Notes on the Condenser Speaker

Radio Record, Volume IV, Issue 1, 18 July 1930, Page 42

Notes on the Condenser Speaker

A New Movement giving Surpris- | ing Realism

By

CATHODE

= JW that at least one coma] mercial receiver employs a re- {| producer the’ operation of Mw. which depends upon the prin33) ciple of electro-static attrac4 tion, it should be of some interest to investigate the theory and possibilities of this type of speaker. As may be guessed, the speaker consists of two metallic plates separated by a thin sheet of insulating material or dielectric. The speech currents are applied between the two plates and traverse the capacity existing between them. A condition for suecessful operation is that the plates shall be polarised; that is to say, one plate must be charged with a direct current E.M.F. of several hundred volts. Where the circuit and power supply employed does not permit of the automatic application of the polarising voltage, it may

readily be obtained from alternating current mains by means of a small transformer-rectifier combination and blocking condenser. The rectifier tube may wel be of the ordinary 201A type, as it is called upon to pass only three or four milliamperes. Constructional Details, "THE unit of a condenser speaker is naturally very thin. This, combined with its ruggedness, makes it a simple matter to disguise it effectively. It may be embodied in the furniture

of the room as a table-top, a wall tapestry, or a screen; alternatively, it may form the top or front panel of a console model radio: receiver. The most successful form of construction as yet adopted consists essentially of (a) a perforated metal back-plate having small corrugations or other undulated surface; (b) 4 flexible dielectric diaphragm stretched over the back plate so as to bridge the corrugations or depressions; and (c) a thin and flexible conductive coating cemented or otherwise secured to the surface

of the diaphragm opposite the backplate. The corrugated back-plate is really the feature which contributes most to the success of the speaker. The writer has a lively recollection of his own early experiments with the condenser speaker. After making the discovery that extremely thin sheet rubber of the toy balloon type was about the owy material for the diaphragm that would give any results at all, and after spending a considerable sum in buying sheets of rubber until one was found ly free of defects to stand the polarising voltage, it was found that anything approaching efficiency was even. then unobtainable. A flat back-plate was in use, of course, the idea of a corrugated plate not then presenting itself, and when the diaphragm placed in contact with the back plate, the possible movement

(limited to the elasticity of the diaphragm) was so small as to render the output of a 210 type valve inaudible at five feet. Somewhat better results Were obtained by stretching the diaphragm a millimetre or so distant from the back plate, but here again there were two factors which militated against success. The first wag that the dimensions of the unit had to be kept very small or the diaphragm touched the back plate. An additional inducement toward small dimensions was introduced by the necessity of drilling countless holes in the back plate to provide the now essential ventilation. . The second factor was the distance and appreciable air space between the back plate and the flexible conductor (which, by the way, consisted of a oth sheet of tinfoil solutioned: to the \rubber sheet). The electrostatic. force existing between the two plates of a condenser varies inversely as the square of the distance between them, and directly as the square of the dielectrie constant of the insulator separating them. Now in the present case not only were the plates necessarily separated by a substantial distance, but the greater part of the dielectric consisted of air, which, of course, has the very low dielectric constant of 1. So one, more disappointed experimenter temporarily abandoned the condenser speaker. Design Considerations, [THE corrugated back plate overcomes the difficulties mentioned in a most ingenious manner, The portions of the diaphragm which bridge the depressions in the back plate are prac- ... tically unrestricted as to movement "sand can readily accommodate them- / Selves to the large amplitude of motion necessary for realistic reproduction of bass notes., Also, a considerable electrostatic force is exerted on those portions of the flexible conductor where the diaphragm is in contact or nearly in contact with the back plate. It is the portions of the diaphragm which are nearly in. contact with the back plate which are most useful. As the diaphragm is attracted under the influence of the electrostatic force the area of contact is increased and the diaphragm rolls. down the slopes of the. depressions. A wedge-edged air space is still maintained adjacent to the point of contact so that the attracted area moves down the slope too. The modern condenser reproducer owes its efficiency almost entirely to this wedge-shaped air space which per- mits a large force to be exerted and at, the same time allows a considera movement of the diaphragm. e depth of the corrugations or undulatiors in the back plate should pre. ferably not be too great, as the efficiency is higher when the depressions are shallow. It is necessary, however, that there shall be sufficient depth to accommodate a fairly considerable movement of the diaphragm. Ventilation must be provided in the depressions of the plate so that trapped air will not restrict the movement of the diaphragm. Commercial speakers usually provide the ventilation in the form of narrow milled slots running at . ight angles to the corrugations, Drilled ~}-holes, provided there are enough of ' them, would be just as effective. The back plate must be fairly stiff or there is danger of it resonatirg and "buzzing" against the diaphragm. The same "buzzing" sometimes arises through the diaphragm not being in close contact with the back plate

