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590 RICHARDSON'S BLUEBOOK OF PROJECTION
quency, the greater the area of the baffle must be. Three cone-type, low-frequency speakers are shown mounted against the bottom of a flat baffle in Figure 144.
Increasing Frequency Range
Figure 144 is a picture arranged to explain one type of multiple speaker installation designed to cover an unusually wide range of frequencies. The large horns at the top of that picture are capable, when used alone, of providing sound of all frequencies between about 60 and 6,000 cycles. This represents the highest standard of theatre reproduction obtainable a few years ago. The white dashed line on the right-hand speaker shows the axis or center of the sound beam emitted. The fact that the center of the sound beam is not the center of the horn must be taken into consideration when the position of this horn is adjusted to secure the best distribution of sound in the theatre. The dynamic unit is shown mounted, face downward,, to the neck of the right-hand horn.
(10) When used as in Figure 144 the large horns are wired (see Page 593) to receive and reproduce only frequencies between 300 and 3,000 cycles. Sounds ranging from 50 to 300 cycles are reproduced by the three low-frequency speakers, called "woofers," which are seen at the bottom of the baffle. The stage itself forms a continuation of the baffle.
(11) Near the top of the baffle board are the two high-frequency speakers, called "tweeters," which reproduce the range between 3,000 and 8,000 cycles. There is less than a foot of baffle above the tweeters. That is enough for them, although it would not be nearly enough for the woofers. The area of baffling required grows less as the frequency becomes higher.
The tweeters shown in Figure 144 are small dynamic speakers equipped with short exponential horns. (The word exponential refers to a mathematical formula for horn design.)
(12) Tweeters of similar appearance are sometimes of the crystal type, neither magnetic nor dynamic, and