| Cinema Cynergy July 2001
Bass Management: Subwoofing . . . One More Time
Last month’s column was my nostalgic look back at the
laserdisc format, but it is now time to get back on track with a discussion on some
butt-kickin’ home theater. What I’m talkin’ ‘bout is the kind of
outrageous, room-shaking systems that drive neighbors crazy and make snobby audiophiles
with their turntables and puny tube amps shake their heads in disapproval. Okay, I am
being facetious, but what I am referring to is home theater’s need for bass…
lots of bass. While it is possible to assemble an enjoyable and musical two-channel system
with bookshelf speakers that might extend down to only 50 or 60Hz, truly reference-quality
home-theater sound necessitates solid bass response down to at least 30 or 40Hz, and
preferably down to 20Hz in all channels.
Do I really need a subwoofer?
Discrete digital surround formats such as Dolby Digital and
DTS require that an ideal home-theater system would have full-range (20 to 20,000Hz) or
near full-range speakers for all channels and a subwoofer capable of frequency response
down to 20Hz for the low frequency effects (LFE) channel. Even if a system has large
full-range speakers for the mains and surrounds, the placement requirements for a center
channel (usually on top of a television) make it very difficult for this speaker to also
possess full-range capability. In fact, for most typical home-theater installations, there
are also space and size considerations for the surround speakers.
Thus subwoofers make a lot of sense not only for the LFE
channel as intended, but also for extending the bass response of speakers in any of the
other channels.
How do I set-up my subwoofer and the rest of my system
to get the most and the best sounding bass?
Because of the complexity of home-theater systems,
confusion often arises concerning the setup of the multitude of speakers that are
required. Considering a typical system which consists of a Dolby Digital/DTS receiver with
five speakers and a subwoofer, the speakers would be connected using the high-level
(speaker) connections and the subwoofer utilizing the line-level (RCA) connection, which
is usually labeled "Subwoofer." This ensures that the LFE channel of digital
soundtracks is properly routed to the subwoofer, as are any other low frequencies that the
bass management directs to it.
The most common and generally best way to connect the
subwoofer to a home-theater system is to connect it to the output labeled
"Subwoofer" on the back of the receiver (or processor).
Most subwoofers can also be connected via the high-level or
speaker inputs. This is accomplished by connecting the subwoofer’s high-level inputs
in parallel to those of any speaker, or any of the receiver’s high-level outputs with
speaker wire just as you would connect any other speaker. This method of connection is
inferior to the use of the line-level connection and will degrade the sound quality of the
subwoofer and will also tend to affect the sound of the speakers that it is connected to.
It also negates bass management, which generally requires that the sub be connected to the
line-level subwoofer output of the receiver.
If your center and surround speakers are capable of bass
response down to at least 70 or 80Hz, set them to "large."
While not an ideal situation, if your center and surrounds
have a frequency response roughly equivalent to that of a decent bookshelf speaker, set
the bass management for those speakers to "large." The fewer channels whose bass
is directed to the subwoofer, the less localizable the bass will be and the less likely
that the subwoofer will become overloaded from having to reproduce too much bass. The
actual bass response of your speakers will depend on how they interact with your room so
you will have to experiment a bit if you are unsure which setting will yield the best
results.
The left- and right-front (mains) speakers require even
better bass response (approximately 30 to 40Hz) to be set to "large."
The bass requirements for the left- and right-front
speakers are greater due to the increased demands placed on them by movie soundtracks,
which routinely contain a lot of bass energy in these two channels. These speakers should
be at least "nearly full range" and have solid response down to 30 or 40Hz for
them to be set to "large." If not, you may be losing considerable bass energy by
directing low frequencies to these speakers. This is just a general guideline and, again,
experimentation is necessary to determine the correct bass-management settings for the
main speakers in your system.
Doug Blackburn, SoundStage!’s Technical Editor,
contends that setting any of the speakers to "Small" is like inserting an
additional crossover in the reproduction chain, which will degrade the sound quality of
those channels. I would agree with this, but mostly I like to have my bass distributed
throughout the room and emanating from the speakers that it was intended for, even if they
cannot quite reproduce the lowest frequencies.
Make sure you have selected the appropriate
bass-management settings on your receiver for your speaker system.
Selecting the correct bass-management settings is critical
to getting the best bass performance from your system. For instance, if you accidentally
select the "No Subwoofer" setting, then a sub connected to the subwoofer output
will not receive a signal and will not reproduce any sound! There have been times when I
have inadvertently changed the bass-management settings of my system and wondered why it
did not sound quite right. It may seem obvious, but checking the bass-management settings
to make sure that they are correct may save you some embarrassment and make your system
sound much better.
Consider adding more than one subwoofer to your system
to get ultimate bass performance.
Because Dolby Digital and DTS soundtracks can have
full-range signals in all of the five main channels, adding a subwoofer to any of those
channels whose speakers do not have adequate low-frequency response will be beneficial.
There are a number of ways in which this can be
accomplished. If you are using a receiver with pre-outputs for each of the channels, you
can simply add a subwoofer by utilizing these line-level connections, or you could use the
speaker-level connections (but only if absolutely necessary). Because adding a subwoofer
to each channel is an expensive proposition, a more cost-effective method would be to add
a single subwoofer to the mains, another for the surrounds, and perhaps a third for the
center channel. These additional subs are meant to complement the LFE subwoofer, which
should not be eliminated even if all the other speakers have full low-frequency response.
Experiment with the placement of subwoofers to get the
smoothest and most accurate bass response.
Subwoofers will interact with the room and some
experimentation is required to yield the best results. Generally, a subwoofer (or any
loudspeaker) will sound louder when placed near a corner, but it will likely also be
"boomier" and less accurate sounding. The further a subwoofer is placed from
corners and other room boundaries, the tighter and more precise it will sound, but its
output may seem reduced or it may even sound lean and thin.
If a subwoofer is used to augment the bass of another
speaker or speakers in a system, it will also interact with those speakers depending on
their distance and orientation to one another. This complicates matters as the subwoofer
is now not only interacting with the room, but also with other speakers in the room,
making placement even more difficult. In this case, subwoofer placement often becomes a
compromise between attempting to minimize the negative effects of these two types of
interactions.
Turning up the house lights
Reference-quality home theater sound (i.e., bass
that can collapse a lung and high volume levels like those you get when you go to the
movie theater) requires powerful, full-range speakers and good subwoofer(s) in addition to
high-quality amplification and processing. You cannot expect a tiny micro speaker system
to be able to recreate the wide dynamic range and room-shaking bass of a professional
theater system. This is not to say that a small speaker system cannot make for a
satisfying home-theater experience, just do not expect it to be able to fill a large room
with extremely high sound-pressure levels and have smooth and extended low-frequency
response.
My own personal recommendation for a real-world speaker
system that can handle the demands of home theater is to use full-range or near full-range
mains, use center and surrounds with reasonable bass response to at least 60Hz or so, and
set all speakers to "Large." A subwoofer that can reproduce low frequencies
solidly down to 20Hz is also required for the LFE channel. This type of system will not
reproduce extreme low frequencies in the center and surround channels, but most movie
soundtracks concentrate the majority of the bass energy in the left and right front and
LFE channels anyway. By compromising a little on the requirements of the center and
surround-channel speakers, there are substantial savings in both cost and space, while
maintaining most of the performance.
...Roger Kanno
roger@hometheatersound.com
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