bel
Sound levelexact by definition
The bel is ten decibels, the original unit from which the decibel was cut. A level in bels is log₁₀(P/P₀) for power quantities. The factor of 10 is exact, the decibel being defined as one tenth of a bel, and the relation is a scaling of the level itself rather than of any physical quantity.
Watch out: The bel survives in one place: appliance sound ratings. Dishwashers, ceiling fans and refrigerators are advertised in bels because the numbers are small and comparable, so a 4.4 bel dishwasher is a 44 dB one. Those figures are sound POWER levels referenced to one picowatt, not sound pressure at a listening position, and they are not what a sound level meter in the kitchen will read.
| 1 B | 10 dB |
Named for Alexander Graham Bell at Bell Telephone Laboratories in the 1920s. It proved too coarse for transmission work almost immediately, since one bel is a factor of ten in power, so the decibel took over within a few years.