DOUBLE COIL, SINGLE CONE LOUD SPEAKER 121 



actance of the compliance Cm is large compared to the mechanical im- 

 pedance Zmi and consequently the entire system moves as a whole. At 

 high frequencies the mechanical reactance of the compliance Cm is small 

 compared to Zmx and the small cone Zmi moves while Zmi remains sta- 

 tionary. By means of this reduction in cone mechanical impedance the 

 range may be extended almost a full octave, depending upon the mass and 

 electrical impedance characteristics of the voice coil. The response char- 

 acteristics of a single coil, single cone loud speaker is shown in Fig. 1.6 A. 

 The voice coil and large cone of Fig. 1 .6B combination is the same as that 

 of Fig. 1 .6A. The high frequency range has been extended about one- 

 half octave without any sacrifice of power handling capacity. 



7.5. Double Coil, Single Cone Loud Speaker^. — The double coil 

 single cone loud speaker. Fig. 7.6C, consists of a voice coil, divided into two 

 parts separated by a compliance, coupled to a single corrugated cone. 

 The larger portion of the voice coil Li, rsi is shunted by a capacitance Ce- 

 At low frequencies the reactance of the capacitance is large compared to the 

 impedance of the larger portion of the voice coil Li, Tei and the mechanical 

 reactance of the compliance Cm separating the two portions of the voice 

 coil is large compared to the mechanical mass reactance of mi and the 

 mechanical impedance Zmi- Therefore, in the low frequency range the 

 action is the same as that of a single coil loud speaker. At high frequen- 

 cies the reactance of the capacitance Ce is small compared to the imped- 

 ance Liy Tei or L2, rE2 and the mechanical reactance of the compliance 

 Cm is small compared to the mechanical reactance of mi. The cone is 

 driven by the lighter portion m^ of the voice coil and the heavy coil mi 

 remains stationary. In the mid range there is a phase difference between 

 the currents in the two portions of the voice coil. A corresponding phase 

 shift occurs in the mechanical system. As a consequence, a smooth over- 

 lap is obtained in going from two coil operation at the low frequencies to a 

 single coil operation at the high frequencies. Above the frequency of ulti- 

 mate resistance the radiation resistance is a constant. In order to obtain 

 uniform output in this range the impedance of the system must be inde- 

 pendent of the frequency. This may be accomplished by embossing suit- 

 able corrugations in the cone which reduce the effective mass reactance. 

 The double coil system reduces the effective mass reactance of the voice 

 coil as compared to a single coil, as well as the electrical impedance at the 

 higher frequencies. A typical response characteristic of this loud speaker 

 is shown in Fig. 7.6C 



3 Olson, H. F., Proc. Inst. Rad. Eng., Vol. 22, No. 1, p. 33, 1934. 



