. The Bell System technical journal . vs. frequency now becomes also an illustration of the shapeof the reverberation time vs. frequency curve which a room shouldhave in order that the loudness of pure tones of all frequencies shalldecay at the same rate. 392 BELL SYSTEM TECHNICAL JOURNAL According to Sabines well known formula the reverberation time isinversely proportional to the number of absorption units in the room sothat, if we assume this, we may immediately infer the shape of thecurve which represents the number of absorption units necessary atany frequency, referred to the amount requ
. The Bell System technical journal . vs. frequency now becomes also an illustration of the shapeof the reverberation time vs. frequency curve which a room shouldhave in order that the loudness of pure tones of all frequencies shalldecay at the same rate. 392 BELL SYSTEM TECHNICAL JOURNAL According to Sabines well known formula the reverberation time isinversely proportional to the number of absorption units in the room sothat, if we assume this, we may immediately infer the shape of thecurve which represents the number of absorption units necessary atany frequency, referred to the amount required at 1000 cycles, toobtain our required condition. These values are plotted in Fig. 3. Ifit should happen that the greater part of the sound absorption in aroom is caused by one particular kind of surface, then the curve inFig. 3 is the shape of the absorption curve that this material shouldhave. A pertinent observation on which every one seems to agree is that if 2 34568 2 34568 10 100 1,000 10,000 FREQUENCY IN CYCLES PER SECOND Fig. 2.—\allies of Af vs. frequency. an auditorium has an unusually long reverberation time and conse-quently is of little use, when empty, it attains excellent acousticconditions when filled with a large audience. In these cases a verylarge part of the absorption is caused by the audience. The absorp-tion of an average audience has been measured by W. C. Sabine ^ andhis results are also plotted in Fig. 3. The close agreement between thiscurve and the one we have obtained from our hypothesis gives con-siderable confidence in our general viewpoint. II. Reverberation Time vs. Volume It is generally accepted that the best acoustical conditions in a roomare obtained when the reverberation time is adjusted to a definite valueknown as the optimum reverberation time. Observations reported inliterature agree that the value of the optimum reverberation time in- ^ Collected Papers on Acoustics, page 86. OPTIMUM REVERBERATION T
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