. Elements of acoustical engineering. Electro-acoustics; Sound. DISPERSION OF SOUND 285 directional loud speakers, the total sound energy density will be the same in all parts of the theatre. Furthermore, the effective reverberation of the reproduced sound (the ratio of generally reflected to direct sound) is the same for all parts of the theatre. The distribution of a reproducing system in a theatre is usually checked by means of a response measuring system. The plan and elevation view of a typical theatre is shown in Fig. The response characteristics for positions yf, 5, C and D on the
. Elements of acoustical engineering. Electro-acoustics; Sound. DISPERSION OF SOUND 285 directional loud speakers, the total sound energy density will be the same in all parts of the theatre. Furthermore, the effective reverberation of the reproduced sound (the ratio of generally reflected to direct sound) is the same for all parts of the theatre. The distribution of a reproducing system in a theatre is usually checked by means of a response measuring system. The plan and elevation view of a typical theatre is shown in Fig. The response characteristics for positions yf, 5, C and D on the orchestra level and E and F on the balcony level are shown in the respective graphs. These characteristics show that. 0 c r- â -, m -^^ 2 ,â / ->N O ^° c ". °5o 100 1000 10000 FREQUENCY C Q 40 V A/ rz '\ Q- ,. \ 0 100 1000 10 FREQUENCY E 000 z ,. ' \ ivA/'' VVv \ 0- ,T \ u (^30 §20 Q. in 10 in 30 O 20 ^ 50 100 1000 10000 FREQUENCY D r^ ' \^ AA/ ^ " 50 100 1000 10000 FREQUENCY F . r^ 'VA/' A^A/ kVvi \ PLAN VIEW Fig. A cross-sectional elevation and plan view of a theatre equipped with a loud speaker for the reproduction of sound motion pictures. The graphs show the response frequency characteristics in various parts of the theatre. it is possible to obtain uniform response in all parts of the theatre by fol- lowing the procedures outlined in the preceding discussions. In sound motion picture reproduction the loud speakers are usually placed behind a perforated screen upon which the picture is projected, Fig. Sound is transmitted through the screen by actual vibration of the screen and by the perforations. In general the transmission by vibration is negligible. The perforations usually consist of small circular holes about a millimeter in diameter. These holes form an inertance and acoustic re- sistance. See Sees. and The resistance of the holes introduces attenuation which is usually small. The acoustic reactance due to the in- ertan
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