ACOUSTIC IMPEDANCE. 9 



The total mechanic impedance z' is made up of se\eral mechanic 

 impedances; namely, 



(1) The mechanic impedance of the diaphragm 



z./ = rj -\- j i 111 jcj — — ) mechanic absohms Z (17) 



(2) The virtual mechanic impedance of the diaphragm ^ due to its 

 motion in the permanent magnetic field 



z„ = / - — = r -\- ' - = sin 13 + / " cos /3 



CO W CO 'CO 



mechanic absohms Z (18) 



The real part of z,., although ^'arying inversely with co, is ordi- 

 narily taken as constant to a first approximation, the working 

 range in co being usually small. 



(3) The acoustic impedance Zi of the air in the chamber behind the 

 diaphragm, and which varies, in some manner similar to (17), as 

 the impressed frequency is varied. 



(4) The acoustic impedance z-2 of the air in front of the diaphragm, and 

 which constitutes the load to be A'aried. 



Consequently, 



z' = Zd + Zy + Zi + Zo mechanic absohms Z (19) 



Keeping tlie impressed frequency constant, z,/, z,., and Zi will renuiin 

 constant; but z-i can be made to vary by changing the acoustic load. 

 The changes in the electric impedance, measured on changing the 

 acoustic load, enable the latter to Ije computed. Moreover, by 

 observing the changes in Z' when the frequency is varied, if allowance 

 can be mafle for the changes thereby produced in z^, z„ and Zj, the 

 variations in the acoustic'impedance Zo may also be computed. 



Measurement of Acoustic Imycdancc of the Air Cohimn in a Tube 

 of Varied Length: A straight smooth fibre tube 87.S cm. long, 3.1 cm. 

 internal diameter, and 3.9 cm. external diameter, was supported 

 horizontally, by metallic strips packed with felt, over a wooden base 

 BB, Figure 2. The tube TT was brought into contact with the co\-er 

 of a regular form of bipolar telephone receiver R, as is shown in detail 

 by Figure 3. The axis of the tube was in line with the axis of the 

 receiver. A hard rubber piston PP slides freely in the tube, and can 



6 Bibliography 16. 



