| Video Noise March 2001
Is One Subwoofer Enough?
So you have your home theater all set up -- your five main
channels and your LFE (Low Frequency Effects) subwoofer. You set the delays for each
speaker based on your distance from each speaker. You balanced the levels with the Radio
Shack sound pressure level meter. You have a nice monitor and DVD player with a good video
cable directly to the monitor so there are zero switching losses in the receiver or
processor. You tried a few digital cables and picked the one that sounded the best to
connect the DVD player to the receiver or processor. And maybe you even added some
accessories like power conditioning, accessory feet, Wave Guides, or power cords.
Now you’re feeling pretty smug about your home
theater. It probably looks and sounds cooler than anything anyone else you know is using
because most people just haven’t got a clue -- they’re still listening to TV
through the built-in speakers. You marvel at the way the quality of the bass can vary from
one DVD to another. The depth charges in U-571 sound nothing like the cannons in The
Patriot. The shotgun blasts in Terminator 2 sound nothing like the pounding
ghost in The Haunting or the bookcases falling in The Mummy. Your friends
always want to watch movies at your place. You’ve got the home theater thing
completely under control.
Now last year was a pretty good year: you got a little
raise, the bonus check is coming next month, and the credit cards are down to zero. You
think it might be time to add a little something to the home-theater system to kick it up
another notch. But you’re not so sure what that might be. There are plenty of
possibilities . . . better amplifiers, a bigger or better video display, better speakers,
better cables, a newer DVD player, maybe one that does DVD-Audio. The list goes on and on,
and $10,000 could be soaked up so fast your head would spin. But you aren’t about to
let (another?) $10,000 that could be saved for your retirement get tied up in your home
theater. But you want to do something to raise the excitement level. You’re
just not sure what would be the most effective thing to do.
Here’s an idea you may not have even considered . . .
more subwoofers. Having one for the LFE channel is a given; you definitely want that. But
the LFE subwoofer, good as it may be, just can’t do the job that additional
subwoofers can do when used with your other speakers. The front main speakers will benefit
from added subwoofers to extend their bass flat to 20Hz. If your main speakers are already
fairly full-range, you may enjoy adding subwoofers elsewhere first.
Front and center
The center channel, surprisingly, can contain significant
amounts of bass in modern DVD soundtracks. The days of the center channel being limited to
100Hz and above are long over. Unfortunately, many receivers and processors delete the
bass below 100Hz in the center-channel-level test tone. When you connect a subwoofer to
the center channel, you will hear no sound coming from the subwoofer when the receiver or
processor is sending the pink noise setup signal to the center channel. If you don’t
have bass in the test signal in your system, the best way to set the subwoofer level is to
use a setup DVD that has full range. The two main DVDs are Video Essentials and The
Avia Guide to Home Theater. You could also use a stereo CD with at least 40, 80 and
160Hz test tones if your receiver or processor has a mono mode that sends everything to
the center channel. Some mono modes "spread" the sound by putting some into the
other front channels. Turn those off, or disconnect them, when setting the
center-subwoofer level. Lacking a mono mode, a five-channel stereo mode would also work.
This mode usually sends left plus right mono to the center while all of the right-side
speakers get right-channel audio and all of the left-side speakers get left-channel audio.
This will put test signals from the CD into the center channel and you’ll be able to
get the subwoofer level set perfectly. Turn off or disconnect the amps to the other
speakers when using the five-channel stereo mode.
When two is not enough
Unfortunately, the product you really want for adding bass
to the front and center speakers in a single subwoofer, to the best of my knowledge,
doesn’t exist: a subwoofer with three input channels. Maybe some observant
manufacturer will pick up on this and produce a subwoofer that can keep up with the new
demands of having five full-range audio channels in movies. But lacking the three-channel
subwoofer, you could get two subwoofers with stereo inputs. Put one near each the left and
right front speaker. Connect each speaker to one of the subwoofer’s inputs. That
leaves one unused input on each subwoofer. Connect the center channel to the unused
connection on both subwoofers. This means the center channel will feed into two
subwoofers and you’ll get an extra bit of room-shaking bass. Some "new"
bass effects will occur that will allow you to feel sound from the left, right, and all
across the front, coming at you through the floor and the furniture. You may not be able
to hear the directionality of bass sound waves in the air, but in a real home environment,
the floors and walls do move and add many directionality cues.
