Dezibel

Dezibel, also known as the decibel (dB), is a logarithmic unit of measurement used to express ratios between quantities, most commonly sound or electrical signal levels. It is one tenth of a bel; the bel is rarely used because the dezibel yields more manageable numbers. The dezibel is a dimensionless unit, defined by the logarithmic ratio of two quantities, such as power or amplitude.

In acoustics, the sound pressure level in dezibels is defined as Lp = 20 log10(p/p0), where p is the RMS sound pressure and p0 is the reference pressure in air, conventionally 20 micropascals at 1 kHz. For sound power, Lw = 10 log10(W/W0), with W0 = 1e-12 watts. In electronics, dezibels express ratios of voltages or currents relative to a reference and follow L = 20 log10(V/Vref) or L = 10 log10(P/Pref). Because dezibels express ratios, the value depends on the chosen reference.

Dezibels are widely used to describe sound levels, loudness, gain, attenuation, and signal-to-noise ratio, as well as the dynamic range of equipment. They enable handling of large ranges of magnitudes in a compact, additive way: when two gains in dezibels are combined, their values add.

Although not part of the SI base units, the dezibel is accepted for use with the SI, and its use is standardized in acoustics, electronics, and telecommunications. Common examples include a quiet room around 30–40 dB, normal conversation about 60 dB, and modern music playback around 90–100 dB SPL; values above ~120–130 dB can cause pain.