AN ELECTROSTATIC LOUDSPEAKER

Technical Field

The present invention relates to electrostatic loudspeakers.

Background of the Invention

A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was, in Australia, known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

Electrostatic loudspeakers use a thin flat diaphragm usually consisting of a plastic sheet, for example such as Mylar, impregnated or covered with a conductive material capable of holding an electric charge, for example such as graphite, located between two electrically conductive grids supported by frames, known as stators, with a small air gap between the diaphragm and stators. The diaphragm, by means of its conductive coating and an external high voltage which is applied to it, is held at a DC potential of several kilovolts with respect to the stators. The stators are driven by the audio signal, the front and rear stators being driven in counterphase. As a result, a uniform electrostatic field proportional to the audio signal is produced between both of the stators. This causes a force to be exerted on the charged diaphragm and its resulting movement drives the air on either side of it, providing an acoustic output consistent with the audio signal.

Electrostatic loudspeakers generally tend not to have a flat amplitude response characteristic across the audio frequency range. In particular, they are known to lack bass response (for eg. for frequencies below about 200 Hz) due, among other things, to difficulty in reproducing low frequencies from a vibrating taut diaphragm with a relatively short deflection range.

An object of the present invention is to provide an electrostatic loudspeaker that gives improved sound output across substantially the audio range.

Disclosure of the Invention

According to a first aspect the present invention provides an electrostatic loudspeaker including: a first stator; a second stator opposite the first stator; wherein each stator is sectioned to provide at least two stator sections each of which includes conductors for receiving an applied audio signal voltage; an electrically conductive diaphragm for each stator section located between the first and second stators such that there is an air gap between the diaphragm and the conductors of each stator section, wherein the diaphragm of each stator section, when electrically charged, is for responding to the audio signal voltage applied to the stators to generate acoustic sound waves; wherein at least one of the stators includes terminals for the diaphragms of each stator section to have separate polarising voltages applied thereto whereby the diaphragms may be differently electrically charged.

The diaphragms for all of the stator sections may be provided by a single membrane such that the diaphragm corresponding to each stator section is a portion of the single membrane. Alternatively more than one membrane may be used to make up the diaphragms.

The polarisation voltage of the diaphragm is directly related to the amount of force stators can impart to that diaphragm. The higher the polarisation voltage, the more force the stators can produce at a given air gap. The restraints are the maximum voltage that can be supplied before the air in the gap begins to ionise and the maximum voltage which can be applied before the diaphragm begins to exceed its deflection limits set by the size of the air gap. The smaller the air gap on a panel the lower the voltage that is needed to achieve maximum deflection before reaching the constraints of the air gap, or ionisation of the air between the stators.

The diaphragms corresponding to each stator section are arranged to have separate polarising voltages applied thereto by way of each being in contact with a terminal that extends about the periphery of that diaphragm. Such a terminal for each diaphragm may be a conductive track that is formed on a frame of the stator on which the diaphragm is mounted.

By dividing the stators into sections having for example different widths, yet retaining a single membrane for the diaphragms or a series of diaphragms joined via attachments to a frame, it has been found that sound output generated by the diaphragms across the audio range can be improved. Thus a stator section can be sized in one dimension (for example its width) relative to the whole dimension of the electrostatic loudspeaker panel to more effectively reproduce a given range of frequencies via that stator section. A stator in an electrostatic loudspeaker that includes several stator sections of dimensions (for example widths) appropriate to their frequency response allows the electrostatic loudspeaker panel to reproduce a wider frequency range with higher output and less distortion than with a non-sectioned stator or a sectioned stator wherein the sections are of the same dimension.

Thus, for enhancing generation and dispersion by the diaphragm of higher frequencies (say above about 400 Hz) of the acoustic sound, an appropriately smaller sized stator section (for example in its width relative to the width of the whole stator) is provided, and for enhancement of the lower frequencies (say below about 100 Hz) a larger sized stator section (for example in its width relative to the width of the whole stator) is provided.

