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What is sound?

Sound is the sensation produced by regular pressure variations that is detected by the human ear. Under normal circumstances these will be transmitted to the ear through air.

A vibrating source eg. vocal cords, a loudspeaker or a motor casing, initiates a succession of compressions and rarefactions of the air molecules near the source. This produces a similar effect on adjacent molecules and energy radiates out in all directions.

Regular disturbance of air may be produced by objects rotating or moving quickly. This is caused by the turbulence set up. Although a wide range of sound frequencies are often produced, most of the sound energy is usually emitted in a few identifiable frequency bands related to the motion of the object. Sound sources in this category include fans, air flowing in ducts and spinning wheels.


The normal frequency range detected by the human ear is often taken as 20 Hertz (waves per second) to 20 kiloHertz. Below 20 Hz (and to some extent above it) the recovery time of the ear is such that it can follow the individual pressure variations. The upper limit is very dependent on the sensitivity of the ear and at normal intensities only healthy young ears detect sound in this region.


For air at room temperature the velocity of sound is commonly taken as 343 m/s.

The frequency (f), wavelength (l) and velocity are related by the equation: c = l x f

Consequently the wavelength across the frequency spectrum varies from about 17m for low frequency sound waves to about 17mm at the upper limit of hearing.