Hearing aids in noisy environments:

Do hearing aids affect risk for NIHL?

Fligor and Chasin

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?)

How eardrum SPL may differ with hearing aids

3 Parameters:

1. Venting

2. Thin tubes

3. Compression/Gain

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

Venting- a low frequency effect

• Venting:– A passive modification that minimizes the

attenuation characteristics below 1000 Hz. (from Kuk and Keenan, 2006, Hearing Review)

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

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

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)

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.)

0

1

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3

4

5

6

7

125 250 500 1000 2000 4000 8000

Log Frequency (Hz)

Dif

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in

Re

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Ea

r A

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(d

B)

#13 tubing (BergerEarlog 18)

1 mm ID thin tube (thisstudy)

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).

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).

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.

Hearing Loss and Hearing Aid Gain

Hearing Loss and Hearing Aid Gain:NAL-NL1 Targets

Freq (Hz) Input Output Gain 55250 55 55 0500 55 60 5

1000 55 72 172000 55 77 224000 55 76 21

Freq (Hz) Input Output Gain 70250 70 70 0500 70 73 3

1000 70 82 122000 70 85 154000 70 87 17

Freq (Hz) Input Output Gain 85250 85 85 0500 85 86 1

1000 85 90 52000 85 92 74000 85 95 10

Industrial Noise at 85 dBA

-12 dB/oct

Industrial Noise at 85 dBA, aided (blue) and unaided (pink)

OAL unaided = 85 dBAOAL aided = 85.43 dBA

Flat 50 dB SNHL: NAL-NL1

Freq (Hz) Input Output Gain 55250 55 64 9500 55 72 17

1000 55 80 252000 55 81 264000 55 79 24

Freq (Hz) Input Output Gain 70250 70 78 8500 70 82 12

1000 70 89 192000 70 90 204000 70 90 20

Freq (Hz) Input Output Gain 85250 85 88 3500 85 91 6

1000 85 98 132000 85 99 144000 85 100 15

Aided (blue) = 85.38 dBAUnaided (pink) = 85 dBA

Flat 50 dB SNHL: NAL-NL1

Freq (Hz) Input Output Gain 55250 55 64 9500 55 72 17

1000 55 80 252000 55 81 264000 55 79 24

Freq (Hz) Input Output Gain 70250 70 78 8500 70 82 12

1000 70 89 192000 70 90 204000 70 90 20

Freq (Hz) Input Output Gain 85250 85 88 3500 85 91 6

1000 85 98 132000 85 99 144000 85 100 15

Aided (blue) = 78 dBAUnaided (pink) = 70 dBA

Flat 50 dB SNHL: NAL-NL1

Freq (Hz) Input Output Gain 55250 55 64 9500 55 72 17

1000 55 80 252000 55 81 264000 55 79 24

Freq (Hz) Input Output Gain 70250 70 78 8500 70 82 12

1000 70 89 192000 70 90 204000 70 90 20

Freq (Hz) Input Output Gain 85250 85 88 3500 85 91 6

1000 85 98 132000 85 99 144000 85 100 15

Aided (blue) = 67 dBAUnaided (pink) = 55 dBA

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.