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Improved Hearing Assessment in Noisy Environments Michael Fisher1,2, Ben Rudzyn1,2, Gordon Jarvis2 and Harvey Dillon1,2 1. The HEARing CRC, 2. National Acoustic Laboratories XX Audiology Australia National Conference 2012 Adelaide creating sound valueTM Introduction The accuracy of pure tone audiometry is dependent on the amount of ambient noise reaching the cochlea. Environmental noise reaches the cochlea by several paths The two main paths being: 1. Air Conduction 2. Bone Conduction creating sound valueTM Fisher et al. 1 Introduction • Environmental noise may mask lower level test tones • Elevated hearing thresholds may result for people who would normally hear these lower level test tones in a quieter environment creating sound valueTM Fisher et al. 2 Introduction The conventional solution to this problem is to use a soundproof booth to attenuate the environmental noise. creating sound valueTM Fisher et al. 3 Introduction There are many situations in which a clinician may wish to conduct a hearing assessment but it is impractical to use a soundproof booth, such as testing in a client’s School Home Workplace creating sound valueTM Fisher et al. 4 Introduction For these situations there are several commercially available devices that reduce the environmental noise such as: • TDH 39 supra-aural headphones with circum-aural enclosures i.e. Audiocups, • ER 3A & ER 5 insert earphones worn with/without earmuffs. creating sound valueTM Fisher et al. 5 Investigation Aim To find the best practical solution to achieve accurate audiometric hearing assessment without employing a soundproof booth. Provide advice on correction factors for bone conduction measurements Provide advice on devices to use SLM creating sound valueTM Provide advice on the Maximum Permissible Ambient Noise Levels Fisher et al. 6 Investigation – Detailed Plan Part 1. Preliminary investigation: Objective testing of external noise attenuation using an acoustic mannequin for: I. Single attenuators – headphones or insert earphones II. Dual attenuators – combinations earmuffs & insert earphones III. Effect of insert earphone delivery signal wire / tube on earmuffs Objective testing (10 subjects) of external noise attenuation using the Microphone in Real Ear (MIRE) technique for single and dual protection Part 2. Subjective evaluation of selected devices (ER-3A and MSA earmuffs): Subjective testing (24 subjects) of the selected devices in terms of: I. external noise attenuation using the Real Ear Attenuation at Threshold (REAT) technique for single and dual protection II. bone and air conduction thresholds for single and dual protection creating sound valueTM Fisher et al. 7 Part 2 – Subjective evaluation Subjective assessment of the selected devices (as determined by objective measurement in Part 1) EARTONE 3A or ER 3A insert earphones Selected Earphone creating sound valueTM MSA 766243 left / RIGHT, High, Yellow, Headband earmuff Selected Earmuff Fisher et al. 8 Part 2 – Subjective evaluation A piece of Libby Horn fits firmly over the surround of the ER-3A nipple and extends to almost the length of the exposed insert-tip’s tube. The piece of Libby Horn is used to prevent buckling of the insert-tip’s tube when the earmuff is placed over the insert earphone. Piece of Libby Horn The tips may be changed without removing the piece of Libby Horn. creating sound valueTM Fisher et al. 9 Part 2 – Subjective evaluation The ER-3A tube should have a gentle curve within the earmuff and should run down the neck. creating sound valueTM Fisher et al. 10 Method – Attenuation evaluation Subjective testing (REAT Method) Diffuse sound field Obtain Thopen Open Ear Subjects: 24 normal hearers Test: Békésy type automatic threshold determination Obtain Thearphone Testing signal: third-octave narrowband noise at audiometric frequencies presented in a diffuse environment Attenuation (Att) equals the difference in threshold (Th) Attearphone = Thearphone – Thopen Obtain Thearphone+earmuff Insert earphones fitted Insert earphones and earmuffs fitted Attearphone+earmuff = Thearphone+earmuff – Thopen creating sound valueTM Fisher et al. 11 Results – Attenuation evaluation Results show the average attenuation of the insert earphones (ER-3A) alone and insert earphones in combination with the earmuff (MSA 