Audio Hardware - Notes 2

The following is taken from the Merriam-Webster dictionary http://www.webster.com.

Main Entry: deci·bel
Pronunciation: 'de-s&-"bel, -b&l
Function: noun
Etymology: International Scientific Vocabulary deci- + bel
Date: 1928
1 a : a unit for expressing the ratio of two amounts of electric or acoustic signal power equal to 10 times the common logarithm of this ratio b : a unit for expressing the ratio of the magnitudes of two electric voltages or currents or analogous acoustic quantities equal to 20 times the common logarithm of the voltage or current ratio
2 : a unit for expressing the relative intensity of sounds on a scale from zero for the average least perceptible sound to about 130 for the average pain level
3 : degree of loudness; also : extremely loud sound -- usually used in plural
The following is an excerpt from WhatIs.com http://www.whatis.com.

decibel

In electronics and communications, the decibel (abbreviated as dB, and also as db and DB) is a logarithmic expression of the ratio between two signal power, voltage, or current levels. In acoustics, the decibel is used as an absolute indicator of sound power per unit area. A decibel is one-tenth of a Bel, a seldom-used unit named for Alexander Graham Bell, inventor of the telephone.

Suppose a signal has a power of P1 watts, and a second signal has a power of P2 watts. Then the power amplitude difference in decibels, symbolized SdBP, is:

SdBP = 10 log10 (P2 / P1)

Decibels can be calculated in terms of the effective voltage if the load impedance remains constant. Suppose a signal has an rms (root-mean-square) voltage of V1 across a load, and a second signal has an rms voltage of V2 across another load having the same impedance. Then the voltage amplitude difference in decibels, symbolized SdBV, is:

SdBV = 20 log10 (V2 / V1)

Decibels can also be calculated in terms of the effective current (amperage) if the impedance remains constant. Suppose a signal delivers an rms (root-mean-square) amperage of A1 through a load, and a second signal delivers an rms amperage of A2 through another load having the same impedance. Then the current amplitude difference in decibels, symbolized SdBA, is:

SdBA = 20 log10 (A2 / A1)

When a decibel figure is positive, then the second signal is stronger than the first signal. When a decibel figure is negative, then the second signal is weaker than the first signal. In amplifiers, the gain, also called the amplification factor, is often expressed in decibels. A circuit amplifies only if the decibel figure for the output-to-input power ratio (SdBP) is positive.

In sound, decibels are defined in terms of power per unit surface area on a scale from the threshold of human hearing, 0 dB, upward towards the threshold of pain, about 120-140 dB. As examples: the sound level in the average residential home is about 40 dB, average conversation is about 60 dB, typical home music listening levels are about 85 dB, a loud rock band about 110 dB, and a jet engine close up is 150dB.

Decibel units are commonly used in audio equalizers, both the hardware kind and the software kind, as a convenient reference point while editing. Boosting an equalizer band whose center point is 1000 by 3 dB means that you have raised the volume level of that frequency band by 3 dB as it relates to the other frequencies in the sound. A typical equalizer has a range for boosting or diminishing a sound level of +/-18 dB.