116 



DIRECT RADIATOR LOUD SPEAKERS 



where ru = mechanical resistance, in mechanical ohms, 



m = mass of the air load, cone and coil, in grams, and 

 Cm = compliance of the suspension system, in centimeters per dyne. 

 Due to the large mass reactance of the direct radiator loud speaker the 



SECTIONAL VIEW 



OF tHE 

 MECHANICAL SYSTEM 



r — 'TRKJ^-^ww-. 



VOICE COIL 

 ELECTRICAL CIRCUIT 



j-fTTOH 



[mo 



EQUIVALENT CIRCUIT 



OF THE 

 MECHANICAL SYSTEM 



FREQUENCY 



B 



100 



FREQUENCY 



Fig. 7.3. Electrical and mechanical circuits of a dynamic loud speaker at the low frequencies. 

 The normal impedance, zen-, and the blocked impedance, zed-, characteristics of the voice 

 coil of a typical dynamic loud speaker are shown in graph A. The characteristics of the 

 resistive, tem-, and the reactive xem, components of the motional impedance zem are 

 shown in graph B. 



motional impedance is very small at the high frequencies and may be neg- 

 lected in calculating the driving force. However, at low frequencies it is 

 a factor in determining the current in the voice coil circuit. 



Figure 7.3 shows a cross-sectional view of a conventional direct radiator 

 loud speaker. The fundamental resonance frequency of the loud speaker 

 occurs at 75 cycles. The normal and damped impedance characteristic 



