|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 listening
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 the past.
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
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
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 with noise.
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.