0 dB means the measured power/voltage/amplitide/loudness is exactly equal to whatever reference it refers to.

Because Bel scale is the logarithmic of power ratio. Hence

dB = 10 log(P/P)

Where the above P is the power of the measured, and the below P is the reference

Because log(1) = 0, when the measured result is equal to reference, the dB is 0

In sound measurement, 0 dB literally means 0 dB SPL or deciBel Sound Pressure Level. dB SPL means the reference is the SPL of the threshold of hearing (20 microPascal). 0 dB means the level is equal to the threshold of hearing.

In digital mixer or any form of digital audio, the Golden rule is that you cannot go above the given highest amplitude because the system will not allow. So the unit used is called dB FS or deciBel Full Scale, referencing the highest grid available for amplitide (equivalence to voltage or SPL). Therefore, the top value is 0 dB

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