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Time coherent topology
An important design goal for the SCS3 which is not shared
with our floor-standing products is placement flexibility. The
requirement that the speaker provide time coherence while
placed either vertically or horizontally and at any height
necessitates that the speaker be symmetrical both horizontally
and vertically and, therefore, that it use either coincident or
D'Appolito driver configuration. The SCS3 uses coincident/
coaxial mounting of a very high output 1" tweeter and a high
1
output 6
/
" woofer.
2
Typical Shelf Speaker
Placed on Side
Off-axis Listeners
(top view)
Two-way speaker positioned on its side
causes the sound from the drivers to reach
the off-axis listener at different times.
Coaxial tweeter response
To maintain time coherence the tweeter must be aligned with
the woofer. There is usually a significant problem with the
response of tweeters which are mounted coaxially in a woofer.
Energy from the tweeter is reflected from the woofer's
diaphragm and causes destructive interference.
25
20
15
10
5
0
-5
-10
20
100
Frequency
25
20
15
10
5
0
-5
-10
20
100
Frequency
25
20
15
10
5
0
-5
-10
20
100
Frequency
The SCS3's woofer diaphragm shape has been designed to
greatly reduce negative effects on the tweeter's response as can
be seen in the third graph. By shaping the diaphragm as a short
tube opening into a shallow flare reflections are almost entirely
eliminated. In addition, the woofer's surround is mounted to the
rear of the diaphragm such that the tweeter does not really "see"
it as a diffraction-causing obstruction. Since the shape of the
woofer's diaphragm has been optimized for the tweeter's
response rather than to minimize its own resonances, it would
not exhibit response as good as it otherwise would. To remedy
this problem the woofer diaphragm is reinforced with very light
and stiff molded polystyrene on its back side.
THIEL SCS3
Off-axis Listeners
Coincident driver mounting results in all
listeners hearing the sound from the
drivers at the same time.
graph shows
the response
of the
SCS3's
tweeter in an
infinite
10K
20K
1K
baffle and
the second
shows the
tweeter's
response
when
coaxially
10K
20K
1K
mounted in
a typical
woofer.
Irregularities
can be seen
above 4KHz
caused by
diffraction
1K
10K
20K
effects.
Frequency response
The graphs below show the frequency response of the SCS3.
The upper graph shows the (normal) on-axis response and
illustrates the very high degree of accuracy; no frequency is
under or over emphasized more than 2 dB.
The second graph shows the on-axis, octave-averaged
response. This curve is representative of the speaker's tonal
balance and shows that the SCS3 is very accurately balanced,
any over or under emphasis below 12 KHz is less than 0.5 dB.
The third graph shows the 30 off-axis, octave averaged
response and illustrates that the speaker's overall energy
response is well balanced, with no large depressions in any area
of the spectrum. This high degree of uniformity is in part the
result of the SCS3's first order crossover system.
On-axis, octave averaged, 30 octave averaged
10
5
0
-5
-10
-15
-20
-25
-30
20
Step response
This graph shows the SCS3's response to a step signal.
Notice that the overall triangular shape is very well preserved
This
with the output remaining smoothly positive until 2.6 ms when it
finally crosses zero due to the fact that the bass response extends
to 46 Hz rather than DC. The irregularities seen in the first few
hundred microseconds are due to the tweeter diaphragm
resonance at the ultrasonic frequency of about 23 KHz.
Waveform accuracy this good can only be achieved with first
order crossovers and time coherent driver positioning.
Time response
The energy-time response of the SCS3 shows that the
speaker's output quickly decays to -40 dB in 1.5 milliseconds,
indicating very clean inter-transient silence. Such performance is
the result of metal diaphragms that have no resonances within
their operating frequency range and very strong cabinet
construction.
5
0
-5
-10
-15
-20
-25
-30
-35
100
1K
Frequency
0.5
1.0
1.5
Time – msec
0.5
1.0
1.5
Time – msec
10K
20K
2.0
2.5
2.0
2.5
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