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Acoustic Problems at the "Waldbuhne: Open-Air Sound Theater in Berlin
By HANS SIMON
Acoustic problems arising in connection with the reproduction of sound films in an open-air theater are discussed. The proper arrangement of loudspeakers as well as the careful adjustment of their beam direction is of utmost importance. It is demonstrated that a uniform sound level with a considerable increase in volume can thereby be attained. Furthermore, as a result of the concentration of sound waves, a substantial increase in the frequency band will occur. The amplifier power necessary for the required acoustic output is calculated.
JL HE OPEN-AIR theater known as Waldbiihne Berlin, built about twentyfive years ago, has now been arranged for the reproduction of sound films. Figure 1 shows the screen with the loudspeaker installations mounted on both sides. The front row of seats is about 164 ft and the back row 394 ft from the screen. For these viewing distances a 36 X 26 ft screen was used. Reproduction of sound films had to be satisfactory for the entire audience of 25,000 persons. To meet this requirement it was essential to achieve adequate synchronization of picture and sound and uniform sound level in all parts of the arena.
Synchronization
It is obvious that acoustic quality in an open-air theater is dependent on very
A contribution submitted on September 23, 1952, by Hans Simon, 6 Xantener Str., Berlin W. 15, Germany.
different conditions from those in a closed room. In an open-air theater all the effects due to reverberation are absent and only the laws for linear and unrestricted diffusion of sound waves need be considered. Their velocity of diffusion, however, will be of particular importance. The long distances sound must travel in an open-air theater tend to produce an undesirable effect on the audience due to the time-lag between picture and sound. Consequently, scenes of rhythmic movement such as a dance would deviate from the rhythm of the accompanying music. Similarly, it would be especially disturbing if the sound produced by a singer were not synchronized with lip movements. It is therefore of the first importance that the phase difference between picture and sound be reduced to a minimum. The solution which proved successful for the Waldbiihne Berlin is described as follows.
As far as is now known, the following
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December 1952 Journal of the SMPTE Vol. 59