Acoustics

Physiological acoustics is the study of the transmission of sound and how it is heard by the human ear. Sound travels in waves, vibrations that cause compression and rarefaction of molecules in the air. The organ of hearing, the ear, has three basic parts that collect and transmit these vibrations: the outer, middle and inner ear. The outer ear is made of the pinna, the external part of the ear that can be seen, which acts to funnel sound through the ear canal toward the eardrum or tympanic membrane. The membrane is highly sensitive to vibrations and also protects the middle and inner ear. When the eardrum vibrates it sets up vibrations in the three tiny bones of the middle ear, the malleus, incus and stapes, which are often called the hammer, anvil and stirrup because of their resemblance to those objects. These bones amplify the sound. The stapes is connected to the oval window, the entrance to the inner ear, which contains a spiral-shaped, fluid-filled chamber called the cochlea. When vibrations are transmitted from the stapes to the oval window, the fluid within the cochlea is put into motion. Tiny hairs that line the basilar membrane of the cochlea, a membrane that divides the cochlea lengthwise, move in accordance with the wave pattern. The hair cells convert the mechanical energy of the waveform into nerve signals that reach the auditory nerve and then the brain. In the brain, sound is interpreted.

In the sonic range of frequencies, the microphone, a device using electrical and mechanical components, is the common method of receiving sounds. One simple form is to have a diaphragm as one plate of an electrical condenser. When the diaphragm vibrates under the action of a sound wave, the current in the circuit varies due to the varying capacitance of the condenser. This varying current can then be used to activate a meter or oscilloscope or, after suitable processing, make an audio tape or some such permanent record.