An amplifier spec, the damping factor is the relationship between source and load impedance, i.e. the amp’s output impedance and speakers’ input impedance. The lower the amp’s output impedance and the higher the speakers’ input impedance, the higher the damping factor. Tube amps with output impedances of 2-4ohms into 4-8ohm speakers naturally have poor damping factors of 1 or 2. Solid state amps with output impedances of 0.05ohms or lower into the same speakers have high damping factors of 100 - 1000. A higher damping factor provides both greater control over a loudspeaker’s drivers and greater immunity to these drivers’ back EMF [electromotive force]. The larger a transducer’s diameter and associated motor structure, the higher the levels of potential back EMF it can produce. Back EMF is thus most significant in bass drivers. Because horn-loaded loudspeakers reach high output levels with significantly reduced driver excursion, such designs generate comparatively low levels of back EMF which makes them more compatible with low damping factor amplifiers such as SETs.
A characteristic of amplifiers; 8 Ohms (the nominal impedance of common loudspeakers) divided by the output impedance. A large damping factor is allegedly good because it gives an amp tighter control over the loudspeaker. This is not true, and a damping factor of 1000 is really not better than a factor of 200, as shown in the section on Thiele-Small analysis.
In power amplifiers, it is a measure of the output impedance of the device. Expressed as a number arrived at by dividing the impedance into 8 ohms. For example, an amplifier with an output impedance of 0.04 ohms would have a damping factor of 8/.04=200. This, and higher numbers are common for solid state amps. Tube amplifiers have much higher output impedances and lower damping factors. In practice, the output impedance of the amplifier has almost no effect on loudspeaker damping, but it can have a significant effect on the frequency response of loudspeakers, most of which have frequency-dependent impedances. Within reason, higher numbers are better.