US3014098A - Transducer - Google Patents

Description

Dec. 19, 1961 c. l. MALME 3,014,098

TRANSDUCER Filed May 8, 1959 2 Sheets-Sheet 2 INVENTOR.

CHARLES l. MALME BY Wy) ATTO R N EYS Uted States Patent ffice 3,014,098 Patented Dec. 19, 1961 3,014,098 TRANSDUCER Charles I. Malmo, Rte. 1, Box 88, Halstad, Minn. Filed May 8, 1959, Ser. No. 811,990 16 Claims. (Cl. 179-111) The present invention relates in general to electrostatic transducers and more particularly concerns a novel electrostatic loudspeaker capable of reproducing the full range of audio frequencies with exceptionally high fidelity at considerable sound pressure levels. Despite its remarkable performance, the novel transducer is relatively small when compared to prior art electrostatic units attempting to reproduce the entire audio spectrum. Details of the development and performance characteristics of an exemplary embodiment of the invention are disclosed in a thesis submitted by Charles I. Malmo to Ithe Department of Electrical Engineering of the Massachusetts Institute of Technology in partial fulfillment of the requirements for the degree of Master of Science and entilted A Wide-Range Electrostatic Loudspeaker. This thesis was made available in the. M.I.T. Engineering Library on lune 25, 1958.

Basically, an electrostatic loudspeaker is in effect a condenser having a vibratable diaphragm forming one of the condenser plates. Electrostatic forces developed between the diaphragm and one or more fixed electrodes are varied in accordance with an electrical signal to displace the diaphragm proportionately and thereby excite a pressure wave. Although the basic principle of the electrostatic loudspeaker has been known for almost a century, it was not until shortly after World War II that practical units were developed.

The increasing popularity of high fidelity sound reproduction created a greater demand for electrostatic loudspeakers. However, nearly all of these loudspeakers are limited to reproducing essentially only the high-frequency end of the audio spectrum. They must, therefore, be used in conjunction with a lowfrequency woofer to form a complete loudspeaker system. This is disadvantageous from the standpoint of the space required, the failure to provide an integrated program source, and the inherent distortion introduced by the use of crossover networks. While some wide-range units are currently being produced, it is diflicult to achieve good low frequency performance at appreciable sound levels with electrostatic units. Moreover, such units are bulky.

Nevertheless, many characteristics of the electrostatic loudspeaker render it intrinsically more suitable for high fidelity sound reproduction than a conventional electromagnetic unit. One major advantage is that the vibration-inducing electrostatic forces are applied substantially uniformly over the entire diaphragm, whereas in electromagnetic units they are applied around the inner periphery of the cone where the voice coil is attached. As a result, when a light diaphragm is used in an electro static unit, the forces derived from the electrical input signal in effect, operate directly on the air adjacent to the diaphragm instead of upon a relatively heavy paper cone. Still another advantage of the electrostatic unit is that the directivity pattern may be better controlled by using a curved or segmented diaphragm. The distribution forces upon the diaphragm lessen the chances of developing resonance effects so that an electrostatic loudspeaker may be expected to have a pressure response as a function of frequency which. is smoother than most cone-type speakers.

Despite these available advantages, prior art electrostatic loudspeakers have a number of disadvantages. The primary shortcoming is the inability to reproduce low audio frequencies. This is caused mainly by limitations on the maximum diaphragm excursion possible without introducing distortion.

Most modern electrostatic loudspeakers operate on a constant charge principle, discussed in detail below. This generally requires a large series resistor between the diaphragm and a high voltage supply which biases the diaphragm relative Ito the fixed electrode. In order to maintain an essentially constant charge on the diaphragm and prevent fall-in (diaphragm collapse) for large excursions, the series resistor must be large enough so that the time constant determined by this resistor and the effective capacity between the diaphragm and fixed electrode is longer than one period at the lowest frequency to be reproduced. However, reproduction of low frequencies requires a large excursion. This in turn dictated a requirement for a large electrode separation with a corresponding small value of speaker capacitance. The requirement for a small capacitance and large time constant imposes a severe limitation on Ithe performance of a conventional electro static loudspeaker. The magnitude of the required series resistance approaches that of the insulation resistance of the medium between electrodes so that much of the bias voltage is developed across the series resistance. In order to establish a sufficiently high bias potential between the diaphragm and fixed electrode, the biasing potential source must be capable of delivering a much higher voltage at higher power levels.

