152 



HORN LOUD SPEAKERS 



where run = mechanical resistance at the horn throat, in mechanical ohms, 

 and 

 Cm\ = compliance of the air chamber. 

 The throat mechanical resistance Tmh is given by equation 8.12. The 

 mechanical compliance, Sec. 4.4, of the air chamber is given by 



r - ^ 



where Ab = area of the diaphragm, in square centimeters, 



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

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

 c = velocity of sound, in centimeters per second. 



8.15 



Fig. 8.4. The efficiency characteristics as a function of the frequency of a voice coil and 

 diaphragm having a mechanical reactance of one ohm at 1000 cycles coupled to a throat 

 of a horn having a mechanical resistance of one ohm and an air chamber having the follow- 

 ing reactances: A. an infinite reactance. B. a capacitance reactance of one ohm at 2000 

 cycles and C. a capacitance reactance of one ohm at 1000 cycles for initial efficiencies of 20, 

 40, 60 and 80 per cent. These characteristics are applicable to other frequencies by multi- 

 plying the frequency by any number and multiplying the mass and the capacitance by the 

 reciprocal of the number. 



Referring to the system shown in Fig. 8.1, it is obvious that the effect of 

 the air chamber will be to reduce the reactance of the system at the high 

 frequencies and thereby increase the efficiency over a wide range. 

 Figure 8.4 shows the efficiency characteristics of a system consisting of a 

 voice coil and diaphragm having a mechanical reactance of 1 ohm at 1000 



