ACOUSTIC IMPEDANCE. 7 



z' contains an internal mechanic impedance Za, and an external acous- 

 tic impedance z, including both surfaces of the diaphragm. If the 

 receiver is operated in vacuo, the external acoustic impedance z dis- 

 appears, and the total mechanic impedance on the diaphragm is 

 reduced to z^. 



The rms. velocity x of a telephone receiver diaphragm, and also 

 the impressed vmf. F, can be measured electrically in the laboratory 

 by mapping out the motional impedance diagram, and measuring the 

 amplitude of vibration at resonance. Consequently, by means of (12) 

 the total mechanic impedance on the disk can be evaluated. If we 

 measure the internal mechanic impedance, we can then find z the 

 acoustic impedance, and ascertain how it varies under different 

 conditions. The measurements described in this paper offer a start- 

 ing point in this direction. 



Brief History of the Method: In preceding researches on the motional 

 impedance of telephone receivers, the receiver has been usually 

 regarded as a motor, and the mechanic resistance of the receiver 

 diaphragm as a motor load, partly due to internal frictions, and partly 

 to external acoustic wave emission. In a few cases, the mechanic 

 resistance was varied, either by operating the receiver in a partial 

 vacuum, or by attaching to the diaphragm a disk vane immersed in a 

 damping fluid.* In the research here described, however, the results 

 obtained have led to changing the function of the tested receiver from 

 a reciprocating motor to a reciprocating generator of sound, with the 

 object of studying the reaction, or change of acoustic load, on this 

 generator, produced by sound reflection and absorption. It is believed 

 that the method here described is susceptible of numerous applica- 

 tions in the science of acoustics. Although the definition of acoustic 

 impedance offered in this paper is believed to be new; yet the concept 

 can be discerned in the writings of various acousticians.^ 



Outline Theory of the Method: It has been shown in preceding pub- 

 lications, that if A is the force factor of a telephone receiver in dynes 

 per absampere Z , a complex or planevector quantity having the 

 lagging slope (3°, and representing the electro-magnetic pull upon the 

 diaphragm per rms. absampere of exciting current I, taken at stand- 

 ard phase, Z' is the motional impedance of the receiver as measured 

 at the impressed frequency, and with a given acoustic load (absohms 



4 Bibliography 9, pages 460-462. 



5 Bibliography 15. Since delivering this paper the authors' attention has 

 been directed to Dr. Drysdale's recent Kelvin lecture (Bibliography 20) where 

 this theory is developed. 



