These speakers are mounted in a rack directly behind the screen and as close to it as is possible without actually touching it. All parts of the rack surrounding the reproducer unit and the baffle itself are covered with sound absorbing material. This material is also used to cover the back of the screen, except where the speaker openings are located. This effectively deadens all sound which might escape back stage, reflect off hard walls and thus through the proscenium opening to the auditorium. This makes sure that the only outlet for the sound is through the portions of the porous screen adjacent to the mouths of the speakers.

Figure 103 shows the housing on the baffle in which the reproducer unit is mounted. The end of this box is removable to give access for servicing. A filter unit is placed here, and is connected across the voice coil. Its purpose is to eliminate undesirable frequencies introduced by needle scratch, etc. The Type PL-22 loudspeaker, used with the PG-13 equipments, is illustrated.

154. Location.- Anywhere from one to four speakers may be used in a theatre, and in some cases, more. The number and arrangement to be used in each particular case is worked out beforehand from the plans of the theatre in order to obtain the best distribution of sound. The speaker unit itself has an effective sound distribution over an area 30 degrees on each side of the speaker axis horizontally, and 15 degrees each side of the axis vertically. When one speaker is used it should be placed at the center, or slightly above the center, of the screen and close to it, as mentioned before. The location of the loudspeakers directly behind the screen gives the best illusion; that is, the apparent effect of speech is such that the actors in the picture actually seem to be speaking from the stage.

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