US1930518A - Electrostatic loud speaker - Google Patents

Description

Oct. 17, 1933. JQ s. HIGH ELECTHOSTATIC LOUD SPEAKER 2 sheets-sheet 1 Filed July 30, 1930 INVENTOR Juf/er1 5./7/9/7.

Oct. 17, 1933.

J. S. HIGH ELECTROSTA'IIC LOUD SPEAKER Filed July 50, 1930 2 Sheets-Sheet` 2 l? Qllnmlm INVENTOR Jwj/n High.

AT'TORNEY Patented @et l?, i3@

, lagp Llllll ELECTRSTATIC lL@ Fill susanna Application lluly 3d, 193th. Serial No. @ldt 6 (Clatronic.` (lDll lili-lll) My invention relates to loud speakers of the electrostatic type and more particularly to features therein for improving the operation. there- In a loud speaker of the type referred to, a large diaphragm is maintained in electrostatic relationship to a relatively immovable plate or plates by means of a direct-current potential impressed across the two, the gas space between l0 the diaphragm and the plate constituting the dielectric. Variations in the impressed potential, as will be produced by superimposing potentials, at audio frequency, upon the direct-current potential, will cause the diaphragm to vibrate according to the variations of the audio-frequency potentials, to reproduce music or other sounds.

For large auditoriums and theaters, a loud speaker, operating on the electrostatic principle, should prove very desirable because of the fact that a speaker of this type may be constructed on a large scale and of a size comparable to that of a movie screen, the depth of a speaker of this size then being but a matter of inches.'

As compared with speakers of the dynamic type, the electrostatic speaker is better adapted, because of its narrow dimensions, for table or wall operation, requires less weight and moving structure, and the expense oi manufacture is materially lower.

In the operation of a speaker of the above specied type, however, as the diaphragm is shifted from its normally stable position, the capacity between the diaphragm and the plate increases, as a result of the reduction in spacing and, consequently, the electrostatic force, because or the direct-current potential acting between the diaphragm and plate, increases. This increase in force tends to stretch the diaphragm to a position favoring a breakdown in dielectric er a direct short circuit of the apparatus.

Various schemes involving the creation of restoring forces have been suggested for combatting this undesirable inherent characteristic, among' which may be cited a method relating to' the production of a spring action at the clamping edges o the diaphragm, but this, obviously, vnecessitates an increase in apparent mass of the diaphragm, giving the diaphragm a natural period of vibration within the ordinary range of signal frequencies, a condition tending toward unfaithful reproduction of transmitted composition, as the diaphragm response is no longer free to follow 'the electrostatic forces set up by the audio-frequency potentials.

lf the apparent mechanical stiness of the ing force is electrical, as distinguished from mechanical.

Another object of my invention is to provide a device of the type described wherein the restoring force is uniformly distributed over the diaphragm surface. A

Another object of my invention is to provide 'a .restoring means, in an electrostatic loud speaker, which shall be independent of diaphragm characteristics.

Another object of my invention is to provide an electrostatic sound translater capable of utilizing an extremely light, and substantially massless, diaphragm, whereby an efficient translation of energy may be obtained.

Another object of my invention is to provide an apparatus of the type described having nondirectional propagation characteristics.

An additional object ofmy invention is to provide a device of the type described which permits of operation with a minimum thickness of dielectric, whereby lower operating potentials may be employed.

Other features involved in my invention will be described and pointed out in the following detailed description of various species of my de- 9@ horizontal view and a vertical View in elevation 105 of another modification,

Fig. 7 is a sectional view illustrating a slight modilcation of the structure shown in Fig. 5. In accordance with my invention, the device dSclOSed in Fig. l comprises a pair of substan- H0 soV . boundaries,

tially flat plates 1 and 3 of semi-conductive material, as, forexample, artificially prepared slate. The plates may be cast or otherwise formed with hollowed out portions 5 and 7, respectively, in one face thereof, the boundaries of said hollowed out portions 5 and '7 comprising a ridge-like structure having arcuate surfaces 9, l1, 13 and 15. Conductive surfaces 17 and 19 are provided on the exposed fiat surface of the plates to which connection may be made from any desired source of electrical potential. 1f desirable, that portion of the cast plate which defines the hollowed out portion may be Aadded as a separate element to an otherwise fiat surface to obtain the desired surface configuration.

