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592 RICHARDSON'S BLUEBOOK OF PROJECTION
containing no coils at all. They use a crystal of Rochelle Salt (see Page 526) which twists when current is applied to it. The twisting motion of the crystal occurs at the frequency of the current causing it, and is proportional to the strength of that current. They need no field excitation, having no field coils, and being of very high impedance they do not disturb the impedance match between the low impedance speech coils of the larger speakers and a low impedance output transformer, when they are connected directly across the transmission line.
The arrangement shown in Figure 144 is not the only one used to reproduce a wide band of frequencies. Some manufacturers use a single cone-type dynamic speaker so designed that it is capable of reproducing a very wide range of sound. Still another speaker arrangement is pictured in Figure 152.
(13) The transmission line between the amplifier output and the loud speakers is commonly arranged to allow for a variable number of speakers, according to the shape and other requirements of the auditorium. In addition, provision is often made for separate volume control for each individual speaker, which assists in securing the best possible distribution.
Amplifier output transformers are often equipped with tapped secondary coils. By making connection to different taps along a coil of this kind impedance match can be secured to any reasonable number of speaker units. A somewhat different arrangement is pictured in Figure 143. The top panel in that picture (the one with the two rows of tap switches) is a "horn control panel," which must not be confused with a "horn field control panel" (Page 588). The panel in Figure 143 is in the horn speech circuit. It contains an autotransformer, the secondary of which is provided with seven sets of taps. One speaker is connected to each set of taps. The panel thus provides individual volume control for each speaker (by adjusting its tap switch), while overall impedance match between the amplifier output transformer and any number of speakers up to seven (including monitor)