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
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