As disclosed in the drawings, a pair of the above plates are arranged with their edges adjacent to each other to define an unrestricted sound chamber 21 and, clamped between them, is a diaphragm 23 of very thin material possessing electrical conductive properties. Aluminum foil is generally known as being a very suitable materialfor the diaphragm, although a thin diaphragm of insulating material made conductive by a coating of conductive material may also be used to advantage. Sound outlets 25 are provided through the plates for obvious reasons.

The electrical'circuit involved in the operation of the above described device comprises an audio transformer 27, thesecondary of which is shown connected between the two plates 1 and 3y through the conductive coatings 17 and 19, a variable resistor 29 being inserted in the connection to each plate. A direct-current-potential source 31 is connected in a lead joining the diaphragm to the midpoint of the transformer secondary to impress biasing voltage on the plates.

If the physical midpoint of the transformer and the electrical center do not coincide or if some differences exist in the physical and/or electrical characteristics of the structure elements, the biasing potential on one plate may not equal that on the other; which it should for enicient operation, with the result, that the diaphragm, because of an unequal pull which would then exist, wolud be distorted out'of its normally desired stable position. Adjustment of the' resistors 29 may then be made to bring the diaphragm back to its desired position.

Referring more particularly to the arcuate surfaces 9, 11, 13 and 15, defining the depression it is noted that their contact with the diaphragm 23 is substantially a linear one, as distinguished from an area contact, and the value of current leakage between the diaphragm and the plates is extremely small. During reception periods, alternating-current potentials, impressed upon the transformer, alternately add to, and deract from, the direct-current potential on the plates, with the result that the diaphragm will vibrate in synchronism therewith.

Assume that, during vibration, an alternatingcurrent pulsation causes the diaphragm to be pulled over to a position such ass illustrated by the broken line in Fig. 1. Because of an increase in capacity on one side and a decrease on the other, an additional pull will be exerted to stretch the diaphragm closer to the iight-hand plate, thereby distorting the output of the speaker and producing a condition favoring breakdown of the dielectric or a short circuit of the device.

However, in my device, the tendency of the diaphragm to vibrate beyond the desired amplitude corresponding to the alternating-current potential component of the electrostatic force, is prevented. The contact of the diaphragm with the plate toward which it has shifted has now changed from that of substantially a line contact to that of an area contact many times the value, as clearly disclosed in the figure. The leakage current through that plate will increase materially, resulting in a substantial drop of potential across the plate sufficient to reduce the electrostatic force tending to swing the diaphragm beyond its desired extent. The control, therefore, becomes automatic in its operation. The slope and the curvature of the control surface may be varied to suit conditions, as it becomes apparent that the sharper the slope the greater will `be the swing of the diaphragm before it can make an area contact of a predetermined value, and the reverse holds true for a more gradual slope.

It will be possible, therefore, to cut down the thickness of the loud speaker bysubstantially reducing the depth of the depressions 5 and "I, giving the proper considerations to the lslope of the control surfaces. In general, the shallower the depth of the depression, the more gradual should be the slope of the control surfaces, as a very quick change in contact area would be desirable under the circumstances.

In order to prevent the polarizing direct-current field from imparting an initial skew displacement of the diaphragm, as illustrated in Fig. 2, the control surface of one of the plates 1 may be given a different slopev than the other, as clearly shown by plate 33 in Fig 3,. This will cause the diaphragm to assume a rest position to the left of thegeometrical axis in the device disclosed, the exact position being determined by the relative values of the contact areas and electrostatic forces existing on each side of the diaphragm.

The modiiication disclosed in Fig. 4 illustrates the use of compressed gas as a restoring force, applied to a single-acting device, that is, one utilizing only one plate. to the speaker in gas-tight relationship thereto to form a compression chamber 37 between the diaphragm 23 and the jacket 35. A gas compressor 39 is adapted, by means of suitable connections, to build up and maintain any desirable pressure within the speaker. The broken lines illustrate the position ordinarily assumed by theT diaphragm without a restoring force, due to the electrostatic force set up by the vdirect-current biasing potential 41. The pressure of the cornpressed gas is adapted to swing the diaphragm back to its desired zero position. If' desirable, the pressure may be increased to produce an outward bulge in the diaphragm, in which case the action of. the restoring force will be very quick, which might be advantageous under certain conditions. l y

In'the forms disclosed in Figs. 1, 3 and 4, the restoring force, it will be noted, is uniformly distributed over the surface -of the diaphragm, whereby uniform operation is obtained, and buckling of the diaphragm is prevented. The dielectric thickness may be reduced to a minimum without danger of the diaphragm buckling or striking th'e. plate and short circuiting the device.

