SYSTEM ADAPTATION 



305 



reverberation time measurements for both Constitution Hall and the 

 Academy of Music were made with the high speed level recorder.^ 

 This instrument measures and plots on a moving paper chart a curve 



O 3.5 



100 200 500 1000 2000 5000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 4 — Reverberation characteristics of Constitution Hall. 



the ordinate of which is proportional to the logarithm of the electrical 

 input furnished to it. When used in connection with a microphone 

 for reverberation time measurements, curves are obtained showing the 

 intensity of sound at the microphone during the period of sound decay. 

 The rates of decay, and hence the reverberation times, are obtainable 

 immediately from the slopes of these recorded curves and the speed 

 of the paper chart. 



Calibration of the System 

 In calibrating the system, a heterodyne oscillator connected to the 

 loud speakers through the amplifiers was used,. The oscillator was 

 equipped with a motor drive to change the frequency, and as the 

 frequency was varied through the range from 35 to 15,000 c.p.s. the 

 sound was picked up with a microphone connected to the level recorder. 

 Continuous curves of microphone response as a function of frequency 

 thus were obtained for several positions in the auditorium, and for each 

 channel independently. These response curves provided a check on a 

 uniform coverage of the audience by each loud speaker, and also 

 provided data for the design of the equalizing networks required to 



