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322 CINEMATOGRAPHIC ANNUAL
the average speaker, based upon the measurements outlined in this lecture. The average speech-power of the average speaker in auditoriums of different sizes is obtained from the curve in Fig. 1. Having determined, from Fig. 1, the probable speechpower of the average speaker in a room of a certain size, and assuming that the speaker maintains this power output for different times of reverberation, it is possible to calculate the resulting loudness in an auditorium of any size or time of reverberation. A typical set of calculations for an auditorium is given in Table I. The values of k! and kr are determined from
TABLE I.
Volume of auditorium = 11,330 cubic meters meters (400,000
cubic feet) . Average speech-power (from Fig. 9) = 54 microwatts.
Time of
Average
Average
Rever
Speech In
Loudness
kx
fer
k\kr
beration
tensity
in db
.50
.665 x 104
38.2
.850
1.00
.850
.75
1.00
40.0
.874
.993
.868
1.00
1.33
41.2
.885
.982
.870
1.25
1.66
42.2
.894
.967
.865
1.50
2.00
43.0
.900
.953
.858
2.00
2.66
44.3
.910
.924
.840
3.00
4.0
46.0
.925
.837
.775
4.00
5.3
47.3
.936
.752
.704
6.00
8.0
49.0
.950
.600
.750
8.00
10.6
50.3
.595
.510
.489
the curves in Figs. 3 and 5, respectively. It is assumed that the auditoriums are of the typical rectangular shape, so that ks=1.0. It also is assumed that the rooms are relatively free from disturbing noise, so that the residual noise is only one-tenth as loud as the speech, and therefore kn will be .96. Equation (1) then becomes:
Percentage Articulation = .922 ki kr. The ordinates of the curves in Fig. 8 were calculated by the use of this equation and a series of tables like Table I.
It is obvious that, for an auditorium of a certain size, the optimal time of reverberation will be the time for which the