This is the logarithmic unit of measurement of sound pressure or loudness. The term Bel was original used in the telephone industry and was named after the inventor of the telephone, Alexander Graham Bell. Since the Bel was quite a large unit of measure and a more precise unit was needed to describe the fluctuations in audio and acoustics. The decibel was used, as it is 1/10 of a Bel. The decibel is a ratio between a known reference level and a measured level. We know that an increase of 3 db is a doubling of the power, but 3 db is barely perceptible to the human ear. A 10db increase would be perceived as twice as loud and a 20db increase would be 4 times as loud. Decibels also describe loss. A –10db loss would be half as loud as the reference level and a –20db loss would be 1⁄4 of the original level.