Decibel Scale

Logarithmic loudness units, hearing currency

Overview

The decibel is a logarithmic unit used to express the ratio between two values, and in acoustics it describes sound levels relative to a reference. An increase of 10 dB corresponds to ten times the sound intensity and is perceived as roughly a doubling of loudness, while an increase of 3 dB doubles sound energy. Typical approximate levels are 30 dB for a whisper, 60 dB for conversation, 90 dB for a lawnmower, over 100 dB in many clubs and concerts, about 120 dB near a jet engine, and above 140 dB for a gunshot. Several scales are used: dB SPL refers to sound pressure relative to a standard reference, dBA applies A weighting to reflect the frequency sensitivity of human hearing, and dB HL, used on audiograms, is referenced to average normal hearing thresholds at each frequency. A hearing loss of 60 dB therefore means sound must be about a million times more intense than normal to be detected. The unit is named after Alexander Graham Bell.