158 



HORN LOUD SPEAKERS 



that is the constants and connections of the network do not vary or change 

 with time. A network which includes a circuit element that varies con- 

 tinuously or discontinuously with time is called a variable network. In 

 some cases the variable elements are assumed to be certain functions of 



O 8 



.001 



5 6 7 8 9 



10 



5 6 7 8 9 



100 



FREQUENCY f CUTOFF FREQUENCY 



Fig. 8.8. The power output of infinite exponential horns, per square centimeter of throat 

 area, for 1, 3 and 10 per cent distortion, as a function of the ratio of the frequency under 

 consideration to the cutoff frequency. 



the time; that is, the variations are controlled by outside forces which do 

 not appear in the equations or statement of the problem. In another type 

 of variable circuit element the variation is not an explicit time function, 

 but a function of the current (and its derivatives) which is flowing through 

 the circuit. 



An example of the latter type of circuit element in an acoustical system 

 is the air chamber capacitance in a horn loud speaker. The excursions 

 of the diaphragm changes the capacitance. The acoustic capacitance of 

 the air chamber of Fig. 8.9 is given by 





V A{d + x) 



8.19 



pc- 



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

 c = velocity of sound, in centimeters per second, 

 V = volume of the air chamber, in cubic centimeters, 



