HT Volume 4.indd - Acoustic Sciences Corporation

HT Volume 4.indd - Acoustic Sciences Corporation
-a five-part article in Home Theater magazine, October 1993 - February 1994
Home Theater Acoustics
Volume Four
Ambiance speakers
are readily becoming
standards in home
s we survey audio systems
for home theater, a trend
appears. We consistently
find one or two subwoofers,
two or three stage speakers,
and two ambience speakers. In the last
two sections, we studied the subwoofer
as it fits into and plays the listening
room. Here we will study ambience
speakers, the kind that are becoming a
standard for home theater.
The Dolby surround signal is a mono
signal usually fed to two speakers
located towards the back of the room.
This signal is unique in audio because
it is rolled off at 100 Hz. This doesn’t
mean that there should be no deep
bass in the ambience effect. It does
mean that the deep bass is generally
understood to have no directionality.
Our head is too small, our ears too
close together, and our hearing too
insensitive to be able to tell which
direction low frequency sounds are
coming from. Remember how no
one worries where the subwoofer
is placed, except for visual or room
mode control? That’s because we can’t
tell where the bass is really coming
from. The way we “know” where bass
comes from is by focusing on where
the upper partials of the bass sound are coming from. The
Dolby surround signal contains the upper partials of the
ambience bass, so we think the ambience bass is coming
from the ambience speakers. But really, it’s the subs and
main speakers that get the signal and do the generating of
the ambience deep bass.
There are two basic kinds of ambience speakers these days,
although more may pop up as time goes on. The first, most
basic type are simply small, book shelf type speakers on
speaker stands or mounted ot the wall. The ambience signal
can be beamed either: directly at you or away
bouncing around the room a bit before it hits you. If the
speaker is aimed directly at you, you will hear it and know
where it is. Our hearing is very sensitive to sounds beaming
directly into one ear. After all, what do we do whet we can
barely hear some sound? We turn our head to the side, so
one of our ears can hear the sound more directly. For the
ambience speaker setup, the orientation of the speaker is a
matter of personal choice and the experiments should be
made. Many people prefer not to hear the ambience signal
directly and their ambience speakers are turned somewhat
towards the wall and face either forwards or backwards.
The second type of ambience speaker is called a surround
speaker and is recommended by the THX people. In this
system, the choice about how we hear the ambience signal
has been made for us. This speaker is mounted high on the
side wall and set up to not beam any sound directly at the
listener. These speakers are specified to be primarily dipole
type speakers. This means that they play backwards and
forwards equally strong, but not at all to their side, which
is, of course, where the listener is located.
The dipole speaker familiar to us in hi-fi is usually a thin
sheet of material that is forced back and forth by either
magnetic or electric fields. The forward wave is exactly
out of phase from the backwards wave. When the sheet
moves forward, a positive pressure wave is sent forward
while a negative pressure wave is sent backwards. Not so
for most surround sound speakers. This
type is often comprised of two dynamic
speakers wired out of phase and playing
back to back. There still is a positive wave
sent out in one direction, while a negative
wave is sent in the other, being equal in
strength but opposite in phase. There are
numerous dipole speakers and the goal
here is not to propose or evaluate which
might be better than the other, if such
would even be possible. The goal here is
to explore the effect on the sound of these
speakers that is imposed by the room in
which they are located.
The dipole type surround speaker is a strange kind of
speaker to the world of audio and it will, without a doubt,
undergo a number of transformations as it evolves into its
mature form. To begin with, it is not a full range speaker
because the surround channel is rolled off at 100 Hz. For
the most part, these speakers nave been a small speaker
cabinet with two speaker baffle boards, one set to face
forward and the other to face backwards. Usually, we
see each panel forward and the other facing backwards.
Usually, we see each panel mounted with a single driver.
Two-way speakers are also used, sometimes with the
tweeter offset from the main driver, other times with
coaxial drivers.
The intent of this style of speaker is to “play forwards and
backwards” so as to illuminate first the room and not first
the listener. This directional effect only works for a limited
frequency range of the speaker. Small-sized drivers are
directional for upper, mid, and high frequency ranges,
but become omni-directional for the lower ranges. This
directionality effect occurs at a predictable frequency based
on the size of the drivers, as well as the cabinet in which
they are mounted.
A good demonstration on the directionality of a speaker
can be achieved by setting a small loudspeaker outside of
the house on a table that is placed in the middle of the
open yard. Then, while keeping some fixed distance away,
walk all the way around the speaker while it is playing
some tune with which you are familiar. You will hear the
full range of sounds of your speaker when you are in front
of the speaker, but as you move to the sides, and especially
when behind the speaker, the highs drop off substantially,
but not the lows. Male vocals, for example, sound pretty
much the same no matter where you are, but sibilance, the
“tsss” sounds, dramatically drop off behind the box.
