Which Loudspeaker Should You Use With Your Amplifier?

Close-up of power amplifier inside rehearsal room

Match Amps to Speakers Without Driving Yourself Crazy

Matching a loudspeaker to an amplifier is a seemingly arcane process, full of exceptions and confusing “rules-of-thumb” bandied about by quasi-mystic front-of-house engineers and MacGyver-like technicians (gaffer’s tape clutched firmly in hand).

For those of us who just want to hook up a simple PA system for our band’s vocalist, or who want to attach a Marshall head to a speaker cabinet without blowing anything up, is there a way to de-mystify the process?

The correct answer, of course, is that  “it depends.” But for most situations, these following 2 guidelines can help you correctly match your loudspeaker to your amplifier.

1. Get the Right Amount of Power From Your Amp

This is easy, although it’s somewhat counter-intuitive. Your amp or powered mixer should be rated at twice the RMS (root mean square) of your speaker.

Why should your amplifier put out more power than your speaker’s power rating?

This is because the rating of your amp applies to peak power, not an average signal. Most loudspeakers can handle signal spikes up to twice their own power ratings.

Although it’s possible to blow your speakers if you’re running a highly compressed or processed audio signal that pushes the average power output closer to the amp’s peak, it’s far easier to fry a speaker by UNDERPOWERING it.

This is because when your amp is struggling to put out enough sound to drive your speaker, it begins to clip when it hits the upper limits of its power range- causing normally smooth sound waves to become square and jagged. And square waves can be deadly to a speaker, regardless of where in your signal chain the clipping is produced.

For this reason you need an amplifier with plenty of headroom- and going for an amp that’s twice the power rating of your speaker is an easy way to do this.

2. Match the Impedance of Your Amp and Speakers

Simple enough- make sure the load handled by your amp (expressed in Ohms) matches the load of your speaker. Use an 8 Ohm speaker with an amp rated at 8 Ohms, a 16 Ohm speaker with a 16 Ohm amp, and so on.

If the impedance doesn’t match, you’re generally safe (with solid-state amps, anyway) any time your amp output impedance is lower than your speaker impedance. A 4 Ohm amp is usually safe driving an 8 Ohm speaker, for instance.

If you have a tube amp, the picture is a bit fuzzier. The jury’s out as to whether a serious upward or downward mismatch is worse on a tube amp (and it depends on the amp), but in general, your best bet is to match your impedance as closely as possible, not going too far in either direction.

Keep in mind that these are general rules. As in the case of almost everything else in the world of pro-audio, exceptions abound.

Any questions? Give us a call at 1-800-263-0112 or send us an email for more sound advice on loudspeakers and other sound reinforcement equipment!