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Test Your Knowledge Of Power As It Relates To Sound Systems
Looking at the many variables and caveats when you assign numerical ratings to amplifiers and loudspeakers
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Here’s a handy quiz for checking your knowledge about audio power.

Rather than submit you to the typical “right or wrong” questions with exact numerical answers, I’ve elected to provide a different means of self-evaluation.

I can tell you up front that the answer to every question is “It depends!” But what’s really being asked is “WHAT does it depend on?”

At first glance it may seem that the question is not even related to audio. Don’t be fooled. Principles are principles.

Each question also paves the way for a short review of the concept. That’s it. Relax. Take out a sharpened number 2 lead pencil and….  Sorry about that.

1) I want to paint my living room walls and need to buy paint. How much will I need if the ceiling height is eight feet?
Obviously estimate of the amount of paint requires more information. What we need to know is the area to be covered which can’t be determined by the ceiling height alone.

The total length of the walls is needed to get the area (length x height).

The paint store would also need to know how many windows are in the walls (they can subtract this area from the total), and how absorbent the surface is (one or two coats?). Only then can the required amount of paint be determined.

It’s equally ridiculous to calculate an amplifier’s output power by using it’s peak voltage rating.

As with the wall, the area of a waveform must be known to determine how much power is generated. This requires amplitude information (like ceiling height) and also knowledge of length (time).

We also need to know how much to subtract for higher crest factors (less intense program - like windows in the wall). And lastly, we need to know how much the load will soak up (porosity of the surface). Think of one coat as eight ohms and two coats as four ohms. And two ohms? Don’t even think about it!

2) Which stock will yield the greatest earnings?
We’ve all learned this one the hard way. Stock A has some high amplitude values, but doesn’t last long. Stock B has lower “highs” but is more consistent over time. Like painting walls and electrical waveforms, it’s all about area.

An amplifier can have a very high peak rating, but may fizzle when loaded for long spans of time (that all-day outdoor show). Make sure that you look at the long-term continuous output power when shopping for amplifiers. Short-term peak ratings are large numbers, but they don’t tell the whole story.

3) Which song will make the loudspeaker hotter?
This should be obvious by now. Grungy, highly compressed rock ‘n’ roll has a much lower crest factor (more area) than an “audiophile” recording of a sitar solo. Both types of music may occasionally light the clip light, but the rock ‘n’ roll is much more likely to toast the loudspeaker.

4) How much must I increase the power applied to a loudspeaker to make it a little louder?
A bunch - 3 dB represents a modest change in sound level, yet a 3 dB increase requires the amplifier to generate twice the power. So every time you turn it up “a little” you are doubling the power to the loudspeaker. No wonder so many loudspeakers succumb to the last song of the evening.

5) How much of an amplifier’s rated power will it likely have to generate in a music playback system?
Not much. Given a typical crest factor of 20 dB for live music, the amplifier’s output power could on average be about 1 watt per 100 watts of rated power.

That kilowatt monster that you bought with the home improvement loan will likely need to generate about 10 watts continuous. If you break out the compressor/limiter you may get this up to 100 watts, but that’s about it.


Comment (1)
Posted by Mike W  on  03/01/11  at  06:15 PM
Best. Amplifier. Explanation. Ever.
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