1. Hearing aids in noisy environments: Do hearing aids affect risk for NIHL? Fligor and Chasin

2. Why would a worker want to use hearing aids? 1.Situational Awareness 2.Communication 3.Cleanliness and fear of loss of hearing aid 4.Others? But what about… Moisture/dust (damages aid), sound levels (further damage hearing?)

3. How eardrum SPL may differ with hearing aids 3 Parameters: 1.Venting 2.Thin tubes 3.Compression/Gain

4. How eardrum SPL may differ with hearing aids Intense low frequency sounds enter directly through vent. –Vent associated resonance Attenuation characteristics of thin tubes vs. #13 tubing. (non vented condition) Compression –Attack times

5. Venting- a low frequency effect Venting: –A passive modification that minimizes the attenuation characteristics below 1000 Hz. (from Kuk and Keenan, 2006, Hearing Review)

6. Venting and low frequency attenuation (from Kuk and Keenan, 2006) Blue 1 mm Green 2 mm Red 3 mm Solid for 6 mm long vent Dotted for 22 mm long vent

7. Venting- a low frequency effect –Vent associated resonance 300-500 Hz of 5-6 dB at the frequency given by: ___________________________ F ≈ 5500 Hz √cross section area/length

8. Thin Tubes (non vented) Found in approximately 56% of new hearing aid fittings Standard hearing aid tubing #13 (1.96 mm ID) Thin tube hearing aid tubing (1.0 - 1.3 mm ID)

9. Thin Tubes (non vented) non-significant (red n=3 thin tubes coupled to silicone unvented molds – the silicone mold without the thin tube) S.D. 5-7 dB …. ns.)

10. Compression/Gain Almost all modern hearing aids use a form of WDRC which serve to reduce gain above a specified set level. Can be single band or multi-band. –We will deal with the single band situation (similar to multi-band but below 1000 Hz).

11. Compression and attack time Attack time is the time interval between the abrupt increase in input level from 55 to 90 dB SPL and the point where the output level has stabilized to within 3 dB of the steady value for an input of 90 dB SPL. (ANSI S3.22-2003).

12. Compression and attack time The ANSI 55-90-55 paradigm will yield typical hearing aid values of 5-50 msec, depending on the design characteristics. BUT,… these same values are significantly shorter (< 1msec) for real life industrial values of 110 dB SPL, so effectively compression can be thought of as instantaneous, at least for intense industrial applications.

13. Hearing Loss and Hearing Aid Gain

14. Hearing Loss and Hearing Aid Gain: NAL-NL1 Targets

15. Industrial Noise at 85 dBA -12 dB/oct

16. Industrial Noise at 85 dBA, aided (blue) and unaided (pink) OAL unaided = 85 dBA OAL aided = 85.43 dBA

17. Flat 50 dB SNHL: NAL-NL1 Aided (blue) = 85.38 dBA Unaided (pink) = 85 dBA

18. Flat 50 dB SNHL: NAL-NL1 Aided (blue) = 78 dBA Unaided (pink) = 70 dBA

19. Flat 50 dB SNHL: NAL-NL1 Aided (blue) = 67 dBA Unaided (pink) = 55 dBA

20. Summary: Thin tube vs. standard #13 hearing aid tubing…. No significant difference. Compression attack time, probably not a significant factor. Due to compression, amplification of sound probably not a significant factor (but this is frequency spectrum dependent). Venting remains the primary factor.