All monitors are mounted about six feet from the floor of the projection room and usually so they are best heard in the space between the projectors. The ideal location is on the end wall to the left of the projectionist when facing the front wall.

152. Phasing of Loudspeakers.-It has been mentioned that the coil moves in a certain direction when both the field current and voice coil current are in given directions. It can therefore be seen that when more than one stage speaker is used, they must be matched so that all field coils are polarized the same and all voice coils are "in phase." This means that their connections must be made so that at any instant the current in all the voice coils is in the same direction. If any field or voice coil connections were reversed, so that two speakers were not in phase, one cone would be moving in while the other moved out when reacting to one particular current impulse. This would produce interference at the line of intersection of the sound beams from the two speakers, and a "dead spot" would result. The speakers have been properly phased at the time of installation, and it is important that none of the connections are ever reversed.

(B) DIRECTIONAL BAFFLES

153. Construction.- All dynamic speakers, with the exception of projection booth monitors, are used in conjunction with a directional baffle, which is a large funnel-shaped wooden form. This unit is six feet long, constructed of soft pine onequarter of an inch thick. The small end, where the dynamic speaker unit is fastened to it, is ten inches square, and this expands until the other end is about three by five feet. Figure 102 shows the type of construction and shape of this unit, as compared in size to the dynamic loudspeaker. Acoustic conditions in the average theatre are such that conditions are best met by the use of such a sound directing device.

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