Décibels

Décibels (dB) are units used to express ratios of power or intensity, most commonly to quantify sound levels. The decibel is a logarithmic unit; it does not measure an absolute quantity by itself but compares two quantities using a reference value. Because it is logarithmic, a small change in dB corresponds to a large multiplicative change in actual power or pressure.

Origin and meaning: The term bel honors Alexander Graham Bell. The decibel is one tenth of a bel. In practice, the decibel is used with various reference standards to express different kinds of levels, which must be stated with the unit (for example dB SPL, dB HL).

Common scales and references: dB SPL (sound pressure level) uses the reference pressure of 20 micropascals in air. Sound pressure levels are given as 20 log10(p/p0). For power, dB uses 10 log10(P/P0). Other common forms include dB HL (hearing level) and dB(A) weighting that accounts for human hearing sensitivity.

Applications and interpretation: Decibels allow comparison of sounds and signals over wide ranges, from thresholds of hearing to levels encountered in daily life or industrial settings. They are used in acoustics, audio engineering, and electronics. Because the scale is relative, the same numeric value in different contexts may have different practical meanings depending on the reference used and any weighting or correction applied.

Examples of typical values: 0 dB SPL is near the threshold of hearing; normal conversation around 60 dB SPL; loud hobby noise around 85 dB SPL; prolonged exposure above 85–90 dB SPL can risk hearing damage; threshold of pain is around 120–130 dB SPL. Measurement requires calibration and, often, weighting to reflect human hearing.