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DIRECT RADIATOR LOUD SPEAKERS 



an acoustic capacitance for changing the output from the horn to the open 

 side of the cone for reproduction of the mid and high frequency ranges 

 (Fig. 7.13). At low frequencies the reactance of the acoustic capacitance, 

 equivalent circuit, Fig. 7.13, is large compared to the impedance Zai at 

 the throat of the folded horn. Therefore, the cone is coupled directly to 

 the horn in this range. The combination of a horn and cone loud speaker 

 mechanism yields high efficiency and smooth response at the low frequen- 

 cies. In the system shown in Fig. 7.13 at 150 cycles the reactance of the 

 acoustic capacitance Cai becomes equal to the throat impedance Zai- 



SECTION A-A 



SECTION a-B 





EQUIVALENT ELECTRICAL CIRCUIT 



/■ 



Fig. 7.13. Sectional views of the combination horn and direct radiator loud speaker. The 

 straight axis equivalent of the folded horn is shown above. In the equivalent electrical 

 circuit of the acoustical system: Mc the inertance of the cone. Cac the acoustic capaci- 

 tance of the cone suspension. Ma and rAA the inertance and the acoustic resistance of the 

 air load upon the front of the cone. Cai the acoustic capacitance of the chamber behind 

 the cone, zai the impedance at the throat of the horn. The driving pressure p in the 

 acoustical system is the force generated in the voice coil divided by the area of the cone. 

 The graph shows the pressure response characteristic of the combination horn and direct 

 radiator loud speaker. The overlap between horn and direct radiator action is shown by 

 the dotted and dashed characteristics. (After Olson and Hackley.) (Courtesy of The 

 Blakiston Company from Olson and Massa " Applied Acoustics.") 



Therefore, above 150 cycles the response from the horn is attenuated. 

 The major portion of the output above 150 cycles issues from the front of 

 the cone and the system behaves as a simple direct radiator loud speaker. 

 The use of a horn as coupling means between the cone and the air makes 

 it possible to obtain large power outputs from a small diameter cone. A 

 small diameter cone is particularly suitable for good efficiency, wide angle 

 distribution and smooth response as a direct radiator loud speaker for the 

 mid and high frequency ranges. A cone with a single coil may be used 

 for reproduction to 7000 cycles. For reproduction to 12,000 cycles a 



