These newer systems have brought environmental factors and frequency content to bear on the measurement of LOUDNESS. See: NOISE, NOISE LEVEL, NOISE RATING, NOISE & NUMBER INDEX, PERCEIVED NOISE LEVEL, TRAFFIC NOISE INDEX. The term became accepted in the 1920s and since then noise measurement has generally come to rely on the decibel scale and others derived from it. ĭecibels may be qualified as dBA, dBB, dBC, indicating the weighting network of the SOUND LEVEL METER with which the measurement was made. See also: HEARING LEVEL, LOUDNESS LEVEL, SOUND LEVEL, SOUND POWER LEVEL, SOUND PRESSURE LEVEL. See: AUDIOGRAM, LEVEL RECORDER, VU METER, ZERO LEVEL VU. In some situations, such as tape recording, a given intensity level is assigned 0 dB, and other levels are measured in negative decibels in comparison to it. Sound Example : Ramp descending at 6 dB per event, followed by a ramp descending at 3 dB.Ġ dB is defined as the THRESHOLD OF HEARING, and it is with reference to this internationally agreed upon quantity that decibel measurements are made. However, note that when the AMPLITUDE of a single sound is doubled, its level rises 6 dB. Thus, two 70 dB cars together measure 73 dB under ideal conditions. When two sound sources of equal intensity or power are measured together, their combined intensity level is 3 dB higher than the level of either separately. The result of this logarithmic basis for the scale is that increasing a sound intensity by a factor of 10 raises its level by 10 dB increasing it by a factor of 100 raises its level by 20 dB by 1,000, 30 dB and so on.

(The physicist Alexander Wood once compared this range from loudest to quietest to the energy received from a 50 watt bulb situated in London, ranging from close by to that received by someone in New York.) See: DYNAMIC RANGE. This entire range of intensities can be expressed on a scale of 120 dB. Thus, the intensity level is the comparison of one intensity to another and may be expressed: Intensity level = 10 log 10 (I 1 /I 0 ) (dB)įor instance, the difference between intensities of 10 -8 watts/m 2 and 10 -4 watts/m 2, an actual difference of 10,000 units, can be expressed as a difference of 4 bels or 40 decibels.īecause of the very large range of SOUND INTENSITY which the ear can accommodate, from the loudest (1 watt/m 2 ) to the quietest (10 -12 watts/m 2 ), it is convenient to express these values as a function of powers of 10. The decibel is defined as one tenth of a bel where one bel represents a difference in level between two intensities I 1, I 0 where one is ten times greater than the other. A unit of a logarithmic scale of power or intensity called the power level or intensity level.

Decibel reference scale