Preferably each stator includes three stator sections of different minimum dimensions, wherein the smallest sized stator section is for enhancing generation by the diaphragm of higher range frequencies (ie., in the order of about 400 Hz and above) of the acoustic sound waves, the largest sized stator section is for enhancing generation by the diaphragm of lower range frequencies (ie., in the order of about 100 Hz and below) of the acoustic sound waves and the in-between sized stator section is for enhancing generation by

the diaphragm of frequencies between the higher and lower range frequencies (ie., in the order of about 100 to about 400Hz) of the acoustic sound waves.

An electrostatic loudspeaker panel is generally square or rectangular and thus will have a height and a width and preferably the stator sections have the same height but have different widths. It is primarily the width of a stator that determines the lower frequency range over which the electrostatic loudspeaker panel will effectively reproduce sound. Thus the narrower the width of a stator section, the higher the nominal frequency response for that stator section of the electrostatic loudspeaker panel and the wider the width of a stator section, the lower the nominal frequency response for that stator section of the electrostatic loudspeaker panel.

Preferably the smallest sized stator section (that is, the one of minimum width) is located between the largest and the in-between sized stator sections. This positioning provides optimal sound positioning for enhanced perception of the entire sound by the human ear.

Additionally, in an electrostatic loudspeaker according to the invention, each stator may be such that there is a different sized air gap between the conductors of each stator section and the diaphragm.

The air gap between the conductors of the stators and the diaphragm determines the amount of polarization voltage that can be applied to the diaphragm of an electrostatic loudspeaker panel.

The effective force available to move the diaphragm is inversely related to the size of the air gap between the diaphragm and the stators. The smaller the air gap, the greater the force that can be developed on the diaphragm. The constraint is the deflection of the diaphragm before it exceeds the air gap. A smaller air gap can produce more sound energy from a given input source than a larger gap. Thus for a stator section of smaller width there may be a smaller air gap than for a stator section of larger width.

Thus, a larger polarising voltage may be applied to a portion of the diaphragm associated with a larger sized stator section having a larger air gap compared to the polarizing voltage applied to a portion of the diaphragm associated with a smaller sized stator section having a smaller air gap.

In embodiments of the invention incorporating different sized stator sections, different sized air gaps and different polarizing voltages applied to the diaphragms corresponding to the stator sections, it is possible to design multiple stator sections on an electrostatic stator which have different frequency ranges but which produce a balanced acoustic output with a flat response over a wide range of frequencies.

According to a second aspect the invention also provides a frame for forming a stator for an electrostatic loudspeaker, wherein the frame is moulded from an insulating material and includes a periphery defined by a first pair of opposite side members and a second pair of opposite side members, and wherein the frame includes at least one sectioning member extending between the members of the first pair of opposite side members to define at least two frame sections having different widths, the frame including means for attaching an electrically conductive grid or mesh respectively to each frame section to thereby provide a stator that includes at least two stator sections of different size.

Preferably the frame includes two sectioning members extending between the members of the first pair of opposite side members to define three frame sections having different widths.

The means for attaching an electrically conductive grid or mesh may include grooves formed in the members of the first pair of opposite side members for receiving, for example via an interference fit or by gluing, rods of an electrically conductive grid.

Also the members of the first pair of opposite side members of the frame may be shaped to provide a defined air gap between a diaphragm when attached to

the frame and a grid when attached to the frame. This shaping of the members may be such as to provide a different sized air gap for each of the frame sections.

The invention also provides a frame as above described having electrically conductive grids attached thereto, thereby providing a stator which comprises at least two stator sections.

For a better understanding of the invention and to show how it may be performed, embodiments thereof will now be described, by way of non-limiting example only, with reference to the accompanying drawings. The figures of the accompanying drawings are not drawn to scale, that is, the dimensions of the various components have been relatively varied for the purposes of clear illustration.

Brief Description of Drawings

Figure 1 schematically illustrates an electrostatic loudspeaker that includes an electrostatic loudspeaker panel (schematically in a side view) according to an embodiment of the present invention to which driving circuitry is connected.

Figure 1 A schematically illustrates a detail of an electrostatic loudspeaker, to show a terminal arrangement on a stator for applying polarising voltages to diaphragms provided by a single membrane.

Figure 2 illustrates a moulded frame for forming a stator for an electrostatic loudspeaker panel according to an embodiment of the present invention.

Figures 3 to 5 are cross-sectional views (not to scale) on respectively section lines Ill-Ill, IV-IV and V-V of Figure 2.

