In practice, the proper execution of a first-order system requires very high quality, wide
bandwidth drivers and that the impedance and response variations of the drivers and the
cabinet be compensated across a wide range of frequencies. This task is complex since what is
necessary is that the acoustic driver outputs roll off at 6dB/octave and not simply for the
networks themselves to roll off at 6dB/octave. For example, if a typical tweeter with a low
frequency roll-off of 12dB/octave is combined with a 6dB/octave network, the resulting
acoustical output will roll off at 18dB/octave. Therefore, in practice, the required network
circuits are much more complex than might be thought.
The result of phase coherence (in conjunction with time coherence) is that all waveforms
will be reproduced without major alterations. The speaker’s reproduction of a step waveform
best demonstrates this fact since, like musical waveforms, a step is made up of many
frequencies which have precise amplitude and phase relationships. For a step signal to be
accurately reproduced, phase, time and amplitude response must all be accurate. Because this
waveform is so valuable, it is commonly used
to evaluate the performance of electronic
components. It is not typically used for speaker
evaluation because most speakers are not able
to reproduce it recognizably. That THIEL
speakers reproduce the step so recognizably is
the result of accurate phase, time and
amplitude response. Figure 5 shows the step
response of the CS.5.
ENERGY STORAGE
Any part of the speaker that absorbs energy will reradiate it later in time in a highly
distorted manner. Although not loud enough to be consciously heard, stored energy causes
significant detrimental effects by obscuring the music’s subtle detail, causing both a reduction in clarity and loss of spatiality. The main
storage mechanisms are the driver diaphragms and cabinet walls, especially the baffle.
To reduce cabinet wall vibration the CS.5 speaker utilizes cabinet walls constructed
with very thick, 1" MDF. Also, to increase the mechanical rigidity and therefore reduce
unwanted vibration, all THIEL drivers incorporate chassis of cast magnesium or aluminum
rather than stamped steel or plastic. The results are shown in Figure 6 where it can be seen
that the output of the speaker falls to -40dB in 1 millisecond and to -20dB in only 400
microseconds. This performance provides very clean reproduction of music’s subtle
information.
4
Time
Output
-
+
Ideal step response
Time corrected fourth order crossover system
First order crossover system
tweeter output
woofer output
combined output
Figure 4
Figure 5 CS.5 step response
Time – msec
0.5
1.0
Output
1.5
2.0
2.5
Figure 5 CS.5 time response
Time – msec
0.5
5
0
-5
-10
-15
-20
-25
-30
-35
1.0
Output — dB
1.5
2.0
2.5
-
-
