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DESIGN GOALS
The technical requirements result in the following major technical design goals:
1. Very uniform frequency response
2. Time response accuracy
3. Phase response accuracy
4. Low energy storage
5. Low distortion
FREQUENCY RESPONSE
In our opinion the human ear is sensitive enough to the balance between component harmonics of musical sounds to detect frequency
balance errors of as little as 0.2 dB if they are over a range of an octave or more. Therefore we believe that extremely accurate frequency
response is an absolute requirement for a truly good speaker. Our design goal was to achieve accuracy in the design prototype of .75 dB
with a production tolerance of .75 dB. The result is a tolerance in every production speaker of only 1.5 dB and a tolerance from speaker
to speaker of only 1.5 dB at all frequencies.
Even more important than the maximum amount of response error at any frequency is the octave averaged, octave-to-octave balance
which has a very high correlation with the perceived tonal balance. Our design goal was to achieve octave-averaged response within 0.5
dB from 200 Hz to 10 KHz. Any deviation more than 0.5 dB is confined to only a narrow frequency range and therefore will have less
effect on the perceived balance.
Achieving these goals requires the use of drivers with exceptionally uniform responses, drivers with very high consistency (so that few
units need be rejected), drastic reduction of usual cabinet diffraction which causes response errors, and an unusual degree of compensation
in the electrical network of even minor driver response anomalies.
Driver Response
The major cause of non-uniform driver response is diaphragm resonance. These resonances are also the major energy storage
mechanism.
In the case of the CS3.6 tweeter, a metal diaphragm is used that is stiff and light enough so the lowest diaphragm resonance occurs
above the range of hearing at 26 KHz. Therefore, there are no resonances in the audible range to cause energy storage or response
irregularities.
The CS3.6 mid driver uses a very effective new method of greatly reducing diaphragm resonance (patent applied). The diaphragm is
constructed of two cones, each with a different shape, which are joined at the rim and at the neck with only air between them. The resulting
three-dimensional structure is drastically stronger than a conventional diaphragm of equal weight. This increased strength causes the
Figure 1 Conventional diaphragm with vibration
Figure 2 Frequency response of conventional
diaphragm
Frequency response magnitude - dB
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log Frequency - KHz
Figure 3 Frequency-time response of conventional
diaphragm
frequency of lowest diaphragm resonance to be
substantially higher and low level vibrations to be
substantially less.
Figure 1 illustrates a conventional diaphragm
and the vibrations that normally cause irregular
frequency response. Figure 2 shows the response
of a CS3.6 mid driver built with a conventional
diaphragm. Apparent in the response is a
depression in the 1 to 2 KHz region followed by
a hump from 2 to 3 KHz, a dip at 4 KHz and a
final peak at 4.7 KHz. These irregularities are
due to diaphragm resonances. Figure 3 is the
frequency response of this driver through time
and illustrates how the resonances cause the
driver's output to ring. It can be seen that the
driver's output up to 5 KHz requires 1 milli-
second (ms) before reducing to the -20 dB floor.
Figure 4 illustrates the new diaphragm.
Figure 5 shows that the response is much more
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uniform than the conventional driver and shows
far fewer irregularities. Figure 6 shows that
diaphragm resonances are virtually eliminated
below 7 KHz. Even the one resonance at 7 KHz
is mild. Below 6 KHz the driver's output is
exceptionally clean, reaching the -20 dB floor in
less than 0.5 ms.
The CS3.6's woofer is the first in a THIEL
product to employ a metal diaphragm. The
anodized aluminum material provides much
higher stiffness and compressive strength than
conventional diaphragm materials. The primary
Figure 4 New double diaphragm
Figure 5 Frequency response of new diaphragm
Frequency response magnitude - dB
45
40
35
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25
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15
10
1
log Frequency - KHz
Figure 6 Frequency-time response of new diaphragm
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