142 BELL SYSTEM TECHNICAL JOURNAL 



have abrupt irregularities due to interference and any two curves so 



obtained will only differ in magnitude. A curve obtained under this 



latter condition would therefore show the response-frequency variations 



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 as these would be observed at any distance greater than feet in 



this same direction. 



While the above facts relate to a piston diaphragm radiator because 

 more definite statements can be made regarding its sound field, similar 

 effects due to the irregular distribution of the sound field are involved 

 in the case of any other loud speaker which does not radiate as a 

 pulsating sphere. Using such piston diaphragm considerations as a 

 basis it has been found possible to predict suitable measuring con- 

 ditions for any particular loud speaker. The fundamental requisite is 

 that the pressure indicator be located at a distance from the loud 

 speaker commensurate with the typical listening distance in order that 

 the response-frequency characteristic shows variations which would 

 normally be observed and which are therefore of interest. If, however, 



the typical listening distance is greater than - feet (where D is 



roughly the diameter of the radiating surface and / is the highest 

 measuring frequency) response-frequency measurements at this dis- 

 tance will show the response-frequency variations at any greater 

 distance in the same direction so that measurements at the greater 

 distances would not be necessary. If the most likely position of a 



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 listener is at a distance less than ^Taq ^^et, the response-frequency 



characteristic obtained with the condenser transmitter at such a 



position will have irregularities due to interference but since these 



irregularities would be heard they should be charged against the loud 



speaker and such a curve would be indicative of the performance. In 



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 this latter case response measurements at a distance greater than jfaa 



feet are sometimes valuable for loud speaker design work in order to 

 distinguish those variations due to poor dynamical characteristics of 

 the loud speaker itself and those due to poor sound field distribution 

 characteristics. 



Measuring Considerations in a Medium with Reflections. — If the 

 sound energy reflected to the condenser transmitter position from the 

 bounding surfaces of the medium is comparable in magnitude with the 

 energy reaching this position directly from the loud speaker, standing 

 waves will exist and the sound pressure may vary greatly with fre- 

 quency at any fixed transmitter position although the acoustic power 



