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 dB_SPL 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

Decibel Media

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  Examples of sound levels in decibels from various sound sources and activities, taken from the "How loud is too loud" screen of the NIOSH Sound Level Meter app

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  A schematic showing the relationship between dBu (the voltage source) and dBm (the power dissipated as heat by the 600 Ω resistor)