HT Volume 2.indd - Acoustic Sciences Corporation

HT Volume 2.indd - Acoustic Sciences Corporation
-a five-part article in Home Theater magazine, October 1993 - February 1994
Home Theater Acoustics
Volume Two
Sophisticated 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
T
here 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 room.
OUR HERITAGE
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.
High end audio strives to achieve a wide “sweet spot” so that the image floats
between the speakers, even if the listener moves off center.
And 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.
SUBWOOFERS
It’s 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.
The subwoofers are an important element in any home theater system and their
placement is critical for a good setup.
The 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.
ROOM MODES
The 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 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.
STANDING WAVES
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.”
One 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 “Cweighted” 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 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. 
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