The Truth About Power Ratings

When you're making comparisons between product specifications it helps to understand what they mean, particularly with amplifier power ratings. Everybody knows that the more watts you have, the better. Right? Ah, if it were only so simple. Here's a guide to the wacky world of watts.

The higher the power (watts) of the amplifier, receiver, or powered subwoofer, the louder and cleaner the speakers will play. Don't worry about small differences in power: in order to get an audible volume difference (a 3dB increase) you need to double the power. So, if you have a 50 watt per channel amplifier, the next significant step up (power wise) is 100 watts per channel.

But beware, because not all watts are created equal. It is common to have two receivers or amps of equal rated power, yet find that one plays louder and sounds better than the other. Why? Some manufacturers measure only one channel operating at a time, rather than all channels driven simultaneously (as you would use it in your living room). Also, standard amplifier tests cannot mimic the same electrical conditions, or load, of an actual loudspeaker. But most of all, specifications cannot measure the quality of sound. So how do you tell which one has the better amplifier section?

Here are a few clues to look for:

Look carefully at the power specifications.
Thorough and meaningful power specs would look something like this: 100W/ ch. @ 8 ohms, with no more than 0.1% THD, from 20-20,000 Hz, all channels driven. In this spec you can tell that the power was measured in the way you will use it: at low Total Harmonic Distortion (anything under .5% is low enough), through the whole audible frequency range (20Hz -20kHz) and with all the speakers playing. A lower quality receiver might quote power like this: 100W/ch @ 8 ohms, at 1 kHz, one channel driven. That's like quoting a car's acceleration as 0-60 MPH in 7 seconds, downhill with a stiff tail wind.
Look for power ratings lower than 8-ohm loads.
(Ohms are a measure of the electrical resistance of the speaker.) Ideally, the amp should be able to pump at least 50% more power into a 4-ohm load than it would into an 8-ohm load. If there is no 4-ohm power rating quoted, chances are that the amp will not drive a 4-ohm speaker. Almost all speakers are less than 8-ohms for some part of the frequency range (impedance varies with frequency). Get a receiver that can safely and robustly drive a 4-ohm speaker.

Nowhere are more games played with power ratings than in powered subwoofers and other self-powered speakers. Recently, the US Federal Trade Commission issued new regulations regarding the power ratings of all amplifiers including those used in self-powered speakers. According to American law, all power specs should be quoted as "Continuous average power into a stated impedance, at a stated distortion over a stated bandwidth," just like we described above. We think it is terrific that there are standardised power testing methods in order to provide consumers with a level playing field and basis for comparison. But it will take some time for all manufacturers to comply with this regulation and revise their specs, so don't be surprised if you see a mix of optimistic and real power numbers as you shop for powered products.

With powered subwoofers and other self-powered speakers, the power rating, whether FTC approved or WLS (When Lightening Strikes), doesn't really tell you much. Why? Because loudspeaker efficiency is by far the most important factor in determining how loud a system will play. Efficiency (a.k.a. sensitivity) is usually given as the amount of sound produced by 1 watt at a distance of 1 metre. A medium efficiency speaker rating would be around 87 dB from 1 watt at 1 metre. A highly efficient speaker might be 90 dB. Each 3dB increase in efficiency doubles the sound output for a given power input. So a 100-watt, 90 dB self-powered speaker and a 200-watt 87 dB unit would produce exactly the same sound output.

So how do you evaluate powered speakers? Right now there really isn't a better way than just listening: how loud does it play before it starts to distort? Ever striving for a better world, we'll continue to list true specifications and are glad to answer any questions you might have, but always be wary when consulting amplifier specs.