The decibel is a logarithmic ratio, because the ear judges level in ratios rather than absolute differences

Human hearing perceives level (and pitch) roughly logarithmically: we compare in ratios, so equal numerical steps mean equal perceived steps. A 10× increase in physical sound power yields only about a doubling of perceived loudness. The decibel encodes this with a logarithmic scale: +6 dB is a 2× change in voltage amplitude, +20 dB is a 10×, and −6 dB is a halving. Because the ear responds to ratios, a given dB change (e.g. 3 dB or 6 dB) sounds like the same amount of change at any starting level, which is why gain controls, faders, and compressor thresholds are all calibrated in dB. A decibel value is relative and meaningless on its own: an absolute level requires a reference (dBu, dBV electrical; dBFS digital full-scale). Common misconception: a dB is a fixed amount of loudness — it is not; without a reference, ‘4.75 dB’ names no absolute level, just as a ‘major sixth’ names no note without a starting pitch.

Examples

+6 dB doubles amplitude whether from −20 to −14 dBFS or −14 to −8 dBFS. On a mixer: 0 dB (unity) passes signal unchanged, −6 dB halves it, −20 dB reduces it to one-tenth. ‘4.75 dB’ alone is meaningless without a reference.

Assessment

Explain why a 6 dB boost is the same perceptual change at any starting level and why faders are calibrated in dB rather than linearly. Calculate how many dB correspond to halving the signal amplitude.