The above features are also incorporated in the modifications of my invention disclosed in Figs. 5 and 6, together with additional refinements. The diaphragm comprises a sheet of A jacket 35 is affixed` reactie paper or cloth 43 slightly impregnated with a conductive substance to render it semi-conductive, and is slidably held between a pair of plates 45 and 47 of conductive material, each plate comprising a slitted or comb-shaped member, the teeth 49 of which constitute individual electrodes having arcuated faces 5l which serve the same purpose as the control surfaces 9, etc., referred to in the other forms. On those portions of the diaphragm exposed between the electrodes of the plates, I arrange a number of parallel strips of conductive material 53, such as a foil, and connect them to a common outlet or bus 55 at the top of the diaphragm. These strips serve to distribute the polarizing potential more uniformly over the semi-conductive sheet. With the construction noted above, it becomes impossible for the metallic portions. of the diaphragm to strike the electrodes to produce a short circuit. Similar results may be obtained by making the diaphragm of conductive material and the plates of semi-conductive material.

To cure an inherent tendency of the electrostatic loud speaker to emit sound predominantly in a single direction, the speaker may be built as the section of a hollow cylinder or similar structure as shown in Fig. 7. The curved outline 57 of the device provides for uniform and eiiicient sound output in all directions along its radii of curvature. It lies within the scope of my invention to build a device partaking of the orm of a complete cylinder, in which case, the volume output will be equal in all directions in a horizontal plane. A loud-speaker in which the diaphragm comprises a spherical surface, which may be complete except for a small area at each end of one diameter, is also within the scope of my invention. The members 51 and 53 would then be lunes of such a sphere. It will be noted that the devices of Figs. 1 and 5 make use oi a variable area of electrical contact between the diaphragm and its supports to minimize the effects of variation of electrical capacity with diaphragm displacement. While I have described these as applied to a doubleacting loud speaker in which the electric eld rllls the space on each side of the diaphragm, it may be desirable to apply the variable-contact-area expedient to certain single-acting loud-speakers, such as that shown in Fig. 4, in which the electric field operates upon but one face of the diaphragm; and such construction is within the range of variations which I contemplate.

It will thus be obvious that I have disclosed structures which full the objects ofmy invention. While I have illustrated and described, in detail, a plurality of such/structures, I do not desire to be limited to the specific details except as is necessitated by the prior art and the appended claims.

I claim as myinvention:

l. In an apparatus of the type described, a plate, a diaphragm in spaced electrostatic relationship thereto, and means for establishing a semi-conductive surface Contact between said plate and said diaphragm and means for causing said semi-conductive surface contact to vary during movement of said diaphragm relative to said plate.

2. In an apparatus of the type described, a plate member, a diaphragm member in electrostatic relationship thereto and normally making a line contact therewith, one of said members being of a semi-conductive material, the other of said elements being of a conductive material, and means for causing said Contact relationship to vary during changes in said electrostatic relationship.

3. In an apparatus of the type described, a plate comprising a slitted structure, a diaphragm electrostatically positioned with respect to said plate, and conductive elements on said diaphragm intermediate the tooth elements of said slitted structure.

4. In an apparatus of the type described, a plate comprising a slitted structure, a diaphragm electrostatically positioned with respect to said plate, conductive elements on said diaphragm lintermediate the tooth elements of said slitted structure, said plate and diaphragm being curved to dene a section of a hollow cylinder.

5. As an article of` manufacture, a plate for an electrostatic loud speaker comprising a substantially nat member of semi-conductive material, a portion of said member defining a depression in one face of said plate, said portion comprising a ridge-like structure having an arcuate surface.

6. In an apparatus of the type described, a pair of plates of semi-conductive material having depressions therein, said plates being arranged face to face to dene a substantially unrestricted chamberv therebetween and a diaphragm of conductive material in said air chamber between said plates and making 'a direct semi-conductive contact with said plates at the edges thereof.

JURJEN S. HIGH.

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