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SPEAKERS 593
is obtained by choosing a correct combination of switch settings.
A suitable combination is found with the help of the following code :
Dial Setting Impedance Index Olunic Resistance
0 1. 15.
2 .63 9.45
4 .40 6.
6 .25 3.75
8 .16 2.40
10 .10 1.5
12 .06 .9
14 .04 .6
16 .03 .45
18 .02 .3
20 .01 .15
Disregarding the "ohmic resistance" column, which is added merely for information, the code is operated by adding up the impedance indices for all the dial settings chosen. Perfect impedance match is secured when the impedance indices total 1, but any total between .75 and 1.25 will give reasonably satisfactory results. Thus, if there are four speaker units backstage, they may all be set at "6," which with "10" for the monitor would give 1.10, a satisfactory figure. However, if considerations of correct distribution require two of the stage speakers to be operated at higher volume than the other two, a setting of 4-4-10-10-, with anything .less than 6 for the monitor, will also serve. A very wide combination of settings, capable of meeting any distribution problem without destroying impedance match, is offered by this panel.
Most speaker transmission lines, however, have much less complicated arrangements, except in the case of such speaker systems as are shown in Figure 144.
With the set-up of Figure 144 the transmission line includes frequency band separators, partly to provide individual volume control of each band of frequencies, and partly to avoid "peak" volume at those frequencies where the speaker ranges overlap. The control of frequency response thus obtainable is very similar in its 'results, although not in the method followed, to the control given by condensers C-2, C-4 and C-7 in Figure