throughout its area. This condition is usually remedied by making the plate slightly convex toward the diaphragm. Any metal may be used for the plate, perhaps the best material for the purpose being the corrugated aluminium used for airplane construction. The Diaphragm. HE material used for the flexible diaphragm of the commercial condenser speakers is one known as "Kylite" and has been specially developed for the purpose. If any misguided experimenter feels disposed to tread the fields of endeavour which the writer has explored with doubtful success, he will find, as ‘has been -previously mentioned, thin sheet rubber of the "toy balloon" variety the best available material. ‘"Kylite"’ is used in a thickness of five thousandths of an inch, but it is doubtful whether rubber can be obtained as thin as this, or, if it can, whether it will stand the application of the polarising voltage. The flexible metal coating on the front surface of the diaphragm presents another difficulty to the experimenter. However, thin and uncreased tinfoil can quite well be used for experimental | purposes, although beaten leaf or a sprayed metal coating is necessary where greater permanency is desired. Fitting a Baffle. L752 any. other speaker, for best results the condenser reproducer demands some form of baffle. However, since the considerable area of the speaker itself acts as a partial baffle, it is permissible to reduce the baffle proper to such a size as to add a margin of 10 inches round the edge of the reproducing unit. The baffle may either be in the same plane as the reproducing unit or, more conveniently perhaps, it may take the form of a rectangular box of which the speaker forms the front. The space at the rear of the speaker is then available for a radio receiver and power pack, although it must not be filled in so solidly as to interfere with the free’ passage of air currents. It is very necessary with this type of speaker that nothing in the baffle or speaker mounting (¢g., wood or sheet metal panels) resonates appreciably at any frequency within the audible range. One particular form of resonance is apt to be troublesome. Speakers of this type are almost invariably built up of small units having dimensions about eight inches by twelve. inches; when such a multi-unit speaker is solidly supported all round its periphery, the whole assembly some. times vibrates as a single diaphragm. and with sufficient amplitude to separate the diaphragmn from the back plate and produce a buzzing sound. The remedy is to divide the speaker by a stiffening member or to attach a brace to its centre; the speaker may also be mounted in damping material-felt or rubber-at the edges. _ The necessity for a source of polaror biasing voltage to maintain a potential difference between the two plates of the condenser speaker has already been touched upon. It is interesting to note that it is a condition for freedom from harmonic or wave-form distortion that the alternating or speech voltages shall be small compared with the polarising voltage. This calls for a high polarising or biasing voltage, as does the fact that the force exerted on the diaphragm varies as the square of the biasing voltage. Nevertheless, there is nothing to be gained by in-

creasing the biasing voltage above about 600 volts, as the diaphragm will be drawn tightly against the back plate and a loss of efficiency result. Between 500 and 600 volts is the best biasing voltage, There is, by the way, no danger of shock from this source, as there’ is practically no current available. -Owing to limitation of space available, the conclusion of. this article and the illustrations must be held over till next week. ,