Audio and speakers
Damping factor
The ratio of speaker impedance to amplifier output impedance, describing how tightly the amplifier controls the motion of the driver.
Damping factor is the speaker's nominal impedance divided by the amplifier's output impedance, including the wire. An amplifier with 0.04 ohms output impedance on an 8 ohm speaker has a damping factor of 200 and can stop a woofer cone quickly after a transient.
The audible effect is subtle above a modest threshold, because the voice coil's own resistance dominates. Where it matters is the cable: a long run of thin wire adds series resistance and can drop the effective value from hundreds to a few dozen.
A damping factor of 20 or more at the speaker terminals is generally sufficient, and beyond about 50 differences are inaudible. That is another reason for the 5 percent wire rule: 0.4 ohms of cable on an 8 ohm speaker caps it at 20 regardless of the amplifier.
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