Radio Broadcast (May 1928-Apr 1929)

All about the DYNAMIC Loud Speaker By JOSEPH MORGAN Jnternational Resestance Company IT IS the purpose of this article to set forth clearly, simply, and without prejudice, the application, performance, method of use, advantages, and disadvantages of the modern dynamic loud speaker. The arrival of a new and successful device upon the market is always attended by exaggerated claims and general misinformation as to its use and operation. This is not so much due to the desire of the manufacturer to further the sale of his product, as to the misguided e/©(|! enthusiasm of the radio fan. That such a condition should exist is deplorable, since the new device is often improperly employed and its good characteristics are discredited. No new instrument is a panacea. Perfect design in a vacuum tube cannot compensate a defective rheostat. ]A perfect loud speaker cannot even slightly ignore the defects of an overloaded amplifier. In fact, the reverse is more nearly true — a poor loud speaker can, to a remarkable extent, overcome such defects! Before considering the dynamic speaker in detail, the reader should have a knowledge of the general problems of loud speaker application and design, sai®0; as well as the specific principle upon which the dynamic type is based. For this purpose the reader is referred to the article by the present writer, titled "All About Loud Speakers" which appeared in the August, 1928, Radio Broadcast. The three most important properties to be considered in the discussion of the technical merits of a loud speaker are: (1) The frequency-response characteristic (2) The efficiency (3) The load capacity We will consider first the frequency-response characteristic. The dynamic type of loud speaker, using a free-edge paper cone, is es sentially what is called an "inertia-controlled diaphragm" loud speaker. This means that throughout the important frequency range the electrical driving force is expended in accelerating the mass of the diaphragm. In other words, the moving structure, which consists of the voice coil and the sound-radiating paper cone, acts as a solid piston. If it were true that the dynamic loud speaker behaved solely in this manner ~KAANY months ago, in " Strays from the Laboratory," -ivl we predicted that the "dynamic" loud speaker would probably be predominant in the radio field during this season. That prediction has been amply borne out. Subsequent comment in "Strays" has compared the performance of these reproducers with other types and presented some information on their operation. This article, written by Joseph Morgan, an engineer in whom we have the highest confidence, does answer practically all of the questions which arise. A careful reading will enable those who are using this type of reproducer to make it better sense their needs and will clarify the minds of those who now feel they are pretty cloudy on the whole subject. — The Editor. throughout the entire audio-frequency range, the frequency-response curve would be very nearly a straight line with the response gradually falling off at the higher frequencies. However, there are a number of secondary factors which must be considered in the determination of the response of such a loud speaker. We shall consider first the mechanical factors. Due to the fact that the moving coil must be supported more or less rigidly with respect to its concentric position in the air-gap of the magnetic field, two or more thin flat metal restraining springs are used, both to maintain the centering of the moving coil and to act as conductors of current to this coil. Further, the base of the cone is fastened to the metal supporting ring by means of a flexible annular ring or washer usually made of leather. These springs together with the leather ring, hold the moving structure quite rigidly with respect to radial movement, but permit very free axial motion. The combination of paper cone, moving coil, and metal springs has its own natural frequency of vibration. Advantage is taken of this natural frequency to obtain a large response at the low-frequency end of the scale, where much energy is required to produce sufficiently intense sounds, due, in part, to the fact that much of the associated apparatus, such as amplifiers, have inadequate low-frequency characteristics. This natural frequency usually occurs somewhere between 20 and 70 cycles per second and varies not only with the make of the loud speaker but also from one loud speaker to another of the same make. In one well-known make of dynamic loud speaker tested by the writer, the resonant frequency in six loud speakers chosen at random varied from 40 to 65 cycles per second. If the 3)®^ resonant frequency were kept within the range mentioned it would have practically no influence upon the response of the loud speaker above 100 cycles per second. THE CHANGE AT 3OOO CYCLES AT ABOUT 3000 cycles per second, the moving structure ceases to act as a solid piston and begins to behave as a conical diaphragm with a fixed outer edge. In that part of the sound spectrum in which the transition takes place from free-edge to fixed-edge action, large irregularities in response are likely to occur, and in the band of frequencies in the fixed-edge region the response is, in general, greater than in the freeedge region. This is due in part to the conical 159