The intensity of sound is expressed in terms of the square of the sound pressure. The bel is a ratio and is equivalent to a lO-fold increase in sound intensity; a decibel (dB) is one-tenth of a bel. Sound is made up of a number of frequencies ranging from 30 hertz (Hz) to 20 kHz, with most being between 1 and 4 kHz. When measuring sound, these different frequencies must be taken into account. In practice a scale known as A-weighted sound is used; sound levels are reported as dB(A). A hazardous sound source is defined as one with an overall sound pressure greater than 90 dB(A). Repeated prolonged exposure to loud noise, particularly in the frequency range of 2-6 kHz, causes first temporary and later permanent hearing loss due to damage to the organ of Corti, with destruction of hair cells and, eventually, the auditory neurones. This is a common occupational problem, not only in industry and the armed forces, but also in the home (e.g. from electric drills and sanders), in sport (e.g. motor racing) and in entertainment (pop stars, their audiences and disc jockeys).
Serious noise-induced hearing loss is almost wholly preventable by personal protection (ear muffs, ear plugs); little treatment can be offered once deafness becomes established.