Damping Factor
Definition
The damping factor of an amplifier is the ratio of the nominal load impedance to the amplifier's output impedance. An output impedance of 0.008 Ω, for example, gives a damping factor of 1000 into 8 Ω. A low source impedance lets the amplifier control the movement of the loudspeaker cone after the signal stops, which mainly affects low-frequency reproduction. The resistance of the loudspeaker cable adds to the output impedance: a 10 m run of 1.5 mm² copper cable has a loop resistance of about 0.23 Ω (out and back), which lowers the effective damping factor from 1000 to about 34 on an 8 Ω load and to about 17 on a 4 Ω load.
In the field
Cable resistance dominates the effective damping factor on long runs, so larger cross-sections, commonly 2.5 mm² or 4 mm², are used for long loudspeaker lines, and amplifiers are often placed close to the loudspeakers. The loop resistance is calculated from the length out and back, the cross-section and the resistivity of copper, about 0.0172 Ω·mm²/m at 20 °C. Keeping it small relative to the load impedance also limits the power lost in the cable.
Sources
- Damping factor (Wikipedia, pointeur secondaire) : rapport impédance de charge / impédance de sortie ; effet de la résistance du câble · en.wikipedia.org
- IEC 60028 : cuivre recuit de référence, résistivité 1/58 Ω·mm²/m (environ 0,0172) à 20 °C
- IEC 60228 : résistance maximale des âmes de câble par section
- Calcul : 20 m × 0,0172 / 1,5 = 0,229 Ω ; 8 / (0,008 + 0,229) ≈ 34 ; 4 / (0,008 + 0,229) ≈ 17