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In audio engineering, 0 dB can mean absolute silence, standard operating voltage, or the catastrophic threshold where digital audio permanently distorts.
A decibel (extdB ext{dB}) is not an absolute unit of measurement like a meter or a gram β€” it is a logarithmic ratio comparing a measured value to a specific reference point (0extdB0 ext{ dB}).
So how do we measure the physical vibrations moving through the air before they ever hit a microphone?

Acoustic Sound Pressure: dBSPL

In physical space, sound waves travel as fluctuating air pressure measured in dBSPL (Decibels Sound Pressure Level).
The reference baseline 0extdBSPL0 ext{ dBSPL} is set to 20Β ΞΌextPa20\ \mu ext{Pa} (20extmicropascals20 ext{ micropascals}), which represents the quietest sound a healthy human ear can detect at 1extkHz1 ext{ kHz}.
Once a microphone diaphragm catches those air vibrations, what happens to the measurement when sound becomes electrical voltage?

Analogue Voltages: dBu and dBV

In analogue audio hardware like preamps, mixers, and compressors, audio exists as an alternating current (AC) electrical signal measured in root-mean-square voltage (extVextRMS ext{V}_{ ext{RMS}}), which is the effective continuous voltage of a fluctuating waveform.
In professional studio gear, dBu references 0.775extVextRMS0.775 ext{ V}_{ ext{RMS}}, derived from the historic Bell Labs standard of dissipating 1extmW1 ext{ mW} of power across a 600Β Ξ©600\ \Omega telephone line. Professional line level operates at +4extdBu+4 ext{ dBu} (1.228extV1.228 ext{ V}). Consumer gear uses dBV, where 0extdBV=1.0extVextRMS0 ext{ dBV} = 1.0 ext{ V}_{ ext{RMS}}, operating at βˆ’10extdBV-10 ext{ dBV} (0.316extV0.316 ext{ V}).