SINGLE COIL, SINGLE CONE LOUD SPEAKER 117 



of the voice coil is shown in Fig. 1 .ZA. The normal impedance, in abohms, 

 of the voice coil is 



"^EN = ZbAT + Zso 7.8 



where Zet> = impedance of the voice coil in the absence of motion, that is 

 blocked, in abohms, and 

 Zem — motional impedance of the voice coil defined by equation 7.5, 

 in abohms. 

 The components of the motional impedance are shown in Fig. 7.35. At 

 the resonant frequency the motional impedance is large because the me- 

 chanical impedance is small. The current in the voice coil circuit may be 

 determined from the driving voltage, the resistance of the generator yeg^ 

 the resistance Yed and inductance Lc of the voice coil and the motional im- 

 pedance Zem- 



The driving force, in dynes, applied to the mechanical system, see 

 Sec. 6.2, is 



Jm = Bli 7.9 



where B = flux density in the air gap, in gausses, 



/ = length of the conductor, in centimeters, and 

 i = current in the voice coil circuit, in abamperes. 



This is the driving force/^if applied to the mechanical system as shown in 



Fig. 7.3. 



The mechanical impedance due to the electrical circuit, from equation 



6.6, is 



Bl 

 zme = 7.10 



This impedance appears in the mechanical system as shown in Fig. 7.3. 

 In calculating the steady state performance the driving force /Af applied 

 to the mechanical system is used and the mechanical impedance due to 

 the electrical system need not be considered. However, in computing the 

 transient response of the system, the damping constant, etc., the mechani- 

 cal impedance due to the electrical circuit must be included. The driving 

 force of the generator in the mechanical system which will produce a force 

 (m across the mechanical system is 



/m.=/m+-^^^^ 7.11 



Zmt 



The increase of impedance of the voice coil, with frequency, in com- 



