of the first things the novice acoustician does upon entering
a room is to deliver a sharp clap of the hands. This is followed
by a grave shake of the head and comments about how bad the
room sounds. Next comes a proposition to fix the room and
the fee. The unsuspecting client then administers a sharp
hand clap, nods the head in agreement, and gives the guru
a retainer. The only problem here is that these people are
busy buying and selling modifications to the sound of their
own hand clap. We don't listen to a speaker while holding
it in our hands, yet we can be tempted to consider acoustics
based on the sound of our own hand clap.
Home theater audio systems have an ambience
channel. It usually delivers a bandwidth-limited (no bass),
mono signal to a pair of speakers that have been mounted high
on the wall and to the side of the listener. If you stand
on a chair and clap your hands in the location of the ambience
speaker, you will hear a very funny and undesirable sound
effect. Is this really something we can hear? If so, do we
want to listen to this sound effect or provide it to our clients?
If not, there might be something we can do about flutter echo
THE ACOUSTIC CLAP TEST
On a practical basis, the only time that a
self-administered and self-audited hand clap is directly relevant
to anything in audio is when the recording engineer is setting
up mikes in a studio. Only in this special circumstance does
the desired audio signal leave from and return to the same
place. Listening to one's own hand clap duplicates this round
trip, acoustic process and thereby is a relevant test. If
someone ever wants to know how a loudspeaker sounds to the
listener, a different technique must be followed, one that
mimics the actual speaker/listener acoustical path.
hand clap contains only high frequencies. For a loudspeaker,
the high frequencies are directional, forward of the speaker
box. To properly administer a hand clap that mimics the high-frequency
beaming pattern of a loudspeaker, the hands must meet at waist
height while the clapper is facing the same direction that
the speaker does. The body of the clapper blocks the expansion
of the clap sound backwards. The listener is no longer in
the clapper position, the listener is now seated in the listening
position. This time, the hand clap is cast forward from the
speaker position and is heard by the real listener. It is
how the listener hears the speaker that counts and not so
much how the speaker sounds to itself, at least in hi-fi playback
In order to properly evaluate the consequence
on the listener of the strange sound we heard when standing
on the chair and clapping our hands overhead and near the
mounting position of the ambience speaker, we must repeat
the test while a listener is seated in the listener's chair.
True enough, in this case, the zing we hear when we clap is
also heard by the listener. And so, is the sound we hear,
good, bad, or inconsequential? Certainly this sound effect
is distracting and that alone is enough to warrant its eradication.
On the other hand, we want to retain an overhead liveliness
so as to promote the ambience signal. We can't sacrifice the
lively quality of the overhead space in the room, yet we must
try to get rid of its distracting effect known as flutter
FLUTTER ECHO/FLUTTER TONES
Before we try to solve our problems, let's
spend some time learning about it. When we administer a hand
clap test while located between a pair of uncluttered and
parallel walls, we hear a flutter echo. It has a "zing"
sound. The flutter echo actually does sound like a tone. The
frequency of the tone depends upon the timing of the flutter.
A flutter echo is how we hear what really is a rapid sequence
of noise pulses. When we clap our hands in the outdoors, we
simply hear the single, sharp pulse of noise we call the clap
sound. If we clap our hands while standing some distance away,
yet facing a wall or building, we will hear a single rapport
of the clap, its echo. Then, if we relocate and stand between
a pair of more nearby and parallel walls, that single pulse
reflects back and forth rapidly between the parallel walls
and we hear what we call a flutter echo.
If the walls are far apart, some 60 feet or
more, we actually hear the flutter sequence of the echo reflections.
