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Tue 26 Dec 06 @ 1:09 am
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Acoustics / Noise
A unit of a logarithmic scale of power or intensity called the power level or intensity level. The decibel is defined as one tenth of a bel where one bel represents a difference in level between two intensities I1, I0 where one is ten times greater than the other. Thus, the intensity level is the comparison of one intensity to another and may be expressed:
Intensity level = 10 log10 (I1 /I0) (dB)
For instance, the difference between intensities of 10-8 watts/m2 and 10-4 watts/m2, an actual difference of 10,000 units, can be expressed as a difference of 4 bels or 40 decibels.
Because of the very large range of SOUND INTENSITY which the ear can accommodate, from the loudest (1 watt/m2) to the quietest (10-12 watts/m2), it is convenient to express these values as a function of powers of 10. This entire range of intensities can be expressed on a scale of 120 dB. (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.)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. 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. Thus, two 70 dB cars together measure 73 dB under ideal conditions. However, note that when the AMPLITUDE of a single sound is doubled, its level rises 6 dB.
