132 



DIRECT RADIATOR LOUD SPEAKERS 



The equivalent circuit of this system is also shown in Fig. 7.11. The 

 phase of the volume currents on the front and back of the cone differs by 

 180°. Referring to the equivalent circuit of Fig. 7.11, the volume currents 

 in the branches 1 and 2 of the equivalent circuit may differ by as much as 

 180° for the case of positive reactances with no resistance in branches 1 

 and 2, and a pure capacitance for the branch 3. The phase will be reduced 



SECTION A-A 



FRONT VIEW 





/ B » 

 -^- /\ > 



I *«• 



-> 



I 



-AV— |— , 



EQUIVALENT CIRCUIT 



Fig. 7.11. Phase inverter loud speaker and the equivalent circuit of the acoustical system. 

 Mc the inertance of the cone. Cas the acoustic capacitance of the suspension system. 

 Ma and rAA the inertance and the resistance of the air load upon the front of the cone. 

 Cav the acoustic capacitance of the cabinet volume. Mr and r^r the inertance and acoustic 

 radiation resistance of the tube. The pressure p of the generator in the acoustic system is 

 the force generated in the voice coil divided by the area of the cone. The pressure response 

 frequency characteristic of the phase inverter loud speaker is labeled B on the graph. The 

 pressure response frequency characteristic of a loud speaker in a typical cabinet having 

 the same volume as the phase inverter is labeled J on the graph. (After Dickey, Caulton 

 and Perry.) (Courtesy of The Blakiston Company from Olson and Massa, " Applied 

 Acoustics.") 



when resistance is introduced. However, the resistance in direct radiator 

 systems is low compared to the reactances in the system, and the constants 

 may be chosen so that the volume currents issuing from the front of the 

 cone and the tube are practically in phase. This system increases the- 

 radiation resistance and decreases the reactance of a direct radiator loud 

 speaker at the low frequencies. The response frequency characteristic 

 of the acoustic phase inverter, as compared to the response obtained on 

 the same loud speaker mounted in a cabinet with the back open, is shown 

 in Fig. 7.11. The low frequency range is extended, cabinet resonance is 

 eliminated and a smoother response characteristic is obtained. 



7.12. Acoustical Labyrinth Loud Speaker^. — The acoustical labyrinth 

 loud speaker consists of an absorbent walled conduit with one end tightly 

 coupled to the back of the cone of a direct loud speaker mechanism and the 



Olney, Benj., Jour. Acous. Soc. Amer., Vol. 8, No. 2, p. 104, 1936. 