This problem Was recognized in an article entitled Distortion in Electrostatic Loudspeakers on page 55 of Wi-reless World for February 1956. Although it is conventional to coat the insulating diaphragm with conducting material, this article suggests the possibility of not coating the insulating diaphragm at all, hoping that the surface resistivity of the diaphragm would be less than its bulk resistivity.

The present invention contemplates and has as an important object the provision of an electrostatic transducer capable of responding to a low frequency electrical input signal in the range below 500 cycles, the lowest upper limit frequency of the frequency range of lower musical notes referred to in Terman, Radio Engineering at p. 860 (1947), by exciting a pressure wave of corresponding frequency in air at substantial sound levels.

It is still another object of the invention to achieve the preceding object with an electrostatic transducer which also excites pressure waves having frequencies in the remaining portion of the audio spectrum.

Still another object of the invention is to provide a full-range electrostatic transducer employing a single vi bratable diaphragm to reproduce the entire audio specltrllm.

Stillv another object of the invention is to provide a fullrange electrostatic transducer in accordance with the preceding object while dispersing the sound over a wide solid angle.

Still another object of the invention is to provide an electrostatic transducer in accordance with the preceding object in which the eects of resonance are minimzed so that the transducer exhibits an exceptionally smooth frequency response characteristic over a wide range of audio frequencies.

Still another object of the invention is to provide an electrostatic transducer in accordance with the preceding objects which is appreciably smaller than the full-range electrostatic transducers attempting to achieve comparable performance characteristics.

According to the invention, a thin insulating diaphragm is coated with a thin layer of resistive material. Means are provided for tautly supporting the diaphragm opposite to one or more fixed electrodes to establish a capacity between the diaphragm and fixed electrode. The resistivity of the thin layer upon the diaphragm is selected so that the resistance measured from an area near the center of the diaphragm to its periphery and the capacity between the diaphragm and fixed electrode determine a time constant which is langer than the period of the lowest frequency signal which is to be radiated.

In a preferred embodiment of the invention, push-pull operation is effected by supporting the diaphragm midway between a pair of parallel fixed electrodes. The diaphragm is preferably circular and the fixed electrodes preferably comprise coplanar irregularly spaced conducting rods. The use of a circular diaphragm minimizes the effects of resonances. The irregular spacing between the rods eliminates the picket fence or acoustic diffraction grating effect. 'Ihe presence of this effect would cause sharp changes in the directivity pattern at certain frequencies whose wavelengths are integrally related to the spacing between rods.

Strips of vibration damping material are attached to the rods to damp mechanical resonance effects in the electrode structure. The second resonant mode of the diaphragm is damped by inserting a pad of sound absorbing material near the center of one fixed electrode.

The use of rods for the fixed electrodes is advantageous because the rods are acoustically transparent and electrically opaque.

Numerous other features, objects and advantages of the invention will become apparent from the following speciication when read in connection with the accompanying drawing in which:

FIG. 1 shows a fixed electrode;

FIG. 2 shows the electrode of FIG. 1 with the center damping material in place;

FIG. 3 shows the other fixed electrode;

FIG. 4 is a view of the diaphragm and supporting assembly;

FIG. 4A is a partial diametrical sectional exploded view through the assembly of FIG. 4; and,

FIGS. 5-7 show stages in assembling the electrostatic loudspeaker unit in a baffle board.

With reference now to the drawing and more particularly FIG. 1, thereof, a view of one fixed electrode shows the side away from the diaphragm. This fixed electrode includes a plurality of irregularly spaced coplanar conducting rods supported in an annular frame 11 of hard plastic or other suitable insulating material. Strips of vibration damping material 12 and 13 are asymmetrically disposed about the horizontal diameter of the electrode. The vibration damping material helps to damp out resonances in the rods. Each rod is divided into unequal sections by the strips 12 and 13. As a result, there is virtually no chance of both strips being at a nodal point of a standing wave on the same rod for a particular frequency.

