audio tracks may be changing how we think of television
audio, thanks to home theater technology. Television
sound has traditionally been viewed as second best to
the video component - but this way of thinking may soon
BY ARTHUR NOXON
There are two
channels of communication involved in television presentations,
the visual channel and the audio channel. Traditionally, the
video component has been technically far superior to the sound
system of the television. A four-inch diameter speaker has,
for many years, been the standard audio component for television.
That should be changing now with the advent of home theater
and sophisticated audio tracks. But old habits are hard to
This work begins with a review of the present
relationship between audio and television. Then we jump towards
the future where high end audio mixes with television in the
home theater. To that end, we study subwoofers, their behavior
and relationships to the listening room.
One would think that the advent of video tape
players would have automatically moved things forward for
television audio. Not so, as evidenced by the abundance of
players with mono audio outputs and the sparsity of hi-fi
grade audio bandwidths, not to mention real stereo/hifi outputs.
Sophistication in video signal processing for tape players
far exceeded any investment into the audio channel.
Manufacturers, salespeople, and consumers
of television systems have developed a long-standing tradition
that sound is a minor, relatively insignificant component
of the television experience. One might compare the standard
television sound system of today to the portable record player
of the '50s. They both have small speakers positioned in empty,
openbacked cabinets and are driven by noisy signal detectors
through very limited bandwidth amplifiers. It's as if early
television adopted for its voice the common audio of its day.
And then, as time went by, television resisted any change
in its audio, as if by claiming that if it was good enough
then, it's good enough now.
Possibly the hottest improvement in television
audio is stereo TV. Now we get to have two four-inch speakers
in an open-back plastic cabinet, one speaker on each side
of the screen. Beyond this, there is a new problem. The stereo
created image overlays on the TV screen only for the person
sitting directly on the centerline of the set. For the rest
of us who sit slightly to the side, the sound falls off the
screen, to the nearest speaker. In the world of high end audio,
there has been a lot of very hard work to achieve a wide "sweet
spot" so that the image continues to float between the
speakers, even though the listener moves off dead center.
They don't use two four-inch speakers, three feet apart and
separated by a glass plate.
now we have home theater, the hottest, most rapidly expanding
market outside of home computers. We have TV stores opening
up home theater rooms in order to stay in business. We have
hi-fi stores opening up home theater rooms to stay in business.
And we have a whole new kind of store opening up, which exclusively
service the home theater business. High definition television
is being enjoined with high fidelity audio in a new kind of
system called home theater.
Unfortunately, this exciting technical evolution
takes place within the context of traditional television performance
values. This tags audio as having no more than a bit part
in the show, when, in fact, it is the audio track that plays
the lead part in home theater. And so, those of us who work
with and enjoy home theater are faced with a major problem
-- reorientation. We need to expect more from home theater
than we are used to getting from present-day TV. Home theater
offers the combined effects of high end audio systems, high
fidelity surround sound tape players, and high definition
TV. We need to learn to expect more from the performance in
home theater than we have ever gotten from our TV sets of
well-known in high end audio that to achieve full potential,
the last link in the audio chain has to be properly set up.
Audio, like any other chain, cannot be stronger than its weakest
link. For modern audio systems, the weakest link is also the
last link, the listening room. That's why the opportunities
for setting up the home theater room mean more than just the
equipment. It means the room acoustics as well. Setting up
a room includes the positioning of the loudspeakers, and no
speaker is more sensitive to room acoustics than the subwoofer.
And so, in the upcoming section, we will begin to study the
relationships that exist between the position of the subwoofer
and the room.
home theater audio system includes subwoofers, main dialogue
speakers, and surround speakers. The very least playback system
should be sort of like a five-channel sat/sub system - one
sub, three dialogue, and two ambience speakers. The higher
end home theater systems sport two or three subs, full-range
speakers for dialogue, and special bipole speakers for ambience.
No matter what the system, subwoofers are an important element
in the complement of speakers needed to present home theater
audio. Placement of subs within the room is critical in the
setup of a good room.
most common enemy of subwoofers is room modes. A room mode
is the organized way that sound is stored in a room. An organ
pipe becomes stimulated into resonance when a thin sheet of
air is blown across a hole at the bottom of the pipe. That
resonance, called a pipe mode, sounds great. A listening room,
like the organ pipe, is an acoustic chamber that can be stimulated
into resonance -- but this time it's done by the air pumping
action of the subwoofer. Room modes cause the subwoofer to
sound very loud for one note, and fairly quiet for another.
