EP0616485A1

EP0616485A1 - Electrostatic transducer

Info

Publication number: EP0616485A1
Application number: EP94301868A
Authority: EP
Inventors: Walter Schmid
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-03-16
Filing date: 1994-03-16
Publication date: 1994-09-21
Also published as: ZA942710B

Abstract

An electrostatic driver (10) comprises an acoustically transparent first electrode (12) an acoustically transparent second electrode (14) spaced apart from the first electrode (12) to define a space between the first and the second electrodes (12 and 14). A charged diaphragm (18) is suspended in the space between the first and the second electrodes (12 and 14). The first and the second electrodes (12 and 14) are partially insulated to generate A.C. and D.C. electric fields dissimilar in strength and form in the said space in which the diaphragm (18) is suspended to optimize the forces moving the diaphragm (18).

Description

BACKGROUND OF THE INVENTION

1. Field of the invention

THIS INVENTION relates to wave propagating and receiving systems. Particularly, this invention relates to electrostatic drivers, particularly for loud speaker, earphones and microphone systems.

2. Brief description of the prior art

Electrostatic drivers in loud speaker and microphone systems are well known. In electrostatic driver, an electric field reflecting the wave form of the input voltage moves a charged diaphragm thus creating waves, typically, sound waves. One type of electrostatic driver in the prior art uses acoustically transparent electrodes, which are connected in such a way that the electrostatic field for direct current and alternating current voltages applied to the electrode is the same in shape, form and intensity. This limits the field strength and output of the electrostatic driver. Another disadvantage is that the conventional electrostatic drivers are restricted in the sense that by increasing the applied voltage, there is a greater tendency for arcing.

OBJECTS OF THE INVENTION

In view of the foregoing, it is the aim of the invention to achieve the following objects, either alone or in combination:

to provide an electrostatic driver upon which a higher voltage and the resultant higher field strength can be applied without arcing;
to provide an electrostatic driver in which the tendency for arcing when the applied voltage is increased is substantially reduced;
to provide novel electrodes for electrostatic driver which are capable of generating dissimilar Ac and DC electric fields; and
to provide a novel diaphragm for an electrostatic driver with improved characteristics.

SUMMARY OF THE INVENTION

According to the first aspect of this invention, there is provided an electrostatic driver which comprises

an acoustically transparent first electrode;
an acoustically transparent second electrode spaced apart from the first electrode to define a space between the first and the second electrodes; and
a charged diaphragm suspended in the space between the first and the second electrodes; characterised in that the first and the second electrodes are partially insulated to generate A.C. and D.C. electric fields dissimilar in strength and form in the said space in which the diaphragm is suspended to optimize the forces moving the diaphragm.

The first and second electrodes may be acoustically transparent grills or perforated plates, and may be straight or arcuate.
The diaphragm may be suspended in the space between the first and the second electrodes by means of a plurality of insulating elongate elements, the insulating elongate elements being stretched on the surface of either or both of the first or the second electrode, and providing precise spacing means between the diaphragm and the inner surface of the electrodes.
Typically, the elongate elements may be insulating strings which provide retaining means for holding and centering the diaphragm between the first and the second electrodes. The insulating strings also create separate sections of diaphragm forming separate driver units, each of which is capable of radiating waves in a slightly different angle and the separate driver units, forming collectively, a substantially improved radiating surface.
Suspension means may be provided for suspending the insulating elongate elements, such as the strings. The suspension means may include holes, provided in the electrodes, through which the strings may be threaded and tensed.
Typically, the strings can be of any shape, dimension or cross-section.
In accordance with an alternative embodiment of the invention, instead of strings or other insulated elongate electrodes, a template may be stamped out of a suitable insulation material to form a pattern providing elongate sections for retaining and suspending the diaphragm in the space between the first and the second electrode, and thereby providing a plurality of multiple radiating units.
The diaphragm may be a single layer diaphragm or may be a diaphragm comprising a single or double film arrangement, the single or double film being coated with high or low resistance material, such as metal film, metal oxide and the like. If the coating is of a high resistance material there is very low distortion in the acoustical output but this coating is susceptible to charge losses due to moisture or pollution. In the case of low resistance coatings a small degree of distortion is induced but the charge loss is attenuated and therefore the use of the coating material is dependent upon the conditions in which the driver is used. Preferably, the coating may be provided between two layers of diaphragm material. In a typical arrangement of the diaphragm, there may be two diaphragm elements, one or both of which may be coated with a high or low resistance material, the coating typically lying between the two layers of the diaphragm elements.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanying drawings, in which

Figure 1 illustrates a schematic view of an electrostatic driver, in accordance with the first aspect of this invention;
Figure 2 illustrates a schematic detailed view of a portion of the driver shown in Figure 1;
Figure 3 illustrates the theoretical considerations relating to the fields generated in the electrostatic driver, shown in Figures 1 and 2;
Figure 4 illustrates a typical film arrangement for a diaphragm of the electrostatic driver, shown in Figures 1, 2 and 3; and
Figure 5 illustrates the different types of electrodes that may be used for constructing the electrodes for the electrostatic driver, illustrated in Figures 1, 2 and 3.

