148 HORN LOUD SPEAKERS 



Zmh = riiiH +JXMH = impedance of the throat of the horn, in 



mechanical ohms, 

 rMH = resistance of the throat of the horn, in mechanical ohms, and 

 Xmh = reactance of the throat of the horn, in mechanical ohms. 

 For initial efficiency considerations, the reactance of the mechanical 

 system is assumed to be negligible compared to the radiation resistance, 

 that is, niA, nzc, wx), Cmi, I /Cms and Xmh are zero. Vma is also negligible. 

 Then 



Zm = rMH = 42—— ^ 8.4 



where Ad = area of the diaphragm, in square centimeters, and 



At = area of the throat, in square centimeters. 

 Substituting equations 8.1 and 8.4 in equation 8.2 



(^^) — 



X 100 



The resistance ^ Ted is given by 



^ED = ~z ohms 8.6 



o 



where q = resistivity of the voice coil material, in ohms, per centimeter 

 cube, 

 / = length of the conductor, in centimeters, and 

 ^ = area of the conductor, in square centimeters. 



Then (5) becomes 



ju = ^^^ ^ ^ ^ X 100 8.7 



The mass of the coil, nic, is 



mc = ^Sp grams 8.8 



where p = density, in grams per cubic centimeter. 



^The voice coil electrical circuit is shown in Fig. 8.1. Ted is the total damped 

 resistance of the voice coil and includes skin effect and hysteresis losses in the iron. 

 L is the Inductance of the voice coil. As shown in Fig. 7.4, the impedance of the 

 voice coil increases at the high frequencies due to the reactance of L and an increase 

 in resistance due to skin effect and hysteresis losses in the iron circuit. In order to 

 simplify these considerations the damped resistance will be assumed to be the 

 same as the ohmic (d.c.) resistance. 



