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