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March, 1930] SURFACE TREATMENT OF SOUND
287
In Table VII the relative noise levels in decibels of each sample is expressed in the terms of the volume level of the constant frequency. This table shows for each case how much additional amplification is required for the sound volume to equal that of the constant frequency record. The constant frequency record was of variable area type and of about 80 per cent modulation which of course is higher than the average modulation in a record of music or speech. On the other hand,
TABLE VII
Volume Level in Decibels Number of Runs
Sample
0
10
20
30
40
50
82
130
91
-25.2
-23.5
-21.5
-21.1
-20.5
-20.4
-19.9
-19.2
92
-24.3
-23.0
-22.0
-21.2
-19.6
-16.6
-14.4
-12.9
93
-23.5
-21.1
-20.5
-20.1
-19.8
-19.6
-19.0
-18.4
94
-25.7
-22.2
-18.4
-14.4
-13.2
-12.5
-11.7
-11.1
95
-25.2
-21.0
-19.5
-18.7
-18.1
-17.6
-16.5
-15.5
96
-27.0
-17.6
-15.8
-14.8
-14.2
-13.6
-12.3
-11.0
97
-25.2
-18.3
-16.1
-14.9
-14.1
-13.6
-12.3
-11.3
100
-23.5
-20.2
-17.8
-16.1
-14.9
-14.0
-12.2
-10.5
101
-25.5
-16.9
-15.9
-15.5
-15.3
-15.2
-15.0
-14.8
102
-25.2
-23.2
-22.1
-21.2
-20.7
-20.3
-19.6
-19.0
103
-23.0
-14.6
-13.3
-12.6
-12.2
-11.9
-11.4
-10.9
106
-23.0
-20.4
-16.3
-14.6
-13.7
-13.1
-12.4
-12.0
107
-24.9
-20.6
-19.8
-19.4
-19.2
-19.0
-18.9
-18.6
110
-23.0
-18.7
-14.8
-13.3
-12.6
-12.2
-11.3
-10.5
111
-23.7
-20 .4
-20.0
-20.0
-20.0
-20.0
-20.0
-20.0
112
-21.9
-18.5
-15.5
-13.9
-13.1
-12.6
-11.7
-11.2
CF
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
the noise level of the best samples is twenty-one decibels down after eighty runs leaving a good margin between signal level and noise level. The noise level of the untreated film, however, is only ten decibels down after eighty runs, in which case the noise would be very objectionable. The values of Table VII are shown graphically in Fig. 4. It is well to emphasize at this point that in actual practice the ratio of signal level to noise level would be considerably greater than that shown in this table. The measuring system used responds to a much greater range of frequencies than any actual sound reproducing system. These measurements therefore include a large amount of noise which would not be reproduced. Also, since these measureicnts were made on clear film, the noise level is somewhat enhanced.