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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.
2.
3.
4.
Situational Awareness
Communication
Cleanliness and fear of loss of hearing aid
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.)
Differences in Real Ear Attenuation
(dB)
7
6
5
4
3
2
#13 tubing (Berger
Earlog 18)
1
1 mm ID thin tube (this
study)
0
125
250
500
1000
2000
Log Frequency (Hz)
4000
8000
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 55
250
55
55
0
500
55
60
5
1000
55
72
17
2000
55
77
22
4000
55
76
21
Freq (Hz) Input
Output Gain 70
250
70
70
0
500
70
73
3
1000
70
82
12
2000
70
85
15
4000
70
87
17
Freq (Hz) Input
Output Gain 85
250
85
85
0
500
85
86
1
1000
85
90
5
2000
85
92
7
4000
85
95
10
Industrial Noise at 85 dBA
-12 dB/oct
Industrial Noise at 85 dBA, aided (blue)
and unaided (pink)
OAL unaided = 85 dBA
OAL aided = 85.43 dBA
Flat 50 dB SNHL: NAL-NL1
Freq (Hz) Input
Output Gain 55
250
55
64
9
500
55
72
17
1000
55
80
25
2000
55
81
26
4000
55
79
24
Freq (Hz) Input
Output Gain 70
250
70
78
8
500
70
82
12
1000
70
89
19
2000
70
90
20
4000
70
90
20
Freq (Hz) Input
Output Gain 85
250
85
88
3
500
85
91
6
1000
85
98
13
2000
85
99
14
4000
85
100
15
Aided (blue) = 85.38 dBA
Unaided (pink) = 85 dBA
Flat 50 dB SNHL: NAL-NL1
Freq (Hz) Input
Output Gain 55
250
55
64
9
500
55
72
17
1000
55
80
25
2000
55
81
26
4000
55
79
24
Freq (Hz) Input
Output Gain 70
250
70
78
8
500
70
82
12
1000
70
89
19
2000
70
90
20
4000
70
90
20
Freq (Hz) Input
Output Gain 85
250
85
88
3
500
85
91
6
1000
85
98
13
2000
85
99
14
4000
85
100
15
Aided (blue) = 78 dBA
Unaided (pink) = 70 dBA
Flat 50 dB SNHL: NAL-NL1
Freq (Hz) Input
Output Gain 55
250
55
64
9
500
55
72
17
1000
55
80
25
2000
55
81
26
4000
55
79
24
Freq (Hz) Input
Output Gain 70
250
70
78
8
500
70
82
12
1000
70
89
19
2000
70
90
20
4000
70
90
20
Freq (Hz) Input
Output Gain 85
250
85
88
3
500
85
91
6
1000
85
98
13
2000
85
99
14
4000
85
100
15
Aided (blue) = 67 dBA
Unaided (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.