| Video Noise January 2001
Subwoofers (and Bass) for Home Theater
Unfortunately, with the way things have shaken out, the job
performed by subwoofers for home theater and music has turned into two completely
different tasks that are no longer available in a single subwoofer product. Therefore,
someone who wants to do bass right for home theater and music is going to need
separate home-theater and music subwoofers. This statement might appear confusing when you
note that there are actually a variety of subwoofers with dual modes, which are often
labeled something like "video" and "music." Indeed, this is a step in
the right direction, but so far I’ve not heard one of these dual-mode subwoofers that
excels in music mode. Certainly they sound better in music mode when playing music, but a
true music subwoofer sounds better still. But nothing (and I really mean nothing), sounds
like a home-theater subwoofer when you are playing a movie with a great soundtrack.
Home-theater subwoofers require a boost in output in the
40-80Hz range to make effects seem real and solid. (Why this is not encoded on the DVDs
themselves is beyond me. If the DVDs simply had the boost encoded where it is needed, one
subwoofer could serve for both music and movies.) Subwoofers for music need linear
response from around 20Hz up to 160-200Hz. Surprisingly, movies are not that dependent on
response below 40Hz or so. When there is sub-40Hz content in the soundtrack, there seems
to be some built-in augmentation to make those frequencies heard and felt, even on
subwoofers that have a hard time going that low. DVD soundtracks played back on music
subwoofers with linear frequency response sound a bit restrained and lack the impact and
serious amounts of air, floor, and wall motion that home-theater subwoofers can generate.
Home theater subs appear simple on the surface, but they
are actually quite complex. It might seem obvious to make all the speakers
"small" and connect an LFE subwoofer and be sure the LFE channel is
"on;" however, "small" mode puts extra crossover circuitry in the
signal path to all the other speakers and those channels then sound noticeably worse.
However, if you leave any or all of the other speakers "large" and they
aren’t flat to at least 40Hz, you will miss some low bass. The best home-theater bass
setup possible is to have one subwoofer for each of the five or six speaker channels plus
one or two additional LFE subwoofers. If that arrangement is too expensive, two subs
running two channels each (main l&r and surround l&r) and a third running mono
(front center only) or stereo (front center and rear center) extends the bass response of
every channel. Of course, you still need the LFE subwoofer so that no bass is lost. Home
theater bass seems easy until you finally realize just what degree of investment in bass
is needed to max-out the bass performance of your home theater.
Connecting the LFE subwoofer
As far as I’m concerned, the only way to
connect your LFE subwoofer is to the LFE output of the receiver or processor. Using any of
the other available connections to your subwoofer will cause some bass loss. How much is
lost depends on how your receiver or processor manages bass, and there are differences
(which can be maddening to try to work around). You really need a DVD like Avia’s Guide
to Home Theater. This contains a "rolling" noise signal that goes from
channel to channel. Since the noise contains all audio frequencies, you listen for the
consistency of the bass portion of the sound as it moves from channel to channel. If the
bass disappears at any point in the circle around the room, you know you have a bass
management problem, or perhaps a problem that arises from not having enough hardware for
bass reproduction from all channels in your system.
Home-theater bass and crossovers you didn’t know
about
First of all, there is the LFE channel itself: turn it on
and most processors insert a crossover that rolls-off bass to all five (or six) of the
other speakers! That means you send the audio signal to all the other speakers through a
crossover and that degrades the sound just a bit. Unfortunately, with this type of
processing, there is no way to get the LFE channel without having this happen. We can only
hope that some day more A/V receiver and surround-processor manufacturers will realize
that rolling-off bass to the other channels when LFE is turned on is not the right
thing to do. If you doubt whether or not this is happening, you can measure it for
yourself using any test CD that has test tones from at least 20Hz to 200Hz. Play the CD
and record the SPL levels with LFE "off." Next, disconnect the LFE subwoofer,
but turn LFE "on" in the processor/receiver menu. Measure bass response again.
You will see that the speakers you just measured now roll-off on the bottom end. The only
way that can happen is from having a crossover inserted in their signal path. If your
processor does not do that, either you did something wrong or you are very lucky to
have latched onto such a processor.
Turn on LFE and make any speaker "small" and you
get two crossovers inserted in the signal path for any speaker that is set to
"small." That sounds worse yet. I strongly recommend never using the
"small" setting. It would only be okay to use "small" if your speakers
are so small that to leave them set to "large" would risk damaging them.
