282 



ARCHITECTURAL ACOUSTICS 



result that there is an optimum reverberation for each auditorium which 

 yields maximum articulation as shown in Fig. 12.1. The obvious solution 

 is the use of sound reenforcing equipment. The articulation for a weak 

 average and loud talker without amplification as compared to amplified 

 speech is shown in Fig. 12.1. By proper selection and placement of the 

 loud speakers the articulation characteristic may be made considerably 

 higher. 



X REVERBERATION TIME IN SECONDS 



REVERBERATION TIME IN SECONDS 



Fig. 12.1. X, the articulation of a speaker in auditoriums of various volumes. Y, the articu- 

 lation of amplified speech, loud, average, and a weak speaker in an auditorium of 400,000 

 cubic feet. 



B. Sound Motion Picture Reproducing Systems'^^. — The resultant sound 

 energy density at the position of the auditor in a theatre depends upon the 

 response and the directional characteristics of the loud speaker and upon 

 the reverberation characteristics of the theatre. From the standpoint of 

 the auditor, it may be said that there are two sources of sound energy, 

 namely: the direct sound, which travels directly from the loud speaker to 

 the auditor, and the generally reflected sound, which is reflected from the 

 boundaries before reaching the auditor. 



In a theatre free from acoustical difficulties, the energy density of the 

 generally reflected sound is practically the same for all parts of the theatre. 

 Therefore, the solution of the problem of achieving uniform energy density 

 is to employ reproducers that will project the same direct sound energy to 

 all parts of the theatre. The example which follows will illustrate how 

 this may be accomplished by employing a directional loud speaker. 



An elevation view of a reproducer in a theatre is shown in Fig. 12.2. The 

 two extreme points to be supplied are indicated as A and E. If the loud 

 speaker were nondirectional, the ratio of the direct sound energy densities 



" Olson and Massa, Jour. Soc. Mot. Pict. Eng., Vol. 23, No. 2, p. 63, 1934. 



