162 



HORN LOUD SPEAKERS 



practically a constant if the voice coil is made longer than the air gap, 

 because as the coil moves into the weaker tufting field on one side it moves 

 into a stronger field on the other side. From the standpoint of efficiency 

 at the higher frequencies this method is not particularly desirable because 

 part of the voice coil is in a weak field. This type of distortion can also 

 be eliminated by making the air gap of sufficient axial length so that the 

 voice coil remains at all times in a uniform field as shown in Fig. 8.11. 

 The latter method is usually used for high frequency loud speakers of high 

 efficiency. Also see Sec. 7.15. 



E. Subharmonic Distortion'^'^''^^. — The distortions referred to above 

 have been concerned with higher harmonics, that is, multiples of the 

 fundamental. It has been analytically shown that subharmonics are 



CROSS-SECTIONAL VIEW 



Fig. 8.12. A system consisting of a mass m\ driven by a crank at a frequency/ and a second 

 mass m^ supported by a spring coupled to mi vibrating with a frequency ^f. The cross- 

 sectional view of the cone shows a similar system and illustrates how subharmonics may 

 be produced by a loud speaker. 



possible in certain vibrating systems. Figure 8.12 illustrates the mech- 

 anism of one type of subharmonic. The driven mass mi at the end of the 

 bar vibrates at a frequency/ while the mass m^ vibrates at a frequency 

 I/. In the same way a cone, Fig. 8.12, will vibrate at a subharmonic fre- 

 quency. The existence of subharmonics in direct radiator loud speakers 

 is well known. However, in horn loud speakers the diaphragms are rela- 

 tively small and quite rigid. Consequently the conditions for the pro- 

 duction of subharmonics are not particularly favor-^ble. Circular corru- 

 gations in the diaphragm or cone may be used to increase the stiffness and 

 thereby reduce the tendency to break into subharmonic vibrations. Also 

 see Sec. 7.15. 



22 Pederson, P. O., Jour. Aeons. Soc. Amer., Vol. 6, No. 4, p. 227, 1935. 



23 Pederson, P. O., Jour. Acous. Soc. Amer., Vol. 7, No. 1, p. 64, 1936. 