In the scientific study of room modes, there
has developed a very unique type of room, a reverberation
chamber. This room is designed for the testing of sound absorbing
materials. A good reverb chamber has very thick, slick, and
heavy walls. Sound is stored for a long time in such chambers.
If you shouted in one of these rooms, you would hear your
voice echoing around for 15 to 20 seconds.
There is a measure for how well sound is stored
in rooms. It is called the RT-60 and seconds are the units
of measurement. RT means reverb time and the 60 stands for
60 dB. RT-60 means the time it takes for the sound to die
away over a range of 60 dB. The dB, of course, is decibel,
the unit of sound loudness. It is no coincidence that the
range of 60 dB corresponds to the range of loudness between
a shout and a barely audible whisper.
The loudspeaker that drives the reverb chamber
is traditionally located tight into a corner of the room and
for good reason. The corner of a room is the single most efficient
place to locate a low frequency driver for the development
of room modes. The speaker can stimulate more resonances from
the corner of the room than it can if located in any other
part of the room. This has to do with the efficiency aspect
of how speakers couple to room modes.
There is another, somewhat significant, reason
that the speaker is located in the tricorner of the reverb
chamber; it is the "horn loading" effect of the
tricorner walls. There is no news in this concept as nearly
all musical instruments have a similar but higher efficiency
exponential type horn which couples their sound generating
system to the air of the room into which they play. Can you
recall listening to someone playing nothing more than a tuba
mouthpiece? It isn't very interesting at all. But plug that
same mouthpiece into a spiral wrapped, exponential horn and
that noise is turned into beautiful sounds.
For the purposes of testing sound absorbing
materials, the acoustical engineers want to stimulate as many
room modes as possible. They also want the mode tones to be
as evenly spaced along the frequency scale.
This is not too strange. For example, the
notes of the musical scale are very evenly spaced. There happens
to be particular ratios of room dimensions that promote evenly
spaced modes. This only holds true if the speaker remains
located in the tricorner of the room.
If the speaker is moved away from the corner,
only some of the modes are able to be coupled to the speaker
and their spacing becomes anything but uniform. The "golden
ratios" for room dimensions are only good if the speaker
is located in the tricorner of the room. Almost no one listens
to a good stereo with speakers located in the corners of the
room. So, on a practical basis and especially for high end
audio, where speakers are carefully positioned away from the
corners, these golden room ratios serve little or no functional
One of the more popular tales in the folklore
of high end audio stems from a basic misunderstanding of the
purpose and limitations of reverb chamber design. Reverb chamber
ratios are all too often quoted as being "ideal"
room dimension ratios because they will "smooth out the
bass." Home theater does have some roots in high end
audio and this tale will eventually begin to circulate in
the world of home theater. It is important for those of us
who work at and enjoy quality audio to avoid being charmed
by magic numbers, unless, of course, they work.
Bass traps (low frequency sound absorbers),
room modes, room dimensions, subwoofers, and their placement
are all intertwined into one composite instrument that generates
sound and delivers it to the room where it is heard by the
listener. Unlike the tuba, where the instrument is in one
place and the listener is in another, the subwoofer/ room
system is so large that it literally engulfs the listener.
This explains an old saying in audio, "For bass, the
listener is not really listening to the speaker, but rather,
listening to the room as it is being played by the speaker."
of the most enlightening experiences found in exploring the
behavior of subwoofers in rooms is witnessing the effects
of standing waves, otherwise known as a room resonance or
mode. We have been studying about modes from the outside.
Next, we dive into the interior of the standing wave. The
first problem we will have is setting up a method to generate
standing waves. Probably it is easiest to simply leave a security
deposit with your favorite high end shop and borrow the signal
generator from their repair department. Be sure to have someone
show you how to feed the signal into your preamp.
Use one channel and one speaker for these
tests because there is nothing but confusion to be gained
by using two speakers. Move a subwoofer into one comer of
your room and put a Radio Shack sound meter in any other corner,
on the floor. String out the signal generator so you can operate
it and still watch the dB needle of the sound level meter.