Figure 6 illustrates a grid in partial view for combining with the frame of Figure 2 for forming a stator.

Figures 7 and 8 are schematic views of sectioned stators for assistance in explaining examples of the invention.

Figures 9 and 10 are output response-frequency curves for illustrating improvements provided by an embodiment of the invention.

Detailed Description of Embodiments

The schematic electrostatic loudspeaker system 20 of Figure 1 comprises an electrostatic loudspeaker panel 22 and driving circuitry 24 for driving the electrostatic loudspeaker panel 22. The electrostatic loudspeaker 22 includes oppositely located first and second stators 26 and an electrically conductive membrane 28 providing one or more diaphragms located between the stators

26 such that there is an air gap 30 between the membrane 28 and each stator 26. Each stator 26 comprises an insulating frame 32 (see Fig. 2 - to be described in detail below) which supports a multiplicity of electrically conductive rods or bars 34 forming grids 36 (see Fig. 6 - to be described in detail below).

Because of the grids 36, the stators 26 are acoustically transparent to acoustic sound waves generated by the diaphragm (membrane) 28. The diaphragm (membrane) 28 is lightly tensioned across and attached to the frame 32 for example by an adhesive.

The driving circuitry 24 includes a step-up audio transformer 38 and a high voltage generator 46. The audio amplifier 24 has input terminals 40 to which an audio signal is applied. Each stator 26 is connected to a respective end of the secondary winding of the audio transformer 38, see connections 42, 44. High tension polarising DC voltages from a high voltage source 46, which is connected to a centre tap of the secondary winding of the audio transformer 38, are connected to the diaphragms of membrane 28, see connections 48 and 52. Circuit arrangements for the electrostatic loudspeaker panel 22 other than as illustrated by Figure 1 may be used.

A feature of the present invention is for the high voltage generator 46 of the driving circuitry 24 to supply different polarising voltages to different portions of

the membrane 28. Thus, for example, in addition to the polarising voltage via connection 48 applied to the section of the membrane 28 between sections 26b and 26c of both stators, the high voltage polarising source 46 may also apply a different polarising voltage to the section of the membrane 28 located between the two stator sections 26a, as shown by connection 52. These membrane sections are the diaphragms of the different stator sections.

In operation, the membrane 28 is electrically charged by the high voltage source 46 and an audio signal voltage is applied to the stators 26 from the audio transformer 38 via the connections 42, 44 to which the diaphragms of membrane 28 are responsive to generate acoustic sound waves.

Each stator 26 is sectioned to provide at least two stator sections, for example three stator sections 26a, 26b and 26c are illustrated by Figure 1 , and the stator sections 26a - 26c are electrically connected together (see connections 50 in Fig. 1 ) such that each stator section 26a - 26c receives the applied audio signal voltage from connections 42, 44. Furthermore each stator section 26a - 26c has a different size in width, that is, each stator section 26a - 26c extends over an area of the membrane 28 that is different for each stator section. In the schematic embodiment shown by Figure 1 , stator section 26a has the largest width, stator section 26c has the smallest width and stator section 26b covers a width of in-between size.

The stator section 26c of smallest width is sized for enhancing generation by its diaphragm of higher range frequencies of the output acoustic sound waves and the stator section 26a of largest width is sized for enhancing generation by its diaphragm of lower range frequencies of the output acoustic sound waves. Although only two stator sections may be provided, it is preferred that three or more stator sections be provided, such as the stator sections 26a, 26b and 26c, wherein the largest width stator section 26a is for enhancing the lower range or bass frequencies generated by the diaphragms of membrane 28, for example up to about 100Hz, the smallest width stator section 26c is for enhancing the higher range frequencies generated by its diaphragm, for example from about 400 Hz and above, and the in-between width stator section 26b is for enhancing

the frequencies between higher and lower range frequencies generated by its diaphragm, for example between about 100 Hz to about 400 Hz, of the applied audio signal.

The air gaps 30 between each stator section 26a, 26b and 26c and the diaphragms of membrane 28, respectively labelled 30a, 30b and 30c in Figure 1 , may also be differently sized. Thus the air gap 30c may be smaller than the air gap 30a, that is, a smaller air gap can produce more sound energy from a given input source than a larger air gap and so a smaller air gap 30c is employed with the smallest sized stator section 26c and a larger air gap 30a is employed with the largest sized stator section 26a.

