TWO-WAY SPEAKER SYSTEMS
Two-way speaker systems have a major inherent limitation due to the fact that one driver is used to reproduce both the bass and the
midrange part of the spectrum. If this driver is made small enough to have upper midrange reproduction free from resonances, colorations
and narrow dispersion, it suffers from inadequate size to move the volume of air needed for good deep bass reproduction. Conversely, if the
driver is made large enough to reproduce bass well, it suffers from poor midrange performance.
There are examples of both of these either/or approaches: two-way systems that provide, by the use of a small driver (5" or less), very
good upper midrange performance but poor bass performance and two-way systems that provide, by the use of a large driver (8" or more),
good bass performance but poor upper-mid performance. However, most high-quality two-way systems (including THIEL's) use a balanced
approach of using an intermediate size driver. These systems can often provide good, but not outstanding, performance in both the bass and
upper midrange areas.
The design goals of the CS1.5 included, in addition to the above mentioned goals, significant reduction of the inherent two-way
limitations. In other words, we wanted to achieve significantly improved bass performance simultaneously with markedly better upper
midrange reproduction. Achieving these goals required the development of a new low frequency driver with improved bass and upper
midrange performance, two characteristics that tend to be mutually exclusive. The concept developed for the new woofer was to utilize a
very unusual short coil/long gap motor system in conjunction with an aluminum diaphragm.
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 was to achieve accuracy in the design prototype of
±
2 dB with a production tolerance of
±
1 dB. The result is a tolerance in every production speaker of
±
3 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
±
1 dB from 60 Hz to 10 KHz. Any deviation
more than 1 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, 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 non-uniform driver response is diaphragm resonances. These
resonances are also the major energy storage mechanism.
In the case of the CS1.5 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 CS1.5’s woofer also employs a metal diaphragm rather than the usual plastic
or paper. The aluminum material provides much higher stiffness and compressive
strength than conventional diaphragm materials. The primary benefit provided is that
the lowest resonance is at 7 KHz, more than an octave above the crossover frequency.
Below 7 KHz 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.
Even though the diaphragm resonance at 7 KHz is more than an octave above the
crossover, its ringing would cause a sonic effect if not corrected. We have therefore
incorporated into the electrical network a notch filter which complements and cancels
the resonance. Figure 1 shows the decay spectra of the woofer. The peak with its
consequent ringing at 7 KHz can be clearly identified. Figure 2 shows the decay
spectra with resonance compensation. Although some trace of the resonance is still
discernible, it has been reduced to a minor level.
A contributing factor in achieving the very high diaphragm resonance of 7 KHz is
the very short voice coil. Usual long coils result in a relatively heavy moving system
which, while not a limitation for woofer-only applications, is a problem in a driver
required to have good upper midrange performance. The longer (and therefore heavier)
the coil is made to improve excursion and bass output ability, the more difficult it is to
achieve good upper-mid performance since the heavier coil excites more strongly, and lowers the frequency of, the moving system
(diaphragm) resonances. Utilizing a short, and therefore light, coil allows the CS1.5’s woofer to achieve a wider bandwidth and therefore
higher quality mid-frequency performance.
2
Figure 1 Time response of woofer without compensation
Figure 2 Time response of woofer with compensation
