R3.6.4
Improved Hearing Assessment in Noisy
Environments – Parts 1 & 2
Project Leader: Michael Fisher
Principal Researcher (Part 1): the late Ben Rudzyn
creating sound valueTM
Aim
Aim of the project –
Investigate methods of improving audiometric hearing assessment
accuracy when background noise is present.
In particular the investigation is focused on evaluating
combinations of insert earphones and earmuffs.
Bone
conduction
Noise
Insert Earphone
Earmuff
Ear canal
creating sound valueTM
Cochlear
1
Objective
Objective / Application –
•Publish advice on how one can improve audiometric hearing assessment
accuracy in noisy environments.
•The intended end users of this advice are people performing hearing
assessment such as audiologists.
Noise
Advice on which insert
earphone to use
Advice on which
earmuff to use
SLM
Correction factors for bone conduction thresholds (for
normal hearers with bone conductor on forehead)
Nearest 1 dB
Nearest 5 dB
250
22
20
dB
Test Tone Frequency Hz
500
1000
2000
16
10
3
15
10
5
4000
-1
0
creating sound valueTM
8000
1
0
Advice on the
Maximum Permissible
Ambient Noise Levels
Advice on correction factors
for bone conduction
measurements
2
Worked performed
Part 1
1.
2.
3.
Background research
•
Papers, Patents & Products
Objective testing of external noise attenuation using an acoustic
mannequin in a diffuse noise environment for:
1. Single Protection – Earmuffs or Insert Earphones
2. Dual Protection – Combinations Earmuffs & Insert Earphones
3. Effect of insert earphone delivery signal wire / tube
Objective testing (10 subjects) of external noise attenuation using
the Microphone in Real Ear (MIRE) technique in a diffuse noise
environment for single and dual protection
Part 2
Subjective testing (24 subjects) of the selected devices in terms of
1. external noise attenuation using the Real Ear Attenuation at
Threshold (REAT) technique in diffuse noise environment for single
and dual protection
2. bone and air conduction thresholds for single and dual protection
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3
Part 2 – Subjective testing
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
4
Part 2 – Subjective testing
Subjective testing
– Attenuation of background
noise
Narrow band
testing noise
in a diffuse
field
Open Ear
24 subjects tested
Testing Noise: 1/3 octave
narrow band at audiometric
frequencies
Attenuation =
difference in thresholds
with and without devices
fitted
creating sound valueTM
Insert
earphones
fitted
Insert
earphones
and earmuffs
fitted
5
Part 2 – Subjective testing results
Attenuation of background noise for an EAR-3A or ER-3A insert earphone
with/without a MSA 766 243 left/RIGHT “High” (yellow) headband
earmuff.
24 Subjects using the Real Ear Attenuation at Threshold (REAT) technique
compared to data on the TDH-50 + MX-41 and Audiocups [Berger 89]
0
125
250
Frequency (Hz)
500
1000
2000
4000
8000
TDH-50 + MX-41*
Attenuation (dB)
10
Audiocups*
20
Insert Earphones
30
40
50
60
creating sound valueTM
Insert Earphones &
Muffs
Limited by bone
conduction
6
Part 2 – Subjective testing results
Maximum Permissible Ambient Noise Levels – for an EAR-3A or ER-3A insert
earphone covered by a MSA 766 243 left/RIGHT “High” (yellow)
headband earmuff.
120.0
125 Hz to 8 kHz
100.0
250 Hz to 8 kHz
80.0
500 Hz to 8 kHz
60.0
40.0
20.0
0.0
31.5
40
50
63
80
100
125
160
200
250
315
400
500
630
800
1000
1250
1600
2000
2500
3150
4000
5000
6300
8000
MPANL in third octave band (dB SPL)
Maximum Permissible Ambient Noise Levels (MPANL's) for
testing hearing thresholds down to 0 dB HL with a maximum
uncertainty of +2 dB
using the selected insert earphones and earmuffs.
creating sound valueTM
Third Octave Frequencies (Hz)
7
Part 2 – Subjective testing results
Maximum Permissible Ambient Noise Levels – for an EAR-3A or ER-3A insert
earphone covered by a MSA 766 243 left/RIGHT “High” (yellow) headband
earmuff.
