Radio Broadcast (May 1929-Apr 1930)

Record Details:

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Input Input _ BALANCED ARMA1 URE TYPE INDUCTOR DYNAMIC rieia supply " MOVING COIL DYNAMIC The above schematic drawings shoiv clearly that the inductor-dynamic loud speaker motor differs both in construction and principle of operation from (he balanced-armature and moving-coil types of loud speaker driving units. A New Trend in Reproducer Design THE INDUCTOR DYNAMIC LOUD SPEAKER By R. H. DREISBACH Farrand Manufacturing Company Anew dynamic loud speaker, the "Inductor Dynamic," has recently made its appearance in the radio field. This is the result of some three years work by C. L. Farrand and has now been perfected to such a degree that it has surpassed the previous hopes of the inventor. This loud speaker, in the author's opinion, has several decided advantages over the moving-coil dynamic; it requires S Position X. / at Rest — *m<t Fig. 1 — If the flux in the two magnetic circuits were 180 degrees out of phase there would be no restoring force. no d.c. excitation for its field and experiments indicate that it is more sensitive and more efficient at the lower frequencies. As its name may imply, the construction and principle of operation of the inductor loud speaker is a new departure from the dynamic loud speaker as known up to the present time. [The term "dynamic" is here used by the author to denote force or motion. Dynamic is usually associated with moving-coil loud speakers but in its general meaning it is equally applicable to all types of loud speakers. — Editor.] Unlike the moving-coil dynamics, the inductor utilizes 4,wo U-shaped permanent magnets to suppl y its fixed magnetic field instead of requiring some external source of energy to supply this field. Instead of the usual moving coil, the armature is of moving iron and is composed of two separate bars connected by tie rods, each bar working between its respective pole faces. Because the armature is of moving iron the reader may be prone to confuse the operation of this loud speaker with that of the balancedarmature type. To alleviate the possibility of this mistake it may be well to point out the disadvantages of the balanced-armature type which are not present in this new loud speaker. In the case of the balanced-armature type, the distance between the armature and the pole face is varied. The magnetic force exerted on the armature varies inversely as the square of this distance and the force exerted on the armature by its soring support varies inversely as this distance. From this we see that an element of distortion has entered into the operation of this type of motor. For the balanced-armature motor to be comparatively sensitive this gap between the armature and pole face must be quite small, but then the spring supporting the armature must be made quite stiff to prevent the armature from "flopping" against the pole face. This necessary stillness impairs the operation of the motor at the lower frequencies; consequently, the design of a balanced-armature motor is a compromise between these two evils. The other obstacles found in the design of DRIVING PIN ARMATURE SPRINGS TIE RODS SPRINGS " The construction of the armature assembly of the inductor loud speaker is clearly illustrate! I above. balanced-armature units concern spring resonances and the fact that the apex of the cone does not move in a straight line but follows an arc. Features of Inductor Type All of these undesirable features are absent in the inductor dynamic. In this new loud speaker the gaps between the armature and the pole faces remain Fig. 2 — In practice, a restoring force (see dotted line) is created by shifting the phase of the fluxes 18 degrees. constant and the area by which they overlap is varied. The restorative force in an inductor loud speaker is magnetic and is a restorative force rather than a restraining force, the latter being the case with the spring of a balanced-armature motor. The inductor unit must necessarily have a spring support for the armature but the primary function of this spring is to hold the gap constant, and not to supply the restorative force but to leave that function to the magnetic force. The driving motor is an induction motor with reciprocating action instead of rotary action. Consider Fig. 3. The armature assembly rides freely between the pole pieces, Pi and P>. The coils, Ci and G;, are connected in series. A current flowing through the windings in the direction indicated will increase the flux through the pole legs Pi and decrease the flux through the pole legs P>. The flux, seeking the path of least reluctance, exerts a greater force on the armature bar At 94 • • JUNE. 1929 •