STEADY STATE LOUD SPEAKER MEASUREMENTS 



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pressure measurement and one which has been found to be quite 

 satisfactory, consists in approximating the volume measurement by 

 rotating the condenser transmitter in a circle 69" in diameter, inclined 

 45° to the horizontal. A mechanism so arranged that the plane of the 

 condenser transmitter diaphragm always remains perpendicular to the 

 loud speaker axis is used and the condenser transmitter is connected to 

 the same voltage indicating system described above. As noted, this 

 indicating system involves a thermocouple and as a result, the meter 

 deflection is very nearly proportional to the square of the input 



500 1000 



FREQUENCY 



Fig. 12 — Curve showing variation with frequency in the effective absorbing 

 power of a felt lined room with respect to a region near the center and relatively near 

 the sound source. Determined from loud speaker measurements in this room and 

 outdoors as shown on Fig. 11. 



voltage. As the condenser transmitter is rotated about the periphery 

 of the circle, therefore, the average meter deflection is proportional 

 to the average of the squares of the transmitter terminal voltages or 

 the average of the squares of the pressures throughout the revolution. 



Fig. 11 is a response-frequency characteristic of the piston diaphragm 

 loud speaker measured in the same room and under the same conditions 

 as the curve in Fig. 9 except with the rotating condenser transmitter. 

 The center of the circle was located at the same point as the condenser 

 transmitter for Fig. 9. 



While rotating the transmitter in this manner does not average the 