Some expensive surround-sound processors perform bass
management in the digital domain. These products are just beginning to appear and
the price point where this capability is available has dropped to the $3000 range as this
article is being written. Doing bass management in the
digital domain avoids the addition of sound degrading analog crossovers. Digital bass
management is definitely preferred if/when it is available and if/when it is done
correctly. Simply having digital bass management does not guarantee it will sound better
than "small" or "LFE on" settings of more typical surround processors,
but digital bass management definitely has the potential to sound better than conventional
bass management.
Don’t forget phase and/or polarity!
Initially, connecting an LFE subwoofer seems simple. Run an
interconnect from the LFE output of the processor or receiver to the LFE input of the
subwoofer and you’re ready to go. Not so fast! You probably also have a phase or
polarity switch to set. You get that correct by playing a test tone that is close to the
crossover frequency, then adjusting the phase and/or polarity to give you the loudest
reading on an SPL meter placed at the listening position. This logic should be pretty
obvious. If the phase or polarity is set incorrectly, there will be cancellation at the
listening position. Cancellation results in a lower SPL reading. When the phase or
polarity is set correctly, you’ll get full output at the listening position. This can
be somewhat tricky to measure and it often helps dramatically to have a helper, especially
if there is a phase knob that allows continuous adjustment. The helper should rotate the
phase knob a little bit, then wait for any change to be measured by the SPL meter. If
there is no change, rotate the knob a little more, etc. until you find the point where the
SPL is the highest. The difference in SPL might measure only a few dB or it could be a
very large difference.
More on the best possible home-theater bass
Okay, you won’t like this, but here we go…
Two LFE subs are better than one if you can swing the
expense. I would like to have one close to the listening position to remove room effects.
The other can be located almost anywhere convenient, perhaps in a room corner to maximize
room loading for even more bass power. However, this arrangement would require two
different delay settings because the two subwoofers would most likely be two different
distances from the listening position. Many processors or receivers will not support
separate delays for two LFE subwoofers. If you cannot independently select the delay for
two LFE subwoofers, experiment with a single delay setting. If you are lucky, there will
not be a big problem with cancellation effects. If there is a cancellation problem the
subwoofers will have to be placed at equal distances to the listening position. In that
case, having both subs close to the listening/viewing area may be slightly preferable to
more distant positioning if you can do that in your room. If not, don’t lose any
sleep over a more distant placement; it will still work pretty well.
Bass and the center channel -- say what?
How do you ever get a center-channel speaker that could be
considered to be "full range?" Well, an add-on subwoofer is probably the path of
least resistance for most home-theater enthusiasts. Many stores, and perhaps even
manufacturers, might laugh at the idea of a subwoofer for the center channel, but just
wait until you hear it! In the days when Dolby ProLogic was our best surround system, a
center-channel speaker with no bass below 100Hz was perfectly fine. Today with discrete
digital sound for the center channel, you can definitely do with a center-channel speaker
that has some decent bass extension. The same logic applies to the surround speakers or
center-rear channel: you can never have too much bass extension when you have discrete
surround channels that are not frequency limited.
Anything else I need to know?
Watch your levels! There is a distinct tendency to
over-drive the subwoofers in a home-theater setup. If you’ve got everything setup
properly, you shouldn’t have to run the subwoofer(s) extra loud. SPL meters often are
less accurate when setting subwoofers using bass signals only. In fact, some SPL meters,
including the Radio Shack meter, can lead you to unintentionally set the subwoofer(s) too
loud. If you sense that you have a bit too much bass even when you use an SPL meter to set
the level of the subwoofer(s), don’t hesitate to back off a bit. Your meter could
easily be guiding you to 5-6dB, too much bass anyway.
You can’t get any precision in balancing your surround
system using only your ears. You really need a sound pressure level meter. The
analog Radio Shack meter (approximately $35) is good enough for all but the most fanatical
enthusiasts. In fact, the more expensive digital Radio Shack meter is not as useful as the
less expensive analog meter. Hold the sound pressure level meter at a 45-degree angle
towards the ceiling and aim it in the general direction of the speaker/channel you are
measuring. Keep the height of microphone close to the middle of your head as you measure
each speaker position. Measure the sound at 70-75dB, no higher and no lower. This will
save your hearing and prevent the noise floor in the room from interfering too much with
the measurements.
Now go have fun with the bass in your home theater!
(Note: This article goes hand in hand with the January
2001 Max dB column
appearing in SoundStage! titled "Subwoofers for Music.")
...Doug Blackburn
db@hometheatersound.com |
