GB2110054A - Directional acoustic transducers - Google Patents

Abstract

An acoustic transducer for producing highly directional characteristics comprises a metallized backplate (18) facing the surface of an electrostatically charged electret foil (10) comprising a metal layer 12 and a charged polymer layer 14. The sensitivity of the electret transducer varies along the length thereof providing a directional response pattern with a main lobe a smaller side lobes. The varying sensitivity can be achieved by varying the width of the metal layer (20) on the backplate, by varying the effective air gap between the foil and the backplate, by varying the charge distribution of the polymer layer or by varying the thickness of the foil. In some embodiments replacement of the electret foil by a d.c. biassed foil is envisaged. <IMAGE>

Description

SPECIFICATION Directional acoustic transducers Technical Field This invention relates to acoustic systems and, in particular, to electret transducers for producing directional response.

Background of the Invention Acoustic arrays comprising a plurality of discrete microphones are useful in producing directional response characteristics, as shown in the copending patent application No.

8,040,183. It is necessary, however, that each microphone in such an array be located precisely during construction of the array. Imprecise location of microphones from desired positions results in substantial degradation of the array response characteristic.

In accordance with the invention there is provided an acoustic transducer comprising a supported foil, sensitivity of the transducer so varying across the surface of the foil as to produce a directional response pattern comprising a main lobe and a plurality of smaller side lobes.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings.

Brief Description of the Drawings Figure 1 shows a disassembled electret transducer; Figure 2 shows a selectively metalized backplate useful in the electret transducer of Fig. 1; Figure 3 shows an isometric view of a part of the assembled electret transducer of Fig. 1; Figure 4 shows an enlarged view of the contact between the electret foil and the backplate in the electret transducer of Fig. 1; Figure 5 shows a response characteristic of the electret transducer of Fig. 3; Figure 6 shows a different embodiment of the electret transducer of Fig. 1; Figure 7 shows an electret transducer, useful in distinguishing actual air gap from effective air gap; Figures 8, 9 and 10 show embodiments of electret transducers made by varying the actual air gap; Figures 11, 12 and 13 show embodiments of electret transducers made by varying the effective air gap;; Figures 14 and 15 show embodiments of an electret transducer made by varying the thickness of the electret foil; Figures 16, 17 and 18 show electrostatic charge distributions on the polymer surface of the electret foil; and Figure 19 shows a further embodiment.

Detailed Description Referring to Fig. 1, there is shown an electret transducer with its component parts disassembled. There is shown an electret foil 10 comprising two layers: an upper metal layer 12, and a lower synthetic resin polymer (such as FEP TEFLON(E)) layer 14. The polymer layer 14 is electrostatically charged to a predetermined value. In one application, the electrostatic charge is uniform over the charged area at - 275 i 3 volts. The metal layer 12, in this application, is about two thousand Angstroms thick; the polymer layer is about 25 microns thick. Also, in the aforesaid application, the electret foil is 21-1/2 centimeters long and 2-1/2 centimeters wide.

The exposed surface of polymer layer 14 of the aforesaid electret foil 10 makes direct contact with the rough surface 1 6 of a selectively metalized backplate 1 8. Selective metalization is obtained by depositing a layer of metal on the naturally rough surface 1 6 of the backplate 18, so that the width, w, of the metal layer varies along the length of the backplate 1 8 in accordance with the relationship

W = L LIJ( 1 (2) where, J1 = Bessel function of the first kind; j=(~ 1)t,2; y = Incur + (r2 - 1)1/21; r = ratio of amplitude of the main lobe to the sidelobe threshold level in the response characteristic, to be described with reference to Fig. 5, hereinbelow; (= normalized displacement of any point on the backplate from the center of the backplate; and L = normalized displacement of the backplate beyond which the width of the metal layer is a constant, K.

For a constant charge density, constant air gaps and constant foil thickness, the sensitivity of the electret transducer at any point along its length is directly proportional to the width of the metal layer 1 6 on the backplate 1 8 at that point.

Alternatively, layer 1 2 of the aforesaid electret foil 10 is selectively metalized so that the width of the metal layer varies along the length of the electret foil 10 according to the aforesaid relationship (1) and (2). In this embodiment of the present invention, the metal surface 1 6 of backplate 1 8 has uniform width along the length of the backplate 1 8. For a constant charge density, constant air gaps, and constant foil thickness, the sensitivity of the electret transducer at any point along its length is directly proportional to the width of the metal layer 1 2 of the electret foil 10 at that point. The response characteristic obtained is substantially the same as the aforesaid response characteristic shown in Fig. 5.

