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These sound measurements were made using continuous tones. The significance of such an increase in the sound level (8 phons) becomes apparent when it is realized that this corresponds to an actual power ratio of 6.3 times. In other words, proper orientation and placement of the speakers resulted in an improvement in acoustic efficiency by a factor of 6.3.
Amplifier Power
Finally, the amplifier installed at the Waldbiihne Berlin may be described. It was considered desirable to create a sound level of 70-75 phons at the back
of the theater. Since it is known that with each 325 ft of sound path loss of sound energy is 5-7 phons, the acoustic power required was calculated to be 8-10 acoustic w. To produce this with the loudspeakers used called for an amplifier power of about 120 electrical w. A potential 1 50-w unit was installed to satisfy special conditions such as the attenuation of sound waves occurring with changes of temperature.
According to the judgment of experts the technical problems arising from the use of the Waldbiihne Berlin as an open-air theater for sound reproduction have been solved quite satisfactorily.
Errata
Raymond Spottiswoode, N. L. Spottiswoode and Charles Smith, "Basic principles of the three-dimensional film," Jour. SMPTE, 59: 249-286, Oct. 1952.
Page 254, column 2, footnote, last line: For: 1 metric p = 10,000/distance in cm. read: Distance in metric p = 10, 000 /distance in cm.
Page 256 : Fig. 2b title, next to last line : For: When tc > te, I — r' > I r read: When // > tci I r' > I r
Page 271, column 2: The equation numbered 11 should be numbered 18.
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December 1952 Journal of the SMPTE Vol. 59