decibelët

The decibelët, or decibels, are a logarithmic unit used to express ratios, most commonly in acoustics to compare sound pressure or sound power. Because they express a ratio rather than an absolute quantity, decibels are not a stand-alone measurement; they describe how one level relates to a reference level.

A decibel is defined relative to a reference quantity. For power quantities, L = 10 log10(P2/P1). For field quantities such as sound pressure, L = 20 log10(p2/p1). In acoustics, sound pressure level (SPL) uses a reference p0 of 20 microPascals, giving Lp = 20 log10(p/p0). When using sound power or intensity, the reference I0 is typically 1×10^-12 W/m^2, yielding Li = 10 log10(I/I0).

Decibels are logarithmic, so a change of 10 dB represents a tenfold change in the referenced quantity (and a 3 dB change is roughly a doubling or halving, depending on context). They are dimensionless in themselves but are commonly applied with a fixed reference. In practice, several specialized forms are used, such as dB SPL for acoustic pressure, dB HL for hearing levels, and dB(A) or dB(C) for frequency-weighted measurements that reflect human hearing.

Common ranges illustrate sensitivity: a quiet room is around 20–30 dB SPL, normal conversation about 60 dB, city traffic 70–85 dB, and high-intensity sounds like concerts 100–120 dB or more. Because decibels are relative, they are always reported with their reference and, when relevant, any weighting or measurement conditions.

Historically, the bel was proposed by Bell laboratories; the decibel, one-tenth of a bel, became the standard unit in electronics, telecommunications, and acoustics, standardized by organizations such as ITU, IEC, and ISO.