| Cinema Cynergy November 2002
Budget Subwoofers
In this month’s Cinema Cynergy
we will consider what to look for in an inexpensive subwoofer and discuss a few issues
that are relevant to using them in a budget home-theater system. Some of the ideas and
tips discussed in this month’s column will require some technical knowledge or at the
least the willingness to learn and experiment. So, continue on if you are prepared to
tinker with your system to make it sound better.
What to expect from a good budget subwoofer
I recently had the pleasure of reviewing the Athena
Technologies AS-P400 powered subwoofer, which provided a tremendous amount of performance
in a relatively compact enclosure and at the remarkably low price of only $400 USD.
However, Athena is not alone in producing a good subwoofer at or near the $500 price
point. These days there are a lot of budget subwoofers out there that have bass response
down to about 30Hz with relatively low distortion and have enough power output to fill a
decent-sized room. It wasn’t that long ago that a subwoofer in this price range would
be fortunate to be able to reach down to 40Hz and usually had less output and more
distortion, which resulted in a boomy sound when pushed to its limits.
When choosing a subwoofer, listen for its ability to
energize a room with sustained bass during rumbling sound effects without overloading and
becoming distorted. Also try listening to some music with complex bass to see if it can
recover quickly and differentiate between different bass frequencies clearly (and not just
emphasize certain frequencies for "one note" bass).
Although budget subs have come a long way, you still cannot
expect them to reach down into the subterranean frequencies of 20Hz and below, or
pressurize a large room as only a very large and expensive subwoofer can. You simply can't
change the laws of physics, so don't expect an inexpensive subwoofer with a relatively
small enclosure, small driver, and low-powered amplifier to attain the same performance.
A few examples
A few overachieving budget subs that I have had firsthand
experience with and highly recommend are the previously mentioned Athena AS-P400, which is
surprisingly well-built and taut sounding, as well as the Paradigm PDR-12. The Paradigm
might not be quite as tight and responsive as the Athena, but it does seem to have a bit
more power to fill a room with sustained rumbling effects.
Some of our Home Theater & Sound reviewers have
recommended the Hsu VTF-2 ($499), and I also recently spent some time with the
excellent-sounding and drop-dead-gorgeous Aperion Intimus S-10 ($499) with its piano-black
gloss finish. But don’t just take my word for it. Go out and listen to some of these
subs, and others, to experience for yourself the type of performance that you can obtain
for a few hundred dollars.
Just hook it up to the LFE output, right?
I’ve previously stated that the best way to connect a
subwoofer to a 5.1-channel speaker system was via the LFE (subwoofer) output of a receiver
or processor with a coaxial (RCA type) cable. In this method of connection, the crossover
control of the subwoofer should be set to its highest position to minimize its effect on
the signal. The receiver should perform any bass management. This is still generally the
best way to connect a subwoofer to a home-theater system, but there are a few things to
consider before settling on this method.
When connecting a subwoofer to the LFE output may not
work
A potential problem with this type of configuration could
arise with budget speaker systems, which often consist of small satellite speakers that
have limited bass response. With this type of system, the receiver is usually called upon
to perform bass management by directing some of the lower frequencies away from the
satellite speakers and re-routing them to the subwoofer. This is normally a good thing,
but it is possible that the cutoff frequency at which the bass is redirected to the sub
could be too low for the speakers being used.
This would occur if your receiver is routing bass
frequencies below 80Hz to the subwoofer, but you are using very small satellite speakers
with bass response down to only 120Hz. This means that your receiver will be sending the
frequencies between 80Hz and 120Hz to the satellite speakers, but those frequencies will
be missing because the speakers are not capable of reproducing them. This will result in a
"hole" in the bass and the loss of those frequencies. While some receivers allow
you to select a higher crossover frequency to avoid this type of problem, most have a
fixed crossover point that is not specified, so you may not even be aware that this is
happening.
Take the high road instead . . .
If you feel that this is happening with your system, you
may be better off setting the main speakers to "large" within the
receiver’s bass management and connecting the subwoofer via the high-level (speaker)
inputs. This will eliminate the loss of bass by sending a full-range signal to the main
speakers as well as to the subwoofer. The subwoofer’s crossover can then be used to
control the bass frequencies being reproduced and the blending with the satellites. Note
that this will only solve the problem of the bass that is missing from the main speakers,
and will do nothing for bass that is missing from the center and surround channels.
However, restoring bass to the main channels is more critical as there is usually more
bass contained in them than in the other channels in most surround mixes.
One problem with high-level inputs is that they sometimes
tend to degrade sound quality when compared to line-level inputs. I have found the
difference in sound quality between the corresponding high-level and low-level inputs of
subs to vary among different models with some, such as the Athena AS-P400 that I keep
referring to, sounding nearly indistinguishable between its two types of inputs. In fact,
I am currently using the AS-P400 connected via the high-level inputs along with five Axiom
M3Ti SE bookshelf speakers for excellent home-theater sound.
. . . Or take the low road
If it bothers you to use the high-level inputs, or the
sound of your system does not sound acceptable to you, you could connect your subwoofer
via the line-level outputs for the right and left mains. This could work, providing your
subwoofer has stereo line-level inputs; many do not, and instead have only a single input
for connection to the LFE output. Your receiver also has to have suitable outputs, which
are found as part of the six-channel "preamp outputs" that are normally used for
connection to an external amplifier.
Don’t fix it if it ain’t broke
Just because you have a hole in your bass response does not
necessarily mean that the crossover point in your receiver is set too high for your
satellite speakers. It is also possible that your speaker system could be experiencing
phase interaction between the satellites and the subwoofer. This could substantially
reduce the amount of bass in the crossover region. Speakers and subwoofers do not simply
cut off the frequencies that they reproduce exactly at the crossover point, but rather
continue to produce some frequencies into each other’s frequency ranges so that there
is an overlap in the sound.
The problem occurs when the same frequencies from the
speaker and subwoofer meet at a distance that is half the wavelength of the sound. When
this happens, the sound waves from the two sources are said to be "out of phase"
and will "cancel" each other out. For frequencies in a typical crossover range
of 50Hz to 100Hz, the wavelength of the sound is approximately 10 to 20 feet, so half of
that distance (5 to 10 feet) is often the distance that separates subwoofers and satellite
speakers in home-theater systems. Simply moving the subwoofer closer to, or further away
from, the satellite speakers will help to alleviate this problem.
Some subwoofers also have a switch that will invert the
phase by 180 degrees, and some even have a continuously variable phase control. Many
budget subwoofers do not have a phase control, so experimenting with the placement may be
the only way to reduce phase anomalies. However, if you are using the high-level inputs on
a subwoofer, you can invert the phase by simply reversing the polarity of the connections
(connecting the red speaker wire to the black input and the black speaker wire to the red
input for both channels).
Parting thoughts
Yes, the best method of connecting a subwoofer to a
receiver or processor is still via the line-level LFE output. That's if all of the
speakers are truly full range and the subwoofer is only reproducing the signal from the
LFE channel of the soundtrack, or if the bass-management settings are fully
compatible with the speaker system you are using. Unfortunately, almost no one has a
system where all of the speakers are truly full range. Thus, there are times when
bass-management settings will be less than optimal for a speaker system -- such as with a
very small satellite speaker system. While there are many extremely capable budget
subwoofers currently available that offer remarkable performance for their price, you
still must take some care in deciding on the method of connection to achieve the best
possible sound from your system.
...Roger Kanno
roger@hometheatersound.com |
