LOUD SPEAKERS AND MICROPHONES 



265 



the fundamental be assumed as the Umit of tolerance of a second 

 harmonic, from equation (1) a throat diameter of about 8 inches is 

 determined.* 



If the radiation resistance at the throat of a horn is not to vary- 

 appreciably with frequency, the mouth opening must be a substantial 

 fraction of a wave-length. This condition calls for an unusually 

 large horn if frequencies down to 40 c.p.s. and below are to be trans- 

 mitted. However, the effect of variations in radiation resistance on 

 sound output can be kept down to a relatively small value if the 

 receiving unit is properly designed. This will be explained in the 



2.8 



2.4 

 2.2 

 2.0 

 1.8 

 1.6 

 1.4 



1.0 

 0.8 

 0.6 

 0.4 



0.2 

 



-0.2 



40 80 120 160 200 240 280 



FREQUENCY IN CYCLES PER SECOND 



Fig. 4— Radiation resistance and reactance of low frequency horn. 



next section. The low frequency horn used in these reproductions 

 has a mouth opening of about 25 square feet. As computed from well- 

 known formulas ^ for the exponential horn the impedance of this horn 



* Since the original publication of this paper, experimental data have been ob- 

 tained which indicate a second harmonic genereition in horns 6 or more db below the 

 value shown by Rocard's equation.' 



