles; one fourth order component at 3f2 – f1 with a yellow circle and fifth order components at 3f2 – 2f1 and 4f2 – f1 by pink circles. The results show that the level of most of the given intermodulation components decreased visibly when the speaker was moved further away, some of them disappearing completely to the noise floor. From the third order components 2f1 – f2 and 2f2 – f1 that are observed in our DPOAE measurements, the level of the component 2f1 – f2 reduced by over 17 dB being visibly indistinguishable from the noise floor (SNR 3 dB) after placing the speakers further away from each other. The level of the component 2f2 – f1 decreased over 15 dB but still has a SNR of 15 dB. Figure 5.10 shows results of the system distortion measurement performed at stimulus frequency f2 = 3660 Hz at the highest level L2 = 65 dB. The left side plot is the result when stimulus producing loudspeakers are placed side by side and the right side plot is the result when one of the speakers is moved away from the other to a 2 m distance. The third order distortion components 2f1 – f2 and 2f2 – f1 observed in the DPOAE measerements do not emerge visibly from the noise floor in either case with maximum SNRs of 2 dB. Two second order intermodulation distortion components at frequencies f2 – f1 and f2 + f1 are marked with red circles to both plots. The other frequency components CHAPTER 5. RESULTS 38 Intermodulation distortion, f2 = 3660 Hz SPL [dB] Speakers next to each other Speakers 2 m apart 60 60 40 40 20 20 0 0 -20 -20 -40 -40 -60 -60 -80 1 2 4 6 8 10 Frequency [kHz] -80 1 2 4 6 8 10 Frequency [kHz] Figure 5.10: Comparison of speaker intermodulation distortion with two different speaker positionings without a test subject. Red circles denote second order intermodulation distortion components. that have high levels above the noise floor are harmonic distortion components and of course, the frequency components of the stimulus itself. The level of the second order component f2 – f1 decreased almost 24 dB and disappeared to the noise floor when one loudspeaker was moved further away from the other. The component f2 + f1 decreased by 32 dB still having a SNR of over 8 dB after relocating the other loudspeaker. It has to be noted that in the case of loudspeakers positioned further apart, there is a remarkable 10 dB decrease in the level of the lower tone of the stimulus entering the microphone inside the reflector. This likely has influence on the levels of the intermodulation distortion components. It is also likely that there is a similar difference between stimulus levels at the point where the ear canal of the test subject is positioned, which should be kept in mind when interpreting the results from the alternate test setup. 5.2.4 Discussion of the DPOAE measurement results Taking into account the system distortion information, frequency components of biological origin could be measured in these measurements, that is DPOAEs. While the system distortion results at frequency f2 = 2745 Hz show that the 2f1 – f2 distortion component is present even without a test subject, it is present with subject A also in the alternative speaker positioning where the given distortion component does not arise from the measurement system itself. Therefore DPOAEs could be measured at least with the alternative test setup at this stimulus frequency. CHAPTER 5. RESULTS 39 In the system distortion measurements at stimulus frequency f2 = 3660 Hz no notable third order intermodulation components emerged. The third order component 2f1 – f2 was measured in the DPOAE measurements from the both subjects and is clearly a DPOAE. Also the component 2f2 – f1 was measured from subject S. Although done with a small sample of subjects and some inherent distortion in the system, these measurements serve as a proof of concept that recording of OAEs is possible from a considerable distance by using an ellipsoidal reflector. Chapter 6 Conclusions A modular ellipsoidal reflector was developed and it’s applicability to measurements of otoacoustic emissions (OAEs) was examined. The response measurement data from the reflector show that clearly the most gain is achieved with the least truncated reflector setups that have the most closed structure and feature the most reflecting surface. These setups are suitable for measurements where the temporal response is not overridingly critical, because there are reflections in the response that take over 10 milliseconds to decay. The least truncated reflector setups could be usable for the measurements of distortion product otoacoustic emissions (DPOAEs), stimulusfrequency otoacoustic emissions (SFOAEs) and spontaneous otoacoustic emissions (SOAEs). The frequency responses of these reflector setups have high resonance peaks which may be inconvenient. The gain with the most closed reflector setup compared to a 3 cm distance free-field response is nearly always higher above about 5 kHz, which makes measurements at this frequency region desireable. Yet, this frequency region is at the very upper end of the frequency range where common OAE measurements are made. With the more truncated setups a similar frequency limit is at even higher frequencies. To test the developed setup to its purpose, OAE measurements were performed to two test subjects of which the author was the other one. DPOAEs could be measured at two stimulus frequencies. These measurements serve as a proof of concept that recording of OAEs from a considerable distance is possible with this kind of a reflector. The comparison of achievable SNR between the developed setup and conventional OAE measurement equipment was not done due to limited time available. Further studies are needed to answer this question. 40 Bibliography [1] M. R. Bailey. Acoustic and cavitation fields of a pressure release ellipsoidal reflector. The Journal of the Acoustical Society of America, 101(5):3138–3138, 1997. [2] C. I. Berlin and L. J. Hood. Current physiologic bases of audiologic interpretation and management. In J. Katz, L. Medwetsky, R. Burkard, and L. Hood, editors, Handbook of clinical audiology, chapter 22. Lippincott Williams & Wilkins, 2009. [3] R. C. Bilger, M. L. Matthies, D. 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