The art of sound pictures (1930)

SOUND TECHNIQUE 239 not easy to note the relative speeds of any particular movement or action. There is one difficulty with reproduction of sound for talking pictures, which a great many people have already noticed and commented upon. As we have previously noted, light and sound travel at different rates of speed. Light travels 186,304 miles a second, and sound, 1,085 feet a second. This means that the rays of light from the screen may reach the eyes of the audience considerably before the air waves of sound from the loud speakers behind the screen reach their ears. It is the same way with a flash of lightning and the thunderclap which follows. You see the light first, and then, a number of seconds later, you hear the sound of thunder which originated at the same time. It is an old rule that you can tell how many miles away a storm is by counting the number of seconds that pass between the flash of lightning and the sound of thunder. If, then, talking pictures are S3mchronized so that the sound and light from the screen reach the main body of the audience simultaneously, there is bound to be some error in the synchronization for people who sit at the sides or extreme back of the auditorium. It is possible, of course, to move the sound track ahead on the film, so that the sounds actually start before the light rays from the pictures to which they correspond. But if this is done, and the pictures are thus accurately synchronized for the people at the back of the auditorium, the synchronization will be wrong for the people in the front rows and middle of the theater. Various devices to correct this difficulty are now being worked on. It is possible that loud speakers may be introduced at