Why is mismatching a weak amp and powerful speakers dangerous to your wallet?
Clipping causes the amp to put out damaging DC (direct current) to the speakers and eventually burns them up.
Your everyday home stereo 100W amp, although rated much higher, typically doesn't run at more than 5 to10 Watts RMS average. It may peak at 80 to 100 Watts, but the average RMS output is far lower than you'd think. But you don't get something for nothing. Let's face it, a race car goes faster than a passenger car because it has a ton more power. Same idea works for speakers. If you want to move a lot of air, as in a pro sound environment, you need a device (speaker) that is big enough and mechanically capable of moving that amount of air. Then, you need a certain amount of power (Wattage) to properly motivate that device. Big device, moving lots of air = lots of power.
And you also want "Headroom" Headroom is basically available power that you don't use most of the time. Then why pay for it? Well, as stated earlier, your home stereo may only average 10 Watts RMS, but on loud passages in the music, it may peak to its full 100 Watt potential. These peaks may only last fractions of a second but without that reserve power, the sound of your music would suffer terribly. .
So what that other 90 Watts you paid for does is prevent your amp from clipping (distorting) during peaks. Think of it as "sound insurance". Headroom is the reason you may see a sound engineer running an amp that seems much too powerful for the speakers. An experienced sound engineer knows that most speakers are blown due to a power amp running out of "clean" power and clipping and not from being "over-powered" with clean Wattage. I have heard it so many times: "why did my speakers blow?!? They can handle 600 Watts and I was only running a 200 Watt amp on them!!" What that person failed to realize was that by badly mismatching his amp and speakers, he ended up pushing the power amp so hard that it was constantly clipping. Clipping causes the amp to put out damaging DC (direct current) to the speakers and eventually burns them up. That is where the old myth that "you can under-power" a speaker comes from. You can't really under-power a speaker as long as the power is clean. But using a low powered amp to try and push high powered speakers will inevitably lead to clipping and damaged speakers. This doesn't mean you can just throw tons of clean power at your speakers and feel safe, it just means you have to think ahead when putting together your system. Don't go over-board or under-board. If you're going to run some 600 Watt mains, then use a quality amp that will deliver between 400 and 800 Watts at the speaker's rated impedance. Go on the high side if you think you will be really pushing the speakers hard so your amp will be able to handle the peaks without distorting. If your speakers will never see anything larger than a coffee-house, the lower powered amp will probably be fine. Just watch and listen for clipping.
So let's recap: You now know how to calculate your total impedance, you know that the best way to get more volume is to add more speakers, and you know how to properly match your amp and speakers so that you have sufficient headroom and prevent clipping. No matter what speakers and amp you use, always keep your eyes and ears alerted for signs of something being pushed too hard. There are usually warning signs long before something goes "boom". If it sounds crisp, clean and punchy, and not distorted, muffled and muddy, you're doing it right.
Now pay no attention to that man behind the curtain...
Later.
D.S
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