If you get an identical second speaker, wire them up in
phase and place them back to back. You’ll hear bass range
the lower registers. Omni is omni and it doesn’t matter which
direction the midbass speaker(s) points.
There seems to be only a couple of rules to follow when
placing the surround dipole speakers. Mainly, they have
everywhere and the sibilance will be heard in two beams,
One forward and the other opposite. Listening directly off
to the side of the speaker pair, you’ll hear the midrange and
bass. Now reverse the phase of the two speakers and listen.
All of the bass drops out, yet the two mid/high back to back
beams remain. To the side, there is a strange drop in all
sound. So it is with the dipole speaker. The dipole effect is
limited to the upper ranges of the speaker because the bass
shorts out, acoustically speaking. At some low frequency,
the dipole speaker simply sloshes air back and forth around
the edges of the speaker and makes no more sound. This
is nc different than listening to a bare speaker and then
mounting it onto a piece of plywood. We increase the
distance between the front of the driver and the back and,
in doing so, give the speaker more range in the bottom end.
Because the surround dipole speakers are fairly small, they
short out at fairly high frequency, around 400 Hz. And, so,
there must be another system in place to generate sounds
below this natural dipole cutoff. There are a number of ways
to accomplish this. The most straightforward way is to use
a single lower frequency driver reversed, large-sized, and
directional midrange drivers. Offset or coaxial tweeters will
accompany these large midrange drivers to get full high
frequency range. The main thing to keep in mind during the
evolution of this style speaker is that the orientation of the
low frequency drivers is irrelevant as to the directionality of
to be placed high on the side walls, directly to either side
of the listener position. They can be positioned in front or
behind the listener somewhat, but must be angled so that
the side of the speaker points to the listener. Above all,
never place them in bookshelves no matter how convenient
it may seem. The honky, tonal resonances this setup
produces will be almost unbearable, not to mention that the
walls of the bookshelf will catch the ambience signal before
it gets to the room. These surround speakers are to fire
along the side wall towards the front and back walls. Next,
there are three factors to be considered in the placement of
ambience speakers -- resonance, self-canceling, and flutter.
Whenever a speaker is placed in a room, it needs to be
positioned so as to minimally stimulate room induced
coloration effects. This is especially true for ambience
speakers because their effects are in direct competition with
the room’s natural ambience for the listener’s attention. If the
ambience speakers are located improperly, they will strongly
stimulate the local room effects and their capability of
generating the desired audio track ambience will be reduced
by the sound masking effects of the room’s acoustics.
We know the ambience speaker is to be located high on the
side wall by the listener. Beyond that, we seem to be left to
our own resources. The lower frequency play of the speaker
can be used to determine the most neutral vertical location
on the side wall. The high frequency characteristics of the
speaker can be used to determine the most neutral front-toback position for the speaker. In the following sections, we
go over the details that determine the most neutral position
for the ambience speaker.
In the previous chapter, we studied how to determine the
most neutral position for the placement of subwoofers in
the listening room. Two factors came up to impact the
coloration of the sound quality. The first and most familiar
was room resonances. We determined that placing the
speaker so as to least stimulate the room resonances would
be most appropriate. In addition, there is the complication
due to placing a speaker near a wall, floor, or corner - a selfcanceling effect. The nearby reflection actually weakens the
strength of the speaker at a certain frequency.
These lessons also apply to the ambience speaker
positioning. The ambience speaker is essentially a single,
mid-bass driver with two reversed phase, mid/hi drivers,
back-to-back. The vertical position of the speaker on the
side wall is determined by the speaker’s low frequency
coupling to the floor/ceiling parallel surface system. We
saw that when the frequency range of the speaker spans
many resonances, the best location for the speaker is at
the 25 percent mark from one end. However, for the
ambience speaker, it is rolled off at least 100 Hz or higher.
This means the first floor to ceiling resonance, typically
at about 70 Hz for an eight-foot room height, cannot be
stimulated. By studying the pressure distribution for the
first three resonances and ignoring the first one, we see
that the minimum position for stimulation of the second
and third resonances lies 20 percent from one end of the
dimension. This means the best, anti-resonant location will
be a distance down from the ceiling that measures about 20
percent of eight feet or 1.6 feet (19 1/4 inches) down from
the ceiling or up from the floor.
