Do You Really Need a
Subwoofer?
One of the most satisfying things
about a good audio system is great bass. I'm not talking about thumpy, boomy,
kid-in-the-muscle-car-at-the-traffic-light bass, but clean low-frequency sound that has
adequate level and -- more important -- that reaches down into the bottom octave of the
audible range.
Speaker design is mostly about bass. Obviously, attention
has to be paid to the other parts of the spectrum, but the box itself is almost entirely
designed for bass performance. In the early days, the only way to get really good
low-frequency output was to put a big speaker in a big box. The result was a speaker (mono
then) the size of a refrigerator.
Much of the effort since then has been to get big-box sound
from a small enclosure and the speaker companies have had some remarkable success in doing
this. But there is a definite limit; it may be too much to ask a box with an internal
volume of less than a cubic foot to put out much in the way of low bass. And yet, as
multichannel home-theater systems become more popular, it's just that sort of speaker most
people are choosing; only the most opulent setups can accommodate five or more full-size
speakers.
One result has been the increasing employment of auxiliary
bass units, or subwoofers. The term is not always accurate, as many of them don't in fact
operate in the bottommost octave of the audio band -- 20Hz to 40Hz -- but instead function
as a normal woofer removed from the rest of the drivers. The common satellite/subwoofer
arrangement used in surround-sound systems depends on there being a subwoofer, as the main
speakers generally don't even try to reproduce the lowest notes.
But even if your system employs full-range speakers, at
least for the main channels, a subwoofer might well be advisable. In fact, their original
purpose was to supplement the low-frequency output of conventional stereo speakers and to
tame some acoustic problems that could arise in listening rooms.
Sound behaves very differently at the extremes of the
audible range. It all has to do with wavelength: the distance from the peak of one sound
wave to the next as it moves through the air. Treble sounds have very short wavelengths,
measured in fractions of an inch at the highest frequencies. Our ears are very sensitive
to when a given sound arrives at each ear; if it arrives at the left ear slightly ahead of
the right, we use that as a clue to place the sound off to the left.
Because the wavelengths are short compared to the distance
between our ears, we can hear such differences readily, and so the careful placement of
midrange and high-frequency speakers is very important if we are to hear instruments or
sound effects in their proper positions. But these speakers need not be very big, so the
small satellite speakers used in many surround-sound systems can be placed optimally with
regard for directionality and imaging without being very obtrusive.
With normal stereo speakers, placement is important for
imaging, too, and since the woofers are usually in the same box, they go along for the
ride, even though that might not be ideal in terms of the portion of the audio range they
have to handle.
The wavelengths in the lowest frequencies are measured in
tens of feet, and that can cause acoustic problems in the sorts of rooms we usually listen
in. The main villain is the "standing wave," where the wavelength of a
particular sound is an even multiple of one of the room's dimensions. Since our rooms are
usually rectangular or square, the sound begins to bounce back and forth between the
parallel surfaces, and because the wavelength has a simple mathematical relationship to
the distance between the surfaces, the peaks and troughs of the sound waves recur at
exactly the same points in the room.
If you stand where the peaks reinforce themselves, the
sound will be incredibly boomy; if it's where the troughs coincide, you will hear
practically nothing. And it’s all different with different notes, which results in
very uneven response in the low frequencies, with some sounds becoming overpowering and
others barely audible.
It's position-dependent, and often moving the speakers only
a slight amount will smooth out these irregularities adequately. Trouble is, moving the
speakers to fix the bass may louse up the imaging.
Fortunately, one aspect of sound in the lowest octaves --
below about 80Hz -- is that they are essentially non-directional, so if you take the
woofers out of the main enclosure and put them where they behave themselves in terms of
standing waves, you normally aren't aware of the bass coming from anywhere but the
location of the main speakers.
By the same token, adding a true subwoofer to a normal
stereo setup can reduce some acoustic problems without requiring you to move the main
speakers, and the sound will blend with the main channels smoothly even if the bass unit
is across the room. But it is generally only necessary in the most intractable of
circumstances: it's a tool to solve a problem that should only be used if in fact you have
that problem. Otherwise, adding a subwoofer just to punch up the bass is likely to result
in poorer sound rather than better.
...Ian G. Masters
ian@mastersonaudio.com
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