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Technology: Geometry
The Acoustical Line-Source
The Important but Generally Ignored "Marriage" Between the
Speaker and the Microphone
The Optimum Speaker Geometry: Why "Faceted" panels
Instead of Curved Panels?
You ask: what is an acoustic line source and how can it improve my life style?
As you will now learn, we are ready to defend to the death, that the line-source is
the optimum acoustic configuration to accurately re-create the musical
performance in your sound room.
Conceptually, the line source is like attaching a magic guitar string (one that can
generate all frequencies of the audio spectrum without prejudice) to one end of
infinity and attach the other end to the diametrically opposite infinity. And, as
luck would have it, the string passes through your ceiling and into the floor at the
precise location where you want your speaker. This would be referred to as a
vertical line source. Please note, we can go into the basement and into the attic
where we can clip off those portions of the line that do not intersect the
room without affecting the sound in the room. This is a serious point that is
entertained in the full white paper.
What does the "line" do? First, all sound rays emanate at perpendiculars to the
line. Thus, there is no vertical dispersion. Doing deep-knee bends in front of the
line would have no effect on perceived sound. Since sound is dispersed only in
the horizontal plane, the line source has another interesting and useful
characteristic. As one walks toward a line source, psycho-acoustically the sound
does not seem to get louder. Now, without going into further technical detail let's
consider what this all means to the perceived sound in your listening room.
If you were given permission to wander around a concert hall while an orchestra
was playing, you would find that as you were nearer to one side of the hall you
would still be able to hear the full orchestra, but the "image" of the orchestra
would have a different perspective than if you sat dead-center. In beholding this
skewed image, the nearer instruments would appear louder than the more distant
ones, but they would not mask the latter. You would still hear them.
Now, let's consider the line-source in the sound room. Remember, its
characteristics? As you walk toward it, it doesn't appear to get louder. Stand
near one line-source speaker and it doesn't drown out its mate. In fact, you get
the same type of perspective that you heard in the concert hall, where each
instrument is in its proper place regardless of where you are. This is staging.
The line-source emulates the staging you heard in the hall. The so called
"sweet spot" (the position in your sound room where reproduction is the
best) is therefore virtually your entire sound room. No "head-vices" are
required to secure your ears to the optimum spot. Get up, move around, and
enjoy the full glory of your sound system without penalty. This is the line
source.
For the sake of making a complete statement, at the opposite end of the
spectrum is the point source. Most other loudspeakers are point sources. Their
characteristics are just the opposite of a line source. As you move toward the
point source, it gets dramatically louder. When you are near a point source
speaker in a stereo system, you will no longer hear the other speaker because of
acoustical masking. Also, energy is radiated vertically, which now opens up
another bag of worms: the problem of "taming" floor and ceiling reflections.
The only listening position possible to obtain proper staging and spectral balance
with point sources is along a line centered between the speakers. You will most
likely need a "head vice" to enjoy optimum performance. In contrast, by having
no perceptible masking effects, the line-source speaker avoids this problem.
Why do designers use point sources? Simply, because it is possible to
make a more compact speaker that is inherently cheaper to build.
The ideal situation would be
where the loudspeaker and the microphone are the same
device, or at least have the same characteristics.
It doesn't take much imagination to visualize the consequences on the staging
(the size of the breadth and depth of the reproduced image) if the spatial
characteristics of the microphone and the speaker are widely different. For
example, contemplate what happens to the image of a piano or the size of the
mouth of a singer if the microphone has a very narrow acceptance angle and the
speaker has a wide dispersion angle, or vice-versa. Staging size is corrupted if
the microphone and speaker characteristics are not similar.
We have studied the type and characteristics of microphones generally used in
professional recording studios. We find that the cardiod (directional acceptance
pattern) is the format most used. Also, we find that most professional
microphones in use have an acceptance angle on the average of around 90
degrees. Therefore, in order to obtain the most accurate staging, we have
designed our speakers to have a 90 degree dispersion angle. As a result, we
have been accused by several well-known reviewers of having the best life-sized,
palpable images of any speaker they have tested. The secret is simple, is it not?
Why don't other speaker designers do this? They are severely limited by the
characteristics of the drivers (raw speakers) they use. We can make the
dispersion angle of our speakers any size we wish, and we wish to emulate the
characteristics of most microphones. A simple principle (principle of reciprocity)
but it works!
There have been some membrane (panel) speakers introduced on the market
that have a truly curved membrane in an attempt to provide smooth horizontal
dispersion of sound energy. Intuitively, this approach appears to be ideal, but
don't be easily deceived.
Consider a horizontal cross- section of a curved membrane. It appears as a
sector of a circle. If the amplifier sends the membrane outward toward the
listener, the membrane tension increases. In fact, for a given fixed displacement,
it increases directly as the angle of the circular arc. A major problem arises on
large excursions, as a "tug-of-war" is created between the weaker electrostatic
forces moving the membrane and the strong tensile strength of the membrane
(mylar has a tensile strength greater than steel on a weight-comparative basis).
Which force would you bet on? In fact, on large excursions, the membrane
cannot reach the full displacement and the peak is clipped. Therefore, those who
would use this approach must necessarily restrict the dispersion angle to a less
than optimum angle, like 30 degrees or less.
The case where the membrane is forced to move away from the listener is the
converse, the membrane loses tension. On large excursions, it can virtually lose
most of its tension and its movements are no longer accurately controlled. Since
the forward and backward movements of the membrane have opposite effects on
membrane tension, an intolerable non-symmetrical distortion results that
produces a "glare" in the sound at realistic levels.
More importantly, since linear displacement is severely limited, low frequencies
must be reproduced by conventional woofers and all of the high-resolution bass
potentially available with electrostatic technology is lost.
Faced with the unacceptable characteristics of the curved membrane, we
developed the faceted single-membrane panel. In more descriptive terms,
this technology results in a piece-wise approximation of a curved surface by
using flat sectors (facets). By using vanishingly small margins between
facets and by judiciously selecting the proper facet width and angle
between facets, a very smooth dispersion curve can be obtained without
a vertical "picket-fence" effect. With this technology, large linear and
symmetrical membrane displacements can be obtained. What's more, there is
no limit to the amount of dispersion angle that can be selected since the angle
chosen does not affect speaker characteristics as in the case of the curved
diaphragm. The result? Full-bodied beautiful high-resolution bass is
obtained without the need of conventional mass-controlled,
low-resolution woofers.
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