Loudspeaker Damping

Author: N.H. Crowhurst

Loudspeaker damping

Another aspect of electro-acoustics concerns the proper coupling of the loudspeaker to the amplifier. Not only does the loudspeaker impedance have to be matched to the amplifier output impedance, but the output impedance of the amplifier affects the operation of the speaker. When the speaker diaphragm starts moving (due to an input signal from the amplifier), the momentum of the diaphragm will tend to keep it in motion even after the drive current has ceased. Diaphragm motion causes the loudspeaker to operate as a microphone, generating voltages in its voice coil. This overshoot movement will generate a further voltage. If the impedance of the amplifier is high, there will be a negligible current in the circuit due to this voltage. If the amplifier impedance is low, however, current flows producing a force that acts to stop overshoot.

This means the source resistance presented by the amplifier will influence the behavior of the loudspeaker. A low source resistance will prevent the voice coil overshooting, while a high source resistance will allow the voice coil to move erratically and affect the transient response of the loudspeaker. This means that the source resistance presented by the amplifier, due to the plate resistance of the output tubes, adjusted according to any positive or negative feedback, will influence the behavior of the loudspeaker.

Variable feedback adjusts the effective amplifier output impedance

The effect of source resistance on loudspeaker behavior is called the damping factor of the amplifier. Because a higher source resistance damps or brakes overshoot less than a low source resistance, the damping factor is given by the load resistance divided by the effective source resistance.

Damping factor

The use of a high damping factor (or a low source resistance) cannot do everything. The electrical damping has to act on the motional impedance of the loudspeaker. (Motional impedance is the impedance reflected back to the loudspeaker terminals due to the movement of the diaphragm.) This impedance is only a small fraction of the total impedance of the loudspeaker, most of which is due to the resistance and inductance of the voice coil.

Speaker damping

For this reason, the loudspeaker is quite inefficient. The greater part of the power delivered to it by the amplifier is expended in the voice coil resistance and merely heats the voice coil, whereas only a relatively small proportion is radiated as sound, due to the motional impedance. Only a correspondingly small fraction of the speaker's output is available for damping. Hence, even an infinite damping factor would have a limited effect on mechanical or acoustic resonance. The resistance of the voice coil (which cannot be eliminated) limits the damping current.