# Decibel

logarithmic unit expressing the ratio of a physical quantity

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 Level Examples
171 dB Next to a loud rifle being shot
150 dB Right next to a jet engine
110-140 dB Jet engine at 100 meters
130-140 dB Where most people begin to feel pain
130 dB Trumpet (a half meter in front of)
120 dB Vuvuzela horn (1 meter in front of), risk of immediate hearing loss
110 dB Gas 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 minutes

## References

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