Here's an explanation of damping for you--- note the last line.
james
DAMPING FACTOR: is a measure of the amplifier's ability to control the woofer, and is measured by dividing the speaker impedance, (normally 8 ohms), into the amplifier's output impedance, (usually in the range of 0.02 ohms). The lower the amplifier's output impedance, the less the amplifier's output level is affected by variations in the speaker impedance. Also, since the woofer's voice-coil can act as a generator, within its magnet structure, the amplifier needs a low output impedance to act as a method of damping the woofer's tendency to keep moving after the signal has stopped. In the example above, the damping factor would be 8/.02 = 400.
Bryston amplifiers have output impedance slightly below 0.01 ohms, and therefore have a calculated damping factor of over 800, (though we conservatively rate them at 500). This parameter is affected by the speaker cable resistance. Even heavy 12- gauge wire has a resistance of about 0.0016 ohms per foot. (Remember we need to double that for twin-lead speaker cable). Thus, it would require only 6.25 feet of 12 gauge per speaker to have a total resistance of 0.02 ohms, (.0016 X 2 X 6.25 = 0.02), cutting a damping factor of 400 in half, to 200. Bryston recommends keeping speaker lead length to a minimum for this reason.
Keep in mind that damping factor is also affected by other real-world impedances, including the speaker-cable resistance, and the varying resistance of the speaker's own voice-coil. The voice-coil of a typical 8-ohm loudspeaker has a DC resistance of between 4 and 6 ohms. This resistance increases with temperature by 0.4%/Deg. C. It would thus require only a 25-degree rise in voice-coil temperature to increase its impedance by 10%. If it started with a DC resistance of 4 ohms, the extra 10%, (0.4 ohms), would reduce the actual damping factor to twenty, (8/0.4=20)!
It is worth noting that it would probably take only about 5-10 Watts to raise the voice-coil temperature by that amount. Add in the likely speaker-cable resistance of about 0.1 ohms, (10 feet of 16 gauge. cable), and it is obvious that the amplifier's contribution to the overall, real-world damping factor of the system is close to nil.
Whether the amp measures 300 or 3,000,000 under ideal conditions, the actual damping factor of the system will almost never exceed 100 anyway.
james
