How to add sound to amateur films (1954)

The same principles apply with films. If you place the loudspeaker above or below the screen, the sound is readily supposed to come from the picture itself. When the speaker is at one side the effect is not so good. An exception arises if you use two loudspeakers, one either side of the screen. This is an eminently satisfactory arrangement, provided you take care to phase them correctly. Incorrect phasing means that one speaker cone moves in as the other moves out. In consequence the sound is very unevenly distributed because in many parts of the room one speaker is trying to cancel the other. When this happens you can put matters right simply by reversing the leads to one speaker. Acoustics The acoustics of the room used determine largely whether you should place the speaker below the screen or above it. When the speaker is below the screen, people in the front row get the full output at close quarters and prevent much of the sound from reaching those at the back. Consequently people at the back may have difficulty in hearing whereas those at the front are nearly deafened. To achieve a more uniform sound distribution, mount the speaker above the screen. The sound then passes over the front rows to reach the back of the audience. When you use this arrangement, incline the speaker very slightly. In this way you can usually get a good proportion of sound to the back without troublesome reflections off beams protruding from the ceiling. When the ceiling is low, however, any such beams are almost certain to produce echoes which will impair the intelligibility of speech. In that case try alternative speaker positions. If these provide no cure, you may have to arrange drapes in front of, or below the beams. Unfortunately, experiments made in an empty hall are a poor indication of the results you will obtain when the audience is seated. Not only does an audience deaden the 148