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 change.
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 break.
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
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
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 the
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
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 purpose.
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.