STEADY STATE LOUD SPEAKER MEASUREMENTS 141 



large compared to the dimensions of the radiating surface. Other- 

 wise, the wave front at the measuring position would not be spherical 

 and the indicated pressure might result largely from the cyclic storage 

 and absorption of energy by the loud speaker in the immediate 

 vicinity of the radiator. With a proper location of the condenser 

 transmitter, however, a response-frequency characteristic provides a 

 useful measure of the dynamical perfection of a loud speaker. 



The second factor which may cause large variations in the response 

 of a loud speaker in a reflection less medium is the change in the space 

 distribution of the radiated sound with frequency. Although the 

 total acoustic power delivered by a loud speaker may be constant, the 

 power density at certain positions in the medium may change greatly 

 with frequency due to the interference of sound originating at different 

 parts of the radiating surface. Unless the radiation from the loud 

 speaker is spherical, this interference phenomenon will result in a 

 concentration of sound power in certain regions in the medium and a 

 diminution in others. The locations of these regions change with 

 frequency, radiator dimensions and the mode of vibration of the 

 radiating surface. 



For the case of a piston diaphragm radiator in a large rigid wall, it is 

 possible to calculate the variations with frequency in the excess 

 pressure at points in the sound field. Such calculations ^ and con- 

 firming experimental data show that in the sound field along the center 

 perpendicular (a line normal to the surface of the piston at the center) 

 to a piston radiator there is a succession of sound pressure maxima and 



minima out to a distance equal to approximately ^yKq f^et (where D is 



the piston diameter and /i is the energizing frequency). Beyond this 

 distance these maxima and minima points disappear and the pressure 

 varies inversely as the distance. If then, the response of a loud 

 speaker with a piston diaphragm is measured with the condenser 



transmitter at a distance less than j—q feet (where / is the highest 



measuring frequency), the response-frequency characteristic will have a 

 succession of peaks and depressions which are independent of but 

 which may be difficult to distinguish from those caused by poor 

 dynamical characteristics of the loud speaker itself. On the other 

 hand, if the condenser transmitter is located at any distance greater 



than j3^ feet, the response-frequency characteristic obtained will not 



^ "The Directional Effect of Piston Diaphragms," Backhaus and Trendelenburg, 

 Zeitschriftf. Techn. Physik., Vol. 7, pp. 630-635, 1926. Also "Theory of Vibrating 

 Systems and Sound," Crandall, pp. 137-149. 



