Results
The end result of reducing diffraction, reducing diaphragm resonances and correcting
response anomalies in the network is a speaker with very accurate tonal characteristics.
Figure 1 shows the on-axis frequency response of the CS.5. It is uniform within
±
2dB
from 23Hz to 17KHz. Subjectively, even more important is the octave-averaged
frequency response. Figure 2 shows this response to be within
±
1dB from 100Hz to
10KHz and within
±
0.5dB from 200Hz to 3KHz. These measurements indicate very
accurate overall tonal balance. Furthermore, as a result of gradual crossover slopes, the
off-axis frequency response of the speaker system is also smooth and well balanced. This
unusual performance is important for producing a uniform amount of ambient energy at
all frequencies, necessary for natural spatial reproduction. Figure 3 shows this octave-
averaged, 30
°
off-axis response to be within
±
1.5dB from 70Hz to 10KHz, showing very
uniform dispersion of energy at all frequencies.
TIME RESPONSE
In most loudspeakers the sound from each driver reaches the listener at different
times causing the loss of much spatial information. One problem caused by different
arrival times from each driver is that the
only remaining dependable locational clue
is the relative loudness of each speaker.
Relying only on loudness information
causes the sound stage to exist only between
the speakers. In contrast to this loudness
type of imaging information, the ear–brain
interprets real life sounds by using timing information to locate the position of a sound. The
ear perceives a natural sound as coming from the left mainly because the left ear hears it first.
That it may also sound louder to the left ear is of secondary importance.
Another problem is that for realistic reproduction, it is important that the attack, or start, of
every sound be clearly focused in time. Because more than one driver is involved in the
reproduction of the several harmonics of any single sound, the drivers must be heard in unison
to preserve the structure of the sound. Since, in most speakers, the tweeter is closer to the
listener’s ear, the initial attack of the upper harmonics arrives a substantial part of a
millisecond before the body of the sound. This delay results in a noticeable reduction in the
realism of the reproduced sound.
To eliminate both these problems the CS.5’s drivers are mounted on a sloped baffle to
position them so the sound from each reaches the listener at the same time. The sloping baffle
arrangement can work perfectly for only one listening position. However, because the drivers
are positioned in a vertical line the error introduced by a listener to the side of the speaker is
very small. Also, the error introduced by changes in listener height are small within the range
of normal seated listening heights provided the listener is 8 feet or more from the speakers.
PHASE RESPONSE
We use the trade mark Coherent Source to describe the unusual technical performance of time and phase coherence which gives
THIEL products the unusual ability to accurately reproduce musical waveforms.
Usually, phase shifts are introduced by the crossover slopes, which change the musical waveform and result in the loss of spatial and
transient information. The fourth-order Linkwitz-Riley crossover is commonly used in high performance speakers and is sometimes
promoted as being phase coherent. What is actually meant is that the two drivers are in phase with each other through the crossover region.
However, in the crossover region neither driver is in phase with the input signal nor with the drivers’ output at other frequencies; there is a
complete 360
°
phase rotation at each crossover point.
Since 1978 THIEL has employed first-order (6dB/octave) crossover systems in all our Coherent Source speaker systems. A first-order
system is the only type that can achieve perfect phase coherence, no time smear, uniform frequency response, and uniform power response.
A first-order system achieves its perfect (in principle) results by keeping the phase shift of each roll-off less than 90
°
so that it can be
canceled by the roll-off of the other driver that has an identical phase shift in the opposite direction. (Phase shifts greater than 90
°
cannot be
canceled.) The phase shift is kept low by using very gradual (6dB/octave) roll-off slopes which produce a phase lag of 45
°
for the low
frequency driver and a phase lead of 45
°
for the high frequency driver at the crossover point. Because the phase shift of each driver is much
less than 90
°
and is equal and opposite, their outputs combine to produce a system output with no phase shift and perfect transient response.
Figure 4 graphically demonstrates how the outputs of each driver in a two-way speaker system combine to produce the system’s output
to a step input. The first graph shows the ideal output. The second shows the operation of a time-corrected, fourth-order crossover system.
The two drivers produce their output in the same polarity and both drivers start responding at the same time. However, since the high-slope
network produces a large amount of phase shift, the tweeter’s output falls too quickly and the woofer’s output increases too gradually.
Therefore, the two outputs do not combine to produce the input step signal well but instead greatly alter the waveform. The third graph
shows how, in a first-order crossover system, the outputs of the two drivers combine to reproduce the input waveform without alteration.
3
10K
Frequency
1K
25
20
15
10
5
0
-5
-10
100
20
20K
Amplitude — dB
10K
Frequency
1K
25
20
15
10
5
0
-5
-10
100
20
20K
Amplitude — dB
10K
Frequency
1K
25
20
15
10
5
0
-5
-10
100
20
20K
Amplitude — dB
Figure 3 30
°
off axis octave-averaged frequency response
Figure 2 On-axis octave-averaged frequency response
Figure 1 On-axis frequency response
Time correction