Referring briefly to Fig. 5, there is shown the overall response characteristic of the electret transducer for 5067 Hz. The response characteristic, as experimentally determined, comprises one main lobe 30 and a plurality of sidelobes 32, 34, 36, 38, 40 each substantially 30 dB below the aforesaid main lobe 30. Other sidelobe patterns will be obtained for different frequencies. In each case, however, the sidelobe or sidelobes will be at or below threshold 35.

The corresponding response characteristic, theoretically determined, is shown in broken lines.

The ratio, r, of the amplitude of the main lobe 30 to the threshold 35, i.e. the sidelobe amplitude, is used as stated in the aforesaid relationship, (1) and (2) to determine the width of the metal layer.

Refering to Fig. 2 briefly, there is shown a plan view of the backplate 18 of Fig. 1. The aforesaid rough surface 1 6 of the backplate 1 8 comprises the metal layer 20 deposited by vaporization, or some other suitable method, on the surface 1 6 of the backplate 18. The backplate 1 8 may be made of circuit board material, often with holes therethrough, or some other electrically nonconductive material.

The width of the metal layer layer 20 is defined by the aforesaid relationships (1) and (2). At points along the length of the metal layer 20, equidistant from and on opposite sides of the center, the width of the metal layer is the same. Beyond a predetermined, normalized length, L, of the metal layer 20, the width remains a constant, K at each end of the metal layer 20.

Referring again to Fig. 1, a metal post 24, making contact with the metal layer 20 on the surface 1 6 of backplate 18, provides a positive electrical lead to a connector (not shown).

Likewise, lead 25, making contact with the metal layer of the electret foil 12, provides a neutral (or ground) electrical lead to the aforesaid connector (not shown). By this means, the acoustic signals impinging on the aforesaid electret transducer are converted to electrical signals, suitable for electrical transmission.

The aforesaid post 24 mates with orifice 26 in a structural member 28, made with brass in one application, for supporting the backplate 1 8. The electret foil 10 is superimposed on the backplate 18, the polymer layer 14 of the electret foil 10 making direct contact with the metalized rough surface 1 6 of the backplate 1 8. The longitudinal sides 27 and 29 of the electret foil 10 are pressed against the sides 41 and 43, respectively, of the structural support member 28 by clamps 42 and 44. A plurality of set screws 46, 48 hold the clamps 42 and 44 in place. Likewise, washers 50, 52 and set screws 54, 56 hold the ends 57, 59 of the electret foil 10 rigidly against the structural support member 28.

The assembled electret transducer may be supported vertically by sliding end 57 into a pedestal (not shown). In another arrangement, the electret transducer may be suspended by end 57, 59 or both 57 and 59 from a ceiling. In yet another arrangement, the electret transducer may be placed on a wall. In each case, the electret transducer is positioned in such a way that the main lobe 30 of Fig. 5 spans a targeted location.

Referring to Fig. 3, there is shown an isometric view of a section of the assembled electret transducer. The same indicia as in Figs. 1 and 2 are used for ease in reference.

Referring to Fig. 4 there is shown a magnified view of the contact between the electret foil 10 and the backplate 1 8 of Fig. 1. The polymer layer 14 of the electret foil 10 makes direct contact with the rough surface 1 6 of backplate 18. Because of the natural roughness of the surface 1 6 of backplate 18, there will be several natural ridges therein for making direct contact with the foil 10, thereby preventing resonant motion in the aforesaid foil 10. Prevention of resonant motion is necessary for avoiding spectral peaks in the frequency response characteristic of the electret transducer. The uneven surface 1 6 provides the necessary air gap between the backDlate 1 8 and the electret foil 10.

Referring to Fig. 6, there is shown another embodiment of the present invention. Electret foil 10, comprising polymer layer 14 and metal layer 12, makes direct contact with backplate 1 8, comprising a metalized rough surface 1 6. Furthermore, the metal layer 1 2 makes direct contact with the metalized layer 16. Polymer layer 14 is electrostatically charged to a preselected voltage level.

Suspended over the electret foil 10 is a second foil 66, comprising a metal layer 68 and a polymer layer 70. Structurally, two insulated end stops 62 and 64 separate polymer layer 70 of the second foil 66 from the polymer layer 14 of electret foil 1 0. Two variations are possible by varying either the width of the metal layers 1 6 and 12, or the width of the metal layer 68 according to the aforesaid relationships (1) and (2) while keeping the width of the remaining metal layers uniform along the entire length of the layer. In each case, the response characteristic obtained is substantially the same as the aforesaid response characteristic: one main lobe and a plurality of sidelobes at or below a predetermined threshold level, as in Fig. 5, hereinabove.

