Lewis L. Spence
The outer portion of the ear called the pinna directs sound waves into the ear canal. The sound waves cause a displacement in the very thin delicate membrane called the eardrum. On the inner side of the eardrum is attached a chain of three bones (the hammer, anvil and stirrup) which has the other end attached to the oval window. This area constitutes the middle ear. The small displacement of the eardrum is transferred along this chain of bones which creates a much larger displacement in the membrane of the oval window. On the other side of this membrane is the inner ear where the cochlea, a coiled device sits in an aqueous solution. Here the vibrations are transformed into electrical impulses which are transmitted to the brain by route of the auditory nerve. The brain interprets the signals.
The human ear is most sensitive to frequencies within the range of 2 and 4 kHz, which to some extent is the result of the ear canal which resonates within this frequency range. We perceive speech mainly in the frequency range of 500Hz to 3000 Hz. A man's voice which is generally lower than a female’s voice because of the lower frequency band due to the size of the vocal tract, is still perceived over the telephone even though most of the low frequencies are filtered out. The recognition is still possible because “our perception mechanism tends to fill them back in”.