But if the walls are closer together, the distinct detail
of the staccato seems to disappear, but only to be replaced
by a new sound, one of tonal quality. If the walls are far
apart, say 60 feet, we hear the slap back at a rate of 1130/60
or 17 times a second and it sounds like the tap-tap-tap of
a true flutter echo. However, if the walls are closer, say
20 feet apart, we will hear that slap back pulse of sound
at a rate of 1130/20 or 57 times per second. When we, the
human listeners, hear a click or noise pulsed at 57 times
a second, our ears/brains are tricked into perceiving a buzz-like
tone of 57 Hz. And so, the flutter echo we hear when the walls
are farther apart becomes a zing-sounding flutter tone when
the walls are closer together.
hi-fi, home theater, and even most recording studios, the
parallel wall surfaces are within the range of 15 to 30 feet
apart. That means we don't hear flutter echoes but do hear
the flutter tones. Flutter tones are sounds that have a low-frequency
character, but they are not to be confused with room modes
which also are low frequency in nature. The control of the
low frequency flutter tones, as we will soon see, is accomplished
with high-frequency type diffusion or absorption. Of course,
control of the low frequency of room modes is accomplished
only by means of larger-sized bass traps, usually best located
in the corners.
The low-frequency flutter tone is a pseudotone
- a trick on our hearing system played by the rapid staccato
of high-frequency noise pulses. Sometimes a careful listener
can become confused as to how a seemingly low-frequency sound
can be eliminated by the introduction of a paper thin reflector
or fabric, especially when common sense leads us to expect
that only those large-sized bass traps should have been needed.
In order to eliminate the detection of a flutter echo pseudotone,
we need only to break up the flutter echo process. It takes
very little scattering or absorption of high-frequency sounds
to break up the flutter echo sequence, and thereby el.iminate
the accompanying impression of the low-frequency sounds of
the flutter tone.
Audio parlor tricks, such as making bass reverberation
disappear with nothing more than a carefully placed scrap
of paper, are accomplished with the magician's classic technique,
a distraction of words and slight of hand. Only this time,
we say that to create the illusion, the hand must be moving
faster than the ear. Actually, the clue to the trick will
be found in the presentation. The guru claps the hands and
says to listen to the low-toned overhang. If you spectral
analyze the energy content of a hand clap, you will find no
energy below 400 Hz, yet the hand clap generates the perception
of typically a 50 Hz sound. It's a great trick. Practice it
and amaze your friends with your superpowers. You could even
start up your own business, selling little tinfoil "bass
traps" and you'll probably even get away with it, for
FLUTTER TONE SCIENCE
If we stand at the end of a long, narrow room
such as a hallway and clap, we will hear the flutter echo
as it returns to us each round trip. If the hall is 20 feet
in length, the flutter echo returns after every 40 feet of
travel. The time for the round trip is controlled by the speed
of sound. In this example, the sound of the clap makes a round
trip some 1130/40 or 28 times a second, which sounds like
the note of 28 Hz, a half octave below the lowest note of
the piano keyboard. However, if we stand in the middle of
the room and clap, we hear a different flutter tone. In this
situation, part of the clap sound travels towards each end
wall. Being in the middle means that each end wall is only
ten feet away. Both sounds return to us after only 20 feet
of travel. They pass by and head off towards the opposite
wall, only to return to us after another 20 feet of travel.
This situation produces a flutter tone of 1130/20 or 57 Hz,
a full octave above the basic flutter tone of the hall.
If we were really doing this experiment, we
would quickly find that we must stand to the side of the hall
so as to let the two end walls have a clear view of each other.
If we stand in the center of the hall, the flutter is quickly
damped out because of the absorption of our body. In this
position, with our back to the side wall, sound travels away
from the clap equally in both directions, up the hall and
down the hall. When we stand at the midpoint of the hall and
clap, the two wave fronts race towards the two end walls,
reach them and reflect back to soon pass by the clapper at
the same time. These two pulses, having arrived at the same
time, are heard as one loud pulse. Positions non reversed,
the two pulses race for the opposite far walk, and again repeat
the course. For this position, the double-strength pulses
are heard every time they make half of a full round trip of
important position to stand at is the end of the hall. We
already know the flutter echo occurs at half the rate as when
we stood in the middle of the hall. But let's look at the
pulse timing detail. Again, two pulses expand from the clapper's
position, one heads toward the far end wall and the other
toward the near end wall. The first reflection, off the near
end wall, hits us after an overall travel of only three or
four feet. It races by and follows the other pulse down the
hall, lagging by six to eight feet. They both hit the far
end wall and return towards the clapper's position. The leading
pulse flashes by and on to hit the nearby end wall. By the
time it again hits the clapper, the lagging pulse also hits
the clapper. This creates the effect of a single-hitting,
double-strength pulse. Then the lagging pulse moves past and
towards the nearby end wall. It reflects and, after a bit,
again passes by while heading for the far end wall. In the
meantime, the leading pulse had already long left the scene,
heading again for the far end wall and a repeat of the cycle.
we have here is a triple pulse event whose timing is that
of a full round trip in the hall. The three pulses are so
close together that they sound as if they were one pulse.
