ACOUSTIC IMPEDANCE. 33 



for linear electric leakance g, and elastic force s for s, the reciprocal 

 of the linear capacitance. 



Table I gives a comparison between electric and acoustic quantities 

 for the case of negligible losses in transmission. 



Table II gives some comparative data for electric and acoustic 

 conductors, still assumed as having no losses ; but freed at the distant 

 end. 



Table III gives similar comparative data for the general case of 

 conductors liaxing linear losses of known values. 



Surge-Impedance Density: In regard to Table I, it may be noted 

 that the acoustic surge impedance density has no counterpart in the 

 ordinary theory of electric lines. 



The value of the surge impedance density, by (49), is v p p^ where p is 

 the density of quiescent air at 0°C and the standard pressure of 10® bars 

 or dynes per sq. cm. The value of jjo =yPo ^^^^Y be taken as 1.41 X 10® 

 bars, and p = 1.276 X lO-l Consequently j = Vl7.99 X 10^ = 42.4 

 mechanic absohms per sq. cm. of tube cross-section, under the stan- 

 dard condition of 1 megabar pressure (10® dynes per sq. cm.) and 0°C. 

 If the actual pressiu-e of the c[uiescent air in the tube is p^ bars, and 

 the actual temperature is t° C, the surge impedance density becomes 



_ 42.4 -Vp^ X 10"® mechanic absohms , . 



Vl + 0.00366 t sq. cm. 



or for values not exceeding 25° C, 



_ 42.4 X V2^ X 10"^ mechanic absohms , , 



^ ~ 1 + 0.00183 t sq. cm. 



A rigid disk diaphragm of S sq. cm. on each face, vibrating with uni- 

 form amplitude over its surface, so as to generate plane waves in the 

 tube indefinitely long, would develop on each face a surge resistance 

 of 



42.4 SV^X 10-3 



z„ = — mechanic absohms Z (72) 



1 + 0.00183 / 



In the case of the receiver used in the reported tests, the diameter 

 of the diaphragm inside the clamping ring was 4.96 cm., the correspond- 

 ing surface area being 19.3 sq. cm. The surge impedance on this 

 surface at 20°C. and 1 megabar pressure, if the diaphragm faced an 

 indefinitely long tube of the same cross-sectional area without acoustic 