For the ambience or surround speakers that are mounted
high up the side walls, the 20 percent down position is
easy. However, for those ambience speakers that are on
speaker stands, putting the speaker 19 inches off the floor
is not a normal thing to do. Most speaker stands are set up
to position the speaker about ear height, 42 inches off the
floor. There is another position, not nearly as good as the
20 percent position, but at least it is a relatively minimal
position. This is at the 40 percent point, where the first and
second harmonic curves cross just below the 50 percent
point. The traditional speaker stand positioning of 42 inches
places the speaker at the 44 percent height point for an
eight-foot high room. It is not easy to change the height of
a metal or even a wooden speaker stand, nonetheless ... we
are at this time concerning ourselves with good acoustics,
not convenience.
Every time the speaker is located near a reflecting surface, the
problem of self-canceling comes up. For a speaker mounted
20 percent down from the ceiling, the self-canceling
frequency occurs at a wavelength that equals four times that
distance or 80 percent of the room height. The wavelength
that goes with an eight-foot high room will be about 6.4 feet,
which corresponds to 1130/6.4 or 177 Hz.
By the way, there will be reinforcement
at twice that self-cancel frequency at 354
Hz and then a cancel at 530 Hz, and so
on. Every 177 Hz there is a self-induced
effect that alternates between cancel and
boost. This is on the order of a four to six
dB magnitude and stops only when the
speaker becomes so directional that it
doesn’t illuminate the reflecting surface,
typically about 600 to 700 Hz.
It is very easy to remedy this self-canceling problem.
Simply, bass trap the bounce back point. But not just any
bass trap will do. The low frequency cut off for the bass
trap should be set about a half octave below the lowest
frequency that needs to be trapped. For 177 Hz, this is
figured as follows: A full octave below 177 is 88 Hz, so a
half octave below is half of 88 or 44 Hz. The half octave
below 177 Hz is 177-44 or 133 Hz. Now that you know
all about it, the simple formula is that the lower half octave
point is 75 percent of the given frequency.
The floor standing ambience speakers seem to luck out
as far as self-canceling effects go. Their drivers will be 39
to 42 inches off the floor and self-cancel at four times
those distances, for the 15- and 14-foot wavelengths. The
frequencies for these are 87 and 80 Hz and both are well
under the 100 Hz cutoff for the Dolby ambience signal. So
these high mounted, floor standing speakers do not selfcancel off the floor. But floor standing speakers tend to be
set up away from the wall. While the floor bounce may
be too far to self-cancel, the nearby wall bounce can be a
problem. We know the omni speaker is rolled off at about
400 Hz. The 1/4 wavelength dimension for this is 8 1/2
inches, which becomes the maximum distance this driver
should be away from the wall and not self- cancel from the
wall bounce.
Why, one might ask, should we be careful of the range of
the bass trap we use? Also, who needs a “bass trap” anyway?
Don’t acoustical foam or wall panel type products absorb
sound and at a lot less cost? The questions are proper to ask
and deserve an explanation. They all involve the balancing
of frequency characteristics, those of the speaker to those of
the absorber.
A speaker loves to be near a corner when reaching for its
lowest registers. The “horn loading” effect due to placing a
speaker near a wall, floor, or corner increases the efficiency
of the speaker in the bottom end, more bass power at no
extra cost. If a bass trap is placed in the corner, we usually
do not want it absorbing the deep bass. We want the
opposite, horn loading to reinforce the deep bass. For this
reason, we need the bass trap to roll off its absorption in the
range where the speaker output is also rolling off and the
benefits of horn loading are being called into action. For
small, full range boxes, this 3 dB down point (50 percent
power) can typically be about 60 Hz. But as mentioned
above for the home theater ambience speakers, the roll off is
set at about 100 Hz or more.
Now we’ll move onto acoustical foam and wall panels.
These fairly common acoustical products are good only
for the midrange and high frequency ranges. This range
includes only the top three octaves of the piano keyboard
and does not include anything in the lower 4 1/2 octaves
of the keyboard. Only bass traps can cover this lower range
of sounds. The middle of the keyboard is C4 at 256 Hz. In
our example, we needed the absorption half power point
to be at 133 Hz and that’s almost one full octave below
middle C. It also is two full octaves below the roll off point
of commodity foam and wall panels. Bass traps are the only
absorptive devices that can correct acoustical
the lower 60 percent of the piano keyboard.
Ambience speakers, like all others, engage the room
acoustics. Because of their limited bandwidth, they do not
couple to the lower resonances of the room. That gives us
the most neutral, anti-resonant position yet for the speaker
position, 20 percent off the floor or down from the ceiling.
Something new has been added to help smooth out the
acoustic space for the speaker - the bass trap - the selfcanceling bounce back point. The best ambience sound is
colorless, except for the ever changing signatures in the
ambience track.
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