Noise, or unwanted sound, is a pervasive occupational health problem. It is a by-product of many industrial processes. Sound consists of pressure changes in a medium (usually air), caused by vibration or turbulence. These pressure changes produce waves emanating away from the turbulent or vibrating source. Sound pressure level is a logarithmic measure of the magnitude or intensity of the pressure change; it is perceived as loudness. Sound pressure level is expressed in decibels, abbreviated dB. Because of the logarithmic scale used to measure sound pressure or noise, a small increase in decibels represents a large increase in sound energy. Technically, each increase of 3 dB represents a doubling of sound energy; an increase of 10 dB represents a tenfold increase, and a 20 dB increase represents a 100-fold increase in sound energy. (For purposes of 29 CFR 1910.95, however, a doubling rate of approximately 5 dB is used. That is, a 5 dB increase in level is permitted each time the exposure duration is decreased by half.) The frequency of a sound is the number of times that a complete cycle of compressions and rarefactions occurs in a second. The descriptor, which used to be "cycles per second," is now hertz (Hz). Frequency is perceived as pitch. Most everyday sounds contain a mixture of frequencies generated by a variety of sources. A sound's frequency composition is referred to as the spectrum.