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OTOACOUSTIC EMISSIONS AT 0.5 kHz: PROPERTIES AND APPLICATIONS W. Wiktor Jedrzejczak, Edyta Pilka, Krzysztof Kochanek, Henryk Skarzynski World Hearing Center, Institute of Physiology and Pathology of Hearing, Warsaw/Kajetany, Poland Otoacoustic emissions (OAEs) are low level sounds originating in the cochlea that can be measured by sensitive microphone in the ear canal. In humans they are used in diagnostic purposes. They are particularly efficient in determining the hearing status in 1-4 kHz frequency range. In the present study OAEs evoked by tone bursts at 0.5 kHz are investigated. Time-frequency properties of the signals are discussed and some clinical applications are introduced. 1. Introduction Otoacoustic emissions (OAEs) were first described by Kemp [1] and are now well established in audiological practice e.g. [2,3]. The most commonly used OAEs are click-evoked OAEs (CEOAEs) and distortion product OAEs (DPOAEs). OAEs can also be evoked by tone bursts, which are short stimuli centered at a certain frequency. Some studies e.g. [4] have shown that the spectral content of a sum of tone burst-evoked OAEs (TBOAEs) centered on the 1–4 kHz region corresponds to that in a CEOAE. The present study investigates TBOAEs centered at 0.5 kHz. The advantage of this particular TBOAE is that it may contain components not present in the CEOAE. The study overviews results from previous studies of 0.5 kHz TBOAEs [5-8]. 2. Material and method The results from normally hearing adult subjects as well as from subjects with some hearing impairment were studied. OAEs were measured in low-noise ambient conditions. Standard click stimuli and 0.5 kHz tone bursts (average amplitude 80±3 peak dB SPL, nonlinear averaging protocol) were used to evoke 260 OAE responses. The tone bursts were two cycles long with equal rise/fall times and no plateau. The initial part of the response was windowed automatically by the system to minimize stimuli artifacts. Window onset was 2.5 ms for clicks and 5 ms for 0.5 kHz tone bursts, and all recordings used an acquisition window of 20 ms. Half-octave-band values of OAE response levels and signal-to-noise ratios (SNRs) were used for analysis. For all parameters the statistical significance of the mean difference between groups was evaluated using the Wilcoxon rank sum test. As a criterion of significance, a 95% confidence level (p < 0.05) was chosen. ICSV22, Florence (Italy) 12-16 July 2015 1 The 22nd International Congress on Sound and Vibration 2.1 Matching pursuit Time–frequency (TF) analysis of the recorded signals was done by decomposing the signals into their basic waveforms. The method of high-resolution adaptive approximation was used, a technique based on the matching pursuit (MP) algorithm [9]. A slight modification was used to account for the asymmetrical character of some components [10]. The modified MP method allowed the CEOAE and TBOAE signals to be decomposed into waveforms of defined frequency, latency, duration, and amplitude. The latency was taken to be the time taken from onset of the stimulus to the maximum point in the waveform envelope. Note that the presence of spontaneous OAEs (SOAEs) can, when using some methods, cause a false shift in evoked OAE latency, whereas MP with an asymmetric dictionary provides estimates that are less prone to SOAEs [10]. 3. Results Figure 1 shows average half-octave band values response levels and SNRs of OAEs evoked by clicks and 0.5 kHz tone bursts. CEOAE magnitudes were greatest over the range 1–4 kHz, decreasing substantially below 1 kHz. Responses from the 0.5 kHz TBOAEs were complementary in that the main components occurred between 0.5 and 1.4 kHz. TBOAEs were larger than CEOAEs (on average more than 5 dB larger) at 0.5–1 kHz (the difference was significant at p<0.05). At 1.4 kHz there was no statistically significant difference between CEAOEs and TBOAEs. As expected, at higher frequencies TBOAE response levels and SNRs declined, while CEOAEs were significantly higher. Figure 1. Average half-octave response levels and SNRs CEOAEs and 0.5 kHz TBOAEs. Figure 2 shows TF properties of CEOAEs and 0.5 kHz TBOAEs, and a clear dependence of latency on frequency can be seen. For CEOAEs, latencies extend from around 5 ms for the 4 kHz band to 10 ms for the 1 kHz band. From 4 kHz to 1 kHz the dependence of latency on frequency is, almost a straight line. For lower frequencies CEOAEs were weak and nearly absent. For TBOAEs latency is only