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.
RESOLUTIONWhat 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 Max NIHL Sensitivity
(kHz) (dB) (dB)
1 1.5 -
2 3.2 0.8
3 5.8 1.5
4 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