Sound intensity and pressure (the physical correlates of loudness) are measured in decibels (dB). A dB is a unitless figure that compares 2 values and is defined as the logarithm of the ratio of a measured value to a reference value, multiplied by a constant:

dB = k log (V_measured/V_ref)

By convention, the reference value for sound pressure level (SPL) is taken as the quietest 1000-Hz sound detectable by young, healthy human ears.* The sound may be measured in terms of pressure (N/m²) or intensity (watts/m²).

Because sound intensity equals the square of sound pressure, the constant (k) for SPL is 20; for sound intensity, 10. Thus, each 20-dB increase represents a 10-fold increase in SPL but a 100-fold increase in sound intensity.

The dB values in the table below give only a rough idea of the risk of hearing loss. Some of them are dB SPL values (referenced to N/m²), whereas others represent peak dB or dB on the A-scale (a scale that emphasizes the frequencies that are most hazardous to human hearing).

Db

Example

0

Faintest sound heard by human ear

30

Whisper, quiet library

60

Normal conversation, sewing machine, typewriter

90

Lawnmower, shop tools, truck traffic (90 dB for 8 hours a day is the maximum exposure without protection†)

100

Chain saw, pneumatic drill, snowmobile (2 hours a day is the maximum exposure without protection)

115

Sandblasting, loud concert, automobile horn (15 minutes a day is the maximum exposure without protection)

140

Gun muzzle blast, jet engine (noise causes pain and even brief exposure injures unprotected ears; injury may occur even with hearing protectors)

180

Rocket launching pad