Electrostatic Acoustic Transducer

Abstract

A high-fidelity electrostatic acoustic transducer comprising a pair of fixed electrodes with a multiplicity of small openings and supported opposite to each other and a vibrating film electret with a conductive layer formed on one side thereof, the vibrating film being held between the fixed electrodes as an acoustic transducer section. An electrostatic field is generated between the electret and one of the fixed electrodes by the electret and thereby the second higher harmonics generated between one of the fixed electrodes and the vibrating film electret are capable of being eliminated on the other fixed electrode.

Description

The present invention relates generally to an electrostatic acoustic transducer or more in particular to an electrostatic acoustic transducer with a simple construction which operates easily to eliminate higher harmonic distortion very effectively thereby to improve the acoustic efficiency.

An object of the present invention is to provide an electrostatic acoustic transducer comprising a pair of fixed electrodes with a multiplicity of openings, said electrodes being supported opposite to each other, and a single vibrating film electret with a conductive layer formed on one side thereof, said vibrating film electret being held between said fixed electrodes.

The acoustic transducer according to the present invention requires no DC voltage as in the conventional acoustic transducer. Simple in construction and easy to operate, the acoustic transducer according to the invention permits higher harmonic distortion to be reduced greatly. In addition, since a very thin plastic vibrating film electret is employed, the acoustic efficiency is improved, resulting in a great commercial value.

The above and other objects, features and advantage will be made apparent from the detailed description taken in conjunction with the accompanying drawings, in which:

FIGS. 1 and 2 are diagrams showing the operating principle of a speaker embodying the present invention;

FIG. 3 is a diagram for explaining the operation of the speaker of FIG. 2;

FIG. 4 is a sectional view of the speaker embodying the present invention; and

FIG. 5 is a diagram comparing the second higher harmonic distortion characteristics of the speaker according to the invention with those of the conventional speakers.

The operating principle of the acoustic transducer according to the invention in the form of a speaker is shown in FIGS. 1 and 2. In these figures, the reference numeral 1 shows a vibrating film electret made of a high polymer such as plastic resin with permanent surface charges. On one side of this high polymer vibrating film electret is coated or deposited by evaporation a very thin layer 2 of conductive paint or metal such as aluminum or silver. The numerals 4 and 4' show fixed electrodes with numerous openings necessary for acoustic radiation, and the numerals 3 and 3' layers of air existing between the vibrating film 1 and the fixed electrode 4 and between the vibrating film 1 and the fixed electrode 4' respectively. As shown in FIG. 1, the AC signal V is applied between the fixed electrodes 4 and 4' after being boosted by the transformer T included in a power circuit. Capacitors C.sub.1 and C.sub.2 are provided for balance and the resistor R for controlling the higher harmonics. FIG. 2 shows an example in which a push-pull transformer T' is employed as a power source.

The operating principle of the above-described embodiment will be now explained in detail. Referring to FIGS. 1 and 2, it is assumed that the distances between the fixed electrode 4 and the conductive layer 2 on the thin vibrating film electret 1 and between the other fixed electrode 4' and the conductive layer 2 are set at d.sub.1 and d.sub.2 respectively. It is also assumed that the thickness of the vibrating film 1 is negligibly small compared with the distances d.sub.1 and d.sub.2. The symbol E.sub.0 denotes an electrostatic field caused by the electret. When the signal v is applied, the circuit of FIG. 2 may be simplified as shown in FIG. 3 for convenience of illustration. The force dF.sub.1 is defined as a force acting on a minute area dS on the vibrating film 1 due to an electric field generated in an airgap between the fixed electrode 4 and the conductive layer 2, and the force dF.sub.2 is defined as a force acting on the minute area dS on the vibrating film 1 due to an electric field generated in an airgap between the fixed electrode 4' and the vibrating film 1. An electrostatic voltage U.sub.0 is generated between the fixed electrode 4' and the vibrating film 1 by the permanent electric charges Q.sub.0 on the surface of the vibrating film electret 1. Since the conductive layer 2 on the vibrating film 1 equivalently acts as being electrostatically grounded, the electric force lines due to the charge Q.sub.0 charged on the vibrating film 1 exist in the airgap 3' but do not exist in the airgap 3. Accordingly, in the airgap 3, no static electricity (D.C. voltage) U.sub.0 due to the charge Q.sub.0 is generated between the vibrating film 1 (the conductive layer 2) and the fixed electrode 4. The force dF.sub.1 which is generated only due to an AC signal e.sub.1 is expressed as

dF.sub.1 = [.epsilon..sub.0 e.sub.1.sup.2 /2(d.sub.1 + .eta.).sup.2 ] dS (1)

where .eta. shows the displacement of vibrating film 1 from its position taken when no signal is applied. .epsilon..sub.0 represents the permittivity of the air.