Referring to FIG. 2, the fixed electrode 10 is shown with a rectangular pad of damping material 4 placed near its center in order to damp out the second resonant mode of the diaphragm. In this mode the central area of the diaphragm vibrates out of phase with the outside area. The pad 14 loads the central area so that cancellation effects are appreciably reduced.

Referring to FIG. 3, there is illustrated a view of that side of the fixed electrode 10 nearest the diaphragm together with a schematic representation of the electrical connection to the different rods. As a result of these connections, the vibrating area of the diaphragm is inversely proportional to the frequency of the applied electrical signal, thereby enhancing the smoothness of the frequency response characteristic. The input terminal 21 is directly connected to the center rods 22 and 23. Resistors 24 and 25 are connected between terminal 21 and rods 26 and 27, respectively. Resistors -31 and 32 are connected from rods 26 and 27, respectively, to rods 33 and 34, respectively.

Resistors 35 and 36 are connected from rods 33 and 34,

4 respectively, to the adjacent groups of two rods 37 and 38, respectively.

Resistors 41 and 42 are connected from the groups of rods 37 and 38, respectively, to the groups of four rods 43 and 44, respectively. Lastly, the resistors 45 and 46 are connected from the groups of rods 43 and 44, respectively, to the groups of ten rods 47 and 48 respectively. The resistors and the capacity between rods and diaphragm effectively form a low pass ladder RC network which delivers the lowest frequency signals to all the conducting rods while the highest frequency signals are applied virtually to only the center rods 23 and 27. As a result, less mass is vibrated as the frequency of the applied input signal increases. This arrangement contributes to the uniform frequency response of the novel transducer over a wide range of frequencies.

Referring to FIG. 4, there is shown a view of the diaphragm mount including the diaphragm, insulating rings, tension adjustment wedges, and corona ring. The diaphragm 51 includes an insulating center portion, such as thin mylar coated with a resistive surface layer on both sides. It has been discovered that this surface layer of resistive material may be sprayed on the mylan A substance having suitable resistivity is the commercially available Stati-Clean made of the Electrovox Company of East Orange, New Jersey and normally used for removing the static charge on plastic records.

The diaphragm 51 is sandwiched between a pair of annular acrylate plastic rings 52 and 52a. The rings 52 and 52a are preferably segmented into quadrants to facilitate assembling the diaphragm mount. The lines between quadrantal segments of ring 52 are visible at 53-56. The outside diameter of ring 52 is slightly less than that of ring 52a.

Four wedges, 61-64 are positioned between segments of the ring 52a. With the diaphragm 51 rubber cemented to the ring 52a, the tension of the diaphragm may be adjusted by varying the radial position of the wedges until the desired tension is obtained. The technique for determining and measuring this tension is discussed in detail below.

After the diaphragm is properly teusioned and sandwiched between the annular rings 52 and 52a, the conducting corona ring 65, which may be hollow copper tubing, is placed around the circumference of the ring 52 in contact with both sides of the coated mylar film. The corona ring 65 includes a high voltage terminal 66 for receiving the biasing potential and delivers charge from the biasing potential source to the perimeter of the diaphragm.

Referring now to FIGS. 5-7 the procedure for assembling the fixed electrodes and diaphragm into a baflie board will be described.

In FIG. 5, the baie board 7'5 is shown with the fixed electrode 10 in a circular opening. An annular ring (not shown) on the other side of the bafe board 71 is in contact with the insulating ring 11. The bolts 72 pass through the ring on the other side of baiiie board 71 and the bathe board. Then the diaphragm assembly shown in FIG. 4 and the other fixed electrode 10 are positioned over the first electrode 10 in that order so that the assembly appears as shown in FIG. 6. Finally, the outer annular wooden ring 73 is placed over the second fixed electrode 10 and bolted in place so that the unit appears as shown in FIG. 7. The input terminal 21 from the respective fixed electrodes and the biasing terminal 66 on the corona ring of the diaphragm mount may be brought out to the rear and are not shown in the drawing. The biasing potential is applied to the terminal 66 while oppositely phased signals are applied tothe terminals 21 to provide push-pull operation of the novel electrostatic transducer.

