A decibel is a measurement of the ratio between two quantities, and is used as a comparator in many branches of physics.

In acoustics, decibels (or dB, for short) are used to indicate the loudness of a single sound in comparison to a fixed reference sound. This reference is the quietest sound audible to the healthy human ear – roughly equivalent to the sound of a mosquito flying 3 m (10 ft) away. This is called 0 dB.

The reason for using decibels as the measurement system for loudness is that our perception of loudness corresponds to a logarithmic curve rather than a simple straight line. Increased loudness is caused by increased air pressure, the consequence of which is that sounds approaching 125 dB will begin to cause pain and sounds over 180 dB will cause damage to the hearing tissue. Because changes in air pressure alter our perception of loudness, the term dB (SPL) is normally used, meaning ‘decibel sound pressure level’, in order to distinguish it from other dB measurements.

The human ear, however, is more sensitive to high frequencies. Noises above a' have a greater impact on the ear than those below. An additional method of dB measurement takes this into account, altering higher-pitched sounds slightly upwards to giver a truer indication of the effect they have on the ear. This measurement is known as dB (A-weighted).

Decibel level alters dramatically with distance and if there is interference from other noises. A string section in a symphony orchestra, for example, is not 45 times louder than a solo violin. It should also be remembered that any instrument can be played so quietly that it is inaudible – it is only in the upper levels of loudness that a limit is reached.