766 243 left/RIGHT) *Also shown for comparative purposes is the average attenuation of the TDH 50 headphones with MX-41 cushions and the Audiocups devices [Berger 89]. Average attenuation (dB) at one octave centre frequencies (Hz) 125 250 500 1k 2k 4k 8k Insert Earphones 31 30 32 33 33 41 42 Insert Earphones & Muffs 35 41 49 44 37 49 45 creating sound valueTM Fisher et al. 12 Results - Maximum Permissible Ambient Noise Levels (MPANL’s) creating sound valueTM Fisher et al. 13 Method – Evaluating the effect on audiometric thresholds Air conduction thresholds are assessed with and without the earmuffs to determine if the addition of earmuffs has any effect on thresholds. Bone conduction thresholds are assessed to determine the effect of the addition of insert earphones alone and insert earphones in combination with earmuffs. Bone Conductor (Centre of forehead) Bone conducted vibration moves ear canal walls as well as cochlear membranes and middle ear ossicles etc Extra vibration of tympanic membrane results from increased sound pressure in the ear canal. This extra sound pressure is due to occlusion of the ear canal by insert earphone and earmuff creating sound valueTM Fisher et al. 14 Method – Evaluating the effect on audiometric thresholds Subjective testing Subjects: 24 normal hearers Open Ear Test: manually determine • air conduction thresholds • bone conduction (unmasked) thresholds Frequencies tested: 125 Hz (air only),250, 500, 1,000, 2,000, 4,000 & 8,000 Hz Step size: • air conduction: 5 dB • bone conduction: 1 dB The correction equals the average difference in thresholds with and without the devices fitted creating sound valueTM Insert earphones fitted Insert earphones and earmuffs fitted Fisher et al. 15 Results – effect on audiometric thresholds There was no effect on air conduction thresholds from placing earmuffs over the insert earphones. There was a considerable effect on bone conduction thresholds from placing insert earphones in the ear canals with/without earmuffs. The change in bone conduction thresholds can be corrected for (assuming no conductive loss) with the following correction factors Nearest 1 dB Nearest 5 dB Correction factors for bone conduction thresholds (for normal hearers with bone conductor on the forehead) (dB) Test Tone Frequency (Hz) 250 500 1000 2000 4000 8000 22 16 10 3 -1 1 20 15 10 5 0 0 Note if the bone conductor is calibrated for mastoid placement then a further correction to the measured thresholds values is required, ISO 389-3:1994 E Annex C provides the following (informative) correction factors. Nearest 1 dB creating sound valueTM Mastoid to Forehead Correction Factor (dB) Test Tone Frequency (Hz) 250 500 1000 2000 4000 -12 -14 -9 -12 -8 8000 -10 Fisher et al. 16 Conclusions 1. Double protection provides significantly better attenuation than single protection using achievable insertion depths 2. The commonly used ER-3A insert earphones perform as well if not better than other insert earphones when used in combination with good earmuffs 3. The combination of the ER-3A insert earphone and a MSA left/RIGHT “High” earmuff enables threshold testing to 0 dB HL in high background noise levels, MPANL’s (minimum one-third octave noise level: Lmax 41 dB SPL at 2 kHz) 4. There is no change in air conduction thresholds as a result of using earmuffs over insert earphones. 5. The change in the bone conduction thresholds due to insert earphones and earmuffs being worn by normal hearers can be compensated for. creating sound valueTM Fisher et al. 17 Summary & Acknowledgements Advice on correction factors for bone conduction measurements Advice on which insert earphone to use Correction factors for bone conduction thresholds (for normal hearers with bone conductor on forehead) EARTONE 3A or ER 3A insert earphones Nearest 1 dB Nearest 5 dB Advice on which earmuff to use 250 22 20 dB Test Tone Frequency Hz 500 1000 2000 16 10 3 15 10 5 4000 -1 0 8000 1 0 Advice on the Maximum Permissible Ambient Noise Levels for a minimum achievable threshold MSA 766243 left / RIGHT, High, Yellow, Headband earmuff SLM Special thanks to Lyndal Carter This research was financially supported by the HEARing CRC established and supported under the Australian Government’s Cooperative Research Centres Program creating sound valueTM In memory of the late Ben Rudzyn Fisher et al. 18