DAMPING FACTOR Tech Made Simple

Te c h M a d e S i m p l e
DAMPING FACTOR
What It Is
How It Works
Loudspeakers have a mind of their own. You send them a signal
and they add their own twist to it. They keep on vibrating after the
signal has stopped, due to inertia. That’s called “ringing” or “time
smearing.”
How does an amplifier control speaker motion? When the loudspeaker cone vibrates, it acts like a microphone, generating a signal
from its voice coil. This signal generated by the speaker is called
back EMF (back Electro Motive Force). It creates a current which
travels through the speaker cable back into the amplifier output, then
returns to the speaker. Since back EMF is in opposite polarity with
the speaker’s motion, back EMF impedes or damps the speaker’s
ringing.
In other words, the speaker produces sound waves that are not part
of the original signal.
Suppose the incoming signal is a “tight” kick drum with a short
attack and decay in its signal envelope. When the kick-drum signal
stops, the speaker continues to vibrate. The cone bounces back and
forth in its suspension. So that nice, snappy kick drum turns into
a boomy throb.
Fortunately, a power amplifier can exert control over the loudspeaker and reduce ringing. Damping is the ability of a power
amplifier to control loudspeaker motion. It’s measured in
Damping Factor, which is load impedance divided by amplifier
output impedance. Let’s explain.
If the speaker impedance is 8 ohms, and the amplifier output
impedance is 0.01 ohms, the damping factor is 800. That’s a
simplication. Since the speaker impedance and amplifier output
impedance vary with frequency, so does the damping factor. Also,
the impedance of the speaker cable affects damping. Thick cables
(with low AWG) allow more damping than thin cables with (high
AWG).
The lower the amplifier’s output impedance, the higher the damping
factor, and the tighter the sound is. A damping factor of 1000 or
greater is considered high. High damping factor equals tight bass.
BOTTOM LINE:
The smaller the amp’s output impedance, the greater is the effect
of back EMF on the speaker’s motion. An amplifier with low output
impedance short-circuits the back EMF, so the back EMF drives the
loudspeaker with a relatively strong current that works against the
speaker’s motion. When the speaker cone moves out, the back EMF
pulls the speaker in, and vice versa.
In short, the loudspeaker damps itself through the amplifier output
circuitry. The lower the impedance of that output circuitry, the more
the back EMF can control the speaker’s ringing.
To prove it to yourself, take a woofer that is not connected to
anything. Put your ear next to the cone and tap on it. You might
hear a low-pitched “bongggg” if the speaker itself is poorly damped.
Now short the speaker terminals and tap again. You should hear a
tighter thump.
Damping factor varies with frequency. As you might suspect, damping factor is most important at low frequencies, say 10 Hz to 400
Hz. For example, in the Crown 2-channel CTs power amplifiers, the
damping factor is well over 3000 from 10 Hz to 1 kHz.
All Crown amplifiers are designed to have high damping
factor. That’s why you can count on Crown amps to
deliver clean, tight kick drum and bass.
©2003 by Crown Audio, Inc.
P.O. Box 1000, Elkhart, Indiana 46515-1000 USA Tel: 574-294-8000
136224-1 2/03