Improved Hearing Assessment in Noisy Environments

1. Improved Hearing Assessment in Noisy Environments Michael Fisher1,2, Ben Rudzyn1,2, Gordon Jarvis2 and Harvey Dillon1,2 1. The HEARingCRC, 2. National Acoustic Laboratories XX Audiology Australia National Conference 2012 Adelaide creatingsoundvalueTM

2. 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: Air Conduction Bone Conduction

3. 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

4. Introduction The conventional solution to this problem is to use a soundproof booth to attenuate the environmental noise.

5. 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

6. 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.

7. Provide advice on correction factors for bone conduction measurements Provide advice on devices to use Provide advice on the Maximum Permissible Ambient Noise Levels SLM • Investigation Aim To find the best practical solution to achieve accurate audiometric hearing assessment without employing a soundproof booth.

8. Investigation – Detailed Plan • Part 1. Preliminary investigation: • Objective testing of external noise attenuation using an acoustic mannequin for: • Single attenuators – headphones or insert earphones • Dual attenuators – combinations earmuffs & insert earphones • 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: • external noise attenuation using the Real Ear Attenuation at Threshold (REAT) technique for single and dual protection • bone and air conduction thresholds for single and dual protection

9. Part 2 – Subjective evaluation Subjective assessment of the selected devices (as determined by objective measurement in Part 1) Selected Earmuff SelectedEarphone

10. 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.

11. Part 2 – Subjective evaluation The ER-3A tube should have a gentle curve within the earmuff and should run down the neck.

12. Method – Attenuation evaluation Diffuse sound field Subjective testing (REAT Method) Subjects: 24 normal hearers Test: Békésy type automatic threshold determination Testing signal: third-octave narrow-band noise at audiometric frequencies presented in a diffuse environment Attenuation (Att) equals the difference in threshold (Th) Obtain Thopen Open Ear Obtain Thearphone Insert earphones fitted Obtain Thearphone+earmuff Insert earphones and earmuffs fitted Attearphone = Thearphone – Thopen Attearphone+earmuff = Thearphone+earmuff – Thopen

13. Results – Attenuation evaluation Results show the average attenuation of the insert earphones (ER-3A) aloneand 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 Audiocupsdevices [Berger 89].

14. Results - Maximum Permissible Ambient Noise Levels (MPANL’s)

15. 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 conducted vibration moves ear canal walls as well as cochlear membranes and middle ear ossicles etc Bone Conductor (Centre of forehead) 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

16. Method – Evaluating the effect on audiometric thresholds • Subjective testing • Subjects: 24 normal hearers • 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 Open Ear Insert earphones fitted Insert earphones and earmuffs fitted

17. 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 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.

18. Conclusions • Double protection provides significantly better attenuation than single protection using achievable insertion depths • The commonly used ER-3A insert earphones perform as well if not better than other insert earphones when used in combination with good earmuffs • 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: Lmax41 dB SPL at 2 kHz) • There is no change in air conduction thresholds as a result of using earmuffs over insert earphones. • The change in the bone conduction thresholds due to insert earphones and earmuffs being worn by normal hearers can be compensated for.

19. Summary & Acknowledgements Advice on correction factors for bone conduction measurements Advice on which insert earphone to use Advice on which earmuff to use Advice on the Maximum Permissible Ambient Noise Levels for a minimum achievable threshold 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 In memory of the late Ben Rudzyn