# 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 1 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 dB as units of sound to indicate consensus on hearing protection. Some examples of sounds are:

Sound Level Examples
> 5000 dB Unknown
600-4000 dB Standing inside a volume max loud speaker
300-600 dB Hearing the take-off while sitting on top of a rocket
280-300 dB Fireworks in Mid-Air
250-280 dB Explosion from 2 feet
200-250 dB Cannon at 12 feet
175-200 dB Saturn Rocket firing
150 dB Airplane at takeoff
130 dB Above this can damage the ear quickly, and pain is felt.
120 dB Diesel engine, ball mill
100 dB Lawn mower, factory
90 dB Band or symphony, above this can damage the ear if heard extensively.
80 dB Police or fire siren, electric shaver
72 dB Modern 10 kW Back Up Generator
70 dB Radio sound level, crowded neighborhood.
50 dB Can normally awaken a sleeping person.
30-35 dB Very quiet conversation, private office noise
20 dB Rustling leaves, whispering
10 dB Sound-proofed room, the minimum most human ears can hear
0 dB Hearing threshold.

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

[1]

## References

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