STEADY STATE LOUD SPEAKER MEASUREMENTS 



157 



relation to the average absorption coefficient of the sound absorbing 

 material of the indoor measuring room. From the ratios of the outdoor 

 to indoor energy densities at different frequencies as determined from 

 Fig. 11, the solid curve on Fig. 12 is obtained. The irregular character 

 of this curve is probably due to the fact that the rotating indoor 

 transmitter did not give an exact measurement of the energy density at 

 each frequency. The trend of this curve, however, is quite definite as 

 is indicated by the dotted curve which is an average curve obtained by 

 inspection. A comparison of this mean curve with the dot-dash curve 

 showing the approximate absorption power of a |" layer of asbestos 

 hair felt ^ indicates an interesting correlation in the trend of the two 



1.0 



2°« 

 o 



Q- 0.6 



5 0.4 



500 1000 



FREQUENCY 



Fig. 14 — Curve showing variation with frequency in the eflfective 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. 13. 



curves. The difference in magnitude is probably due to sound 

 diffusion at the walls resulting in the ratio of the energy reflected to the 

 energy direct from the source being smaller than would be the case 

 were the measuring region located close to the absorbing surface. 



The solid curve in Fig. 13 is a response-frequency characteristic of 

 the 115 cycle cut-off exponential horn obtained with the rotating 

 condenser transmitter under the same conditions as the curve in Fig. 

 11. The center of the horn mouth was placed at the same position as 

 was the center of the diaphragm of the 3|" diameter piston radiator. 

 The dotted curve in Fig. 13 is the outdoor characteristic obtained with 

 the condenser transmitter on the horn axis at a distance of 12'. It 

 will be noted that these indoor and outdoor curves diverge at the low 



^ See "Collected Papers on Acoustics," W. C. Sabine, p. 213, Fig. 4. 



