Speakers: A Rich Vein
of Mythology
It's probably inevitable that when a basically technical
field like audio is directed at consumers whose enthusiasm often exceeds their grasp of
engineering, misconceptions can arise. All the components that go to make up a stereo
system have had their share of folklore, but the loudspeaker seems to have attracted far
more misunderstanding than other devices.
Selecting a speaker is hard enough without the burden of
the sort of misinformation that a lot of potential buyers, salesmen, and some dealers
carry around with them. Here, therefore, are some firmly held beliefs that you can safely
ignore when buying or setting up a speaker system.
A well-known brand name is an assurance of quality.
It may be an assurance of construction quality, but
it is certainly no guarantee that a speaker will sound good. There are innumerable
instances of companies that have gained their reputations from clever advertising and
promotion rather from any discernible audio quality, or from innovations made many years
ago that have not been improved upon in the intervening time.
A more expensive speaker is better than a cheaper one.
Of all the components in audio, the speaker exhibits
the poorest price-vs.-quality relationship. There are, indeed, extremely fine speakers
carrying high price tags, and terrible ones at bargain prices; but by the same token, the
number of high-ticket dogs are legion and excellent speakers with moderate price tags are
reassuringly common.
The bigger the speaker, the more bass it will produce.
There was a time when this was more or less true,
but speaker technology has progressed a great deal over the years, and most of the
development has been directed toward making smallish speakers sound like big ones.
Nowadays, size and bass performance bear practically no relation to each other until you
get down to very compact enclosures.
Of course, in trying to pump those low-frequency decibels
out of small boxes, speaker designers have had to make some compromises. Some of the
old-style behemoths that take up most of an average living room can produce wall-rattling
levels from one watt of amplifier power or less; to get the same volume from a bookshelf
speaker, you may need 60 or 100 watts. Most listeners are willing to provide this sort of
power rather than put up with the inconvenience (and price) of the huge speakers.
A speaker rated at 50 watts should be driven by a
50-watt amplifier.
This proposition would seem to be logical, and
indeed it would be if there were any real way of wattage-rating a speaker. As it is, such
numbers are meaningless, and are only included because speaker manufacturers feel that
some sort of amp/speaker correlation is needed by the buyer. Some of the better
manufacturers make an earnest effort to provide useful numbers -- minimum and maximum
powers, for example -- but the fact that there is no consistency from one maker to another
destroys the value of any such rating as a method of choosing one speaker over another.
Specifications are an important guide to speaker
performance.
With some audio components specifications are a very good
comparative guide to how equipment will perform. Because of this, speaker manufacturers
feel some pressure to include specs, even though few of them would claim that they mean
very much.
Actual performance data are all but meaningless because
there are no standards for measuring them, and because there is no agreed method of
representing on paper how a speaker will sound. Even if there were, it would document a
speaker's performance in one particular acoustic environment; at least half of a speaker's
sound is a result of its surroundings, however, and the speaker's designers and measurers
have no way of knowing what your listening room is like.
The problem is compounded by the desire to come up with
single-number specs. A specification such as "Impedance: 8 ohms" fails to take
into account that even though 8 ohms may be the average impedance, it may dip much
lower than that at some frequencies.
An audio dealer is the best guide to making a choice.
Well, he may be. The majority of dealers know their
products and are adept at discovering what you want and what may suit you. He's unlikely
to recommend a competitor's product, of course, but that can't be held against him.
But never forget that he is in business, and that
absolutely perfect sound in your listening room may be secondary to his sales and profit
figures this month. There are many possible reasons for his recommending a certain
speaker: it may indeed have the best sound, but it may also have the highest markup, or it
may have been on the shelf too long -- if he doesn't sell it to you, he may never sell it.
Or he may have just given up the brand and want to clear out his inventory of the product
as quickly as possible. Also remember that he is probably a stereo buff himself, and as
prey as anyone to preconceived notions and myths about loudspeakers.
On the other hand, like any retailer, he wants repeat
customers, so he will go to considerable lengths to make sure your needs are satisfied if
he hopes to stay in business long.
Because we all hear differently, choosing a speaker
is simply a matter of personal preference.
What we may think of as an informed audio choice often has
little to do with sound quality. Rather, even the most experienced of us can be swayed by
a speaker's appearance or brand name or by the musical selections used to demonstrate it.
