Radio broadcast .. (1922-30)

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Data on Electrical and Acoustical Characteristics ELECTROSTATIC LOUD SPEAKERS By F. J. SOMMERS and G. E. MATTOS THE PURPOSE of this article is to de- scribe some interesting features in connection with a recent type of electrostatic loud speaker and to discuss its electrical and acoustical characteristics as determined by engineering tests. At the present time, as is generally con- ceded, the electrodynamic loud speaker, which makes use of a moving coil in a strong magnetic field, is the most powerful and efficient sound reproducer that engi- neering science has developed. It is not only a satisfactory loud speaker from the engineering standpoint, but its even re- sponse, good tone quality, and ability to transform comparatively large audio- frequency currents into sound, have made it exceedingly popular with the radio public. Despite this tendency toward loud speakers of the electrodynamic type, interest has lately been aroused in certain electrostatic loud speakers now in the course of development, which, because of their simplicity, may be manufactured at a comparatively low cost. Engineers in general are, for good reasons, inclined to look with disfavor upon any device which transforms electri- cal energy into mechanical through the medium of electrostatic attraction and re- pulsion alone. Everyone knows that the sources of power loss in such devices are many, that electric charges are prone to leak off before they can be put to use, and that even where the best dielectrics and best design are used, the loss of energy due to dielectric hysteresis and corona effects is liable to be considerable. If high volt- ages are used, these losses are greatly magnified, though they are also present when comparatively low voltages are employed. While many claims are being made in favor of electrostatic loud speakers, at the present time there is little engineering data available as to their characteristics, and it was for this reason, coupled with the widespread interest in such devices, that the tests described in this article were undertaken. Mechanical Construction The mechanical construction of the loud speaker tested is as follows: The back plate (see Fig. IA) is a disc of 24 gauge sheet-iron thirty inches in diameter, Undergraduates, University of Santa Clara whose periphery is bent up to form a flange half an inch wide. In this back plate are punched some 2000 quarter-inch holes, the holes being punched so that there is a sharp bur at the back of the plate, but none in front. In addition to this, the back of the plate is "dished-in" half an inch, as shown in the figure. Over this back plate is stretched a membrane of varn- ished silk two-thousandths of an inch thick, and a layer of imitation gold foil is applied to the outer surface of the mem- brane. An enlarged cross-section of the loud speaker is shown in Fig. IB, which shows the type of hole punched in the back plate, as well as the relation of the varnished silk and the gold foil with re- spect to the back plate. In using this loud speaker, it may be 8+ CIRCUIT- A SPEAKER J Fig.l CIRCUIT- C Fig. 2 connected to the amplifier in a number of ways—three of which are shown in Fig. 2. Referring to Fig. 2, circuit A is one in which no external source of bias voltage is used. In circuit B, bias potential is sup- plied by means of a battery. Circuit c is the same as circuit B, except that the resonant frequency of the loud speaker and effective inductive reactance of the output transformer have been altered by placing a condenser in series with the loud speaker. These circuits were chosen for tests be- cause the results obtained could be ana- lyzed more easily with respect to the effect of the resonant frequencies of the loud speaker circuit and the effect of various bias voltages. For the sake of clarity in explaining its characteristics, the theory of the _loud speaker in question will first be outlined. It can be seen that the electrostatic loud speaker is nothing more than an electro- static condenser, one of whose plates is fixed, and the other is free to vibrate. When the condenser charges, there is an attrac- tive force which pulls the diaphragm more closely to the back plate. When it is dis- charged, there is a certain restoring force supplied by the elasticity of the diaphragm which pulls it back to its initial position. It can be seen, therefore, that the dia- phragm will vibrate in accordance with voltages across the loud speaker. Source of Sound It might be thought that the foregoing was the entire theory of the loud speaker, and that the holes in the back plate were merely for the purpose of allowing air to escape. On the contrary, we have found that very little of the sound comes from vibration of the diaphragm as a whole, but that most of it comes from a more intense vibration taking place in the parts of the diaphragm immediately over the holes. It is proposed by the authors that this in- creased vibration is partly due to distor- tion of the electrostatic field about the holes, in such a way that there exists a large difference of potential between the sharp burred edges of the holes, and the parts of the diaphragm over the holes. The electrostatic field about the holes is by no means uniform, and changing the shape of the holes may greatly alter the distribution of electrostatic flux lines be- tween the membrane and the back plate. This increased vibration is also partly due to the curvature of the holes from the front inward. As shown in Fig. 4, the vibration of the membrane may be thought of as a progressive process. As the dia- phragm rolls inward over the hole in Fig. 4, it can be seen that there is always a com- paratively large force on such parts as at A for position I and B for position 2. In addition to this, the authors have found that when large holes are used, the loud speaker responds more easily to the lower frequencies, while if smaller holes are used, it responds better to the higher frequencies. From theoretical considerations, the response of the loud speaker with change of frequency may be said to depend mainly upon the following factors: mechanical resonance points in the back plate caused by the particular construction used; the size of hole used; the shape of hole used; the electrical resonance effects due to the circuit used in connecting the loud speaker to the amplifier; and the magnitude of the bias potential applied to the loud speaker plates. It can be proven both mathematically and by experiment that harmonic dis- XI* o f Z-12 -DYNAMIC -ELECTROSTATIC o o o ooo o o o o ooo tfl <O C- tOGIO FREQUENCY IN CYCLES o o o o Fig. 3 106 • • DECEMBER 1929