ACOUSTIC IMPEDANCE. 



17 



ance (z^ + Zi> + Zi) at the impressed frequency of the measurement 

 (921 o^). This leads to the inference that if the acoustic impedance 

 in the tube and in front of the receiver diaphragm could be com- 

 pletely removed, the mechanic impedance left in and behind the 

 diaphragm would be the vector impedance OA. This would cor- 

 respond to a vector motional impedance Oa = 165 V 48?1 on Figure 

 8. Oa is therefore the inferred motional impedance for the ease of 

 removal of all acoustic load from the front of the diaphragm. The 

 addition'of the acoustic load of the air column in the tube is a vector 



600 too Toco 1300 1400 \eoc 



RESISTANCE .DYNES PER KINE 



ifso jssr 



14«0 



Figure 9. Graph of Total Mechanic Impedance on Receiver Diaphragm 

 obtained by inverting the graph of Figure 8 and multiplying by A'-. Heavy 

 curve connects observation values. The dotted curves represent the com- 

 puted values. 



with its origin at A, Figure 9. The spiral locus 1 2 4 .... 62 is the 

 spiral of vector impedance of the tube acoustic conductor, with its 

 distant end sealed at distances of 1 2 4 .... 62 cm. from the dia- 

 phragm, or more strictly from a^ plane 1.5 mm. in front of the dia- 

 phragm, where the tube may be considered as beginning. This spiral 

 impedance conforms fairly well with the familiar expression 



Zq tanh 8a dynes per kineZ (20) 



where z^ is the "surge impedance" of the tube, or the acoustic imped- 



