Projection engineering (Sept 1929-Nov 1930)

Page 22 Projection Engineering, June, 1930 Excellence in Auditoriums* Possibility of Obtaining Apparent Loudness Without Overloading Vacuum Tubes and Vibrating Parts of Recording System, by Livening the Recording Stage By W. A. MacNair IN basis, our judgments of the acoustics of auditoriums appear at first thought to be qualitative and diverse rather than quantitative and similar. But we get an indication that this may not be altogether so when we find that there is some approach to unanimity of opinion regarding what auditoriums are "good" and what "bad." When several observers listen to sounds in an auditorium whose acoustic properties are changed from time to time, they will concur to a significant degree upon one acoustical arrangement as "best." Their agreement leads us to look, among the various physical parameters whereby acoustic properties are the sound from a standard 1000-cycle source to decay to the threshold of audibility after the source is cut off. In the range auditorium volumes in which optimum reverberation time varies from one to two, the optimum value of the new parameter only varies from 1.0 to 1.6, effecting considerable progress toward constancy. Recently, however, it has been shown that one-half the product of this time and the "loudness"1 set up by the standard source constitutes a parameter which has a fixed value for all auditoriums acoustically best, whatever their sizes. More exactly, this parameter is the integral of instantaneous loudnesses caused by the standard .0 8 6 .4 © ""*<D v|) 8 6 ® 12 8 2 c « < 3 > VOLUME IN CL BIC FE :t t i 5 i o o o' o o Fig. 1. The optimum rev e r beration time of a room varies with its volume, as shown (for 512 cycles) by the e x perimental work of (1) Watson, (2) Lifshitz, and (3) P. E. Sabine, and by (4) a theoretical calculation. measurable, for some quantitative index corresponding to their aesthetic judgment. "Were such an index discovered, auditoriums could with its help be confidently engineered by physical procedures to meet aesthetic criteria. The first parameter so used was "reverberation time" — the time required for a sound in a room to decay sixty decibels in power level. Several investigators have reported the optimum reverberation times for auditoriums of various sizes ; the plots of their results are shown in Fig. 1. All find that the optimum time increases with the size of the auditorium, extending from about one second for a few thousand cubic feet to two seconds for a million cubic feet. For a true physical criterion of "bestness," however, some quantitative acoustic feature of an auditorium should be used which has the same value for all auditoriums in their best acoustical arrangement. Only then could it be supposed that the physical quantity underlying the aesthetic judgment had been found. A further step toward such a quantity can be made by choosing, instead of the reverberation time, the time taken by source, taken over the period between the time the source is cut off and the time the sound becomes inaudible. Its optimum value is 32.6 for a 1000-cycle tone. The meaning to be attached to this discovery is that, when a sound is cut off, it is the combination of its loudness and the time of its audibility that is sensed. Since the quantitative combination of these two factors has the 1 The sensation level of a sound is defined as its power level in decibels above its poiver level at the threshold of audibility. The loudness of a sound is defined as its sensation level, for tones of 1000 cycles; and, for tones of other frequencies, as the sensation level of a 1000-cycle tone which is judged equally loud. same value for all ideal auditoriums, the aesthetic sense concerned must resemble a ballistic instrument, attentive only to the value of the parameter and indifferent to how the factors combine to attain that value, whether by a loud tone decaying rapidly or a faint tone slowly. Common experience bears out this theory in a direct way, and points to an important practical application of it. Staccato notes of equivalent intensity can be heard better in acoustically "live" than in acoustically "dead" rooms. Indeed a stream of speech or music can be made to sound considerably louder, without augmenting its peak amplitudes, by merely "livening" the auditorium. Up to a certain point, persistence can be made to take the place of intensity in improving the audibility of sounds, but if they are too much prolonged, successive sounds will incoherently overlap and apparent loudness will be gained at a too great expense of articulation. It is the ideal balance of loudness and persistence that is specified by equating the new parameter to its optimum constant. In recording sound pictures, therefore, it will be possible to attain apparent loudness, without overloading the vacuum tubes and vibrating parts of the recording system, by livening the recording stage. With the aid of the new parameter, the curve of optimum reverberation times for 1000 cycles in rooms of various volumes can be derived by purely methematico-physical methods. That the values thus computed agree closely with those experimentally determined is shown in Fig. 1. Among other results which the parameter has assisted to secure is the curve of optimum reverberation times for tones of different frequencies in a room of any particular size. Since the parameter is specified in terms of loudness and its time of de Fig. 2. The ratio of rates of decay of loudness and of power varies with the frequency. 2b 2 A 2.2 20 1.8 1 6 1.4 1.2 V 08 04 02 O * Bell Laboratories Record, March, 1930.