ACOUSTIC PHASE INVERTER 



131 



condition is fulfilled above the resonance frequency of the system. Below 

 the resonance frequency the output falls off rapidly with frequency. A 

 consideration of the equivalent circuit shows that the fundamental reso- 

 nance of the system is influenced by both the acoustic capacitance of the 

 suspension system and the acoustic capacitance of the cabinet volume. 

 The response frequency characteristic with and without the back on the 



SECTION A-A 



FRONT VIEW 



Mc II Ma r^;^ ' 



1 



100 



FREQUENCY 



EQUIVALENT CIRCUIT 



Fig. 7.10. Back enclosed 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 vaa the inertance and acoustic resistance of the air load upon the front of the cone. 

 Cav the acoustic capacitance of the cabinet volume. 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 back enclosed cabinet is labeled B 

 on the graph. The pressure response frequency characteristic with the back removed is 

 labeled A on the graph. 



cabinet is shown by the graph of Fig. 7.10. It will be seen that the re- 

 sponse with the back enclosed cabinet is superior to the conventional open 

 back cabinet. This is in general the case, providing the fundamental 

 resonance frequency of the cone is made sufficiently low. The low fre- 

 quency response of the open back cabinet falls off more rapidly with de- 

 crease in the frequency than that of the closed cabinet. This is due to the 

 fact that the open back cabinet acts as a doublet while the closed back 

 cabinet acts as a simple source. See Sees. 2.2 and 2.3. The power output 

 of a doublet for constant velocity is proportional to the fourth power of 

 the frequency while the power output of a sim.ple source is proportional 

 to the square of the frequency. As a consequence, the power output of 

 a back enclosed cabinet will be independent of the frequency above the fun- 

 damental resonance frequency of the system. 



7.11. Acoustic Phase Inverter^. — The acoustic phase inverter consists 

 of a direct radiator loud speaker mechanism mounted in a back enclosed 

 cabinet with a pipe coupling the cabinet volume to the air (Fig. 7.11). 



^ Dickey, Caulton and Perry, Radio Engineering, Vol. 8, No. 2, p. 104, 1936. 



