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BELL SYSTEM TECHNICAL JOURNAL 



squares of the pressures throughout a volume, it does give a measure 

 which is closely proportional to the power density. This is apparent 

 from a comparison of Fig. 11 with a curve on the same loud speaker 

 measured outdoors shown on Fig. 7 and replotted on Fig. 11 for 

 comparison. Note that these two curves very closely coincide 

 between 1,000 and 3,000 cycles. Below 1,000 cycles and above 3,000 

 cycles the uniformly greater response indoors can be explained in the 

 following manner. 



As given by equation (8) above, the average energy density in a 

 room resulting from a loud speaker emitting sound power at a constant 



1-20 



50 



100 



500 1000 



FREQUENCY 



5000 



Fig. 13 — Response-frequency characteristic of 115 cycle cut-off exponential horn 

 with moving coil type receiver. Measured in highly absorbing room 12 feet from 

 horn mouth with rotating condenser transmitter. 



rate is inversely proportional to the absorbing power of the room. 

 Inasmuch as the two curves in Fig. 11 were obtained with the same 

 loud speaker with the condenser transmitter located at the same 

 distance and at approximately the same relative position in the 

 medium, the difference in the two curves would appear to be due only 

 to the difference between the indoor and outdoor absorption. As- 

 suming for the outdoor case a fictitious bounding surface making an 

 enclosure of the same shape and size as the indoor room and that the 

 energy striking this fictitious surface is completely absorbed (as would 

 be the case) the area element of the factor "a" in equation (8) becomes 

 the same for the indoor and outdoor tests, and the ratio of the indoor 

 energy to the outdoor energy would therefore bear some simple 



