ACOUSTIC i:mpedance. 27 



of resonance. ^^ Such an instrument would ha^'e 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. Relati\'ely feeble reflected sovmd waves, 

 falling on the diaphragm, would thus appreciably affect the motional 

 impedance, and enable the corresponding acoustic impedance varia- 

 tion to be 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 

 ^'ib^ating in a fluid. 



(3) Acoustic impedance can be measured at a telephone-receiver 

 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 ^-ibration 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 receiver diaphragm, under 

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

 cal of its motional impedance, multiplied by A^, the square of the 

 vector forcefactor of the instrument. 



(7) The mechanic impedance graph, under varied frequency, of any 

 receiver 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, bv measuring its 

 effect upon the motional impedance in the neighborhood of the 

 resonant frequency. 



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

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

 is Bibliography 7, page 1,3.3 and Fig. 11, where the receiver had a resonant 



sharpness of about 320. 



