As 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.
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.
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 from.you/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.
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 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
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 the lower registers. Omni
is omni and it doesn't matter which direction the midbass speaker(s)
There seems to be only a couple of rules to follow when placing
the surround dipole speakers. Mainly, they have 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,
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-to-back position for the speaker. In the
following sections, we go over the details that determine the most
neutral position for the ambience speaker.
ANTI-RESONANCE AND SELF-CANCELING
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 self-canceling
effect. The nearby reflection actually weakens the strength of the
speaker at a certain frequency.
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.
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 self-cancel 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.
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 problems.in the lower 60 percent of the piano
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 self-canceling
bounce back point. The best ambience sound is colorless, except
for the ever changing signatures in the ambience track.