Decibel

Decibel is a logarithmic unit used to describe the ratio of the signal level - power, sound pressure, voltage, intensity, etc.

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Most signal systems - as sound power or sound intensity, human speech, sonar, microwaves, radio signals and fiber optics - can be described by

·         transmitting power

·         transmission path loss

·         receiver sensitivity

Transmitting power, path loss and receiver sensitivity are absolute power values - Watts in the SI system.

The Definition of Decibel

Decibel is a logarithmic unit used to describe the ratio of the signal level - power, sound pressure, voltage or intensity or several other things.

The decibel can be expressed as:

decibel = 10 log( P / P_ref ) (1)

where

P = signal power (W)

P_ref = reference power (W)

A decibel is one-tenth of a Bel, a seldom-used unit named for Alexander Graham Bell, inventor of the telephone.

Note! Doubling the signal level increases the decibel with 3 dB ( 10 log (2) ).

If we know the decibel value and the reference level, the absolute level can be calculated by transforming (1) to:

P = P_ref 10^{(decibel / 10)} (2)

Example - Sound Intensity and Decibel

The difference in sound intensity of 10^-8 watts/m² and 10^-4 watts/m² (10,000 units) can be calculated in decibels as

ΔL_I = 10 log( (10^-4 watts/m²) / (10^-12 watts/m²) )

- 10 log( ( 10^-8 watts/m²) / ( 10^-12 watts/m²) )

= 40 dB

Increasing the sound intensity by a factor of

·         10 raises its level by 10 dB

·         100 raises its level by 20 dB

·         1,000 raises its level by 30 dB

·         10,000 raises its level by 40 dB and so on