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ARCHITECTURAL ACOUSTICS 



lecting system no longer increases; this is called the steady state condition. 

 Therefore, at a given point in a room there are two distinct sources of sound, 

 namely: first, the direct and, second, the generally reflected sound. For 

 rooms that do not exhibit abnormal acoustical characteristics it may be 

 assumed that the ratio of the reflected to the direct sound represents the 

 effective reverberation of the collected sound. 



Consider a sound collecting system. Fig. 12.14, the efficiency of reception 

 of which may be characterized as a function of the direction with respect 

 to some reference axis of the system. (The nondirectional collecting sys- 



Fig. 12.14. Sound collecting system in a studio. Graph ^^ shows the reverberation time and 

 the absorption coefficient of the boundaries of a typical studio. Graphs 5, C and D show 

 the energy response for the direct and reflected sounds for various microphones as follows: 

 B. Nondirectional microphone. C. Bidirectional velocity or unidirectional microphone. 

 D. Ultradirectional microphone. 



tem is a special case of the directional system in which the efficiency of 

 reception is the same in all directions.) The output of the microphone 

 may be expressed as 



e = ^pMrP) 



12.14 



where e = voltage output of the microphone, in volts, 



p = sound pressure, in dynes per square centimeter, 

 ^ = sensitivity constant of the microphone, and 

 ^ = angle between incident pencil of sound and the reference axis 

 of the microphone. 



If the distance, in centimeters, between the source of the sound and the 

 collecting system is D, the energy density at the microphone due to the 

 direct sound is 



Ed = 



DHtc 



12.15 



