Decibel

A decibel (or dB) measures ratios of power or intensity. It expresses them as an exponential function. One bel is a power ratio of 10:1, and is divided into ten decibels. An increase of three decibels is approximately a doubling of power. Decibels are often used in measuring telecommunication signals. With electric audio signals, there are several decibel units, relative to several bases. For example, dBm is relative to one milliwatt.

It's been said that the smallest difference humans can hear is 0 dB and is related to the Absolute threshold of hearing, although this is very subjective at best.

History

The bel unit was named after Alexander Graham Bell. This unit was so rough that it is more typical to use the decibel, which is one bel divided by ten. Before bels, there was the Transmission Unit (TU).

Examples and Protection

Often, decibels are used to say how loud a sound is relative to the threshold of hearing. The decibel is not an SI unit. The table here uses dBSPL as units of sound to indicate consensus on hearing protection. Some examples of sounds are:

Sound LevelExamples
171 dB Next to a loud rifle being shot
150 dBRight next to a jet engine
110-140 dBJet engine at 100 meters
130-140 dB Where most people begin to feel pain
130 dB Trumpet (a half meter in front of)
120 dBVuvuzela horn (1 meter in front of), risk of immediate hearing loss
110 dB Gas chainsaw
100 dBJack hammer
80-90 dBTraffic on a busy roadway
60-80 dBPassenger car
40-60 dBNormal conversation
20-30 dBVery calm room
10 dBLight leaf rustling, calm breathing
0 dBHearing threshold right next to ear

Hearing protection can also be used to shield from ear damage. This table gives some safe limits for the level of sound so that the ears are not damaged.

DecibelsMaximum Exposure Time
908 hours
926 hours
954 hours
973 hours
1002 hours
10290 minutes
10560 minutes
11030 minutes
11510–15 minutes
1203–5 minutes

[1]

References
  1. Pocket Ref, General Sciences, pages 322-323.
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