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BELL SYSTEM TECHNICAL JOURNAL 



speaker obtained outdoors with the condenser transmitter at the same 

 relative position in the medium gives an idea of the magnitude of the 

 effect of room reflections even under comparatively favorable indoor 

 measuring conditions. The same frequencies were measured in both 

 the indoor and outdoor curves and no attempt was made to locate all 

 the irregularities in the indoor curve. 



Fig. 10 shows an indoor curve of the 115 cycle cut-off exponential 

 horn measured in the same room and under the same conditions at a 

 distance of 12'. The variations in this latter curve as compared to the 

 outdoor curve shown on Fig. 2 appear to be less than in the case of the 

 piston type loud speaker, probably because the horn is more directive, 



50 



100 



500 1000 



FREQUENCY 



5000 



10000 



Fig. 11 — Response-frequency characteristic of 314" piston diaphragm loud 

 speaker. Measured in highly absorbing room 12 feet from diaphragm with rotating 

 condenser transmitter. 



resulting in a larger difference between the direct and the reflected 

 sound energy at the transmitter position. 



The variations in these two indoor response-frequency characteristics 

 resulting from reflections could have been reduced by making the 

 measurements in a much larger room treated with sound absorbing 

 material in the same manner. Such a room for loud speaker measuring 

 purposes is not usually available, but if sufficiently large and of a 

 suitable shape does afford the most satisfactory indoor measuring 

 conditions especially at very low frequencies. 



Another method of obviating the effects of reflections is to measure 

 the mean square pressure throughout a suitable volume as discussed 

 previously. A practicable means of making such a mean square 



