ACOUSTIC CAPACITANCE 79 



d = thickness of the sht normal to the direction of flow, in centi- 

 meters, 

 / = width of the sHt normal to the direction of flow, in centimeters, 

 w = length of the slit in the direction of the flow, in centimeters, 

 CO — 27r/", and 



/ = frequency, in cycles per second. 

 In equation 5.2 the resistance varies inversely as the cube of d and the 

 inertance inversely as d. Therefore, practically any ratio of inertance to 

 resistance may be obtained. The magnitude may be obtained by a suit- 

 able choice of w and /. A slit type of resistance may be formed by using a 

 pile of washers spaced by small shims. Another form consists of a spiral 

 of tape with adjacent turns very close together. The former may be used 

 as a shunt resistance in a line and the latter as a series resistance. See 

 Sec. 4.9. 



5.5. Inertance. — Inertance is defined as 



M = =f22 5.3 



where S = area, in square centimeters, over which the driving pressure 

 acts to drive the mass, in grams. 

 The impedance of various types of systems will be considered in Sees. 5.7, 

 5.8 and 5.9. The imaginary part of these expressions is due to the inertance 

 of the systems. 



For closed systems the resistance term of Sees. 5.7, 5.8 and 5.9 

 should be omitted because there is no radiation. In this case the entire 

 impedance is positive reactance. The reactance term of equations 5.1 

 and 5.2 is due to inertance. 



5.6. Acoustic Capacitance. — The most common type of acoustic capaci- 

 tance used in acoustic systems consists of a cavity or volume with rigid 

 boundaries. The linear dimensions of the enclosure are assumed to be 

 small compared to the wavelength. 



From equation 1.21 the sound pressure is 



p = pc^s ■ 5.4 



where p = density of air, 



c = velocity of sound, and 

 s = condensation. 

 The condensation, from Sec. 1.3D, is 



dV 

 s = -y 5.5 



