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102
SOUND MOTION PICTURES
sound. Such a room is acoustically "dead" and undesirable. A little reverberation is necessary to satisfy our established auditory taste and habit. The desired amount of reverberation is found empirically to increase with the size of the auditorium.
Because of the pioneer work of Sabine it is possible to define the "reverberation time" of a room (perhaps somewhat arbitrarily and artificially) as the time required by a sound of specified intensity to die away to inaudibility. This standard intensity is a sound ordinarily painful to a normal ear at close range, and is difficult of reproduction. Fortunately, its use is not necessary in ordinary practice, for since Sabine's day the "reverberation time" of a room is a matter of calculation rather than experiment. The method of making the calculation I will explain later.
Experience with a number of existing auditoriums of acceptable acoustic quality makes possible the formulation of the following table, in which the acceptable limits of the standard reverberation time are expressed for rooms of different volume.
Table I
ACCEPTABLE LIMITS
ACCEPTABLE LIMITS
VOLUME OF
ROOM
IN
OF
REVERBERATION TIME IN SECONDS
VOLUME OF
ROOM
IN
OF
REVERBERATION TIME
IN SECONDS
CUBIC
CUBIC FEET
FEET
Half Audience
Maximum Audience
Half Audience
Maximum Audience
10,000
O.9-I.2
O.6-O.8
400,000
2.1-2.3
I.7-2.O
25,000
I.0-I.3
0.8-I. I
600,000
2.3-2.6
1.8-2.2
50,000 100,000 200,000
1. 2-1. 5
1. 5-1.8
1.8-2.0
O.9-I.3 1. 2-1. 5 1 .4-I.7
800,000 1,000,000
2.5-2.8 2.6-2.9
I.9-2.3 2.1-2.5