164 HORN LOUD SPEAKERS 



G. Power Handling Capacity and the Amplitude of the Diaphragm ^^. — 

 The maximum allowable amplitude of the diaphragm is another factor 

 which may determine the maximum allowable power output. The power 

 output, in watts, of a horn loud speaker in which the diaphragm is ter- 

 minated in an acoustic resistance is 



P^BE^M)^,^-. ■ 8.21 



where p = density, in grams per cubic centimeter, 



c = velocity of sound, in centimeters per second, 

 / = frequency, in cycles per second, 



d = maximum amplitude from its mean position, in centimeters, 

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

 Ah = area of the throat of the horn, in square centimeters. 



The amplitude of various diameter diaphragms coupled to a horn throat 

 of one square inch for one acoustic watt output is shown in Fig. 8.14. 



8.4. Horn Loud Speaker Systems. — A. Single Horn, Single Channel 

 System. — The single horn, single channel system consists of a single horn 

 driven by a single diaphragm. A diaphragm coupled to an exponential 

 horn constitutes the simplest and most widely used system. The efficiency 

 characteristic of a simple exponential horn coupled to a diaphragm and coil 

 having a mass ratio of 2 operating in a field of 22,000 gausses is shown in 

 Fig. 8.15. Two efficiency characteristics are shown with initial efficiencies 

 of 80 per cent and 50 per cent. Although it is possible to obtain reasonably 

 high efficiency over a wide range with a single flare horn coupled to a dia- 

 phragm, the efficiency can be increased by employing a multiple flare horn. 



To obtain maximum efficiency in a horn loud speaker at any frequency, 

 the effective reactance of the entire system should be equal to the effective 

 resistance. This, in general, means that to obtain maximum efficiency the 

 throat resistance of the horn should be proportional to the frequency, 

 since the reactance is primarily mass reactance and, therefore, proportional 

 to the frequency. The surge resistance of the exponential horn is inde- 

 pendent of the frequency. However, the acoustic resistance ^^ of a mul- 

 tiple flare horn increases with frequency as shown in Sec. 5.23. Therefore, 

 the efficiency is higher over a wide range than in the case of a horn with a 

 single rate of flare. The efficiency characteristic of the multiple flare 

 horn described in Sec. 5.23 coupled to a diaphragm and coil having a mass 

 ratio of 2 operating in a field of 22,000 gausses is shown in Fig. 8.16. This 



25 Massa, F., RCA Review, Vol. 3, No. 2, p. 196, 1938. 



26 Olson, H. F., Jour. Soc. Mot. Pict. Eng., Vol. 30, No. 5, p. 511, 1938. 