Mid frequency of one-third
octave band
Maximum permissible ambient noise levels (MPANL's)
Lmax dB SPL
Test tone frequency range
Hz
125 Hz to 8 kHz
250 Hz to 8 kHz
500 Hz to 8 kHz
31.5
79
89
101
40
77
87
98
50
74
84
95
63
70
80
92
80
68
78
89
100
64
74
86
125
60
71
83
160
56
63
80
200
54
54
76
250
54
54
72
315
56
56
71
400
58
58
64
500
60
60
60
630
56
56
56
800
54
54
54
1000
52
52
52
1250
48
48
48
1600
45
45
45
2000
41
41
41
2500
45
45
45
3150
48
48
48
4000
53
53
53
5000
6300
8000
54
54
54
54
54
54
54
54
54
NOTE - Using the values given, the lowest hearing threshold level to be measured is 0 dB, with a maximum uncertainty of + 2 dB due to the
ambient noise. If a maximum uncertainty of 5 dB due to ambient noise is permitted, the values may be increased by 8 dB.
creating sound valueTM
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Part 2 – Subjective testing
Bone conduction correction factors
Bone conduction thresholds will?
> reduce when wearing insert earphone and earmuffs
Bone
Conductor
(Centre of
forehead)
Insert Earphone
Earmuff
Ear canal
Bone conduction
moves ear canal walls as
well as cochlear
membranes and middle
ear ossicles etc
Extra vibration of tympanic
membrane resulting 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
Correction of bone conduction thresholds is required …
creating sound valueTM
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Part 2 – Subjective testing results
Correction factors for bone conduction thresholds with EAR 3A or
ER 3A insert earphones inserted with/without wearing a MSA
766 243 left/RIGHT “High” (yellow) headband earmuff.
Correction factors for bone conduction thresholds (for
normal hearers with bone conductor on forehead)
Nearest 1 dB
Nearest 5 dB
250
22
20
dB
Test Tone Frequency Hz
500
1000
2000
16
10
3
15
10
5
4000
-1
0
8000
1
0
NB The above correction factors correct for the effect of the insert earphones and
earmuffs and are appropriate if the bone conductor is calibrated for placement
on the forehead. If the bone conductor is calibrated for placement on the
mastoid then difference between forehead and mastoid presentation also needs
to be corrected for in addition to correction for the effect of the insert earphones
and earmuffs.
creating sound valueTM
10
Conclusions
Conclusions:
1. Double protection provides significantly better attenuation than single
protection using achievable insertion depths
2. Leakage due to cable / tube under muff is relatively minor
3. The commonly used ER-3A insert earphones perform as well if not
better than any other insert earphone when used in combination with
good earmuffs
4. Active earmuffs appear to offer little benefit over good passive
earmuffs when used in combination with insert earphones.
5. 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)
6. 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
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Summary & Acknowledgements
A reminder of what it is all about
Mid frequency of onethird octave band
EARTONE 3A or ER 3A
insert earphones
Noise
Advice on which insert
earphone to use
Advice on which
earmuff to use
MSA 766243 left / RIGHT,
High, Yellow, Headband earmuff
Nearest 1 dB
Nearest 5 dB
250
22
20
dB
Test Tone Frequency Hz
500
1000
2000
16
10
3
15
10
5
4000
-1
0
8000
1
0
250 Hz to 8 kHz
500 Hz to 8 kHz
79
89
101
40
77
87
98
50
74
84
95
63
70
80
92
80
68
78
89
100
64
74
86
125
60
71
83
160
56
63
80
200
54
54
76
250
54
54
72
315
56
56
71
400
58
58
64
500
60
60
60
630
56
56
56
800
54
54
54
1000
52
52
52
1250
48
48
48
1600
45
45
45
2000
41
41
41
2500
45
45
45
3150
48
48
48
4000
53
53
53
5000
54
54
54
Advice on the
Maximum Permissible
Ambient Noise Levels
6300
54
54
54
8000
54
54
54
Advice on correction factors
for bone conduction
measurements
National Acoustic Laboratories
This research was financially
supported by the HEARing CRC Ltd
established and supported under
the Australian Government’s
Cooperative Research Centres
Program
creating sound valueTM
Test tone frequency range
125 Hz to 8 kHz
NOTE - Using the values given, the lowest hearing threshold level to be measured is 0 dB, with a maximum
uncertainty of + 2 dB due to the ambient noise. If a maximum uncertainty of 5 dB due to ambient noise is permitted,
the values may be increased by 8 dB.
SLM
Correction factors for bone conduction thresholds (for
normal hearers with bone conductor on forehead)
Maximum permissible ambient noise levels (MPANL's)
L max dB SPL
Hz
31.5
the late
Ben
Rudzyn
12