Sound is the mechanical vibration of a gaseous, liquid or solid elastic medium through which
energy is transferred away from the source by progressive sound waves.
This is the strict physical definition of sound. More generally we restrict the term sound to
be pressure variations which can be detected by the human ear.
Sound pressure level (SPL) or sound level is a logarithmic measure of the effective sound
pressure of a sound relative to a reference value. It is measured in decibels (dB) above a standard reference level.
The standard reference sound pressure in air or other gases is 20 �Pa, which is usually considered the threshold of
human hearing (at 1 kHz).
The human hearing mechanism requires that the variations occur at least 20 times a second, but
not more frequent than 20 000 times a second. The number of variations per second is called the frequency of the sound
and is expressed in hertz (Hz). Hence, the human hearing is able to hear sound with frequencies ranging from 20 Hz to
20 000 Hz, the latter often written as 20 kHz. Of course, pressure variations with frequencies lower than 20 Hz should
also be regarded as sound. They are normally referred to as infrasound. Sounds with frequencies higher than 20 kHz lie
above the audible region and are referred to as ultrasound.
Note: The propagation speed of sound in air at ordinary temperatures is approx. 343 m/s,
corresponding to about 1224 kilometres per hour. Given the speed of sound propagation we are able to calculate the
wavelength at a certain frequency by using the following relation:
c = f � L
in which c is the speed of sound,
f the frequency and L the wavelength.
An inspection of this relation will reveal that the
wavelength is inversely proportional to the frequency, i.e. low frequency means a large wavelength and vice versa.
Examples: at 20 Hz the wavelength is 17 m, while at 20 000 Hz it is merely 1.7 centimetres.