ACOUSTIC IMPEDAXCE. 27 



of resonance. ^^ Such an instrument would June a relatively large 

 motional-impedance circle, and a small change of impressed fre- 

 quency would carry the vector from one quadrantal point, through 

 resonance, over to the other. Relatively feehle reflected sound waves, 

 falling on the diaphragm, would thus appreciably affect the motional 

 impedance, and enable the corresponding acoustic impedance varia- 

 tion to l)e measured. 



Summary of Results. 



(1) Acoustic resistance, reactance and impedance are defined. A 

 technique for their measurement is described. 



(2) Acoustic impedance is offered to any disk or diaphragm steadily 

 vibrating in a fluid. 



(3) Acoustic impedance can be measured at a telephone-recei^'er 

 diaphragm, but can be considered as existing in a fluid at any surface 

 that can be drawn, and across which sound is steadily transmitted. 



(4) When the amplitude of vibration of a diaphragm is not uniform 

 over its surface, the acoustic activity over its surface can be evalu- 

 ated in terms of the mean square amplitude over the surface. 



(5) Acoustic impedance at the diaphragm of a telephone receiver, 

 carrying an alternating current, can be determined by measuring 

 changes in the motional impedance of the receiver under changes of 

 acoustic load. 



(6) The total mechanic impedance at a recei^'er diaphragm, under 

 any steadily impressed a-c. frequency and acoustic load, is the recipro- 



' cal of its motional impedance, multiplied by A-, the sc^uare of the 

 vector forcefactor of the instrument. 



(7) The mechanic impedance graph, under varied frec^uency, of any 

 receiA'er whose motional impedance diagram is a circle through the 

 origin, consists of an infinite straight line parallel to the reactance axis. 



(8) By applying a telephone receiver to one end of an air tube, or 

 to the mouth of any air chamber, the acoustic impedance of the tube 

 or chamber can be determined in this condition, by measuring its 

 effect upon the motional impedance in the neighborhood of the 

 resonant frec^uency. 



(9) In the case of a long straight cylindrical air-tube with smooth 

 walls, attached to the cap or cover of a receiver, the stationary sound- 

 is Bibliography 7, page 133 and Fig. 11, where the receiver had a resonant 



sharpness of about 320. 



