SONIC REALISM
described. It simply sounds as if the
choir is in front of the listener. (It is
also useful to close your eyes because
your brain knows where the sound is
coming from. By closing your eyes,
there is no conflict between what your
eyes and ears are telling you.)
It is unfortunate that there does
not seem to be an objective manner
in which to measure this effect.
However, there are two subtle and
unconscious physiological effects
that have been noticed when l
istening to solo singing voices that do
not occur with conventional speaker
systems. The first is the listener’s
breathing tends to match the singer’s.
The second is that the listener tends
to mouth the words to the song.
Like tapping a foot in time with
the music, these effects are variable,
but the fact that they do not occur
with ordinary speaker systems seems
significant.
The last piece of evidence is
somewhat more objective. My wife
entered the room while I was working
at my computer with the stereo on
to discuss an unrelated matter. She
was not aware of the details of the
speakers. A song then started with the
words, “Hi. How are you?” She
whirled around, clearly startled and
said “What ...?” This spontaneous
reflexive response is telling. It is, in
BUILDING THE SPEAKERS
Cost and performance were the
major issues that were considered
when designing the buried tweeter
speakers. A closed/sealed box (rather
than ported) was required to ensure
that all the sounds came from one
opening. This required speakers suitable for such an enclosure. The determination of the Thiele/Small parameters necessary for the enclosure were
determined with the aid of a modeling
program included in the book Great
Sound Stereo Speaker Manual (2nd
ed.) by Weems and Koonce (
McGraw-Hill, 2000). This is a very useful book
for anyone building speaker systems.
After considerable effort, Sony 12
inch Xplod (XS-L121P5) automotive
speakers were selected. These had the
proper Thiele/Small parameters and
were very attractively priced at a local
electronics superstore as close-out
items (suggested retail about $199
each; sale price of $35 each). These are
rated at 350 watts RMS and 1,200 watts
peak. The voice coils are only four ohms
each, but using two in series provides
the standard eight ohm impedance.
The tweeter chosen was a piezo
horn driver (Motorola/CTS KSN1142A)
attached to a standard diffraction horn.
This driver can handle 400 watts RMS
and only cost about $12. Note that
horn tweeters generally have much
smoother frequency response characteristics than the “Bullet” type circular
tweeters. Round dome tweeters have a
smoother frequency response
than the horn type but cannot
FIGURE 1. The basic crossover network for
the buried tweeter speaker system is a
four-pole, 2,000 Hz low-pass filter. The piezo
tweeter series capacitor reduces the output
by about 4 dB for better balance.
handle the power. It is difficult to find
these KSN1142A tweeters now. High
power tweeters are not common.
The reason for the use of
high-power speakers is the concern
about distortion at high output levels.
Most home speaker systems are rated
at 50-100 watts RMS. However, their
distortion increases significantly at
half-power or less. Often this is not
enough power to provide realistic
volume levels. Driving them at half-power makes the music sound ”loud”
similar to a person’s voice when
shouting. There is a sonic difference/
distortion. With high-power speakers,
the music gets louder but doesn’t
sound as if the speakers are shouting.
The crossover network is technically not required because the piezo
tweeter has a high impedance at low
frequencies. However, it was desired
to keep as little sound of the same
frequencies as possible from exiting
from both the woofer and tweeter. For
this reason a sharp-cutoff, four-pole, 2
kHz crossover network was employed.
This is shown in Figure 1.
The enclosure was built with 3/4
inch thick, maple, cabinet-grade
plywood. The standard method of
screws and glue was employed. All
corners used an internal corner-framing member and the screws were
driven from the inside (see Photo 4 for
an example of the standard internal
construction methods). No fasteners
are visible from the outside. The top
and bottom are mitered to the sides.
This involves a three-way miter at each
corner. Such a detail cannot be accomplished without the careful use of a
table saw. Other simpler methods can
be employed but minor changes to the
enclosure will be required. The
absolute dimensions of the enclosure
are not critical and can be varied
somewhat. (Enclosure building techniques are presented in the Weems &
Koonce book.) Some experience in
woodworking is suggested before
attempting to build these, or any,
speaker enclosures. Sound quality is
directly related to construction quality.
The speaker panel was not glued in
place so that access was possible to the
internal blind screws. However, this
panel vibrated at high drive levels. This
problem was corrected by placing a
piece of weather-seal foam tape between
the panel and the mounting members.
June 2007 63
