Sound is measured in decibels (dB), a logarithmic scale that compresses the enormous range of human hearing into manageable numbers.
The Decibel Scale
The decibel is a logarithmic ratio, not an absolute unit. For sound pressure level (SPL): 0 dB is the threshold of hearing (20 µPa). Each 10 dB increase sounds roughly twice as loud. 1 bel = 10 dB.
Common Sound Levels
Quiet library: 30 dB. Normal conversation: 60 dB. Vacuum cleaner: 70 dB. Rock concert: 110 dB. Threshold of pain: 130 dB. Jet engine at 30 m: 140 dB.
Why Logarithmic?
Human hearing covers a range of about 1 to 10,000,000,000,000 in power ratio. The log scale compresses this to 0–130 dB. A 3 dB increase doubles the sound power; a 10 dB increase means 10× the power.
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Frequently Asked Questions
Is the decibel an absolute unit?
No. dB is a ratio. For sound, dB SPL references 20 µPa (threshold of hearing). Always check what reference is being used.
How much louder is 80 dB than 60 dB?
80 dB has 100× the sound power of 60 dB (20 dB difference = 10²). It sounds about 4× louder to human perception.