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Outer/Middle-Ear Filtering

The outer ear modifies the sound waves in transferring the acoustic vibrations to the ear drum (tympanic membrane). It consists of a partially cartilaginous flange (the pinna) which includes a resonant cavity (the concha) at the entrance to the ear canal (external auditory meatus). The ear canal terminates at the ear drum, which may be considered as the interface of the outer- and middle-ear. The resonances of the outer-ear increase the sound pressure at the ear drum, particularly in the range of frequencies in man of 2-7 kHz - a band pass function (Pickles 1988).

Sound reaches the inner ear or cochlea via three extremely delicate middle-ear bones - the malleus, incus and stapes - which form an interconnecting link between the ear drum and the input port of the cochlea. The transformer action of the middle-ear helps to match the impedance of the air in the ear canal to the much higher impedance of the cochlea fluids. In doing so the middle-ear also increases a band of frequencies, and is thus another band pass function. Both the outer- and middle-ear band pass functions combine to give an approximately flat-topped band pass function. This function is modelled in the CRL as a Pre-Emphasis band pass filter.


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