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Microphone Month 3

dB

Abbreviation for Decibel. It literally means one tenth of a bel. The bel is a unit of measurement named after Alexander Graham Bell. The bel had its origin in the Bell Telephone Labs, where they needed a convenient way to express power losses in telephone lines as power ratios. Because the bel is a power ratio of 10, and this is a rather large ratio, it is convenient to divide it into tenths of bels, or decibels. Decibels are used all over the place in audio measurements and specifications partly because it is very easy to express extremely large or small values with decibels due to their logarithmic nature. The full depth of the topic is well beyond the scope of this writing (we will cover more in the future), but suffice to say that the decibel always refers to a ratio of two values. It is never an absolute value. Thus when we speak of losses and gains in audio we use dB to quantify those values. If a signal goes into a unit and is output at a lower level it is said to be down by some value of dB. Decibels come in many variants: dB PWL, dB SPL (also known as Sound Pressure Level), dBm, dBu, dBV, dBv, and more.

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