It looks like I found the answer to part of my question here.
http://www.onhifi.com/features/20021001.htm
How Many Watts is That Speaker?
If there's one area of hi-fi that's rife with misinformation, it's that tricky region surrounding the loudspeaker. Understanding loudspeakers and their place in a good sound system isn't rocket science, but it can be difficult -- primarily because of all the things that people know that are simply not true.
I know I don't have to actually come out and say this (but that won't stop me!), but the true measure of a loudspeaker is how accurately it reproduces the signal it receives from the amplifier -- not how loud it plays. (Of course, they are called "loudspeakers," so they should -- at least -- be able to reproduce music at realistic volumes.)
As a reviewer, I frequently have exotic loudspeakers around the house, and when non-audiophiles catch sight of, say, the Dynaudio Evidence Temptations or the Thiel CS7.2s in my living room, they almost always say the same thing: "I bet those play really loud!"
Both do, of course, but that's not what makes them so good.
Another frequent misconception about loudspeakers -- one that is encouraged by hi-fi marketers of dubious probity -- is the whole subject of a speaker "having" watts. If you've looked over the sales circulars in your local paper or tried to choose a system from online specifications, you'll know exactly what I mean. The ads trumpet 200W loudspeaker! -- but when you read the fine print, you see that these boxes contain a 4" woofer and .75" tweeter. Obviously, those aren't speakers designed to be paired with 200W amplifiers -- what gives?
The most honest answer is that speakers are not rated by watts -- at all, ever. A watt is the standard unit of power, and it is used to specify the rate at which electrical energy is dissipated (or rather, the rate at which electromagnetic energy is radiated, absorbed, or dissipated). As we all know, the amplifier manufactures the energy in a stereo system, so what's up with 200W loudspeakers?
It's a piece of information that started out as quite useful and got perverted by manufacturers who were trying to make their products look better than they were. The specification's original intent was to help hi-fi hobbyists match speakers to amplifiers capable of driving them, not, as is now commonly believed, to prevent people from blowing up loudspeakers with amplifiers that are too powerful for them.
Of course, it is possible to destroy loudspeakers with too much power -- and it's also possible to destroy a loudspeaker using an amplifier with too little power. Confused yet?
The reality is that just about any loudspeaker can be driven by just about any amplifier. (Don't e-mail me with exceptions -- I'm risking a small amount of inaccuracy in order to make as broad a point as possible.) However, it is possible to damage loudspeakers several different ways by playing them too loudly, and that's why you need to know how much power a loudspeaker can handle.
Loudspeakers are usually blown by abuse, plain and simple -- we're talking about operator error, folks. And while there are exceptions, such as equipment failure that sends a massive DC pulse through a voice coil, most loudspeaker disasters can be avoided if you pay attention to what you're hearing. (But that's a topic for another column.)
The real information you want from a speaker's power rating is how much energy can be dissipated in its voice coils without damaging them. This can be expressed as a function of watts, but there are several different ways of doing it.
By law, this figure must be expressed as either peak power or continuous (RMS) power. The more accurate of these is RMS power, but that's also the lower number, since peak power is twice the RMS power. So a speaker that can be driven safely by a 75Wpc amplifier can be described as a 150W loudspeaker. What's interesting is how seldom the companies who advertise peak power ratings alert you to that fact. Kind of sneaky, huh?
Most honest audio manufacturers cite RMS power figures -- and identify them as such. Even more to the point, most honest manufacturers realize that a voice coil's capacity to dissipate wattage isn't the best way to tell which amplifiers work best with their loudspeakers, so they recommend a range, not a number. "Appropriate for amplifiers between 75-150W," they might say.
This works better than calling it a 150W speaker, but it still doesn't guarantee that you won't blow up a loudspeaker with an amp -- even an amp rated lower than a speaker's power rating. That's because there's more than one way to burn up a voice coil. And while it's certainly true that delivering 600W into a 150W speaker could fry the coil, that doesn't mean that a 600W amplifier will fry a voice coil.
Most of the time (with most loudspeakers), it only takes a watt of two of output power to drive a loudspeaker to moderate levels. However, music is a dynamic art form -- that means it changes loudness and intensity. It also moves in time -- you might almost say that music is time, divided creatively. So, as music flows and changes, different demands are placed upon an amplifier. Certain passages with, say, huge drum thwacks or other massive instantaneous power demands, might require immediate delivery of far-higher power outputs -- and, believe it or not, it is because of the need to deliver massive power for those fractions of a second that 600W amplifiers exist. For most of their lives, those behemoths just cruise along putting out 1W or 5W or maybe even as much as 20W, no more.
Of course, there's no defense against abuse. If you insist on playing your speakers as loud as they'll go, you can manage to burn 'em out, no matter what kind of amp you're using. The only real defense against frying a loudspeaker is to listen closely to how it sounds. You'll hear clues -- maybe not the instant the speaker or amp starts to distort, but long before damage is done. Telltale sounds include the sound of a woofer rattling its basket or the hazy, whitish sound of a tweeter going hypersonic. Never assume that turning a system up will make anything sound better -- chances are it won't.
Ironically, more tweeters are probably fried from too little power than from too much. When you try to push a loudspeaker with an underpowered amplifier (or fill a huge room with sound or point your speakers out the window and get "full" sound in your yard), you are likely to drive a modest amplifier into "clipping."
We say an amplifier "clips" when it is asked to produce more power than it can deliver without distortion. What really happens is that the amplifier's output voltage exceeds the power supply's voltage. As a result, the signal gets chopped off at the top and bottom of its waveform -- the signal becomes more like a square wave. A square wave contains a greater proportion of high-frequency harmonics than an undistorted audio signal, which means that tweeters tend to work much harder trying to reproduce distortion -- and more work means more heat, and heat is what kills voice coils. So a lot of speakers aren't killed by too much power, but by too little power as a loudspeaker attempts to reproduce the uncontrolled high-frequency harmonics of amplifier clipping.
There's another little glitch in the whole issue of watts and speakers: You must rely upon the integrity of the company issuing the ratings. Simply because a driver can dissipate an enormous amount of wattage, it does not mean that it requires all that power. A speaker that can survive a 200W encounter may not sound best with a 200W amplifier.
Most of the better speaker builders out there are extremely proud of their products -- and this pride frequently manifests itself as a form of understatement. If one of these companies makes a loudspeaker that can dissipate the heat generated by a 200W amplifier, it might suggest that it be paired with an amplifier that outputs between 25-100W (RMS watts, of course) because that is what sounds best. A manufacturer less concerned with accuracy could legitimately claim that the same loudspeaker could withstand 400W (peak, of course).
Strictly speaking, both statements are accurate. But which company would you rather do business with -- the one that was careful to tell you what sounds best or the one straining to impress you? The trick is figuring out which one to trust -- it's a bit like that old paradox about the two tribes: one could not tell a lie, while the other would not tell the truth. You need to ask precisely the right question to determine which is which and, in this case, the question is what kind of watts?
Actually, beyond broadly matching your speaker to your amplifier -- which involves a lot of other questions such as room size and speaker sensitivity, which we'll get around to soon -- what you really want to know isn't how loud that speaker will play, but how good it will sound when you play it. Which, as always in matters of taste, comes down to what you want.
You won't find that on a chart, so listen carefully, ask lots of questions, find a store or manufacturer or friend you can trust, and remember to have fun.
Speaking of which, if you want to watch a certain middle-aged audio critic turn purple and sputter incoherently, all you have to do is ask him, "How many watts is that speaker?"
...Wes Phillips
wes@onhifi.com
