Decibel

The decibel (symbol: 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 Level  Examples171 dBNext to a loud rifle being shot 150 dB  Right next to a jet engine 110-140 dB  Jet engine at 100 meters130-140 dBWhere most people begin to feel pain130 dBTrumpet (a half meter in front of) 120 dB  Vuvuzela horn (1 meter in front of), risk of immediate hearing loss110 dBGas chainsaw 100 dB  Jack hammer 80-90 dB  Traffic on a busy roadway 60-80 dB  Passenger car 40-60 dB  Normal conversation 20-30 dB  Very calm room 10 dB  Light leaf rustling, calm breathing 0 dB  Hearing 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.

 Decibels  Maximum Exposure Time 90  8 hours 92  6 hours 95  4 hours 97  3 hours 100  2 hours 102  90 minutes 105  60 minutes 110  30 minutes 115  10–15 minutes 120  3–5 minutesPocket Ref, General Sciences, pages 322-323.

References

Category:Sound
Category:Acoustics