Document j2rX6nYwJOdJLYg89rZxrmDZ

Industrial Noise comes a disability only if defined as such by law This is true of the term "occupational disease" also We may be correct m saying that a loss of function is occupationally in duced, but legally that loss of function need not be an occupational disease What might be classed as an occupational disease or a dis ability m one state will not necessarily be so classed in another Most of the recent problems encountered in establishing equitable compensation laws for noise-induced heanng loss are a result of provisions declaring that occupational dis ease may be compensable Heanng loss un questionably reduces a man's capacity, but if the legal cntena used in many states for establishing disability are applied to noiseinduced heanng loss, it is not legally a dis ability While there is no date of miury or, in most cases, is there a loss of wages, there is no question that heanng loss can be occupa tionally induced Since the late 1940's, courts have fre quently awarded financial compensation for heanng losses even though no specific date of injury could be shown and the claimant had suffered no wage loss, the standard bench marks used since the laws were promulgated dunng the decade between 1910 and 1920 This increased emphasis on social loss caused by heanng impairment has put increased pressure on industry to worry more about conservation of heanng than compensation as a result of its loss The Ear and Hearing Anatomy of the ear The ear is among the most complex of the sensory organs Its dynamic range of re sponse is enormous Sounds so faint they might well have been caused by the Brownian motion of molecules can be heard, and yet man can also hear without distortion sounds that are a million, million times more intense Man also possesses the ability to distinguish some 300,000 tones of different frequency and/or intensity and to analyze complex sounds into their component parts The ear, that mechanism which is capable of such diversity, is a delicate, complicated structure It is divided into three parts the external ear, the middle ear, and the internal (inner) ear (Fig 45-1) The external ear The external ear acts as a conducting channel for sound It is bounded outside by the auricle or pinna, and inside by the external layer of the drumhead or tympanic membrane, and consists of the au ricle and the external auditory canal This canal is about 25 millimeters long and is S-shaped Sound waves traveling down this canal strike the eardrum The middle ear. Anatomically, the middle ear is bounded externally by the inner layer of the drumhead and internally by the mucous membrane which covers the bony wall just external to the inner ear Within the middle ear cavity is a linkage of three ear bones or ossicles the malleus (hammer) the incus (anvil), and the stapes (stirrup), and two small muscles This linkage transmits the vibra tions to the oval window--one of the five openings in the middle ear cavity The middle ear has three functions It transmits energy from the sound vi brations in the air column of the external auditory canal, by means of the drumhead ossicular linkage, and oval window, into the fluid contained within the cochlea (de scribed under "inner ear") It is apparent from the degree of heanng loss that exists when the drumhead and ossicular chain are not present in an ear, that vibrating air par ticles alone do not exert enough force on the oval window to transfer adequate amounts of acoustic energy to the cochlear fluid The surface area of the drumhead plus the ossicu lar chain, however, with its attached muscles and ligaments, and the footplate of the stapes all act as a complex impedance-matching device effecting an efficient transfer of acoustic energy from airborne sound waves to sound waves m the fluid medium of the inner ear (The flattened bony footplate of the stapes fits the oval window and acts as a hinge, allowing the stapes to move m trap door fashion The round window is very small and is located below and in front of the oval window, and is covered by a thin membrane resembling the drumhead ) It protects the inner ear from the shock of intense sounds of low frequencies by the reflexive action of the middle ear muscles These muscles tense the drumhead and os sicular chain thus reducing the amplitude of 1510