The decibel (dB) is a measure of sound intensity; that is, the magnitude of the fluctuations in air pressure caused by sound waves. The decibel scale is logarithmic, not arithmetic. This means that a doubling of sound intensity is not represented as a doubling of the decibel level. In fact, an increase of just 3 dB means twice as much sound, and an increase of 10 dB means ten times as much sound.

A sound pressure level of 0 dB represents the threshold of hearing in the most sensitive frequency range of a young, healthy ear, while the thresholds of tickling or painful sensations in the ear occur at about 120 to 130 dB.

Decibels are usually measured with a filter that emphasizes sounds in certain frequencies. The "A" filter (dBA) is the one most frequently used. The "C" filter (dBC) puts more weight on low-frequency sounds such as the bass in amplified music.

The perception of loudness by the human ear is not directly proportional to the decibel level. For example, a sound 10 dB greater than another is not perceived as being ten times as loud but only about three times as loud.

The intensity of noise diminishes with distance. Outdoors, and in absence of any close reflecting surface, the effective decibel level diminishes at a rate of 6 dB for each factor of two increase in distance. For example, a sound measuring 100 dB at 10 metres would be 94 dB at 20 metres, 88 dB at 40 metres, and so on.