Damping factor

The damping factor in an electrical circuit gives the ratio of the impedances of two electronic devices, the load impedance Z_load and the source impedance Z_source.

The damping factor DF is:

For audio power amplifiers this source impedance Z_source (also: output impedance) is generally smaller than 0.1 Ω (ohms), and can be seen from the point of view of the loudspeaker as a near short-circuit. This will very rapidly absorb any unwanted currents induced by the mechanical resonance of the speaker's voice coil, acting as a very effective 'brake' on the speaker (just as a short circuit across the terminals of a generator will make it very hard to turn), thus keeping it under control.

This is called voltage bridging. Z_load >> Z_source.

The loudspeaker's load impedance (input impedance) of Z_load is usually around 4 to 8 Ω

Solving for Z_source:

One can therefore calculate the small output impedance, hidden in the word damping factor in the manual datas. Note that modern amplifiers, employing relatively high levels of negative feedback, generally exhibit extremely low output impedances - one of the many consequences of using feedback. Thus "damping factor" figures in themselves do not tell you very much about the quality of a system, and given the controversy that has surrounded the topic of feedback for many years, may actually be seen as a mark of poor quality, if excessively high, for those who subscribe to the view that high levels of feedback are a bad thing.

Incidentally, maintaining a good damping factor is why decent quality (that is, very low resistance) speaker cables are part of every good system, as cable resistance adds to the source impedance. However, this subject brings its own controversies; in general the most gains are made for a modest increase in outlay over the very cheapest of cabling - beyond that is where the controversy lies.

See also

• Impedance
• Input impedance
• Output impedance
• Voltage divider
• Audio system measurements

External links

• Calculation of the damping factor and the damping of bridging
• Description of damping factor