As state hereinabove, the sensitivity of the electret transducer at any point along its length is directly proportional to the width of the varying metal layer of the electret transducer at that point. The width of the varying metal layer of the electret transducer, in accordance with the present invention, is given by the aforesaid relationships (1) and (2).The sensitivity +(x) of the transducer at any distance, along its length, from the center of the electret transducer is given by the relationship

where +(x) = sensitivity of the electret transducer at a distance x from the center thereof; (r(x) = the spatial charge distribution; S,,,(x) = the effective air gap thickness to be described more fully hereinbelow; S1(x) = the actual air gap thickness, to be described more fully hereinbelow; w(x) = width of the metal layer, stated by the aforesaid relationships (1) and (2); P0 = atmospheric pressure; A = area of plane surface 16 of electret transducer in Fig. 2, hereinabove; += permittivity of the electret foil, +0 = permittivity of air; and S(x) = thickness of the electret foil.

The aforesaid response characteristic is calculated, theoretically from the relationship

where, 1 = length of electret transducer from the center to one end; - 1 = length of electret transducer from the center to the other end; {(x) = sensitivity of the electret transducer at any distance x from the center thereof; k = acoustic wavenumber; and R(8) = response to the electret transducer at any angle H formed by the angle between an incident acoustic wave and the surface of the transducer.

Referring to Fig. 5 again, there is shown in solid lines 30, 32...40 the response characteristic actually measured. The corresponding theoretically calculated response characteristic, from relationship (4), is shown in broken lines.

As stated in relationship (3), hereinabove, the sensitivity of the electret transducer is directly proportional to the width of the metal layer, the thickness of the electret foil, the thickness of the effective air gap, and the spatial charge distribution, i.e., the electrostatic charge on the electret foil. The sensitivity of the electret transducer is also inversely proportional to the thickness of the actual air gap. Thus, by varying the aforesaid parameters, one at a time, in accordance with the aforesaid relationship (1), the desired response characteristic shown in Fig. 5 is obtained.

Referring to Fig. 7, there is shown a device useful in understanding the aforesaid terms: actual air gap and effective air gap.

A backplate 72 having uniform thickness, t, rests on a ridge 74 machined into the cylinder 78 of radius rO, at a distance, h,, from the bottom surface 80 thereof. The backplate 72 has perforated therethrough a plurality of apertures 82, each of diameter, h,. Resting at the top of the cylinder 78 is an electret foil 84, of thickness, S, at a distance, S1, from the top surface of backplate 72. Because of the weight of electret foil 84 and because of vibrations caused by sound impinging on the foil 84, there will be some insignificant variations in the distance, Si, of the foil 84 from the backplate 72.

The actual air gap is the volume 86 of air between the electret foil 84 and the backplate 72.

As stated hereinabove, because there is only an insignificant deformation in the electret foil 84, the thickness or depth of the actual air gap, S,, is substantially constant. The actual air gap affects the electrical behavior of the system. The closer the electret foil 84 is to the backplate 72, the higher will be the output signal produced by electret foil 84.

The effective air gap is the sum of the volume of air in the back cavity 88, the volume of air in the plurality of apertures 82 and the actual air gap 86. The thickness or depth of the effective air gap is given by the relationship

where, h0 = depth of the volume of air in the back cavity 88; n = number of apertures 82; h, = diameter of each of the apertures 82; t = thickness of the backplate 72; rO= radius of the cyliner 78; and S, = thickness of the actual air gap.

The effective air gap affects the mechanical behavior of the electret transducer. The larger the effective air gap, the higher will be the deflection of the electret foil 84 for the same incident acoustic pressure thereon. Thus, the effective air gap determines the mechanical stiffness of the electret transducer but does not affect the electrical properties of the electret transducer.

Whereas Fig. 7 shows a cylinder 88 for teaching the meaning of the terms actual air gap and effective air gap, the same principles apply to the rectilinear electret transducer of Fig. 1 Referring to Fig. 8, there is shown an electret transducer manufactured by varying the thickness of the actual air gap along the length of the aforesaid electret transducer. The aforesaid variation in the thickness of the actual air gap is realized by the use of a plurality of posts 90 for separating the electret foil 92 from the backplate 94. The plurality of posts 90 have heiahts along the lenath of the electret transducer determined bv the relationshiD (w(x))- ' (6) and (K)-1 (7) where, w(x) is the aforesaid relationship (1) and K is the aforesaid relationship (2).

