FINITE EXPONENTIAL HORN 



91 



h = distance from the apex to the throat, in centimeters, 

 p = density of air, in grams per cubic centimeter, and 

 Za2 = acoustic impedance of the mouth, in acoustic ohms. 

 The impedance of the mouth of the horn is usually assumed to be the 

 same as that of a piston in an infinite baffle. In this case the mouth im- 

 pedance Za2 is given by equation 5.12. 



The impedance characteristics of a finite conical horn is shown in Fig. 5.4. 



■ 25CM.— ») 





1 yvvvvv:: 



' ' III 



Fig. 5.4. Throat acoustic impedance characteristics of a conical and an exponential horn 

 having the same throat and mouth area, ta, resistive component, xa, reactive component. 



5.20. Finite Exponential Horn ^4. — The acoustic impedance at the 

 throat of an exponential horn is 



Zai = 





5.51 



where pi = pressure at the throat, in dynes per square centimeter, and 

 Ui = volume current at the throat, in cubic centimeters per second 

 The acoustic impedance at the mouth of the exponential horn is 



C/2 



ZAi 



5.52 



where p^ = pressure at the mouth, in dynes per square centimeter, and 



U2 = volume current at the mouth, in cubic centimeters per second. 



From equations 5.46 and 5.47 the expressions for pi, Ui and p2 and U2 



at the points 1 and 2 corresponding to the throat and mouth may be sub- 



24 Olson, H. F., RCJ Review, Vol. 1, No. 4, p. 68, 1937. 



