Damping factor is a specification most commonly associated with amplifiers, but since it concerns the amplifier/speaker relationship, a quick tutorial is in order.
Damping Factor (DF) is the amplifier’s ability to control speaker motion once a signal has stopped. Technically speaking, Damping Factor is the ratio of nominal loudspeaker impedance (the impedance the loudspeaker is rated at) to total output impedance of the system driving the loudspeaker, including the amplifier and cables.
Damping Factor changes with frequency (as does impedance) and is most noticeable at lower frequencies. A high DF typically results in a tighter, more controlled bass, which is usually (but not always) more desirable from a listener’s point-of-view. Low DF results in soft or fat bass.
Damping factors over ten are acceptable with numbers in the 50-100 range being a good average, but you may sometimes see numbers as high as 200 or 300 or even up into the low thousands.
Here’s How Damping Factor Works
The voice coil of a speaker becomes a current generator that produces a flyback current that is sent to the amplifier (sort of a reverse answer to the current the amplifier sends the speaker). Every time the voice coil moves it creates this flyback current. If the load presented by the amplifier to this current is very low (high DF) this current is rapidly dissipated causing the force of the voice coil to diminish. The speaker diaphragm and surround assembly also make up a resonant system that will produce ringing (or unwanted resonances) within the operating range of the speaker, but rapid damping acts like a brake on the voice coil, therefore reducing the resonances. To complicate things further, voice coil impedance is complex, meaning it changes with frequency, temperature and power level.
Calculate Damping Factor
Take the nominal speaker impedance and divide it by your amp’s DF specification. Let’s say your amp has a DF of 300 and you have an 8 Ω speaker, your output impedance is 0.027 Ω. Then add the nominal impedance of your cable. A twelve-foot cable presents roughly .0016 Ω per foot. Double that to account for the “out” and “return” impedance from the amp to the speaker (flyback current). If we have a 20’ run on 12AWG cable we have 0.064 Ω of cable impedance (40 x .0016 = .0064).
Add 0.064 to 0.027 and you’ll get a system output impedance of 0.091 Ω which is super-low. Then divide your speaker impedance by the output impedance (8/0.091) and you get a Damping Factor of 88 which is really good.
Thicker cable presents less impedance, so it is always a good idea to use as thick a cable as is practicable, especially for long runs.
The higher the speaker impedance the higher the damping factor. Of course, the lower the speaker impedance the more efficiently an amp will work, so like everything in audio, it’s a trade-off.
Solid-state amplifiers use a large amount of Negative Feedback (NFB) to eliminate distortion but older solid-state amps (and even some current ones) and vacuum tube amps don’t use a lot (or any) NFB which will give the amp a lower Damping Factor.
Some vintage amps have controls that change the Damping Factor allowing the user to choose a sound they prefer through changes to the DF.
Extreme DF numbers don’t always translate into “better” sound. This is where our ears come in – some amps and speakers match very well to create a DF that results in a very pleasing sound, while other combinations fall short. Another reason you simply can’t pick the “best” amp for a particular speaker.