124 DIRECT RADIATOR LOUD SPEAKERS 



citance Ce is small compared to the impedance of either the heavy coil 

 Li, VEi or the light coil Li, r^i and practically all the current flows in the 

 light coil. The mechanical impedance of the compliance Cm is small com- 

 pared to the mechanical reactance of jtii or the mechanical impedance 

 of Zmi- Therefore, at high frequencies the small cone 2^1/2 is driven by the 

 light coil mi and the heavy coil mi and the large cone zmi remain station- 

 ary. This system is equivalent to two separate loud speakers. The ad- 

 vantage, of course, resides in the fact that only a single field structure of 

 the size required to accommodate a coil the size of the two coils is needed. 

 Such a field structure coil system and large cone would be required to 

 give equivalent performance at the low frequencies. The response fre- 

 quency characteristic of a double cone, double coil loud speaker is shown 

 in Fig. 7.6D. This system provides a means of obtaining good response 

 to 14,000 cycles. At the same time, the diameter of the large cone 

 can be chosen so that moderate power handling capacity as a direct radi- 

 ator loud speaker may be obtained at the low frequencies. 



The directional characteristics of a vibrating piston are given in Sec. 2.7. 

 These characteristics show that if the effective diameter of the cone de- 

 creases inversely with respect to the frequency, the directional character- 

 istics will be independent of the frequency. The effective diameter of 

 practically all direct radiator loud speakers decreases at the higher fre- 

 quencies. The effect can be accentuated by corrugating the cone as 

 shown in Fig. 7.6C A very close approximation to uniform directional 

 characteristics may be obtained in the double cone loud speaker by a 

 suitable selection of cone diameters. 



7.7. Mechanical Networks for Controlling the High Frequency Response 

 of a Loud Speaker. — In general, in radio and other forms of sound repro- 

 duction it is desirable to attenuate the response above a certain high fre- 

 quency limit. In some cases, it may be desirable to attenuate a certain 

 band as, for example, 10,000 cycles in radio reproduction to eliminate the 

 adjacent channel beat note. Electrical networks and filters are usually 

 quite costly compared to mechanical filters for certain applications in sound 

 reproduction. It is the purpose of this section to describe the construction 

 and performance of several mechanical networks and filters for suppressing 

 certain frequency bands or for attenuating the high frequency response 

 of a loud speaker. 



A relatively light weight, 8-inch loud speaker was chosen for these tests. 

 This type of loud speaker is used in small radio receivers. Due to the 

 small mass of the cone and coil the response is well maintained at the high 

 frequencies. The principles involved are applicable to all loud speakers. 



