AUDIBLE FREQUENCY RANGES 



617 



audible frequencies of the sounds were included in the range between 

 the cut-offs thus delineated. Sound sources were: Musical instru- 

 ments — tympani, bass drum, snare drum, 14" cymbals, bass viol, 

 'cello, piano, violin, bass tuba, trombone, French horn, trumpet, bass 

 saxophone, bassoon, bass clarinet, clarinet, oboe, soprano saxophone, 

 flute, and piccolo; male and female speech; noises — footsteps, hand 

 clapping, key jingling. These tests are described in Part I. 



Measurements of the relation of reproduced frequency range to the 

 quality of orchestral music, as judged by a number of experienced 

 listeners, are reported in Part II. Tests of this kind must be used in 

 establishing acceptable frequency ranges. 



Part I 



Apparatus 

 The reproducing equipment was built by the Bell Telephone Labora- 

 tories especially for fundamental studies of speech and music quality. 

 Fig. 1 is a block diagram of the circuits involved in these tests. The 



TRANSMITTER 



I FILTER 



'control 



I SWITCH 



^ I 



e^Tj 



VOLTAGE AMP. 



AND 

 CAIN CONTROL 



rx 



TRANSMITTER 

 ROOM 



D=] 



LOUD 

 SPEAKER 



P 



INDICATING 

 CHART 



Fig. 1 — Schematic circuit diagram of essential apparatus. 



electro-dynamic microphone was mounted in a 5' square baffle placed 

 near the center of a large soundproof room, 29' X 29' X 13' in size, 

 which had a reverberation time of about one second for frequencies 

 between 60 and 4000 cycles. The microphone amplifier, mounted at 

 the rear of the baffle, raised the microphone output to a level that per- 

 mitted satisfactory switching operations without objectionable surges. 

 In another room the filters, their switching circuits, and the main 

 amplifiers were set up. The attenuator shown in the "filter-out" cir- 

 cuit was used to compensate for the losses in the transmitted bands of 



