Journal of the Society of Motion Picture Engineers (1930-1949)

Fig. 1. The Waldbiihne Berlin Open-Air Sound Theater method is the only one that can be used satisfactorily to reproduce sound films for large audiences. The method used takes advantage of the ability of human eyes and ears to perceive optical and acoustic phenomena belonging together as synchronous when the time-lag between them is not too great. According to experience the time-lag must not exceed 0.1 sec*; whether the picture or the sound precedes is unimportant. Therefore, if at A (Fig. 2) there is perfect synchronization, then virtual synchronization will occur for all viewers seated within a range of 112 ft (i.e. 0.1 sec) each side of A. Hence the radius of virtual synchronization is 224 ft. Since the value of 0.1 sec may not be exceeded for the reasons stated above, the synchronization area of 224 ft must be taken as the limit beyond which no satisfactory * The normal limit used in synchronization practice. reproduction of sound films is possible. Therefore, the use of an open-air theater for sound will always be limited to a linear distance of 1 72 ft either side of the calculated line of synchronization. Satisfactory vision in this area is dependent solely on the size and illumination of the screen and, therefore, on the light output of the projector. The vertical section of Waldbiihne Berlin (Fig. 2) shows that the distance of A from the screen corresponds to 0.25 sec (sound path = 276 ft). To obtain synchronization at A the sound produced by the loudspeaker must precede the picture by 0.25 sec. This may be accomplished by using a special copy of the film or by altering the length of the film loop between the film gate and the sound head of the projector. Since this is a problem of construction it will not be discussed further. In actual practice, this method of preceding picture by sound produced Hans Simon: Open-Air Sound Theater 513