146. General Requirements.-The last step in reproducing the recorded sound of a disc or film record on
the Photophone reproducing equipments is by means of the loudspeaker. One or more of these units are
mounted on the stage of the theatre, behind the picture screen, and are so located as to direct the sound
produced by them through the screen to all parts of the theatre auditorium. Another loudspeaker,
known as the monitor, is located in the projection room, and is operated whenever the equipment is
being used, so that cues may be followed by the projectionist.
All Photophone reproducers use the same type of loudspeaker, both for stage units and for monitors in
the projection rooms. Those on the stage are used in conjunction with a directional baffle, discussed
later in this chapter.
A loudspeaker must be capable of setting up vibrations in the surrounding air which will range in
frequency from about fifty cycles to six or seven thousand cycles per second if natural reproduction is
to be obtained. The air can only be set into vibration if some solid object is moved in it, thus causing
these movements to be transmitted to the air. Therefore, the f unction of a reproducer is to set a body
which is in contact with the surrounding air into rapid vibration so that its vibrations correspond at all
times to the frequency of the original sound, as reproduced electrically by the amplifier. Since it is
impracticable to set a heavy object into vibration over the necessary wide range of frequencies, the
vibrating element of the reproducer must be light in weight so that it follows faithfully all variations of
the electrical impulses from the amplifier.
To produce the desired volume of sound, the vibrating mechanism must have a large surface, because
the distance through which it moves is limited, and in addition, the driving mechanism for the
reproducers must be capable of changing the electrical sound current into mechanical force without
introducing an appreciable variation or distortion. That is, the driving force produced must be many
times greater than the resisting forces tending to hold the movable parts in place.
The ideal mechanism for a faithful reproducer (loudspeaker) will consist of a very light weight piston
which moves as a whole. The action of such a device is described in Chapter I. It is not practical to
construct a mechanism as shown in Figure I which would be sufficiently light and stiff, but a cone
shaped contrivance, as used in loudspeakers, answers the purpose admirably because it can be made
both light and rigid.