COMPLETE REPRODUCING SYSTEMS 



309 



designed to have a very low reverberation time to prevent echoes and reduce 

 the effect of extraneous noise generated in the studio. In the case of sets 

 consisting of small rooms the acoustics of the set masks the acoustics of 

 the sound stage. In the early days of sound motion picture recording it 

 was customary to make the sets of acoustic materials having good trans- 

 mission at the low frequencies and high absorption at the high frequencies. 

 In this way it was possible to keep the reverberation time of the set very 

 low. With the advent of directional microphones it has been possible to 

 use conventional materials for the construction of sets. 



D. Synthetic Reverberation. — The reverberation time of studios may 

 be changed and controlled within certain limits by means for varying the 

 absorption. The amount of control that may be obtained by varying the 

 amount of absorption by means of hard panels which cover the absorbing 



TRANSDUCERS WITH PROGRESSIVE 

 DELAY AND ATTENUATION 



Fig. 12.17. 



Schematic arrangement of a system for introducing synthetic reverberation in 

 reproduced sound. 



material or other similar systems is limited. Furthermore, the reproducing 

 conditions may also require additional reverberation. Where the rever- 

 beration time of reproduced sound is far below the optimum value, the 

 reproduction may be enhanced by artificially adding reverberation. Rever- 

 beration in a room consists of the multiple reflection of a large number of 

 pencils of sound. Each pencil of sound suffers a decrease in intensity 

 with each reflection. These conditions can be simulated by the system 

 shown in Fig. 12.17. The amplified sound is passed through a number of 

 transducers with progressive delay and progressive attenuation. These 

 transducers may be a series of pipes with a loud speaker at one end and 

 a microphone at the other. Or, these transducers may be a series of pick- 

 ups on a phonograph record or on a magnetic tape.^^ The amount of 

 reverberatiiDn may be controlled by varying the direct and reverberant 

 attenuation. 



12.4. Complete Reproducing Systems. — A. Telephone. — The tele- 

 phone is a sound reproducing system consisting of a carbon microphone 

 (sometimes termed the transmitter), a telephone receiver, and a battery. 



31 Wolff, S. K., Jour. Soc. Mot. Pic. Eng., Vol. 32, No. 4, p. 390, 1939. 