This combining effect is well-known in pro and high-end audio.
It is called the Haas effect, after the scientist who did
a lot of work in this area of hearing. What he found is that
when high-frequency reflections, such as those in the hand
clap arrive within ten to 15 ms (thousandths of a second),
they fuse together and sound as one.
Next, we take a few steps down the hall and
repeat the hand clap test, listening for any changes in the
sound of the flutter tone. If we moved five feet off the end
wall, the two pulses would be 20 feet apart and heard as separate
pulses because they arrived outside the sound fusion time
period. However, the same sequence of events still occurs.
The only difference is the separation of the two distinct
and small pulses. In the middle position, double-strength
pulse effect still occurs. As we change positions along the
length of the hall, we change the timing of the discrete echoes
that make up the flutter tone. We also find that as we approach
the middle of the hall, the two single echoes get far away
from the double pulse and closer to each other. When they
are within about six feet of each other, the fusion effects
begin and the two pulses start sounding as if they were one
and the upper octave flutter tone is heard. Get just a few
feet off dead center of the hall and the upper octave disappears
and the lower flutter tone begins to reappear.
timing of the two separated pulses is what accounts for the
changing of the character of the flutter tone. As we move
closer to either of the end walls, the timing between the
two separate pulses gets closer together, sandwiching the
double-strength pulse until the end wall is reached and they
are essentially all on top of each other. As we move closer
to the center of the hall, the timing between the two separate
pulses again gets smaller. This time, they do not sandwich
and are as far as possible from the double-strength pulse.
Finally, at the center, the time between them goes to zero,
creating a second, double-strength pulse.
All the pulses contain energy, the same amount
of energy. Whenever they return to the clapping position,
together they combine into a stronger, double-strength pulse.
Even more, when they arrive at the clapper's position within
six feet of each other, they still combine into a single,
double-strength pulse. When a clap originates within three
feet of an end wall, all of the pulses arrive at effectively
the same time and the result is heard as a four-times stronger,
low-frequency flutter tone. Then again, if the clapper is
within three feet of the middle of the hall, the separated
pulses arrive close enough together to combine and double
up in strength. Either of these extreme conditions is about
as easy to detect.
the two separated pulses are not close to the doubled-up pulse,
the lower flutter tone is quieter, less noticeable to detect
and that is good. Also, when the separated pulses are not
combined due to a midpoint clap position, the upper octave
flutter tone is not heard. That is also good. Clearly, we
now know that the most non-stimulating position for flutter
tone generation will be more than four feet away from either
end wall and a few feet off the center of the room. By experimenting,
additional information is developed. Anywhere in the end third
of the room seems to strongly stimulate the lower flutter
tone. The thirdway point seems to stimulate the third octave,
along with the fundamental flutter tone. The middle of the
room really generates the second octave flutter tone within
a foot or two of the center point.
our 20-foot room as an example, the ambience speaker ought
to be located ahead of the 1/3 point, but two to three feet
off the center. That puts it at about seven to eight feet
off either end of the room, probably the rear wall for home
theater. As a general rule, the ambience speaker can be placed
38 percent of the room length off the back of the room. This
position will ensure that minimal flutter tone coloration
is introduced into the room.
This section has been intended to be a baseline
guide for the anti-flutter tone positioning of the surround
speakers. To this, we next add some enhancement devices to
both increase the presence of the ambience signal and to continue
to reduce the telltale presence of flutter tones in the home
DIFFUSION OF FLUTTER
In addition to positioning the speaker to
weakly stimulate the distracting flutter tones, another element
of acoustics can be brought into the battle and put to good
use. Diffusors are devices or surfaces that scatter sound.
