Frequency Response - Thiel CS6 Technical Information

FREQUENCY RESPONSE

Since frequency response errors are a measure of tonal imbalances which alter music's tonal characteristics, we believe that accurate
frequency response is an absolute requirement for a truly good speaker. Our design goal for the CS6 was to achieve accuracy in the
design prototype of 1 dB and a production tolerance of 1 dB. The result is a tolerance in every production speaker of 2 dB and a
tolerance from speaker to speaker of 2 dB at all frequencies.
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, 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
perceived tonal balance. Our design goal was to achieve octave-averaged response within 0.5 dB from 50 Hz to 15 KHz. Any deviation
more than .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 very uniform responses, reduction of usual cabinet diffraction which causes
response errors, and compensation of driver response anomalies in the electrical network.
Driver response
The major cause of nonuniform driver response is diaphragm resonances. These
resonances are also the major energy storage mechanism. In the CS6 all three driver
diaphragms are constructed of anodized aluminum which provides much higher stiffness and
compressive strength than conventional diaphragm materials. The primary benefit is that the
lowest internal resonance is much higher than with other materials. Below this lowest
resonance there are no resonances to store energy and cause ringing. An additional benefit is
that the aluminum's higher compressive strength results in more of the energy of a transient
attack being transferred to sonic output rather than being absorbed in compression of the
diaphragm material. In the case of the CS6's tweeter the lowest diaphragm resonance occurs
above the range of hearing at 22 KHz. The lowest diaphragm resonances for the other
drivers are 3 KHz, and 6 KHz, putting them 2.5 and 1 octave above their respective
crossover points of 500 Hz and 3 KHz. So, in every case, each driver has no internal
resonances in its operating range to cause response irregularities and colorations of the
speaker's tonal response.
Figure 1 shows the frequency response (in an infinite baffle) of the CS6's drivers. You
will notice that in each case the response is virtually perfect below the primary diaphragm
resonance.
Diffraction
Diffraction causes frequency response and time response errors and
therefore a reduction in tonal, spatial, and transient fidelity. Diffraction
occurs when some of the energy radiated by the drivers is reradiated at a
later time from cabinet edges or other sudden change of environment. For
musical signals that remain constant for a few milliseconds, diffraction
causes, by constructive and destructive interference, an excess of energy to
Cabinet-edge diffraction
tweeter
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the listener at some frequencies and a
deficient amount of energy to the
listener at other frequencies.
Diffraction also causes all transient
signals to be radiated to the listener a
second (and possibly a third) time,
smearing transient impact and
distorting spatial cues.
To greatly reduce diffraction the
CS6 employs a baffle that is curved
at the edges so energy radiated along
the baffle can continue into the room
without encountering abrupt edges.
Figure 2 compares the response of
the tweeter and mid drivers in a
conventional square-edged cabinet and in the CS6's cabinet with the target response. It can be seen
that response imperfections are reduced by approximately 75% in the mid driver's bandpass and in
the response of the tweeter below 6 KHz.
Coaxially mounting the tweeter and mid driver would normally be another source of
diffraction of the tweeter's energy. The upper diagram on page 4 shows a tweeter mounted in a
normally shaped mid diaphragm. The conical shape of the tweeter's environment causes response
irregularities shown in the upper graph of Fig. 3. To solve this response problem the CS6 uses a
Figure 1
Woofer, midrange and tweeter driver responses
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Figure 2 Target response, response with rounded-edge cabinet and
response with square edge cabinet for tweeter (top) and mid drivers.
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