| Video Noise March 2004
Assessing the Bass Performance of Surround-Sound
Processors and Receivers
As far as home-theater gear has come in recent years, there
are still way too many surround processors and receivers that are flawed in how they
manage bass. The most common such error I’ve seen is when the processor removes bass
from speakers you’ve programmed as Large when the subwoofer is On. This problem can
insert a crossover in the Large channels, which begins rolling off the bass at 40Hz or so
in each Large channel and redirecting it to the subwoofer. This wouldn’t be so bad if
you had the option of routing some bass into the low-frequency effects (LFE), or
subwoofer, channel, or leaving all the bass in the Large channels -- but typically,
you’re not given that choice. If you have full-range main speakers (20Hz-20kHz), you
won’t be making full use of their bass capabilities.
In addition, inserting the necessary crossover in the
signal path for the Large speakers degrades sound quality. Although the crossover is
typically in the digital domain, in my experience it audibly degrades the sound from the
Large channels compared to the sound you get with the LFE channel off. Typically, you get
less resolution of space, echo, and fine detail. The resulting sound is duller, flatter,
and less interesting.
In an ideal home theater, the best possible setup is
to have all your speakers "full-range" -- in other words, every speaker
in the system would be able to produce sound from the deepest, 20Hz bass to the highest,
20kHz treble. With an ideal surround processor or receiver, you’d set every channel
to Large, and all the bass going to any channel would stay in that channel no matter what
other settings you might make. You’d also want an LFE subwoofer, because that’s
a dedicated channel unto itself. If there were no sub, redistributing the LFE bass to the
other Large channels would dilute the listening experience. There will always be
degradation of the sound when sound is redistributed to other channels, even if this is
done in the digital domain. Obviously, you wouldn’t want your ideal system
compromised by inappropriate bass management.
You have little or no ability to know in advance if any
pre-pro or receiver you may purchase has this anti-feature or not. Reviewers overlook it
consistently, failing to report whether or not such a product creates bass-management
problems. But with a simple tool and a little time, you can find out yourself whether or
not the next surround processor or receiver you buy has this problem.
How a calibration DVD can help
The tool you need is a test DVD that has at least
five-channel and LFE bass test tones. The tones should cover at least a range of 20-80Hz.
Ovation’s Avia: Guide to Home Theater contains the needed bass test tones;
Ovation’s newer and less expensive disc, Sound & Vision Home Theater Tune-Up,
does not.
It doesn’t matter how the receiver or surround
processor is set up at the dealer’s, as long as you can get close enough to the
subwoofer to hear if it’s doing anything. You’ll need to access the surround
processor’s or receiver’s setup menu. Set one of the channels to Large -- the
front left or right speaker is probably the best choice for this test. Whichever speaker
you set to Large, choose the same channel in the Avia: Guide to Home Theater
disc’s bass test-tone menu (with the other channels’ test tones off).
You’ll want to send a signal of close to 30Hz to the channel you’ve set as
Large.
With the subwoofer On and the test tone playing in the DVD
player, move your ear near the subwoofer to hear if it’s emitting any bass. If you
hear bass without your ear close to the sub, don’t assume it’s from the
sub -- low bass response can fool you. Put your ear very close to the subwoofer so
that you can be certain that it’s not playing back the 30Hz tone. If the surround
processor or receiver is doing the right thing, the sub should be making no sound at this
point.
However, if you do hear bass in the LFE channel when a
single Large front channel is receiving a 30Hz test tone, the surround processor or
receiver is rerouting some bass from that channel to the LFE channel.
Surround processors and receivers make this mistake because
of well-meaning attempts by manufacturers to second-guess your system and setup. They
assume your Large speakers aren’t that large, and that they’ll benefit
from the deepest bass being moved to the LFE sub. That might work well if your main
speakers are floorstanders but not truly full-range. But if your main speakers have
built-in subwoofers, or if you’ve augmented them with subwoofers, those channels are
truly full-range. They don’t need to be second-guessed by the surround processor or
receiver.
Good bass management is hard to find. Good luck in your
search!
...Doug Blackburn
db@hometheatersound.com |
