Subwoofer Placement
Now that we've discussed
bass management, we need to think about where to put the bass -- or at least where to
put the subwoofer.
Everybody knows that bass is non-directional, so you can
just put it anywhere, right?
That would be nice, but -- in a word -- no.
In the sense that deep bass radiates equally in all
directions (to the sides and rear of the speaker just as freely as in the direction that
the driver is facing), bass is omnidirectional. But we're not dealing with a
theoretical driver in a theoretically perfect room when we attempt to place a subwoofer
for optimal sound; we're confronting a real speaker in a real room. And everybody knows
that reality bites.
Most subwoofers these days are pretty well-engineered --
it's the room that creates the problems. Well, it's really the interaction of the room
with the bass tones reproduced by the subwoofer, so it's pretty nearly impossible to
separate the two. In fact, there's a saying in audio engineering that you never listen to
a subwoofer, you listen to the way the subwoofer plays the room.
So what's the problem with rooms? It's those darned
parallel walls, mostly.
Four walls …
When a sound is reflected back and forth between two
parallel surfaces (say, a room's side walls), it creates room modes or eigentones,
which are sound waves that interfere with one another by producing a series of peaks and
valleys in the sound-pressure levels (SPLs). Confused yet?
(This is an oversimplified version of complex acoustic
concepts, in order not to get too bogged down in jargon. If you want more detail than I go
into here, just Google "standing waves," "room modes," or
"eigentones" -- and knock yourself out.)
Basically, the room has alternating areas of sound-pressure
peaks (emphasis) and nulls (drop outs). These are called peaks and nodes.
It's hard to explain, but easy to hear -- just put your subwoofer down any place in your
room and play a track with a consistent bass beat and walk around the room. You'll hear
the bass get stronger and weaker based not on the music, but on your location within the
room. Actually, it's based on your location within the room and the subwoofer's
placement within it.
Smoothing out the room's reaction to the subwoofer is the
tricky part, but it is possible -- as long as you don't just put the subwoofer down
in the first place it fits.
First, let's get rid of a few pieces of "common
knowledge" that are far more common than knowledgeable. We’ve already discarded
the "put it anywhere" school of thought, but there are two more pieces of
"good" advice we need to get rid of: put it in a corner and park it in
front of a wall.
You do get loud bass in room corners because of boundary
reinforcement, but it's not a subtle effect and it doesn't deliver uniform bass boost.
You get flatulent one-note bass that does nothing to enhance either music or films. Of
course, if that's what you want, go for it -- but you don't have to settle for
something so crude.
Placing a subwoofer in front of a wall does the same thing
as sticking it in a corner, only slightly less so. Boundaries are not your subwoofer's
friend.
If you don't want your subwoofer in the corner and you
don't want it in front of a wall, then you should place it halfway between the corners --
say, in the middle of the room, right? Unfortunately, no -- the key to minimizing standing
waves is staying away from symmetry. (It's also important if you wish to preserve domestic
tranquility -- very few significant others think highly of the "subwoofer in the
middle of the room" school of decorating.)
So what do you do?
Try this!
First, try to place your subwoofer as close as possible to
your main speakers. The sub's sound will integrate more closely with the speakers' sound
that way. If that's a problem for any reason, consider placing the sub close to you
(this trick works especially well in really big rooms and really small
rooms).
Actually, if you want to draw a map of the best places in
the room for your subwoofer, consider putting it in your listening position. Since
bass radiation is non-directional, if you play your strong bass-beat track with the sub in
your chair and then walk through the room, you can make note of the spots where it
sounds particularly good.
Mark them with low-tack masking tape, and you'll have
several places where you can place your subwoofer. (If you don't end up with any
good-sounding spots, you might want to change your favorite listening position by a foot
or so and then try again.)
Once you have a home for your subwoofer, you're going to
need to adjust it to blend with your loudspeakers. If you have an SPL meter, you can use a
test disc with test tones to roughly adjust the levels and low-pass filter. (I use the
Stereophile Test CD 1 because I'm used to its warble tones, but almost any audio-test
CD will have bass tones you can use.)
If you don't have a meter, you can do a pretty good job by
ear (and final adjustment is always done by ear, even if you do have an SPL meter).
What are you measuring and listening for? Even response. If
you're using an SPL meter, you want all the tones to measure more or less the same
(you'll never get ruler-flat response). If you're doing it by ear, you want the tones to
sound the same -- again roughly, since some tones will be emphasized and some
subdued.
The first adjustment you need to make is to set the
subwoofer's low-pass filter -- and you'll set this based on the type of speakers you have.
If your main loudspeakers are "Large," you'll only want to reproduce the lowest
bass through your sub, so you should set the crossover somewhere between 60Hz and 80Hz. If
your speakers are "Medium" or "Mid," you'll want to cross over
somewhat higher -- between 80Hz and 100Hz. If your speakers are designated as
"Small," set the crossover as high as it will go (between 120Hz and 150Hz,
usually).
Now, listen to your bass tones. These will usually be a
series of tones going up in pitch by fixed amounts from 20Hz to 200Hz, and you should
listen to them and (if you have a meter) measure them. Try the series again after you have
raised or lowered the crossover point, and continue until you are satisfied that you are
getting a reasonably smooth response from the bottom to the top of the range.
You'll never get a perfectly smooth response, but you want
it to be reasonably uniform -- especially right at the crossover point.
Paradoxically, you'll have it adjusted properly when you can't
hear when the subwoofer takes over. A very good final test is to play some male speech (a
radio newscast is perfect) -- ideally, it will not sound overly chesty.
I can hear it now
Most subwoofers allow you to adjust "polarity."
Get a friend to help with this one. Listen to your steady bass-beat disc again and have
your friend adjust the polarity switch. It's set properly when you hear the strongest
bass.
Here's the tricky part -- tuning in the polarity might
change things you've already adjusted, so go back and listen to the bass-tone sequence
again and make sure everything is still relatively smooth. Sometimes it won't be and
you'll need to go through it all again. But don't worry -- you're almost there.
After you're satisfied that you've got the bass tones as
smoothly integrated as you can, it's time to listen to some music. But don't just listen
to bass-heavy music; listen to some vocals and solo instruments, as well. On most
material, you shouldn't even be aware of anything coming from the subwoofer at all -- the
sound should seem to come from your main speakers.
But turn the sub off completely and you should hear the
sound get smaller and less full. When you can't wait to turn the subwoofer back on -- even
on a solo acoustic-guitar recording -- you've got it set up right. Well, for music,
at least.
Some listeners may still want a tad more slam for
home-theater use. In that case, almost all subwoofers have volume controls -- and canny
listeners even mark two spots on the volume control: one for music, one for movies. As for
myself, I just set it for optimum music performance, which is what I'm pickiest about, and
leave it there.
But I'm weird that way.
...Wes Phillips
wes@onhometheater.com
Next time, we'll look at a question you may not have
even thought to ask: How many subwoofers do I want? Acoustics-maven Floyd Toole's answer
might surprise you!
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