Download Improved Hearing Assessment in Noisy Environments

Improved Hearing Assessment in Noisy
Environments
Michael Fisher1,2, Ben Rudzyn1,2, Gordon Jarvis2 and Harvey Dillon1,2
1. The HEARing CRC, 2. National Acoustic Laboratories
XX Audiology Australia National Conference 2012 Adelaide
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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:
1. Air Conduction
2. Bone Conduction
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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
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Introduction
The conventional solution to this problem is to use a
soundproof booth to attenuate the environmental noise.
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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
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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.
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Investigation Aim
To find the best practical solution to achieve accurate
audiometric hearing assessment without employing a
soundproof booth.
Provide advice on correction
factors for bone conduction
measurements
Provide advice on
devices to use
SLM
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Provide advice on the
Maximum Permissible
Ambient Noise Levels
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Investigation – Detailed Plan
Part 1. Preliminary investigation:
Objective testing of external noise attenuation using an acoustic
mannequin for:
I.
Single attenuators – headphones or insert earphones
II.
Dual attenuators – combinations earmuffs & insert earphones
III.
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:
I.
external noise attenuation using the Real Ear Attenuation at
Threshold (REAT) technique for single and dual protection
II. bone and air conduction thresholds for single and dual protection
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Part 2 – Subjective evaluation
Subjective assessment of the selected devices (as
determined by objective measurement in Part 1)
EARTONE 3A or ER 3A
insert earphones
Selected
Earphone
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MSA 766243 left / RIGHT,
High, Yellow, Headband earmuff
Selected
Earmuff
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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.
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Part 2 – Subjective evaluation
The ER-3A tube should have a gentle curve within the earmuff and should run
down the neck.
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Method – Attenuation evaluation
Subjective testing
(REAT Method)
Diffuse sound field
Obtain Thopen
Open Ear
Subjects: 24 normal hearers
Test: Békésy type automatic
threshold determination
Obtain Thearphone
Testing signal: third-octave narrowband noise at audiometric
frequencies presented in a diffuse
environment
Attenuation (Att) equals the
difference in threshold (Th)
Attearphone = Thearphone – Thopen
Obtain Thearphone+earmuff
Insert
earphones
fitted
Insert
earphones
and earmuffs
fitted
Attearphone+earmuff = Thearphone+earmuff – Thopen
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Results – Attenuation evaluation
Results show the average attenuation of the insert earphones (ER-3A) alone and
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 Audiocups devices [Berger 89].
Average attenuation (dB) at one octave centre frequencies (Hz)
125
250
500
1k
2k
4k
8k
Insert Earphones
31
30
32
33
33
41
42
Insert Earphones & Muffs
35
41
49
44
37
49
45
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Results - Maximum Permissible
Ambient Noise Levels (MPANL’s)
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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
Conductor
(Centre of
forehead)
Bone conducted vibration
moves ear canal walls as
well as cochlear
membranes and middle
ear ossicles etc
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
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Method – Evaluating the effect
on audiometric thresholds
Subjective testing
Subjects: 24 normal hearers
Open Ear
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
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Insert
earphones
fitted
Insert
earphones
and earmuffs
fitted
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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
Nearest 1 dB
Nearest 5 dB
Correction factors for bone conduction thresholds
(for normal hearers with bone conductor on the forehead) (dB)
Test Tone Frequency (Hz)
250
500
1000
2000
4000
8000
22
16
10
3
-1
1
20
15
10
5
0
0
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.
Nearest 1 dB
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Mastoid to Forehead Correction Factor (dB)
Test Tone Frequency (Hz)
250
500
1000
2000
4000
-12
-14
-9
-12
-8
8000
-10
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Conclusions
1. Double protection provides significantly better attenuation than single
protection using achievable insertion depths
2. The commonly used ER-3A insert earphones perform as well if not
better than other insert earphones when used in combination with
good earmuffs
3. 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 at 2 kHz)
4. There is no change in air conduction thresholds as a result of using
earmuffs over insert earphones.
5. The change in the bone conduction thresholds due to insert earphones
and earmuffs being worn by normal hearers can be compensated for.
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Summary & Acknowledgements
Advice on correction factors
for bone conduction
measurements
Advice on which insert
earphone to use
Correction factors for bone conduction thresholds (for
normal hearers with bone conductor on forehead)
EARTONE 3A or ER 3A
insert earphones
Nearest 1 dB
Nearest 5 dB
Advice on which
earmuff to use
250
22
20
dB
Test Tone Frequency Hz
500
1000
2000
16
10
3
15
10
5
4000
-1
0
8000
1
0
Advice on the Maximum Permissible
Ambient Noise Levels for a minimum
achievable threshold
MSA 766243 left / RIGHT,
High, Yellow, Headband earmuff
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
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In memory
of the late
Ben Rudzyn
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