146 HORN LOUD SPEAKERS 



the efficiency depends primarily upon the flux density, the mass and the 

 density-resistivity product of the voice coil, the mass of the diaphragm, 

 the ratio of the diaphragm to the throat area, the dimensions of the air 

 chamber, the area of the diaphragm and the voice coil temperature. Some 

 of the factors are interrelated and others are independent; as a consequence, 

 it is impossible to depict in one set of characteristics the effect of the various 

 parameters. Therefore, the design of a horn loud speaker is usually a long 

 and tedious task. The labor is further increased when economic consid- 

 erations are involved. It is believed that a general consideration of the 

 problem, together with a series of characteristics, is valuable for initiating 

 the design of a loud speaker and for facilitating the determination of the 

 ultimate constants. The throat impedance and directional characteristics 

 of a large number of representative horns were given in Sees. 5.19 and 2.8. 

 From these characteristics it is possible to interpolate the characteristic of 

 practically any horn and thus eliminate considerable initial work in the 

 design of a horn loud speaker. It is the purpose of this section to consider 

 the effect of the various parameters, referred to above, upon the efficiency 

 of a horn loud speaker and to include characteristics depicting the influence 

 of these parameters upon the performance. 



A. The Relation between the Voice Coil Mass, the Load Resistance and the 

 Initial Efficiency. — Initial efficiency is the ratio of sound power output to 

 electrical power input in the system in which the mechanical reactance is 

 negligible and in which all the mechanical resistance may be attributed to 

 radiation. In most loud speakers the mechanical reactance of the vibrating 

 system is negligible in the upper low frequency range. Near the cutoff of 

 the horn the reactive component at the throat of the horn is relatively large. 

 Furthermore, the mechanical reactance due to the stiffness of the diaphragm 

 may be comparable to the other mechanical impedances in the system. 

 Nevertheless, the starting point in most horn loud speaker designs is a 

 determination of the initial efficiency. This is logical because the mechan- 

 ical reactances referred to above are usually chosen so their effect upon the 

 efficiency characteristic in the upper low frequency range is very small. It 

 is the purpose of this section to discuss briefly the factors which influence 

 the initial efficiency and to include a family of curves showing the effect of 

 the flux density, the voice coil mass, the throat area, and the diaphragm 

 diameter upon the initial efficiency. The motional impedance Zem^ in 

 ohms, is given by (see Sec. 6.2). 



%EM = -^^ X 10-9 8.1 



