I Am So the Boss of You --
Bass Management Made Easy
I probably get more questions about integrating subwoofers
into a home-theater system than about any other essential of setup. It can be
confusing. It doesn't have to be.
One of the biggest causes of this confusion was intended to
be its solution -- that's the "subwoofer output" of the typical A/V receiver. It
makes sense: You see an output labeled Subwoofer or LFE (low-frequency effects), you'd
figure you should connect a subwoofer to it. Well, not necessarily.
The problem is bass management -- or, more properly,
the lack of a consistent standard for it. Almost all powered subwoofers have their own
adjustable low-pass filters, which means that you can set a frequency below which
they will function. Most subwoofers allow you to choose a point between 60Hz and 150Hz --
the subs will ignore higher frequencies and just handle the ones below the one you’ve
set.
One problem is that many A/V receivers also have low-pass
filters, and sometimes those filters are set higher than the ones in the subwoofers
themselves. Some A/V receivers don't allow you to set an LFE filter below 100Hz.
Conceivably, you can be outputting a filtered signal into another filter -- a better, more
adjustable one.
Higher-end A/V receivers and processors permit you to
disable the built-in filter -- or, at the very least, select its frequency yourself. Some
manufacturers even provide unfiltered subwoofer outputs that bypass the internal LFE.
There's another problem with some A/V receivers and
processors: Some models don't output any bass signal in two-channel mode. Then your
subwoofer won't work, for the simple reason that it isn't getting any signal.
That's okay if you don't want to use your subwoofer when you're listening to music, but
chances are you won't find that too pleasing a prospect.
What to do?
First, check the rear panel of your A/V receiver or
processor. If it has an unfiltered output, use it. Even if it does, you'll need to check
the manual to confirm how it handles stereo bass. If you're dealing with a higher-end
product, such as a separate preamp-processor, the chances are higher that it will have
flexible bass-management options.
If you have a processor that directs low bass to the left
and right speakers in two-channel mode, or if you have an unfiltered output, speaker setup
is relatively logical: just connect the output to the sub. But if your processor isn't
so cooperative, don't despair -- there are workarounds.
Almost all subwoofers have two sets of inputs -- a Low
Output (line-level) set and a High Output set. If your receiver doesn't give you the
bass-control options you need, you'll have to trick it -- and the easiest way to do that
is to use the speaker-output option. That means you connect the subwoofer to the FL and FR
speaker outputs of your amplifier, then connect the speakers to your sub's high-level
(speaker) outputs. Then you trick your processor by misinforming it when you run its
speaker setup function.
Most processors have a setup program called Speaker Setup
or Speaker Selection, usually accessed via the remote control. You then follow a
step-by-step procedure on your video display. You’re generally asked if your speakers
are Large or Small. Which you choose tells the processor how to portion out the bass
signals. If you select Large, low frequencies are sent to the speakers. If you select
Small, the processor generally sends the speakers only the sound above a specific
frequency (this can be arbitrarily fairly high). Most processors also ask if you have a
subwoofer -- once again, sometimes it’s best to fib a bit.
If you've connected your subwoofer to the FL and FR speaker
outputs, you should tell your processor that the FL and FR speakers are Large, and that
you do not have a subwoofer. The processor will then direct all bass frequencies to
those speakers. This will give you the maximum grunt, and your subwoofer will be doing its
job.
If your receiver has only a filtered line-level output,
however, things won’t always be so clear-cut.
Here's the problem: If you tell your processor that your FL
and FR speakers are Large, it will send all the bass in those channels to those speakers
and none to the subwoofer. You don't want this -- even large speakers with built-in
subwoofers benefit from the help of an external sub. First, you need to determine if your
sub has an unfiltered input (a few do). If it does, use it.
What you'll probably have to do, however, is use your
receiver's filtered output and your subwoofer's filtered input. First, turn the sub's
variable low-pass filter as high as it will go -- you want to make it as transparent as
possible. Next, tell your processor that your FL and FR speakers are Small. Your processor
will now send all of the bass signals for those speakers to your sub. You should probably
also tell your processor that your center-channel speaker is Small, too -- the pre-pro
will then send all of the center’s bass to the sub as well.
Surround effects speakers are not typically intended to
reproduce deep bass, so they tend to be small -- and should be designated as Small when
you set up your processor.
You're probably feeling pretty comfortable by now, so
naturally, there's a complication: THX.
If your receiver or processor is THX Certified, it
is required (by the folks at THX) to use a very tightly specified type of bass management.
In THX mode, your receiver or processor will employ active low-pass filtration on all
of your speakers, shunting all the sound above 80Hz to the speakers and everything below
80Hz to your sub.
If your entire system is THX Certified, this can work
extremely well. By the same token, THX is great if you play only movies on your system.
However, if you have full-range loudspeakers up front (FL and FR or all three front
channels) -- or if you listen to a lot of music with your multichannel system -- you may
not wish to use THX's filtration system.
The answer's simple: You don't have to use THX processing.
Simply choose your Dolby or DTS option. (THX is an extension of Dolby Digital processing
that works within DD's specs; it’s not a separate decoding scheme.)
Okay, you've got your processor properly set up and your
subwoofer connected to it. Now where the heck are you going to put that sub?
We'll get to that next time.
...Wes Phillips
wes@onhometheater.com
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