Audiogram and Middle Ear Construction

Abstract

To repeat man is a low-frequency form among mammals. The hearing capabilities in man were thoroughly examined by Fletcher and Munson (1933), using a large number of test subjects; a number of later studies confirmed their results on the frequency response of the human ear. Sensitivity is greatest in the frequency range of 3 kHz to 4 kHz; the upper limit of hearing is at ~ 20 kHz in children, but decreases with age toward presbycusis. Thus, for adults the upper limit of hearing is at ~ 15 kHz. Although the methods employed to determine the hearing capabilities in mammalian animals are widely different and both direct and indirect, it is nevertheless clear that only a few species do not have a hearing range extending into the range of ultrasound (frequencies above 20 kHz). One such indirect method was worked out by Fleischer (1973a, 1976a, 1976b), who demonstrated that the structure of the inner ear — especially the suspension system of the basilar membrane — depends on the hearing capability. From all the results available, elephants, camels, perhaps sloths, and various forms of moles apparently have similar, low-frequency characteristics of their ears. All of these species have a freely mobile type of the malleus-incus complex and tympanic membranes larger than those of others with comparable size of the inner ear.