I'm the pro audio guy at a music store in Iowa, and my office is in the back corner of our electronic repair shop.

Because I get to overhear the sad tales of woe from the repair shop customers, I've come to the conclusion that the most frequent cause of blown loudspeakers is not too much amplifier rated power, but rather too little.

Overpower a loudspeaker, and it will definitely blow.

A loudspeaker with an 800-watt program power handling rating is going to fry if it's fed anything beyond that, sustained for a fairly short period of time.

This is totally understandable. The loudspeaker isn’t designed to accommodate the power it's being given, so the excess current builds up additional heat very quickly in the voice coil, magnet structure, and speaker frame. In no time at all, the voice coil fails and the loudspeaker is toast.

What is confusing to many is how it can be possible to blow an 800 watt (program) loudspeaker with a power amp that has a lower rated output. It’s fairly simple: if the power amp is being pushed beyond its limit due to the demands of the presentation (outdoors is a great example; rock and roll is another), the amp will start clipping, which will result in audible distortion in the program material. If the system operator doesn’t notice the distortion and back off the level, speaker damage will result.

We’ve all seen an oscilloscope at one time or another; it is the device that shows electrical current as a sine wave. The sine wave appears as a line across the screen that looks like very smooth waves in the ocean.

When a power amp is pushed to its rated limit or beyond, the upper and lower limits of the sine wave seen on the oscilloscope will swing above and below the limits of the amp, and the smooth upper and lower limits of the wave on the scope looked chopped off, hence the term “square wave”.

Amplifier power ratings are measured with a continuous sine wave at the threshold of clipping (defined arbitrarily using a Total Harmonic Distortion figure at either 1khz or full range at 20-20khz). The rating of the amp does not indicate the maximum power output of the amp, but rather the maximum clean power output of the amp. Understandably then, when the amp is pushed beyond its rated power, it does not sound as clean but it is still delivering additional power to the speaker.

By way of example, there is a very popular sound reinforcement loudspeaker that will handle 400 watts continuous/800 watts program/1600 watts peak. There is also a very popular amp on the market that will deliver 775 watts of clean power (measured 20hz-20khz) into an 8 ohm load. Let’s presume that our system operator owns both products, and has a rock n roll gig outdoors. He doesn’t have a sound man or woman, but operates his own rig from the stage.

Because sound dissipates so readily outdoors, and because the crowd is large, loud and boisterous, and because the system owner/operator is on stage behind the main speakers, the system gets turned up to past the point where clipping begins. The operator can’t hear the clipping because he’s behind the mains and his monitors are fairly loud.

According to an amplifier engineer who was kind enough to lend his expertise to this article, the amplifier in use can quite possibly provide an additional 25-50% of power output once past the point of clipping. Let’s choose 30% of 775 watts for our example. 30% additional power results in 1107 watts now being delivered to the speaker (program rating of 800 watts) and in fairly short order the show is over. The voice coils can’t tolerate the additional heat buildup and will fail.

The following Monday morning, the sad system owner shows up at our repair shop, explaining that this is a warranty repair because his speakers are rated 400/800/1600 watts and his amp is rated at 775 watts, so he couldn’t possibly have overpowered the speaker.

And so we tell the story again….

The moral of the story is to be certain that both your speakers and your amp have performance specs that can go significantly beyond the levels you intend to operate. Doing so is one of many factors that will help to ensure you of clean trouble free operation of your system.