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Test Your Knowledge Of Power As It Relates To Sound Systems

Here’s a handy quiz for checking your knowledge about audio power. Know that right upfront, the answer to every question is “It depends!” But what’s really being asked is “WHAT does it depend on?”

6) So why buy a big amplifier? After all, they’re expensive!

Loudness and generated power are all about area. Clarity is all about headroom. If program peaks get clipped by a small amplifier, it sounds like trash. Amplifiers must be oversized relative to their average output power by a factor of 10 to 100 to allow for signal peaks. This translates into 10 dB to 20 dB of headroom.

If you have deep pockets, go for 20 dB. If not, spend some of the money saved on a hard limiter to make the program peaks “fit” though the amplifier. All of this makes sense only if you look at power using the decibel.

7) Which subwoofer is better? One that handles 100 watts or 500 watts?

You simply can’t tell from the power rating alone. It’s just a “waste disposal” number. How much sound can each produce? This is the efficiency rating. A 15-inch bass horn sitting in a corner and consuming 100 watts continuous could easily be much louder than an 18-inch in a sealed box hanging in free space and consuming 500 watts continuous.

It’s not what’s fed in, it’s what comes out. See the stock market question (number 2, above) for an object lesson on this.

8) Can the loudspeaker’s power rating be trusted? The “Killbox 5000” is rated at 5 kW and the “Lighttones 100” is only rated at 100 watts.

Most reputable manufacturers use standardized power testing to rate their loudspeakers. These methods define the type of waveform, time duration, crest factor and a few other metrics.

These tests are designed by experts to simulate real-world demands on loudspeakers. They tell you how long the loudspeaker survived under the described conditions. Since they can’t possibly know how the end user will use (or abuse) the product, they can’t guarantee that you won’t blow it up.

Power ratings are very useful for comparing the relative differences between products from the same manufacturer. When using them to compare one brand to another, make sure that the ratings are based on the same standard (i.e. AES, EIA-426B, etc.).

And even when the standards are used, the “honor system” governs the writing of spec sheets. Since overrated loudspeakers generally don’t kill or maim people, the government doesn’t require validation of power ratings.

Only in “Audiotopia” are there “data police” that check all the ratings. You can pinch yourself to wake up now.

Also remember that large power ratings can be achieved with resistors (which don’t produce much sound!). That “Killbox 5000” might have a couple of water-heater elements as part of its crossover network.

9) What happens if my power amplifier is too large?

It’s better to have an oversized amplifier than an undersized one, provided that one stays within the thermal limits of the loudspeaker. A bigger amplifier is less likely to clip the signal.

But there are limits here, too. A piston can only travel so far before it becomes non-linear. If you hook a zillion watts up to a loudspeaker to provide 40 dB of headroom, and then someone drops a mic, you may see impressions of all of the cones in the metal grills. Over-excursion kills fewer loudspeakers than heat, but it must still be considered.

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10) Should I use a rubber band or a chain to pull my boat trailer?
I had to get one in regarding reactance. There’s a big difference between the impedance of a real loudspeaker and that of the non-inductive load resistors used to test many amplifiers.

Reactive loads reflect power, and the amplifier has to deal with this. Amplifiers with extended bandwidths are often unstable into reactive loads. If presented the choice between a “20 Hz to 40 kHz” bandwidth spec and a “DC to gamma rays” spec, I would pick the former.

Pencils Down…
How did you do? Hopefully this series has provoked some thought. Power ratings are useful for getting a general idea of the performance of a device, but as I have shown, there are many variables and caveats when you assign numerical ratings to amplifiers and loudspeakers.

Don’t give these numbers and more or any less attention than they deserve. They are but one piece of the puzzle, and probably not the most important piece.

Also, don’t be afraid to do your own power testing. The A/B comparison of two products is still the best way to tell the difference between them, regardless of what the numbers say. ZZ Top at full volume might be a more relevant power test for your system than pink noise.

Just be sure going in which party is paying for the toasted voice coils.

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