DECIBEL (dB)
 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.) See: DYNAMIC RANGE.

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.

Sound Example: Ramp descending at 6 dB per event, followed by a ramp descending at 3 dB.

0 dB is defined as the THRESHOLD OF HEARING, and it is with reference to this internationally agreed upon quantity that decibel measurements are made. 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.

Decibels may be qualified as dBA, dBB, dBC, indicating the weighting network of the SOUND LEVEL METER with which the measurement was made. 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.

These newer systems have brought environmental factors and frequency content to bear on the measurement of LOUDNESS. The PHON scale attempts to account for the subjective response of the ear to loudness, which is not possible with the decibel measurement of intensity. See also: EQUAL LOUDNESS CONTOURS.

See INVERSE-SQUARE LAW for variation of decibel measurement with distance, and SOUND PRESSURE LEVEL for scale according to which decibel measurements may be combined. Appendix D gives a conversion chart of voltage and power ratios to decibels.

 Threshold of hearing 0 dB Motorcycle (30 feet) 88 dB Rustling leaves 20 dB Foodblender (3 feet) 90 dB Quiet whisper (3 feet) 30 dB Subway (inside) 94 dB Quiet home 40 dB Diesel truck (30 feet) 100 dB Quiet street 50 dB Power mower (3 feet) 107 dB Normal conversation 60 dB Pneumatic riveter (3 feet) 115 dB Inside car 70 dB Chainsaw (3 feet) 117 dB Loud singing (3 feet) 75 dB Amplified Rock and Roll (6 feet) 120 dB Automobile (25 feet) 80 dB Jet plane (100 feet) 130 dB

Typical average decibel levels (dBA) of some common sounds.

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