The home theater ambience speakers are located high on the
sidewalls and directed to illuminate the upper outside areas
of the front and back walls. The first idea about scattering
sound tends to be directed to these areas. Why not add a curved
or otherwise irregular surface to these areas of direct illumination?
As it is, we can hear the flutter tone that
comes from the ambience speaker because its multiple reflecting
wavefront not only shuttles back and forth between the front
and back walls, but the wavefront expands while doing so.
What we hear is the expanding edge of the flutter echo circuit.
Now if we add diffusion to the end walls, we will certainly
reduce the time that the flutter tone is sustained because
the diffusors are redirecting some of the flutter energy away
from the flutter circuit at each reflection. This redirected
energy is not absorbed but scattered more fully into the room.
That means that the listener is getting an even stronger flutter
tone signal than before. Not only does the listener hear the
expanding edge of the flutter echo, but now additionally hears
the scattered sound off the diffusor. Ironic as it seems,
adding diffusors to the end walls is a trade-off treatment
with mixed results. The flutter tone becomes louder but shorter-lived.
It is a change, but is it an improvement? Better, worse, or
merely different, this now is something for you to decide
look at another technique. The flutter echo runs back and
forth along the length of the room, hugging the upper sidewall/ceiling
corner. Sound-scattering devices can be placed along the upper
sidewalls of the room. Again, sound is depleted from the flutter
echo circuit. As energy from the flutter echo is redirected
into the room, the flutter echo lifetime is reduced. However,
this time the scattering takes place between the end wall
reflections and not in lumped reflections off the end walls.
These deflectors can be slightly angled down
so as to not only kick the reflection to the side, but also
downwards. After all, the listener is nearer the floor than
the ceiling. Such deflectors are sometimes called ambience
kickers in the professional world of recording studios. Another
aspect in the setup of these kickers is their spacing. Just
as the regular timing of end wall reflections manifests itself
to us as a flutter tone, regular timing of reflections off
the deflectors can also create a flutter tone. Additionally,
we don't want to place the deflectors so that their signal
arrives at the same time as any of the regular flutter echo
signals. In such a case, the work accomplished would be minimally
different from that by diffusors on the end walls.
Clearly, we won't want the deflector to be
located the same distance towards the front of the room as
the distance the ambience speaker is to the rear wall. This
would give the same timing to both reflections being received
at the listener's position. The side scattering deflector
has to either be in front of or behind this position. Since
the ambience speaker is located about 38 percent off the back
wall, the ambience kicker should avoid the location of 76
percent off the rear wall. As a first guess, we could locate
it almost halfway between, about 52 percent off the rear wall.
This produces two new reflections spaced out between the timing
of the end wall reflections. The strength of these reflections
will be similar to the end wall reflections because of the
longer distances involved.
Another deflector could be placed about halfway
between the ambience speaker and the rear wall. This one will
produce a reflection that arrives somewhat before the rear
wall reflection and helps to fill in that big time gap. How
many other such ambience kickers can be installed is not so
easily predicted. The side fill they produce and its value
to the listener belong, in a large degree, to the listener's
taste and judgement.
The sonic impact produced by upper sidewall
diffusors is quite different on two levels. First, the scattering
reflections are distributed all around the listener rather
than coming from just in front of and behind the listener.
This more diffuse "source" of the ambience signal
seems to promise to be more supportive and involving for the
surround sound effect. Second, is the relief provided due
to multiple reflections that crop up in between the end wall
reflections. These intermediate reflections spoil the perception
of the otherwise clear and distinct end wall reflections.
The result is that distributed, upper sidewall deflectors
produce a signal that masks out the flutter tone. The result
is a lively, diffuse, and colorless ambience signal.
Over the last two sections, the dipole ambience
speaker has been shown to best be placed about 38 percent
of the room length off the back wall, and 20 percent of the
room height down from the ceiling. Located directly above
it there needs to be a bass trap good through 100 Hz. Along
the upper sidewalls there should be distributed a set of ambience
kickers. Attend to these details and the ambience speakers
can safely play into your. room without inducing coloration
or distracting distortions. Only then can the true shading
and hue of the signal on the ambience sound track be heard.