The meter should be set at 80 dB and on "fast" and
"C-weighted" for best results. Start with the lowest
frequency at about 20 Hz. Slowly raise the tone of the generator
and watch the meter. You may have to change meter scale settings
or adjust the speaker volume to get the needle to stay somewhat
on scale. As you raise the frequency, you are performing a
frequency sweep. You can sweep up or down the frequency scale.
Often, the first sweep range of the signal
generator will be 20 to 200 Hz. This contains for all practical
purposes the entire bass range. And it's the only range you'll
need for subwoofers as they are usually rolled off at about
85 Hz. For more fun, you can raise the roll off point of the
sub to its highest value, about 150 Hz. Then use the frequency
sweep controls of the signal generator and watch the needle
of the sound level meter rise and fall as you change frequencies.
Remember as you do this that the speaker volume control is
not being touched, only the tone is changing. Despite the
constant power to the speaker, you will see the sound levels
in the corner of the room rise and fall as much as 15 dB between
adjacent peaks and valleys.
Adjust the signal generator so as to choose
a peak, in the 50 to 60 Hz region. Then get up and slowly
walk around the room, noticing the peaks and valleys of sound
that have filled the room. Next, pick up and carry the sound
meter to observe the strength of the peaks compared to the
strength of the valleys as you again move around the room.
Move to your listening chair and sit. Locate the sound level
near your ear and compare the reading there with those elsewhere
in the room. Hold the sound meter at arm's length and slowly
wave it around while keeping an eye on the meter. The indicator
needle of the meter will rise and fall with position. However,
if you move too quickly, the meter will begin to pick up the
sound of the air rustle and your readings will become polluted
By now, you are beginning to really understand
that some bass. tones can be much louder than others. And
further, that any particular tone can sound louder or quieter,
depending on where you might be sitting. Although we didn't
explore one other variation, it doesn't take much imagination
to expect that if we sat still and moved the speaker, again
we would hear peaks and valleys as it is dragged about the
room. You could put the subwoofer on a furniture dolly and
have someone pull it out of the corner and across the room
while you sit and listen to the build up and fall of sound
as the speaker moves in and out of efficient coupling zones.
Our hearing of bass is so sensitive that you
can actually hear someone else walking across the room. Dial
in a resonance at about 100 to 115 Hz. Find a spot in the
room where there is almost no sound. It's called a "suck
out" for some unknown reason. Find it and stay there.
Then have someone slowly walk around the room. You will hear
the strength of the sound field come and go as the person
moves. Use the meter to measure this sound level shifting.
It's as if the room was nearly filled with large balloons;
and, as the person walked around, these balloons were shoved
aside, circulating out of the way. You could detect the movement
as it shifts the positions of a few nearby balloons. You can
even hear if someone opens or closes a door, as if the balloons
fell out of the room.
All of this discussion about room modes is
applicable, due to the fact that the typical listening room
is fairly small. If you move the speaker and listening test
onto the sand dunes, there will be no reflections and no modes.
Put the speaker in a deep forest, and there will be many reflections
off the big trees, but still no modes. Put the speaker in
a huge hall or even a normal-sized movie theater and still,
no modes to speak of.
Only small rooms have room modes and only
small rooms sound like they have room modes. This remains
one of the biggest problems in high end audio and home theater
systems. Wonderful audio tracks played over great electronics
and speakers are reduced to overwhelming boom and mud, due
to the coupling of speakers to the various small room modes.
So, what do we do about this inevitable mess? Well, we certainly
don't want to place the speaker in some position that stimulates
all of the room modes, whether they are evenly spaced or not.
Our needs are not that of the acoustical testing engineer.
For high end and home theater audio playback, we want the
subwoofer to be located preferably so that it couples not
to some, but actually none of the room modes.
On one hand, we have the science of acoustics
which gives us a set of magic numbers that enable a cornerloaded
speaker to efficiently couple to all possible modes in a room
and even more, have the modes spaced as evenly as possible.
It should be no surprise that this same science can give another
set of magic numbers, one that prevents the speaker from efficiently
coupling to create the room modes. It is only those magic
numbers which can create the anti-mode type, high end audio,
and home theater room setups that we are really interested
in knowing about and working with. Next time, we will study
the anti-mode method of speaker placement.