Transcript MECHANISMS OF HEARING

MAKING INDUSTRIAL
AUDIOMETRY
WORTHWHILE
Robin Howie
Robin Howie Associates
CONVENTIONAL
TEACHING
Noise Induced Hearing Loss
(NIHL) is a loss in sensitivity,
primarily at about 4 kHz
FUNCTIONS OF HEARING
To detect alarm sounds
To communicate
To locate sound sources
To enjoy sounds
FUNCTIONS OF HEARING
Of the above functions, the ability
to understand speech is probably
the most important in our society
SPEECH
English speech can involve rapid
simultaneous changes of frequency
and intensity
SPEECH
The ear’s ability to distinguish
small differences in both frequency
and intensity permits speech to be
understood
SELECTIVITY
The ear is able to detect frequency
differences of the order of 1-2%
between two frequencies
It is this ability which allows us to
“hear” a solo singer against the
background of a full orchestra
SELECTIVITY MECHANISM
Current theories suggest that the
IHC are signal detectors which
form a positive feedback loop with
the brain and with the OHC
supplying energy, so increasing
gain and selectivity
SELECTIVITY
Noise induced hearing loss reduces
both sensitivity and frequency
selectivity
SELECTIVITY
Many persons with sensori-neural
hearing loss can understand
speech well in the quiet but have
progressively greater difficulty as
background noise levels increase
NOISE INDUCED HEARING
LOSS
Persons with noise induced
hearing loss often have sufficient
hearing sensitivity to be aware that
someone is speaking but may have
insufficient selectivity to be able to
decipher speech signals in noisy
environments
AUDIOMETRY
What is the purpose of
audiometry?
Directive 2003/10/EC
Article 10 – Health surveillance
10(2) A worker whose exposure
exceeds the upper exposure action
values [85 dB(A)/140 Pa] shall have
the right to have his/her hearing
checked ...
Directive 2003/10/EC
Article 10 – Health surveillance
10(2) The objectives of these checks
are to provide early diagnosis of
any loss of hearing due to noise,
and to preserve the hearing
function.
RESOLUTION
What resolution is required to meet
the above objectives?
Hearing check requirements
To enable the objects of Article 10
to be achieved, hearing tests must
reliably detect early NIHL in
individuals caused by exposure
to 85 dB(A)
Required audiometric
sensitivity
The objects of early diagnosis and
preservation of hearing mean that
< 50% of the maximum NIHL at
critical frequencies must be reliably
detected in individual subjects
Required audiometric
sensitivity
Each individual test must therefore
allow < 25% of the maximum
NIHL at critical frequencies to be
reliably detected
Required audiometric
sensitivity
Fre’ncy
(kHz)
1
2
3
4
Max NIHL Sensitivity
(dB)
(dB)
1.5
3.2
0.8
5.8
1.5
6.7
1.5
Sensitivity of conventional
audiometry
“… with careful, well-conducted audiometry …
changes of 10 dB between audiograms …
should be regarded as possibly significant.”
“… the accuracy could be increased two-fold by
repeating the audiogram four times …”
Burns (1973)
Audiometric technique which
achieves the required
sensitivity
Protocol
Test only subjects without ear wax, ear infection,
congestion or recent exposure to high noise
levels
Two tests per subject per session
Test at 1, 2, 3, 4, 6 and 8 kHz only
Give subjects soft drinks prior to and during the
test to help clear their Eustachian tubes
Ensure subjects wear “quiet” clothing
Equipment
Ear inserts rather than ear phones
Test tones presented in 1dB steps rather
than 2.5 or 5.0 dB steps
Record increasing SPL thresholds only
Record seven thresholds at each frequency
for each ear
Data analysis
Analyse the last 5 of 7 increasing SPL
thresholds only
Record thresholds on spreadsheets, so
enabling ANOVA between current and
previous audiogram(s)
Analyse results for 4 or 6 kHz “dips”
Sensitivity of modified
technique
Differences of 1-2 dB between test
sessions can be resolved with trained
subjects
Details of modified
technique
Howie, Gardiner and Watt (1998)
OTO 98 020 on HSE website
HOWEVER
The critical consequence of NIHL is loss of
frequency selectivity rather than loss of
sensitivity
Loss of sensitivity is a poor predictor of loss
of selectivity
SPEECH AUDIOMETERY
We should evaluate speech audiometry as a
hearing surveillance techqique
SPEECH AUDIOMETERY
Speech audiometry is a direct measure of
the generally most important hearing
function