. Elements of acoustical engineering. Electro-acoustics; Sound. 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
. Elements of acoustical engineering. Electro-acoustics; Sound. 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. The latter method is usually used for high frequency loud speakers of high efficiency. Also see Sec. 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. 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 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. , 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. 22 Pederson, P. O., J
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