ELECTROMECHANICAL TRANSDUCERS 



above. They have electrical and mechanical resistance, mass and comphance. 

 In many cases the mechanical impedance is determined almost entirely by 

 the mass of the moving parts and the compliance of the control spring; here 

 the impedance of the electrical supply will have little effect. If, however, 

 particular efforts have been made in the design of an efficient transducer to 

 achieve, for example, a low output impedance, all these efforts may be 

 nullified if the transducer is fed from an unsuitable (i.e. low) impedance 

 electrical supply. In other cases, the abihty to introduce a mechanical 

 resistance by suitable design of the driving circuit is of great value in achieving 

 a good frequency response. 



Moving-iron transducers — The most elementary forms of moving-iron 

 transducer are the solenoid and the electromagnet {Figure 33.23). As drawn. 



Soft_ 

 iron 

 plunger 



Soft iron 

 yoke and 

 armature 



Figure 33.23 Elementary moving-iron transducers: (a) solenoid; (h) electro- 



magnet 



they are of no value except as actuators of an 'on-off' variety. This is 

 because any inward movement of the armature increases the tractive force 

 since the reluctance of the magnetic circuit is reduced; the situation is 

 unstable, and the armature is eventually fully attracted. If a control spring 

 is provided on the armature over a small range of current in the coil 

 corresponding movement occurs, but any greater current will cause the 

 armature to move to its limit. Furthermore, since the tractive force is 

 proportional to the square of the current in the coil, gross distortion of any 

 applied signal results. 



By a simple modification (Figure 33.24) the moving-iron transducer can 

 be made reasonably linear, and therefore a practical device. A stiff control 

 spring is still necessary to prevent the armature locking in ; this spring gives 

 the transducer a high output impedance (i.e. approximately constant move- 

 ment regardless of load). The ordinary moving-iron headphone is an example 

 of this type of transducer, and lends itself conveniently to modification for 

 biological use. 



Moving-coil transducers — The unit shown in Figure 33.13 can be used 

 directly as an electromechanical transducer, and if the suspension is 

 sufficiently flexible it has a very low mechanical output impedance. Consider- 

 able force is available from moving-coil transducers, typical values being 

 5 X 10^ dynes (500 gwt) for a unit designed specifically as a transducer, 

 and 2x10^ dynes (200 g wt) for a transducer made from an ordinary 



493 



