At its most simple level, sound is a rapidly fluctuating change in some medium – typically, air. Hosken says “sound is caused by vibrations in the air,” but this only describes one medium.
Generation
Vibrations in the air need to be initiated in some way. Some action has to initiate, or start this fluctuation in air pressure. For instruments, that action could be plucking a string, or blowing on a mouthpiece and reed, or buzzing one’s lips into a mouthpiece, or drawing a bow across a string. All of these actions will initiate a vibration.
Propagation
Once a vibration has started, it must travel through the air. Vibrations travel through the air by a process called propagation. The vibration causes air molecules around the vibrating body to compress in state of relative positive pressure. Since compression cannot be maintained without constant energy, the compression releases and causes a state of relative negative pressure called rarefraction. It’s best to think of air (or any other sound transmitting substance) as “springy.” When a spring is pulled it wants to return to its starting point, but the energy in the spring causes it to move past its starting point. And the cycle continues until the energy has died out.
Through a process of compression and rarefraction, vibrations travel through the air from a point of initiation out in all directions. Individual air molecules only move back and forth within a small (minute) area. Compression one area leads to rarefraction in adjacent areas. Rarefraction leads to compression in adjacent areas. Through this process of propagation, sound travels from one point to another.
Reception
In order to hear sound, vibrations need to reach a receptacle – our ear. The pinna of the outer ear helps to collect vibrations and focus them into the ear canal. The ear canal, also part of the outer ear, amplifies the frequency range from 2,000 – 5,000 Hz because of its shape and length. We are most sensitive in our hearing to that frequency range, which improves the understandability of speech and other necessary sounds.
Through the ear canal vibrations reach the tympanic membrane (eardrum), which vibrates three small bones known as the ossicles. These bones pass the vibrations from the tympanic membrane to the oval window of the cochlea.
The cochlea comprises the inner ear. It is an organ that is filled with fluid that vibrates in response to sound, triggering movement in tiny hair cells along the basilar membrane that correspond to different frequencies. These hair cells in stimulate nerve receptors that transmit signals to the organ of Corti.
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