Bauhaus-Universität Weimar

Titel:
Collected Papers On Acoustics
Person:
Sabine, Wallace Clement
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit39364/178/
THEATRE ACOUSTICS 169 
ference system of the stage and cupola by the rest of the auditorium 
would be very slight. 
The interference system on the stage in question being deter¬ 
mined wholly by the floor and cupola, it may be computed, and in 
the preparation of the chart was computed, by the so-called method 
of images. The sound reflected from the floor comes as from a virtual 
image as far beneath the floor as the mouth of the speaker is above 
it. Each of these produce real images by reflection from the interior 
of the cupola. Bearing in mind that these real images show the 
phenomenon of diffraction and some astigmatism, and taking into 
account the phase of the sound as determined by reflection and by 
distance, the calculation is laborious but not difficult. It involves 
but the most familiar processes of geometrical optics. 
The disturbing effect of this interference system is not so great 
when the speaker is well in front of the center of curvature of the 
cupola, and of course it is almost always more or less broken by the 
stage properties, as indicated in Figs. 3 and 4. Nevertheless, it is 
well to bear in mind that the quarter sphere form, as indicated in the 
diagrams, is neither necessary from the standpoint of illumination 
nor desirable from the standpoint of acoustics. Acoustically a flatter 
back with sharper curvature above and at the sides is preferable. 
It should be repeated that the interference system is established 
only when the tones are sustained, in this case over one-tenth of a 
second, and is more of an annoyance to the actor on the stage than 
to the audience. With shorter tones it becomes an echo, and in this 
form is quite as annoying to the audience as to the actor. It should 
be added that the interference changes with change of pitch, but 
preserves extreme maxima and minima for a central position in a 
spherical or partly spherical surface. Finally in music, since sus¬ 
tained tones occur more than in speech, the interference is more dis¬ 
turbing. The effect of such spherical stage recesses on music is 
shown by those otherwise unusually excellent auditoriums, Orches¬ 
tra Hall in Chicago, and the Concert Hall at Willow Grove Park 
near Philadelphia.
        

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