Having described the physical structure of the novel transducer, an understanding of certain principles related to the suitable choice of parameters is helpful in practicing Ithe invention. As indicated above, the greatest difficulties in connection with providing a full-range electros tatic loudspeaker are encountered in reproducing the lowfrequency range. Reproducing low frequencies requires the movement of -a relatively large volume of air. This may be accomplished by utilizing a `diaphragm with a large surface area or allowing the diaphragm to make large excursions. Prior art full-range electrostatic transducers have generally followed the large surface area approach to provide units which are rather bulky.

In accordance with the present invention, a circular diaphragm having a dia-meter of but 20 inches and separated from each lxed electrode by approximately 1A inch was found to be satisfactory. lt will be immediately recognized that this is only slightly larger than the diaphragm of many electromagnetic woofers The circular shape of the diaphragm has a number of advantages. First, the resonant frequencies of a circula-r diaphragm are not harmonically related. This helps to prevent additional harmonic distortion when a speaker is driven with a signal including harmonic distortion and having a fundamental frequency corresponding to one of the lower resonance frequencies of the diaphragm. Second, corona discharge is prevented because the diaphragm is charged from a circular corona ring at high potential, thereby avoiding sharp corners subject to corona discharge.

The diaphragm material is preferably mylar plastic film approximately .00025" thick. This material is preferred beouse it is light, stable and has great strengnh. The weight of the diaphragm in the exemplary embodiment wherein the diaphragm is 20 in diameter, is approximately equal to that of a sheet of air 7 millimeters thick. Consequently, if the fundamental resonance of the diaphragm is established at a suiciently high frequency, the higher order resonances are completely damped by the resistive component of the air load, thereby facilitating the achievement of a very smooth frequency response characteristic. lf the :resonance of the fundamental or grav/est" mode is established at 20 cps., it will have the very beneiicial effect of increasing the eticiency of the transducer at the end of the audio spectrum Where it is ordinarily found wanting.

The design procedure used to calculate the required diaphragm tension is given below.

The fundamental frequency in vacuum is n1 T 1.01:.386; CPS.

Where a=radius of diaphragm in cm.

T=tension in dynes/ cm.

Mylar The resonant frequencies in air are considerably lower than those in vacuum due to the accession to inertia of the air load. The shift in frequency of the fundamental mode can be easily calculated by using correction terms.

(the inertial mass of the air load on a membrane operating at its gravest mode, assuming a spherical wave front and an infinite bafile mounting) a=25 cm.

for

f' C f' .14

for

f=20 cps. fv=143 eps.

This gives a required tension of 7.82)(104 dynes/cm. or gms/cm. for the diaphragm specified.

As indicated above, thediaphragm 51 is stretched between two acrylic plastic annu-li 52 and 52a, the former having a smaller outside diameter than the latter. Typically these outside diameters are 27.5 and 28" respectively. The diaphragm is held to the plastic annuli by ru'bber cement. Its tension is adjusted by means of the wedges 61-64 inserted between the quadrants of the larger annulus 52a. The smaller annulus 52 is held together by the copper corona ring 65. Ilhe annulus 52 is lightly tacked to the diaphragm 51 with a small amount of rubber cement to lend stability to the assembly without interfering with the tension adjustments.

As indicated above, both sides of the diaphragm are coated with resistive material. In order to deliver charge to both of these surface layers, the corona ring 65 contacts both sides of the diaphragm. This is accomplished by slitting the diaphragm for a short distance radially at short intervals about the circumference of the diaphragm to form a consecutive array of radial flaps. Alternate aps are then brought over the corona ring 65 'while the remaining flaps remain between the corona ring 65 and the annular ring 52a.

The diaphragm tension may be measured through tihe use of a relationship for the displacement of a circular membrane under a uniformly distributed normal force.

