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TIME RESPONSE
In most loudspeakers the sound from each driver reaches the listener at different times causing the loss of much spatial information.
Therefore, with most speakers the only dependable locational clue is the relative loudness of each speaker which 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
Time correction
PHASE RESPONSE
We use the trade mark Coherent Source to describe the unusual technical performance of
our products. This phrase is descriptive of the 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 changes the musical
waveform and results in the loss of spatial and transient information. During the past decade
the fourth order Linkwitz-Riley crossover has risen in popularity and it 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, 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.
There is a type of crossover system that does not introduce any phase shift or time smear,
although it is difficult and expensive to execute. This crossover is the first order (6dB/octave)
system that THIEL has employed since 1978 in all our Coherent Source 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 results by keeping the phase shift of each roll-off
to 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. 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 15 graphically demonstrates how the outputs of each driver in a two-way speaker
system combine to produce the system's output. 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 quickly and the woofer's output increases only 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.
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.
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 as much as two milliseconds before the body of the sound. This
delay results in a noticeable reduction in
the realism of the reproduced sound.
To eliminate this problem the CS3.6
mounts the drivers on a sloped baffle to
position them so the sound from each
reaches the listener at the same time.
This 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, because the driver spacing is small compared to the wavelength at the crossover
frequency, 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.
Figure 14 shows the group delay, the measure of time error, of the CS3.6 from 200 Hz to
20 KHz. For all frequencies above 300 Hz the delay is less than 0.5 ms. The general trend is
toward zero delay at higher frequencies.
Figure 14 Time response
Transfer Function Group Delay - msecs
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
1
Frequency - KHz
Figure 15
-
Time
Ideal step response
Time corrected fourth order crossover system
First order crossover system
10
tweeter output
woofer output
combined output
4
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