Loudspeaker (Jan-Aug 1931)

Horns and Speaker Units By D. B. McGOWN, A. P. S. No. 7 The function of the loud-speaker, is as its name suggests, to produce loud enough sound so that all mav hear. The fact that the loudspeaker is actually a motor is a conception that is slightly different than ordinary but it is a fact. A motor is actually any device that changes energy from one form to another, as for example a heat motor that changes the heat of explosive gasoline and air mixed by the carburetor, into mechanical energy produced by the motion of a piston in a cylinder. Other forms of motors will suggest themselves to anyone, but in the case of a loudspeaker the driving unit is an electric motor which operates on speech energy alternating current, a n d which drives a diaphragm, which produces sound by the motion of the diaphragm. The sound actually results from a series of alternating compressed and rarified air conditions, almost in the form of layers, which result in vibrations in the air, and which when they reach the ear produce the result we call sound. The pitch of this sound is determined by the frequency of the alternating current supplied to the unit; the volume of the sound depends on the magnitude or amount of energy delivered to the receiver, and the tone quality is a result of the number of combinations of various frequencies which simultaneously reach the unit, and which may or may not be a “pure” or complex tone, which is determined by whether or not there are harmonics and overtones of the basic frequencies present. Thre are two general types of loudspeakers in general use in theatre reproducing systems. Both operate on the same principle, whereby a coil of wire is held by a diaphragm between the poles of a powerful electro T -iv e it ty j i x magnet, and if the said coil is fed with the speech current, the coil will displace itself backwards and forwards, exactly in accord and in propcrtion to the current supplied as alternating current energy from the Electric type of horn unit drives a small metal diaphragm which produces practically a uniform pistonlike motion, and which drives an aircolumn composed of a long horn, commonly known as an “exponential” horn. This air-column serves as an actual mechanical load for the speaker unit, and requires considerable energy to drive it. as the inertia if a column of air twelve or fifteen feet long is considerable, as obvious, when it is realized that the entire aircclumn moves in and out of the horn slightly with the motion of the diaphragm. The other type of unit uses a much larger diaphragm, which is composed of a paper or other non-resonating cone, with the driving coil at its apex, and which drives a relatively large column of air thru a short distance, the cone being free to move back and forth slightly due to a leather or other soft flexible support for the edges. This is the type horn that is used with the RCA Photophone and several other systems. When the unit is mounted in a short tapering square wooden horn, the approximate size of the horn unit at one end and spreading out to several feet square at the outer end, the result will closely approximate the same as obtained with the longer air column system described above. In any type of horn unit, the two most important things to be obtained are the reproduction of all frequencies equally, over the entire audio range, and (what is a dependent function) the obtaining of reason