In this case, the displacement measurement will be at the center of the membrane, so the equation reduces to The tension of the diaphragm is, therefore,

The added weight, W, consisted of several thin disks of very flexible rubber with the same outside diameter as the diaphragm. A small hole was punched in the center of each disk to allow the diaphragm displacement to be measured with a depth micrometer. For small displacements, the weight of these disks was assumed to be evenly distributed over the surface of the membrane.

Constant charge operation is preferred because distortion is minimized. This can -be better understood by recognizing that electrostatic lforces develop along the line joining a pair of opposite charges. If the charge density along the diaphragm remains substantially constant, the forces are uniformly distributed along its surface. However, if the charges shift position as the diaphragm vibrates, then greater forces will be developed on the surface area having the greatest charge concentration.

Prior -art devices generally achieve constant charge open-ation by placing a resistor in series with the diaphragm. While the charge density will not change appreciably under such conditions if the diaphragm displacement is small, large excursions will tend to cause the charge 4to migrate toward the center of conventional conductively coated diaphragms. This occurs because the field strength increases as the gap between the diaphragm and fixed electrodes decreases. However, by coating the diaphragm with material of the proper resistivity, the charge remains essentially stationary during large excursions. Moreover, -the full potential of the biasing source is applied to the edge of the diaphragm instead of being partially dropped across a series resistor.

As indicated above, the diaphragm is coated with a material of high resistivity before mounting in order to efliciently provide constant charge operation. The resistivity of the coated surface may be calculated by assuming that the diaphragm shape at maximum excursion is approximately paraboloidal. If the surface resistivity is too low, the charge will be redistributed cylically over the diaphragm surfaces. The quantity of interest is the resistance of the surface as measured from an area -near the center of the diaphragm to the conducting corona ring 65 at its periphery. It can be shown that this resistance R is determined by loge I3 Where u, is the surface conductivity and not the conductivity per unit length, r2 is the radius of the corona ring 65 and r1 is the radius of a very small circle at the center of the diaphragm wherein the surface concentration of charge is small compared to the total surface charge distributed over the diaphragm so that with a proper resistivity negligible charge enter-s or leaves this small circle during diaphragm excursions. In the exemplary embodiment, r2 is 14" and r1 was assumed to be an inch. The surface resistivity is selected so that the center-to-edge diaphragm resist-ance (on each side of diaphragm) combines with the capacity between diaphragm and an adjacent fixed electrode to define a .time constant approximately 10 times longer than the period of 4the lowest frequency it is desired to reproduce. The capacity between electrode and diaphragm is approximately that of a parallel plate condenser with an area equal to that of the diaphragm opposite a fixed electrode and lhaving the same spacing therebetween. This was calculated to -be 270ML for the exemplary embodiment of the invention. When 20 cps. is selected as the lowest Afrequency to be rcproduced, the preferred value of center-to-edge resistance is greater than 108 ohms.

If the insulating diaphragm Were coated with conducting material or if the diaphragm were made from conducting material, this is also the order of magnitude of the resistance that would have to be inserted ibetween the bias supply and the diaphragm to insure constant charge operation. Clearly, any current leakage from the diaphragm would cause most of the bias voltage to be dropped across the series resistor and severely reduce the acoustic output of the electrostatic loudspeaker. The required resistivity may be computed from the above considerations by using the above equation. This resistivity was calculated to be of the order of 5 X 109 ohms.

After spraying the resistive coating on the plastic lm, it is desirable to measure the uniformity of lthe coating. This may 4be accomplished by placing two pennies upon the diaphragm surface and measuring the resistance therebetween Iwith a megohmmeter. The pennies may then be moved about the diaphragm while maintaining the separation therebetween constant. The resistivity from these measurements is given by loge ohms

tween the centers is maint-ained const-ant at 4", the reading on the megohmmeter should be 4.1 X 109 ohms.

After the resistive coating is applied to the plastic film, the film may 'be fastened to the insulating rings, trimmed and tensioned Ias previously described. The diaphragm is preferably evenly stretched around .its periphery, the combination of the rubber cement and tension adjusting wedges enabling this to be accomplished with a minimum amount of operator skill.