Back-door bass
A subwoofer can be added to the rear-surround channels to
extend their bass flat to 20Hz. We are well past the days when the lowest frequencies you
would hear from the rear speakers are above 80Hz or 100Hz. Many of today’s
best-sounding DVDs have significant low-frequency content in the rear channels. Recently,
I’ve been surprised by how much audible interest is added by having a subwoofer in
the back of the room producing powerful low-frequency information for the rear-surround
channels. The added sonic cues from bass originating in the rear of the room adds
excitement to the presentation that you can’t obtain any other way. Based on recent
experiences, I find it would be difficult to decide whether to add a subwoofer to the
center channel or rear-surround channels first.
What to expect
The first thing I notice when using multiple subwoofers is
low frequencies with a solidity that can’t be delivered by the LFE channel alone. You
also get bass that moves around the room . . . its physical presence moves, literally, around
the room. It’s easy to notice the bass coming from behind you and from the front left
or the front right -- not in the air, but in the floor and in the furniture. When I have
four subwoofers working, I feel bass in my back, prominently, when the rear subwoofer is
active. There is a different quality to the bass when it originates from two locations in
the room. The rear subwoofer, for example, is pumping away, but so is the LFE subwoofer;
yet, I still feel and hear the bass localized behind me. Part of this, no doubt, comes
from having the rear subwoofer well integrated with the rear-surround loudspeakers. When
the subwoofer and speakers are well integrated, the smaller loudspeakers provide distinct
localization cues since bass transients always start much higher in frequency than the
fundamental tones. Having the bass reproduced in more than one location in the room seems
to impart higher degrees of realism to the reproduced bass. Bass seems more solid and
substantial while conveying a better sense of pitch and power when there are two or more
sources. Part of the reason for this may be that room interactions are more favorable when
there are multiple source locations. Another part is likely the result of moving more air
when there are multiple sources producing the bass. The bass becomes a believable
"presence" in the room. I can’t tell you how many times since starting to
use four subwoofers that I’ve heard trucks in a DVD soundtrack, which were not
visible in the frame, and thought UPS was in my driveway with another box of home-theater
gear.
Unexpected help
If you are thinking about adding a rear-center channel to
your system, some of the add-on boxes include a "subwoofer out" RCA jack, which
permits the use of a second LFE subwoofer in the rear of the room. The subwoofer gets all
of the bass that would have gone to the left and right surrounds as well as bass going to
the new rear-center channel. I find these "subwoofer out" jacks to be about 50%
of the value equation for add-on rear-center-channel boxes. The rear-center channel is
nice if you have space behind you and your existing surround speakers. If your existing
surround speakers are on the back wall, the rear-center thing is not going to work for
you. Adding the rear-center channel is subtler than adding a rear subwoofer. So you can
see why I find the "subwoofer out" RCA jack so significant.
Subbing up
You probably already know that good bass is expensive. It
can easily become the most expensive portion of a home theater if you are not careful. The
ultimate bass setup would be dual LFE subwoofers plus one subwoofer for each of the five
other channels. If you have a rear-center channel, your ultimate bass system would include
a subwoofer for that also. There are steps along the way to "ultimate" bass that
are definitely worthwhile. One additional subwoofer is great, but if you already have an
LFE subwoofer, it is really hard to decide where to put the second subwoofer. The answer
will vary from system to system. Adding two subwoofers makes things interesting. I’d
put one in front and one in back. Add three subwoofers and it’s easy to know where to
put them all: two up front on the left and right with the center channel going to both of
them, and the third would go in the back. Be sure you understand the connections you will
need to have on the additional subwoofer(s) for the various connection methods. Some
subwoofers are very flexible with many inputs, while others accept only an LFE line-level
input. You’ll have very specific connection needs for extending the bass of existing
loudspeakers. Using a receiver versus using a processor with external amplifiers can also
influence what connections you need for the subwoofers.
Once all the questions are answered and you have the new
subwoofers connected, I think you are going to enjoy the results.
...Doug Blackburn
db@hometheatersound.com |