There is a growing body of evidence, however, showing that if we eliminate these factors,
even unsophisticated listeners will gravitate to the same speakers, and that these are the
ones that show up best in conventional measurements. It is, of course, extraordinarily
difficult to filter out all the extraneous influences, but it's worth giving it a try --
the more you approach each speaker with an awareness of the unimportant factors that might
sway your decision, the closer you will come to a good one. The evidence would seem to
deny that we all "hear differently"; it is likely, however, that we all react
differently to things that have nothing to do with audio quality, but that color our
appreciation of it.
Bass reflex speakers are superior to acoustic suspension
(or vice versa).
This myth is basically the result of some historic
one-upmanship between speaker companies in the late 1960s and early 1970s. There were two
main schools of speaker design in the U.S.: the "New England Sound" and the
"West Coast Sound."
The West Coast Sound was reputed to enhance rock music. Its
sonic characteristic was a very forward, bright midrange coupled with bass that was
powerful, or "punchy", but which didn't go down very low in frequency. The New
England Sound had smooth, deep bass, but was notable for its muted treble. This
characteristic was deemed to be most suitable for classical music. When truly flat
speakers ultimately arrived on the scene, the deficiencies of both these
"sounds" were apparent, and they quickly disappeared.
But they did leave a legacy. The New England companies were
committed to the acoustic suspension design, the West Coast manufacturers to the bass
reflex. The enclosures naturally came to be associated with the particular
"sounds," and audiophiles who liked one set of tonal characteristics tended to
become advocates of the type of speaker that had produced it. In reality, it is possible
to make superb speakers by either method, and the best of each breed are virtually
indistinguishable from one another.
Three-way speakers are better than two-way.
Different parts of the frequency spectrum require different
treatment by speakers, so multi-driver speakers have become all but universal. Many are
two-way systems, in which high- and low-frequency sounds are delivered by units optimized
for each part of the band. It would seem to make sense, then, to further divide the audio
band into as many different chunks as possible, giving each one a driver optimized for it.
Four-way, and even five-way, systems have appeared from time to time, but usually the
practical limit is three. Three-way systems are usually sold as premium speakers but, as
it turns out, this is mainly because they are more expensive to make, not because they
have any inherent advantage in sound quality.
The most difficult aspect of speaker design is matching the
various drivers. Obviously one part of the spectrum should not be louder than another.
Also, each of the drivers encroaches upon the territory of its neighbor to some extent,
and it's a major part of the speaker designer's art to make sure that where frequencies
are being shared, the two drivers behave as one -- no mean trick.
If solving the problems of one crossover point is
time-consuming and expensive, dealing with two or more is that much more complicated, and
increases the likelihood of unpleasant glitches in the sound. To be sure, a three-way in
which these problems have been solved my theoretically be superior to a similarly
successful two-way, and probably can handle more power. But in the real world, the chances
of a two-way outperforming a three-way are quite substantial.
Too much amplifier power may damage a speaker.
Truly gargantuan peaks, such as that made by turning up the
volume too high at the beginning of a compact disc, or cranking the FM tuning knob across
the dial with the muting off, can indeed blow a speaker, but this will happen even if the
amplifier is a modest one. Similarly, sustained high-powered sounds can cause damage.
In normal operation, however, your amp will be coasting
along at a couple of watts or less most of the time, producing high power levels only for
very short times on musical peaks. The only difference between a low-power and a
high-power amplifier in such a situation is that the latter will produce the peaks with
less distortion.
The danger is not that you will overpower your speakers,
but that you will underpower them. Any amplifier has an absolute limit as to the
amount of power it can produce (called the "clipping level"). When it reaches
that level, the waveform simply flattens out, producing the same level no matter how much
higher the input signal tries to drive it. This flattening out is a momentary amount of
direct current (DC), and while it is happening the speaker's voice coil is motionless and
offset from the magnetic gap that normally surrounds it and absorbs heat. In addition, the
clipped signal contains much more high-frequency energy than would occur in a normal
musical signal.
Usually all this happens only briefly, and while the
distortion may be audible, no physical harm is done. But if it were to last for an
appreciable length of time, as it would if you were overdriving a low-power amp, the voice
coil would burn out. Tweeters are particularly susceptible to this, both because of the
delicate wire used in their coils, and the unusually high levels of high-frequency energy
during clipping. A high-powered amp will clip very rarely; as the power available is
reduced, however, the amount of time spent clipping increases, along with the risk of
damage.
...Ian G. Masters
ian@mastersonaudio.com
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