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The decibel (dB) is a logarithmic unit, often used to measure sound pressure. Decibel means 1/10 of a Bel, after Alexander Graham Bell. There is no absolute decibel scale, so the decibel is used to measure sound in comparison to another sound, i.e, the 0 dB reference. This is defined as a sound pressure level of .0002 microbar, or 20 micropascals: commonly the threshold of hearing a sound with a frequency of 1 KHz.