Havi-ng discussed the design considerations of the diaphragm, it is appropriate to discuss similar criteria relative to the fixed electrodes. The spacing between the conducting rods is related to the electrode-to-diaphragm distance. Basically, the object is to space the electrodes suiliciently close to be the electrical equivalent of a solid plate, but suiiiciently far apart so that the structure is the acoustical lequivalent of an open window. 'I'his can be accomplished because the capacitance of a parallel array of conducting rods spaced a distance d above a conducting plate is approximately equal lto that between two parallel plates until the distance between the conductors becomes appreciably larger than the separation distance d. Since the spacing between the fixed electrodes is V2", la mean wire spacing of 1/2 achieves these resuits.

The acoustic resistance of the resulting grid structure is negligible at all frequencies of intert. The conducting rods Iare not mounted at regular intervals of 1/z", but instead are offset slightly in a random order from this mean spacing to prevent an acoustic diffraction grating effect. That is, if the spacing were regular, the structure would become very directional at certain kfrequencies having a wave length integrally related to such spacing. As indicated above, it is preferable to connect an impedance element between adjacent groups of conducting wires so that more of the diaphragm vibrates as the frequency of the applied signal decreases.

The means for driving this loudspeakerv is not a part of this invention. It Vmight be driven by a suitable high voltage step-up transformer insulated .to withstand the high biasing voltage and driven from a conventional amplifier. In making actual tests on the exemplary embodiment, a high output voltage push-pull audio Aamplifier was utilized to directly drive the loudspeaker without a transformer. Details of the circuitry are in the aforesaid thesis. In this circuit, the output stage included a pair of 2G53 triodes receiving plate current from a plate volt-age supply of 8,000 volts.

The bailie construction for this unit is not 4a part of the invention. However, it is evident that well-known techniques may be utilized to provide a bale for the novel loudspeaker so that the back-wave does not cancel the forward wave at low frequencies.

The capabilities of the novel transducer itself have been established as a result of tests made in an anechoic chamber wherein measurements showed the frequency response characteristic to be within i5 decibels from approximately 70 cycles to 15,000 cycles at an average sound pressure level of 75 db above standard reference level on the axis 6 feet from the diaphragm. Moreover, there was no visible distortion in the pressure wave generated at this level in response to an electrical signal of l5 cps.

The specific arrangement of rods, diaphragm and baie described herein are only for the purpose of illustrating an exemplary embodiment. The inventive concepts may be embodied in many different forms. For example, different shapes of diaphragms or configurations of fixed electrodes may be employed wit-hin the inventive concepts. More specifically, when the preferred circular diaphragm is employed, it may be advantageous to utilize concentric conducting rods for the fixed electrodes instead of the vertical rods. Different materials may be employed for the diaphragm and the resisitive surface coating. The supporting structure of the diaphragm and fixed electrodes may be deviated from. Numerous other modifications of and departures from the specific embodiments described herein may be practiced by those skilled in the lart without departing from the inventive concepts. Consequently, the invention is to be construed as limited only by the spirit and scope of the appended claims.

What is claimed is:

1. An electrostatic transducer responsive to a low frequency signal in the lower range of audio frequencies below 500 cycles comprising, a thin diaphragm of insulating material having a thin surface layer of resistive material, a fixed electrode opposite said diaphragm, said diaphragm and said xed electrode forming a capacitor having a capacitance, the product of said resistance established by said layer of resistive material with said capacitance being a time constant of said capacitywith the resistance between a small area near the center and the edge of said surface layer being larger than the period of said low frequency signal, conducting means for establishing electrical contact with the periphery of said diaphragm, and means for applying biasing and A.C.Y signal potentials between said conducting means and said fixed electrode, said time constant being sufficiently large so that said resistive layer prevents the charge pattern established by said biasing potential from migrating along said diaphragm when the frequency of said A.C. signal is above said low frequency.

2. An electrostatic transducer in accordance with claim 1 wherein said diaphragm is circular, and further comprising, means suspending said diaphragm solely from its periphery to maintain said diaphragm-in radial tension.

3. An electrostatic transducer in -accordance with claim 1 wherein said time constant is at least ten times larger than said period.

