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406 CINEMATOGRAPHIC ANNUAL
switch. As each power amplifier is independent even to having its own speakers, on all installations of theatres seating over 750 all equipment is duplicated. A rack having two voltage amplifiers and two power amplifiers is shown in Fig. -4.
The loudspeaker used is the electro-dynamic cone. It consists of a parchment cone with a small coil affixed to its apex, which is slipped loosely around the core of a cylindrical electromagnet excited by a direct current from its own power amplifier. When the signal current passes through the small coil, its magnetic reaction with the electro-magnet vibrates the parchment in synchronism with the signal current. This vibratory motion acts on a column of air and becomes sound. The cones are mounted on baffles aiding the reproduction of the lower frequencies. In reverberant houses these baffles are made directional.
Given a good commericial reproducing equipment, perfectly adjusted, and a theatre with good acoustics, we still have the variables of film condition and projection. The crackling noises we hear from a film record are known as ground noise. Some of it is recorded on the film, having been picked up when the record was taken or some added by faulty amplifiers. Much of it, however, is caused by dirt on the sound track in the form of small specks. This can be eliminated by cleaning the film. Additional noises may be added by improper patching. Whenever a patch is made it should be painted as shown in Fig. 5, so that the change in light entering the photoelectric cell is gradual, thereby causing little or no sound. With sound the changeover from reel to reel becomes very important so that no dialogue is lost. It is becoming practice in release prints to have a scene at the end and beginning of each reel in which no dialogue occurs so changeovers can easily be made without danger of losing dialogue. No sound feature should be shown without being rehearsed so that it may be checked for dirt, splices, changeovers, and volume. Many people differ on what volume should be, but in general it is agreed that the volume should be such that the persons on the screen speak at a level which gives the audience the illusion that the sound is coming from the action on the screen. For instance, we see a closeup of an actor speaking and, if the sound is too weak, there is no illusion because the sound seems to be coming from a point far behind the screen. Conversely, if the voice of a person back