ICSV 14 Cairns • Australia 9 - 12 July , 2007 COCHLEAR MECHANICS , THE ACOUSTIC REFLEX AND NOISE INDUCED HEARING LOSS

In a noise environment devoid of impulsive sounds the ear responds as a linear system to levels sufficient to destroy it in response to energy input. In such environments a long time average Aweighted measurement will provide a suitable measure of exposure. In an environment characterised by impulsive sounds, however, it is proposed that the response of the ear is determined by the response time as well as the on-time of the acoustic reflex and the durability of the hair cells. Pickles[1] has stated, “the reflex is too slow to protect the ear against impulsive noise,” which is true of a single impulse, however, in an environment characterised by many impulses the effectiveness of the reflex may also depend upon the durability of the hair cells, as well as the reflex on-time. It is proposed that the acoustic reflex can respond quickly enough to reduce sound transmission enabling the ear to respond as a linear system in the low frequency range to 2 kHz however, it cannot respond quickly enough to enable linear response in the high frequency range above 2 kHz. Consequently, in the high frequency range of 4 kHz, where the ear is most sensitive, the ear responds to impulsive sounds as a non-linear system where destruction of the ear is proportional to peak pressure. On this basis the well known“4 kHz dip” may be explained.