2
DESIGN AND ENGINEERING FEATURES
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. 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 0.2dB 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 for the CS.5 was to achieve
octave-averaged response within
±
1dB from 100Hz up to 10KHz with even tighter tolerance within the midrange from 200Hz to 3KHz.
Therefore, any deviation more than these limits 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, drivers with high consistency (so that few units need be
rejected), reduction of usual cabinet diffraction which causes response errors, and an unusual degree of 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. All THIEL tweeter diaphragms are constructed of 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 much
higher compressive strength results in almost all the energy of a transient attack being transferred to sonic output rather than being
absorbed in compression of the diaphragm material. In the tweeters the lowest diaphragm resonance occurs above the range of hearing at
26KHz. Therefore, there are no resonances in the audible range to cause energy storage or response irregularities.
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 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 reduce diffraction the CS.5 employs a grille board that fits around (rather than on) the baffle
and one that is curved at the edges so energy radiated along the baffle can continue into the room
without encountering abrupt cabinet edges.
Off-axis response
In addition to on-axis response accuracy, it is also important that the off-axis response be even,
without major dips, for two reasons. First, listeners may be located far from the optimum position
and therefore will be hearing the speaker as it performs off-axis. Secondly, off-axis response is an
indication of the uniformity of the speaker’s total energy response. Since the total energy (in all
directions) radiated from the loudspeaker determines the amount of reverberant energy in the room,
it is important that the off-axis response be uniform to
avoid changes in perceived character and spatiality at
different frequencies.
Most speakers with high-slope crossover systems cannot maintain uniform off-axis
response because the dispersion of a driver narrows as frequency increases toward the
crossover frequency. Above the crossover frequency the radiation of the next driver is again
wide since it is operating at the low end of its range. First-order crossover systems have an
advantage in this regard. Since a significant part of the total energy below the crossover point
is radiated by the upper driver, the narrowing of the dispersion of the lower driver has much
less effect on the total output. Speakers with first-order crossover systems therefore can
achieve a more uniform off-axis response.
Cabinet-edge diffraction
tweeter
First-order system off-axis
High-slope system off-axis
First-order system on-axis
High-slope system on-axis
