SONIC REALISM
PHOTO 3. The center channel speaker uses
coaxial speakers (seen as two small
circles under the grill) to achieve
reasonable consistent phase output.
This is a much simpler design and can
probably be used for the left and right
speakers with good results. However,
this was not tested. (Note that a trim piece
was used to cover the plywood edge.)
However, it was realized that the
individual instruments were localized
on a line between the two speakers. It
was also noted that it was easy to hear
both speakers even when standing
close to one of them. So while there
were some sonic differences from
the conventional approach, they were
not significant.
Upon further consideration, it was
realized that a third speaker placed in
front of the listener might be useful.
The idea was that since the human
hearing system uses both phase and
amplitude for localization, a center
channel could provide additional
amplitude cues. A “center” signal was
one that had an equal amplitude in
both stereo channels. Adding the
channel signals together meant that
the center channel would provide
twice the amplitude (+ 6 dB) as either
channel alone. This should be enough
to significantly assist in localization,
which should also enhance the reality
of the sound.
So, a third speaker was built.
However, because of a number of
issues (including parts availability) the
third, center-channel speaker was
much more modest. It is shown in
Photos 3 and 4, and consists of a
three-way automotive speaker with
coaxial drivers. The enclosure is a
simple box and is also described in
more detail in the sidebar. This
center speaker design is much easier,
smaller, and much less expensive than
the buried tweeter design. It probably
would be adequate for the left
and right channels, but this has not
been tested.
This speaker was driven by a
separate amplifier that was just the
sum of the left and right channels.
(The left and right earphone outputs
of the main amplifier were connected
together and used as the input for
the center channel amplifier.) This
speaker was placed about seven feet
directly in front of the listener. Note
that all the speakers had a direct “line
of sight” to the listener. (A rear
channel using an out-of-phase left and
right mixture has been considered,
but not tested.)
The results of this addition
were stunning. Excellent sound
localization through 180 degrees
occurred. When listening to
orchestral or choral music, each
individual instrument or voice
PHOTO 4. The inside of the center
channel speaker. This shows the
standard construction technique
of using internal corner-framing
members. Screws and glue are
used for every joint (except the
back panel) for a solid, air-tight box.
had a precise and separate location.
There was also a depth that placed
different instruments in front and
behind others. Curiously, all three
speakers acoustically disappear. Note
that the program material is ordinary
stereo CDs and FM broadcasts.
(Although some recordings are better
than others. This is discussed in more
detail below.)
An A/B switch was incorporated
to disable the center channel. This
provided some interesting information. It appears that it sometimes
takes a fraction of a second for the
localization to appear and disappear.
(Unlike turning a speaker on and off,
where the sound is immediately
heard or not.) This delay suggests
that there is significant mental
processing that occurs. This is
precisely what was expected and
suggested in the previous article:
The brain has an auto-correlation
function (or its equivalent) that is
used for phase analysis.
Speech reproduction is the most
important aspect of sonic reality.
There are several reasons for this.
First, speech is a fundamental aspect
of human nature. It is important
to us. Speech is also extremely
common. We all spend a lot of
time each day talking and listening.
Finally, unlike most music, speech is
mostly heard live. Even at “live”
events, microphones and amplifiers
are often used. Again, multiple
drivers destroy the phase
information. Thus, “live” events are
actually electronically reproduced
stage events. True “live” events are
fairly limited. Professional orchestras
and amateur/school bands and
choirs are the most common. Even
jazz is being augmented with
electronic amplification. In short, we
all know when speech sounds real,
but music is a different matter.
It is not surprising that the
most dramatic results occur when
reproducing voices. When a chorus is
reproduced, each voice is distinct and
has a precise location. There is also a
quality of realism that cannot be
62
June 2007
