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 Max NIHL Sensitivity (kHz) (dB) (dB)

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 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