shown for the 0.5–2 kHz range since at higher frequencies there was practically no response (see Figure 1). ICSV22, Florence, Italy, 12-16 July 2015 2 The 22nd International Congress on Sound and Vibration Figure 2. Average latency of CEOAEs and 0.5 kHz TBOAEs. In Figure 3 examples of SNRs for CEOAEs and 0.5 kHz TBOAEs are shown. This subject had hearing thresholds better than 20 dB HL for frequencies up to 0.5 kHz and higher than 50 dB HL in the remaining frequencies. It may be seen that SNRs for CEAOEs did not exceed 0 dB. On the other hand, 0.5 kHz SNR was better than 3 dB for broad-band analysis and even higher than 8 dB for band-limited values. Figure 3. Examples of SNR values for CEOAE and 0.5 kHz TBOAE, for a partial deafness subject (normal hearing for frequencies up to 0.5 kHz and severe hearing loss above 0.5 kHz). Global values as well as halfoctave band values around 0.5 kHz are shown. 4. Discussion The properties of OAE parameters such as reproducibility, response level, and signal to noise ratio in relation to different frequency bands have been investigated in several studies e.g. [11,12]. Usually it was the case that CEOAEs were not efficient at 0.5 kHz. The same property was reproduced in the present study. On the other hand, 0.5 kHz TBOAEs provide additional information to CEOAEs about the frequency range below 1 kHz. The apparent imperfection of 0.5 kHz TBOAE measured here is that they have maximum around 1 kHz and not close to 0.5 kHz. This is probably because of spectral splatter from the short tone burst stimulus and from rapidly falling responses of the cochlea and the recording system at low frequencies. From the previous works that used 4 cycles burst it might be concluded that it is better ICSV22, Florence, Italy, 12-16 July 2015 3 The 22nd International Congress on Sound and Vibration in evoking response that has narrower spread in frequency and has maximum closer to 0.5 kHz. However such option is not available in current commercial equipment used in this study. 0.5 kHz TBOAE is more reliable than the CEOAE in the case of activity at low frequencies. The 0.5 kHz TBOAE is also a promising tool for the detection of emissions in patients with deep, highfrequency hearing loss when click stimuli do not produce OAEs. REFERENCES 1 Kemp DT. Stimulated acoustic emissions from within the human auditory system. J Acoust Soc Am. 1978 Nov;64(5):1386-91. 2 Lonsbury-Martin, B. L., Whitehead, M. L., Martin, G. K. (1991). “Clinical applications of otoacoustic emissions,” J. Speech Hear. Res. 34, 964-981. 3 Hatzopoulos S, Prosser S, Mazzoli M, Rosignoli M, Martini A. (1998). “Clinical applicability of transient evoked otoacoustic emissions: identification and classification of hearing loss,” Audiol Neurootol. 3, 402-18. 4 Elberling, C., Parbo, N. J., Johnsen, N. J., and Bagi, P. (1985). “Evoked acoustic emissions: clinical application,” Acta Otolaryngol. Suppl. 421, 77-85. 5 Jedrzejczak WW, Piotrowska A, Kochanek K, Sliwa L, Skarzynski H. Low-frequency otoacoustic emissions in schoolchildren measured by two commercial devices. Int J Pediatr Otorhinolaryngol. 2013 Oct;77(10):1724-8. 6 Jedrzejczak WW, Kochanek K, Trzaskowski B, Pilka E, Skarzynski PH, Skarzynski H. Tone-burst and click-evoked otoacoustic emissions in subjects with hearing loss above 0.25, 0.5, and 1 kHz. Ear Hear. 2012 Nov-Dec;33(6):757-67 7 Jedrzejczak WW, Lorens A, Piotrowska A, Kochanek K, Skarzynski H. Otoacoustic emissions evoked by 0.5 kHz tone bursts. J Acoust Soc Am. 2009 May;125(5):3158-65. 8 Jedrzejczak WW, Kochanek K, Sliwa L, Trzaskowski B, Pilka E, Skarzynski P, Skarzynski H.Otoacoustic emissions for the evaluating the low-frequency hearing of patients considered for Partial Deafness Treatment.J Hear Sci 2012; 2(2): OA86-91. 9 Mallat SG, Zhang Z. Matching pursuit with time–frequency dictionaries. IEEE Trans. Sign. Process. 1993; 41, 3397-415. 10 Jedrzejczak WW, Kwaskiewicz K, Blinowska KJ, Kochanek K, Skarzynski H. Use of the matching pursuit algorithm with a dictionary of asymmetric waveforms in the analysis of transient evoked otoacoustic emissions. J Acoust Soc Am 2009; 126: 3137-46. 11 Gorga, M. P., Neely, S. T., Bergman, B. M., Beauchaine, K. L., Kaminski, J. R., Peters, J., Schulte, L., Jesteadt, W. (1993). “A comparison of transient-evoked and distortion product otoacoustic emissions in normal-hearing and hearing-impaired subjects,” J Acoust Soc Am, 94, 2639-2648. 12 Lichtenstein, V., Stapells, D. R. 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