Decibel (dB)
Definition
The decibel (dB) is a logarithmic unit that expresses the ratio between two levels, never an absolute quantity. An increase of +6 dB doubles the voltage, +20 dB multiplies it by ten, and +3 dB doubles the power. A level of 0 dB therefore means no difference from the reference, not silence. Each suffix names its reference: dBu refers to 0.775 V and dBV to 1.0 V. Human hearing spans from 0 dB SPL, the threshold of hearing (20 µPa), to the threshold of pain, usually given as 120 to 140 dB SPL.
In the field
Professional line level is +4 dBu and consumer line level is −10 dBV, a difference of about 12 dB. Connecting one to the other without a level converter leaves the signal too weak and noisy in one direction (consumer output into professional input) and overdriven in the other.
Related lessons
Sources
- Ballou, Handbook for Sound Engineers, p. 54 : dBV référencé à 1 V, dBu à 0,775 V, dB SPL à 20 µPa ; +6 dB double la tension et quadruple la puissance
- IEC 60027-3:2002 : le décibel, unité de niveau logarithmique (rapport à une référence) · webstore.iec.ch
- ITU-R V.574-5 (2015) : usage du décibel et du néper, niveaux relatifs à une référence · itu.int
- Line level (Wikipedia) : +4 dBu ≈ 1,228 V, −10 dBV ≈ 0,316 V, écart ≈ 11,8 dB · en.wikipedia.org
- SFU Handbook for Acoustic Ecology : seuil de douleur vers 120–140 dB SPL · sfu.ca