Moving-Coil Speaker Manufacture

Radio Record, Volume III, Issue 24, 27 December 1929, Page 26

Moving-Coil Speaker Manufacture

An I nteresting Process --_- [% this modern age of mechanical achievement we are inclined to take everyday machinery and apparatus for granted rather than pause a moment and consider in detail its complicated construction. An interesting case in point is the modern dynamic loudspeaker, and the following description of the manifold and intricate details involved in its construction is both interesting and enlightening. The initial operation in the construction of these speakers is the shaping of the field cases. . This is accomplished by means of complicated and powerful presses which bend and shape stout steel casing as easily as a child moulds clay. It is essential that these ma-. chines should be accurate to a hairbreadth, for even a slight ‘variation in the field casing would produce a great variation in the "driving force," and thus a tremendous distortion in the sound from the speaker. In the construction of many of the parts the raw matérials.are fed to a line of machines, each of which has its own particular operation to perform, and thus finally the completed speaker part is produced. For example, the cone housings pass through the many operations necessary-the blanking of the steel, the forming operation, and finally by many stages to the last, precise blow that makes-them just the right height, the right diameter, and the eorrect angle. The manufacture of a completed speaker from raw materials necessitates over 200 inspections, as carried out in the Magnovox factory.. An inspection takes place after every machine operation, and in addition there are many inspections during the assembly of the unit. One of the most interesting phases of the manufacture lies in the automatic lathes, which turn, cut, drill, tap and finish cores without a workman near. : The problem of dynamic speaker design is a fascinating one, and explains why some speakers: are superior to others, though all are operated on the basis theory. In a.theoretical dynamic speaker the diaphragm would be massless, of infinite size, totally rigid, and would operate Without friction and with no load except the resistance of the air. In addition, there is the problem of keeping. the electrical factors constant. A speaker. should furnish a pure resistance load equal to twice the output valve impedance. These, and other theoretical specifications, which practically are impossible to adhere to, are ingeniously overcome by designers of dynamic sperkers by making the errors compensate for each other. ‘Thovgh it is impossible to obtain 4 weightless diaphragm, it can be ap-

proximated by the use of a light Paper cone which is both strong and /rigid. The weight is compensated for by suspension. Similarly, other irregularities in design are corrected by further compensating’ factors. The most important object to be attained in dynamic speaker manufacture is, of course, perfect reproduction, This is. governed’. by -many © factors, . chief among which is the strength of‘ the field coil, which is determined chiefly by the dimensions of the airgap in which the moving-coil operates. The whole design and constructions of Magnavox dynamic speakers is the result of many years of research and experimentation; seldom do _ people pause to appreciate the wonderful packground of inventive und-reative. reasoning that lies behind thes#t everyday appliances which we buy in a store, set on the table, and then forget about, except to enjoy their performances, ,