DETAILED DESCRIPTION OF THE PREFERRED

EMBODIMENTS

Referring to the drawings, an electrostatic driver, in accordance with this invention is indicated generally by the reference numeral 10.
The electrostatic driver 10 comprises an acoustically transparent first electrode 12, an acoustically transparent second electrode 14, uniformly spaced apart from the first electrode 12 to provide a space 16 between the first and the second electrodes. A charged diaphragm 18 is suspended in the space 16.
The first and second electrodes 12 and 14 can be acoustically transparent grills or perforated plates.
The diaphragm 18 is suspended in the space 16 by means of a plurality of insulating elongate elements 20, the insulating elongate elements 20 being stretched on the surface of the first or the second electrode 12 or 14, and providing precise spacing between the diaphragm 18 and the inner surface of the electrodes 12 and 14.
The elongate elements 20 are insulating strings which provide retaining means for holding and preferably centering the diaphragm 18 between the first and the second electrodes 12 and 14. The insulating strings 20 also create separate sections of diaphragm forming separate driver units, each of which are capable of radiating waves in a slightly different angle and the separate driver units, forming collectively, a substantially improved radiating surface, as particularly seen in Figure 2 of the drawings.
Suspension means (not shown) are provided for suspending the strings 20. The suspension can include holes (not shown), provided in the first and second electrodes 12 and 14, through which the strings 20 are threaded and tensed.
The strings 20 can be of any shape, dimension or cross-section.
In accordance with an alternative embodiment of the invention, instead of strings or other insulated elongate elements, a template (not shown) can be stamped out of a suitable insulation material to form a pattern providing curved elongate sections for retaining and suspending the diaphragm 18 in the space 16, and thereby providing a plurality of multiple radiating units, as particularly shown in Figure 2 of the drawings.
The diaphragm 18 can be a single layer diaphragm or can be a diaphragm comprising a single or double film arrangement, as particularly shown in Figure 4 of the drawings. The single or double film is coated with a high or low resistance material, such as metal film, metal oxide and the like. Preferably, the coating is provided between two layers of diaphragm material. In a typical arrangement of the diaphragm, as shown in Figure 4 of the drawings, there are two diaphragm electrodes 22 and 24 which sandwich a coated layer 26 therebetween. The use of a double- layered diaphragm results in a significant higher resistance to arcing the other corona effects which are detrimental to the output and working of the electrostatic driver. The use of the double film arrangement therefore results in higher acoustical output. Further, the provision of the conducting layer sandwiched between the double film diaphragm is protected from atmospheric influences, such as corrosion and moisture.
The construction of the electrodes 12 and 14 is schematically seen in Figure 5. Typically, the electrodes are partially insulated electrodes in which the insulator layer is represented in Figure 5 by numeral 26 and the conductive layer by numeral 28. The presently known electrodes for electrostatic drivers are constructed in such fashion that the D.C. and the A.C. electrostatic fields between the electrode and the diaphragm are of the same form. The construction of the electrode for the electrostatic drivers, in accordance with this invention, is novel in that the electrodes are shaped differently and generate direct DC and AC field lines as illustrated in Figure 3 of the drawings. The function of the AC field is to move the diaphragm 18 and thus create sound waves. The DC electric field is a biasing field necessary to avoid distortion. As can be seen in Figure 3, the AC and DC electric fields, represented by numerals 30 and 32, are not in the same shape, form and intensity. The DC field is mainly created by the uninsulated outer surface 34 of the electrode 36, whereas the AC field is transferred through the insulation medium into the gap X between the electrode and the diaphragm. The DC is created at the base of the electrode, and moves through the gap Y between the electrode and the diaphragm. Because the AC field moves through a narrower gap X than the DC field, which moves through the gap Y, the field strength of the AC field is higher, and the electrostatic driver has a higher output. Concurrently, the arcing characteristics that can be applied when turning the voltage to maximum remains unchanged.

Claims

1. An electrostatic driver which comprises

an acoustically transparent first electrode;

an acoustically transparent second electrode spaced apart from the first electrode to define a space between the first and the second electrodes; and

a charged diaphragm suspended in the space between the first and the second electrodes; characterised in that the first and the second electrodes are partially insulated to generate A.C. and D.C. electricfields dissimilar in strength and form in the said space in which the diaphragm is suspended to optimize the forces moving the diaphragm.

2. An electrostatic driver as claimed in Claim 1, in which the first and second electrodes are acoustically transparent grills.

3. An electrostatic driver as claimed in Claim 1, in which the first and second electrodes are acoustically perforated plates.

4. An electrostatic driver as claimed in any one of the preceding claims, in which the first and second electrodes and the diaphragm are flat planar elements.

5. An electrostatic driver as claimed in any one of Claims 1 to 3, in which the first and second electrodes and the diaphragm are arcuate.

6. An electrostatic driver as claimed in any one of the preceding claims, in which the diaphragm is suspended in the space between the first and the second electrodes by means of a plurality of insulating elongate elements, the insulating elongate elements being stretched on the surface of either or both of the first or the second electrode, and providing precise spacing means between the diaphragm and the inner surface of the electrodes.

7. An electrostatic driver as claimed in Claim 1, in which the diaphragm is suspended in the space between the first and the second electrodes by means of a plurality of spaced apart insulating strings which provide retaining means for holding and centering the diaphragm between the first and the second electrodes.

8. An electrostatic driver as claimed in Claim 7, in which the insulating strings create separate sections of diaphragm forming separate driver units, each of which is capable of radiating waves in a slightly different angle and the separate driver units, forming collectively, a substantially improved radiating surface.

9. An electrostatic driver as claimed in Claim 1, in which the diaphragm is a double film diaphragm coated with high or low resistance material.

10. An electrostatic driver as described herein substantially with reference to the accompanying drawings.