1. Noise can impair hearing by its intensity as well as by its duration. A single loud sound or explosion if occurring suddenly and unexpectedly is injurious.
2. The hearing loss is at first a reversible process but prolonged and sustained exposure to noise leads to irreversible changes. What at first appears to be a manifestation of fatigue may ultimately result in pathological changes.
3. Noise injures the most sensitive and vital parts of the organ of hearing.
4. The resulting deafness in certain noisy occupations constitutes what one might term an occupational hazard.
5. Not every human ear is injured by loud continual noise to the same extent. The healthier the organ of hearing, the less harmful is likely to be the effects of noise.
6. If hearing becomes impaired and it invariably is manifest at first in the high tones, the individual cannot hear whistles or tones of a high pitch. Prolongation of exposure to the effects of noise will result in the degenerative process of other cells of the cochlea and other frequencies will be impaired.
7. Impairment of hearing caused by noise is gradual in development and hardly noticed by the individual. Only examination by an otologist with reliable equipment at his disposal will reveal the initial loss.
8. An individual suffering from any ear disease or hearing impairment should be seen by an otologist if engaged in a noisy occupation. If engaged, frequent checks should be carried out.
9. If a worker finds that a noisy occupation impairs his hearing or affects his general well being, he should choose different work.
10. Every endeavour should be made to limit noise. It includes the proper distribution of machinery and suitable contrivances for the suppression or reduction of noise such as sound proofing.
11. Ear wardens shoa11 be worn in noisy occupations. Plugging the ears with cotton wool alone is not sufficient. A close fitting comfortable ear warden with an attenuation of 30 decibels should be used.
12. A partial hearing loss greater than would be expected on the basis of age alone seems to be statistically evident among workers in very noisy industries, including certain branches of military service. It is possible that many cases of definite hearing loss may be due to a single exposure to a louder than usual sound and not to a cumulative effect of more moderate exposures.
13. Present evidence does not indicate that air borne ultrasonic vibrations constitute a practical hazard to hearing or produce any specific effects on the nervous system or sense organs.
14. Sound above 120 decibels stimulates the sense of touch and may cause temporary and possibly permanent damage to hearing. Levels above this are harmful to the ear and exposure without special protection should be avoided.
15. Intense sound may give rise to a disturbance of equilibrium. This is regarded as a manifestation of sound intensity rather than one of frequency.
16. Special equipment, properly calibrated and maintained is necessary for the measurement of noise intensities, analysis of noise, and evaluation of hearing losses.
This Thesis is based on personal work and research carried out by the author since his appointment as Consultant in Otorhinolaryngology to the Royal Air Force. Some of the observations contained in this thesis are based on several papers and reports which have been published from time to time.
