It should be noted that although an amplifier delivering 200 watts per channel is appropriate for use with a full-range loudspeaker rated at 220 watts program, subwoofer enclosures tend to require a bit more power to generate their lower frequencies. In other words, a single mono subwoofer rated at 200 watts should be paired with an amp offering absolutely no less than 200 watts.

In fact, a good rule of thumb when dealing with subwoofers is to employ an amp rated somewhere in the vicinity of 20 percent higher than the subwoofer. With that in mind, a 250-watt amplifier would be ideal in this application.

Another factor to be aware of is the impedance load that the amplifier is being required to drive. Impedance is the resistance to the flow of the audio signal by the loudspeaker. Measured in ohms, it is usually called "nominal impedance" by most loudspeaker manufacturers.

A mismatch in impedance between an amplifier and loudspeaker can result in poor performance, in the form of improper frequency response and excessive distortion. Serious damage to one or both components is also a very real possibility.

Most professional loudspeakers are rated at either 8 ohms or 4 ohms, although a number of power amplifiers are specified to drive loads ranging from 16 ohms to 2 ohms. You'll notice that the lower the impedance, the greater the amplifier power rating. More power is required to overcome the increased resistance.

For example, a loudspeaker delivering 110 watts at 8 ohms will also likely be specified to deliver about 150 watts at 4 ohms. Consider our prior discussion of power handling, and you quickly see why its important to note a loudspeaker's impedance specification in the matching process.

It's usually best to avoid 2-ohm operation, which can unnecessarily stress an amplifier, and also can require much larger (heavier gauge) cable to effectively and safely transport the load.

And speaking of cable, why go to all of the trouble of properly matching loudspeakers and amplifiers if the link between them is subpar? Most manufacturers recommend using heavy gauge (#14-#10), professionally manufactured cable in all applications. A good rule of thumb is the heavier the cable, the more efficient it will be as a conduit between the devices.

(Thanks to the gang at EAW for their input on this topic.)

Related articles on PSW:

Loudspeaker Primer 1 – The basics.
Loudspeaker Primer 2 – Methods of selection.