The polarising voltage via connection 48 is applied to a terminal that contacts membrane 28 and extends about the periphery of the section of the membrane 28 between the stator sections 26b and 26c and the polarising voltage via connection 52 is applied to a terminal that contacts membrane 28 and extends about the periphery of the section of the membrane 28 between the stator sections 26a. This physical separation between the terminals for applying the two polarising voltages is schematically indicated by reference 53 on Fig. 1. Figure 1 A schematically illustrates the connections 48 and 52 to, respectively, a terminal 49 formed on the insulating frame 32 of a stator (to be described in detail below with reference to Figure 2) which provides the stator sections 26b and 26c, and a terminal 51 formed on the insulating frame 32 which provides the stator section 26a. The arrangement is such that the polarisation voltages on the diaphragm section within each terminal 49 and 51 effectively remain at their nominal values whilst an area of membrane 28 between the two terminals is at an average of the two polarising voltages, however this is a non-working portion of the membrane 28 which is fixed to the frame 32 as indicated by reference 55. Generally, a higher polarising voltage may be applied to the section of the membrane 28 associated with the stator sections of larger size, that is, in Figures 1 and 1A a higher voltage is applied via connection 52 (because stator section 26a is the largest in size) than is applied via connection 48. Other arrangements for surrounding terminals such as 49 and 51 can be provided depending on what polarising voltages are to be applied to which

stator sections, for example a surrounding terminal could be provided corresponding to each stator section. Also, the terminals (such as 49 and 51 ) need not be closed loops, that is, the loops may include a gap whereby they extend at least substantially about the periphery of the diaphragm for that stator section.

Figure 2 illustrates an insulating frame 32 for a stator 26 according to an embodiment of the invention. The frame 32 is moulded from a plastics material, such as polyvinylchloride (PVC) or polypropylene (PP) and thus can be very accurately dimensioned. The frame 32 can be formed by injection moulding or casting. It includes a first pair of opposite side members 54 (i.e. top and bottom members 54 the length of each of which define the width of the electrostatic loudspeaker panel) and a second pair of opposite side members 56 (i.e. side members 56 the length of each of which define the height of the electrostatic loudspeaker panel) forming a peripheral structure. Intermediate between the side members 56 and extending between the top and bottom members 54, are sectioning members 58 and 60. The sectioning members 58 and 60 divide the frame 32 into three sections, namely a large width section bounded by members 54, 56 and 60, and a small width section bounded by members 54, 58 and 60 and an in-between width section bounded by members 54, 56 and 58. The frame 32 also includes cross-brace members 62 extending between the side 56 and sectioning members 58, 60. The top and bottom members 54 and the cross-brace members 62 are moulded to have parallel grooves 64 formed therein for receiving, with an interference fit, the bars 34 of a grid 36 (to be described below with reference to Fig. 6). Figs 3 to 5 (which are schematic not to scale cross-sections on section lines Ill-Ill, IV-IV and V-V respectively of Fig. 2 - that is, the relative dimensions have been exaggerated for clarity of illustration) indicate the grooves 64 by dashed lines behind the sectioning lines (thus the cross-sections Ill-Ill, IV-IV and V-V are not through the grooves 64).

The face of the top and bottom members 54 behind that shown by Fig. 2, that is the face 66 illustrated by Figs. 3 to 5, is moulded to have a step 68 and this step 68 defines, for each section of the frame 32, a different distance, respectively x, y and z as shown on Figs. 3, 4 and 5. The face 70 of each cross-brace member

62 is set back from the level of face 66 of the top and bottom members 54 by the same distance, respectively x, y and z for that section. The grooves 64 have a defined depth and the distance between the bottom of each groove 64 and the faces 68 and 70 is accurately known. Thus when a membrane 28 is attached to the stator frame 32 on the faces 66 of the top and bottom members 54 (and on corresponding faces of the side members 56 and the sectioning members 58 and 60) with rods or bars 34 of grids 36 (to be described with reference to Fig. 6) positioned within the grooves 64, then different sized air gaps 30a (see Fig. 5), 30b (see Fig. 3) and 30c (see Fig. 4) are defined.

