120 DIRECT RADIATOR LOUD SPEAKERS 



In the case of the first system and referring to Figs. 7.1 and 7.2 it will 

 be seen that uniform response and adequate power handHng capacity may 

 be obtained by employing a large and a small single coil, single cone loud 

 speaker unit. The filter system may consist of an inductance in series 

 with the low frequency loud speaker and a capacitance in series with the 

 high frequency loud speaker and the two resulting circuits connected in 

 parallel with the power amplifier. See Fig. 1 .SA. Due to the large 

 electrical reactance of the large voice coil of the low frequency loud speaker, 

 see Fig. 7.4, it has been found that for most applications the inductance 

 in series with the low frequency loud speaker may be omitted. On the 

 other hand, if a more elaborate filter system is required the circuit of 

 Fig. 8.18 may be used. 



Referring to Sec. 7.2 and Figs. 7.1 and 7.2 it will be seen that uniform 

 response and adequate power handling capacity may be obtained by 

 employing a large number of small cones driven by light coils. The con- 

 stants of the system may be determined from the frequency range. Then 

 the number of units may be determined from the required power output 

 and the allowable excursion together with equation 7.5 or Fig. 7.2. An 

 arrangement of seven small loud speakers mounted in a flat baffle with 

 the voice coils connected in parallel is shown in Fig. 1 .SB. The voice coils 

 of the loud speakers may of course be connected in parallel, series or 

 series-parallel. In order to obtain better high frequency spatial distribu- 

 tion the units may be inclined at various angles, for example, the units 

 may be mounted so that the resulting vibrating surface approximates that 

 of a sphere. 



The multiple loud speaker systems described above possess certain 

 disadvantages. The radiating surfaces must be separated by a finite 

 distance with the result that the system may exhibit peculiar directional 

 characteristics in the overlap region where the sound radiation issues 

 from both loud speakers. The same is true of the small units at the 

 higher frequencies. The cost of two or more field structures is in general 

 more than that of a single structure. 



7.4. Single Coil, Double Cone Loud Speaker ^. — A typical single coil, 

 multiple cone loud speaker, Fig. 7.65, consists of a single coil coupled to 

 two cones. In this system an increase in frequency range is obtained by 

 reducing the impedance of the diaphragm by coupling a smaller cone to 

 the voice coil at the high frequencies. The two cones Zmi and Zm2 are 

 separated by a compliance Cm- At low frequencies the mechanical re- 



2 Olson, H. F., Jour. Acous. Soc. Amer., Vol. 10, No. 4, p. 305, 1939. 