On the other hand, the force dF.sub.2 which is generated by the superposition of the electrostatic voltage U.sub.0 due to the electric charges Q.sub.0 on an AC signal e.sub.2 is

dF.sub.2 = [.epsilon..sub.0 (U.sub.0 + e.sub.2).sup.2 /2(d.sub.2 - .eta.).sup.2 ] dS (2)

therefore, the combined force applied to the area dS of the vibrating film 1 is

dF = (dF.sub.2 - dF.sub.1)dS

= .epsilon..sub.0 /2{([U.sub.0 + e.sub.2 ]/[ d.sub.2 - .eta.]).sup.2 - (e.sub.1 /[ d.sub.1 + .eta.]).sup.2 } dS (3)

from this equation (3) is obtained the force f applied to the unit area, which is

f = dF/dS = .epsilon..sub.0 /2 {([U.sub.0 + e.sub.2 ]/[ d.sub.2 - .eta.]).sup.2 - (e.sub.1 /[ d.sub.1 + .eta.]).sup.2 } (4)

The most significant feature of the invention is that the second higher harmonics in equation 1 are reduced from second higher harmonics in equation 2, thereby lessening the second higher harmonics greatly. This fact is apparent from the equations 3 and 4. From the equation 4, alternating element F.sub.A of the force F applied to the whole of the vibrating film is obtained when d.sub.1, d.sub.2 >> .eta., as

F.sub.A = (.epsilon..sub.0 U.sub.0 S/d.sub.2.sup.2) e.sub.2 + (.epsilon..sub.0 /2d.sub.2.sup.2. e.sub.2.sup.2 - .epsilon..sub.0 /2d.sub.1.sup.2 . e.sub.1.sup.2) S + {.epsilon..sub.0 (U.sub.0 + e.sub.2).sup.2 /d.sub.2.sup.3 + .epsilon..sub.0 e.sub.1.sup.2 /d.sub.1.sup.3 } Y (5)

where Y = .intg..intg..sub.S .eta. dS.

In the equation 5, assuming that d.sub.1 = d.sub.2 = d and e.sub.1 = e.sub.2 = e,

F.sub.A = (.epsilon..sub.0 U.sub.0 S/d.sup.2) e + (.epsilon..sub.0 /d.sup.3) (U.sub.0.sup.2 + 2U.sub.0 e + 2e.sup.2) (V/j.omega.) (6)

where V is the volume velocity and equals j.omega.Y. It is understood from the equation 6 that the higher harmonics are capable of being sharply reduced if the relation e/U.sub.0 << 1 is satisfied.

As can be seen from the above description, the acoustic transformer according to the present invention employs a sheet of plastic vibrating film electret with a conductive layer on one side thereof, whereby an AC-derived force is generated in one of the opposite fixed electrodes by superimposing an AC signal upon the electrostatic voltage U.sub.0 due to the permanent surface charge Q.sub.0. Higher harmonics generated at this time are eliminated by the other fixed electrode.

An electret capacitor speaker is shown in FIG. 4 as an example of the present invention. In this figure, the reference numerals 1 to 5 denote like components as those shown in FIGS. 1 and 2. The numeral 6 shows a ring of metal or plastics to which a vibrating film is attached by fusion or supersonic wave or by the agency of a high polymer bonding agent. The numerals 7 and 7' show fastening means of plastic for applying tension to the vibrating film 1, the fixed electrodes 4 and 4' being secured to the fastening means 7 and 7'. The numerals 8 and 9 show a bolt and a nut respectively for securing the whole transducer, and numerals 10 and 10' take-out portions of lead wires made of conductive paint or printed member for the fixed electrodes 4 and 4', which are connected to the lead wires 12 and 13. The conductive layer 2 of the vibrating film 1 is in contact with the lead wire 11. The resistor R as shown in FIGS. 1 and 2 is very high in resistance value, and may be replaced by the contact resistance between the conductive layer 2 and the lead wire 11. In FIG. 5 the higher harmonic distortion characteristics of the speaker constructed as above described are compared with those of the conventional speaker of the electrostatic type. It is obvious from the figure that the second higher harmonic distortion characteristic A according to the present invention is superior to the like characteristic B of the conventional speaker. The characteristic curve C shows an output sound pressure level according to the present invention.

Claims

What we claim is:

1. An electrostatic acoustic transducer comprising a pair of fixed electrodes each with a multiplicity of openings, said electrodes being supported opposite to each other, and a single vibrating film electret with a conductive layer formed on one outer surface thereof facing only one of said electrodes, said vibrating film electret being held between said fixed electrodes and separated therefrom by an air layer.

2. An electrostatic acoustic transducer according to claim 1, in which said pair of fixed electrodes are connected with the output terminals of the secondary winding of a boosting transformer.

3. An electrostatic acoustic transducer according to claim 1, in which said pair of fixed electrodes are connected with the output terminals of the secondary winding of a boosting transformer, a pair of series-connected capacitors is inserted between said output terminals of said boosting transformer, and a junction point between the pair of capacitors is connected through a resistor to said conductive layer.

4. An electrostatic acoustic transducer according to claim 1, in which said conductive layer is connected with an intermediate terminal of the secondary winding of a push-pull transformer, and the end terminals of said secondary winding of said push-pull transformer are connected with the fixed electrodes respectively.

5. An electrostatic acoustic transducer comprising an annular frame, a single layer vibrating film electret having a conductive layer formed on one outer surface and attached to the upper surface of said frame, a pair of fixed electrodes between which said vibrating film electret is interposed, said conductive layer being in spaced facing relation to only one of said electrodes, and a fastening means for supporting said annular frame and said fixed electrodes, said fastening means being provided with a conductive layer and terminals electrically connected with said fixed electrodes.