Figure 2 indicates that the section of frame 32 of smallest width that will form the higher frequency response, stator section 26c, is between the largest width bass response stator section 26a and the in-between width stator section 26b. This is to provide a relatively high output aperture with wide dispersion. That is, if the higher frequency response stator section 26c is located on a side, the sound balance of each channel will not be centralised, that is, high frequencies of the output sound will appear to be off to the side. The illustrated positioning for the higher frequency response section 26c gives enhanced perception for the entire sound.

Figure 6 illustrates portion of a grid 36 for force fitting into the grooves 64 of the stator frame 32 of Figure 2. The grid 36 comprises parallel steel rods or bars 34 which are mechanically and electrically joined by top and bottom cross-rods 35 for example by welding. The rods 34 and 35 are coated with and electrically insulating material, for example, a nylon, for example by spraying, brushing or dipping. The rods 34 and 35 may be about 2mm in diameter and have an insulating coating about 0.5 mm thick, thus giving an overall diameter of about 3 mm. The rods 34 are spaced such that about 60% of the area of the grid 36 is open thereby ensuring substantial acoustic transparency. The rods 34 cover about 40% of the area thereby ensuring adequate driving force on the diaphragm.

Three grids 36 are provided, one for each of the three sections of the insulating frame 32, and can be press fitted onto the frame 32. The rods 34 are an

interference fit within the grooves 64 and thus the grids 36 are retained in the frame 32. The separate grids 36 are electrically connected together as would be routinely known by a person skilled in the art (schematically illustrated by connections 50 in Figure 1 ). Also persons skilled in the art will routinely know how to provide appropriate terminals in an assemblage of the first and second stators 26 with a membrane 28 for the audio signal voltage connections 42 and 44 and the polarising voltage connections 48 and 52 to the terminals 49 and 51 for the membrane 28.

Embodiments of the invention may employ stator constructions other than as described with reference to Figs. 2 - 6. Also, other than three stator sections may be provided, for example only two, or possibly four or more. Also, other than two polarising voltages for the diaphragms may be used, for example a single voltage may be sufficient for some sectioned stators, or possibly more than two polarising voltages could be applied.

Examples of electrostatic loudspeaker panels that have been constructed are:

A panel of three stator sections (see Fig. 7) with dimensions a = 90mm, b = 40mm, c = 120mm and h = 715mm or 1450mm. The 90mm section has a diaphragm to grid air gap of 1.7mm and a polarisation voltage of 3.5 KV; the 40 mm section has an air gap of 1.2mm and polarisation voltage of 3.5 KV; and the 120mm section has a gap of 1.9mm and polarisation voltage of 3.5 KV.

A panel of three stator sections (see Fig. 7) with dimensions a = 235mm, b=35mm, c=135mm and h=715mm or 1450mm. The 235mm panel has an air gap of 2.6mm and polarisation voltage of 4 KV; the 35mm section has a gap of 1.2mm and a polarisation voltage of 3.4KV; and the 135mm section has a gap of 1.9mm and a polarisation voltage of 4KV.

A panel of 4 stator sections (see Fig. 8) with dimensions a=235mm, b=35mm, c=95mm, d=145mm and h=1135mm. The 235mm section has an air gap of 2.6mm and a polarisation voltage of 4KV; the 35mm section has a air gap of 1.2mm and polarisation voltage of 3.5KV; the 95mm section has an air gap

1.7mm and a polarisation voltage of 4KV and the 145mm panel has an air gap of 2.0mm and a polarisation voltage of 4KV.

Fig. 9 illustrates an output-frequency response curve for a prior art electrostatic loudspeaker. This figure is provided to give a basis for comparison with an output-frequency response curve, shown by Fig.10, for an electrostatic loudspeaker according to an embodiment of the invention.

Fig. 9 illustrates a significant drop in the treble response, that is, for frequencies above about 8KHz. Also figure 9 illustrates declining output from a frequency of about 300Hz and below.

The figure 10 output-frequency response curve is for an electrostatic loudspeaker having stator sections, air gaps and polarization voltages of sizes comparable to the second example of a panel as described above. The treble response is fully recovered. There is also an improvement at all frequencies below 300Hz. These improvements are plainly evident from a visual comparison of the two curves.

The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the scope of the following claims.