4. An electrostatic transducer in accordance with claim l and further comprising, a second ixed yelectrode opposite said diaphragm, means for supporting said diaphragm midway between said fixed electrodes, means for establishing a high biasing potential between said diaphragm aud both said fixed electrodes, and means for applying oppositely phased signals between said center electrode and said fixed electrodes.

5. An electrostatic transducer in accordance with claim 4 wherein said diaphragm is circular, and further comprising, means suspending said diaphragm solely from its periphery to maintain said diaphragm in radial tension.

6. An electrostatic transducer in accordance with claim 1 wherein said fixed electrode comprises irregularly spaced coplanar conducting rods, the mean separation between rods being of the same order of magnitude as the distance between diaphragm and fixed electrode so that said fixed electrode is acoustically.transparent.

7. An electrostatic transducer in accordance with claim 6 and further comprising, a plurality of serially connected resistors, and means for coupling adjacent groups of said rods to adjacent junctions of said serially connected resistors.

8. An electrostatic transducer comprising, a thin vibratable diaphragm, a fixed electrode including a plurality of coplanar conducting rods opposite said diaphragm to establish a capacity therebetween, an input terminal connected to a first of said conducting rods nearest the center of said diaphragm, a resistor connected between said first rod and a group of others of said conducting rods, and

means for applying a signal between said diaphragm andv said input terminal to cause a larger area of said diaphragm to vibrate as the frequency of said signal is lowered.

9. .An electrostatic transducer in accordance with claim 8 wherein the spacing between said rods is irregular to prevent acoustic diffraction grating effect.

10. An electrostatic transducer in accordance with claim 8 and further comprising, strips of damping material in contact with said rods dividing each rod into sections of unequal length to prevent resonant'vibration of said rods.

11. An electrostatic transducer in accordance with claim 8 and further comprising, a pad ofsound` absorbent material positioned near the center of one of said fixed electrodes to damp the second resonant mode of said diaphragm.

12. A full-range electrostatic loudspeaker comprising, a thin circular kdiaphragm of insulating material coated on both sides with a thin layer of resistive material, means for solely peripherally supporting said diaphragm in tension, a circular conducting corona ring in contact with both sides of said coated diaphragm near the circumference thereof, a pair of fixed electrodes parallel to said diaphragm on opposite sides thereof.

13. An electrostatic loudspeaker in accordance with claim 12 wherein said fixed electrodes each comprise a plurality of coplanar conducting rods, means for maintaining the spacing between said conducting rods nearly the same but deviating randomly from an average interval approximately equal to the separation between said fixed electrodes.

14. An electrostatic loudspeaker in accordance with claim 13 and further comprising strips of vibration damping material in contact with said rods generally transverse to the length of said rods and dividing each rod into unequal sections.

15. An electrostatic loudspeaker in accordance with claim 14 and further comprising a pad of sound absorbent material located near the center of, but spaced from said diaphragm for damping vthe second resonant mode thereof.

16. An electrostatic transducer comprising, a thin vibratable diaphragm, a fixed electrode including a plurality of coplanar conducting rods opposite said diaphragm to establish a capacity therebetween, means for maintaining the spacing between said conducting rods nearly the same, but deviating randomly from an average interval, said average interval being sufliciently large so that said fixed electrode is acoustically transparent, said interval being sufiiciently small so that said fixed electrode is electrostatically opaque, and means for applying biasing and A.C. potentials between said diaphragm and said fixed electrode.

References Cited in the file -of this patent UNITED STATES PATENTS UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,914,098 vDemIm-n 19,1961@ Charles I. M-alme It is hereby certified that error appears'in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column l, line 64, before "foroes'f :insert of columnf 5, lines 46 to 48, the equation should appear' as shown below instead of as in the patent:

same column. 5, lines v60 to 62 the equation should appear as shown below instead of as in the patenti f g l fVA l+ mi. I

Signed and sealed this lOth dayof July 1962.,4

(SEAL) Attest:

ERNEST W. SWIDEE- l f DAVID L. LADD-l Attesting Officer v l i Commissioner of Patentsv

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