That is, the sensitivity at any point along the electret transducer is inversely proportional to the height of the posts, at that point. At any point on the backplate 94, the posts are equally high along the width of the backplate 94 at that point. Alternatively, the posts are replaced by raised ridges which have constant height and run along the width of the backplate.

The backplate 94 has a thin metal layer as wide as the backplate 94 on the surface 96 thereof facing the electret foil 92. Alternatively, the entire backplate is made of metal. The electret foil 92 comprises a metal layer 91 and an electrostatically charged polymer layer 98.

Polymer layer 98 faces the backplate. A negative lead 97 from the metal surface 91 of the electret foil 92 and a positive lead 99 from the metal surface 96 of the backplate 94 are terminated on a connector 100.

Fig 9 shows an isometric view of a part of the electret transducer in Fig. 8, showing the details of the posts 90 on the backplate 94 with a portion of the electret foil 92 cut away.

In another embodiment of the present invention, the metal layer 91 of the electret foil 92 is affixed to the metal layer of the backplate 96, so that the two metal layers make direct contact.

Suspended over the electret foil 92 is a second foil (not shown), the two foils being separated by the aforesaid posts 90.

Referring to Fig. 10, there is shown another embodiment of an electret transducer obtained by varying the thickness of the actual air gap. The actual air gap thickness between the electret foil 102 and the backplate 104 is realized by varying the thickness of the backplate 104. The thickness of the electret foil 102, however, remains constant along the entire length thereof.

Consequently, the heights of the plurality of posts 108 vary along the length of the aforesaid electret transducer. The posts 108 provide structural support for the electret foil 102. The thickness of backplate 104 varies along the length of the electret transducer according to the aforesaid relationships (6) and (7), i.e., the sensitivity of the electret transducer at any point thereon is inversely proportional to the thickness of the backplate thereat.

The surface 110 of backplate 104 is coated with a metal layer extending the entire width of backplate 1 04. Alternatively, the entire backplate 104 is made of metal. Electret foil 102 comprises two layers: a metal layer 101 and an electrostatically charged polymer layer 112. A neutral (or ground) lead 111 from the metal surface 101 of electret foil 102 and a positive lead 11 3 from the metal surface 110 of backplate 104 are terminated at a connector 114.

Alternatively, the metal layer 101 of the electret foil 102 is attached to the metal surface 110 of the backplate 104 so that the two metal layers are in direct contact. A second foil (not shown) is suspended over the backplate 104. This second foil is supported by the aforesaid posts 108.

In each electret transducer shown in Figs. 8, 9 and 10, the response obtained therefrom is substantially similar to the response characteristic in the aforesaid Fig. 5.

Referring to Figs. 11, 1 2 and 1 3 there are shown three separate embodiments of electret transducers, in each case obtained by varying the effective air gap thickness. In each of the aforesaid three embodiments, there is an electret foil superimposed on a backplate, similar to the electret transducers in Figs. 1 and 4, hereinabove. Because the invention is embodied substantially in the backplate, only the backplate will be described.

Referring more particularly to Fig. 11, there are shown an electret foil 121 and a backplate 116. The aforesaid electret foil 121 is superimposed directly on backplate surface 118 which is naturally rough. A plurality of equal diameter holes 1 20 are drilled to various depths through surface 118 of backplate 116. Because the aforesaid electret foil 121 is placed directly on surface 118, the actual air gap thickness is substantially constant along the length of the backplate 11 6. Consequently, by referring to the aforesaid relationship (5), the effective air gap thickness is directly proportional to the depth of the holes 1 20.

By varying the depths of the aforesaid holes 120, the effective air gap thickness is given by the aforesaid relationships (1) and (2). The sensitivity, 4'(x), at any point along the electret transducer is directly proportional to the effective air gap thickness at that point. The sensitivity, 1p(x), is stated by the aforesaid relationship (3). The response characteristic is calculated, theortically, from the aforesaid relationship (4). The response as calculated from relationship (4) and as actually measured are substantially similar to the response characteristics shown in Fig.

5, hereinabove.

Referring to Fig. 12, there are shown an electret foil 1 23 and a backplate 1 22 comprising a plurality of apertures perforated therethrough, useful in realizing another electret transducer. The diameters of the apertures vary so that the volume of air in the apertures varies directly in proportion to the aforesaid relationships (1) and (2), i.e., the effective air gap thickness varies directly in proportion to the aforesaid relationships (1) and (2).

Referring to Fig. 13, there are shown an electret foil, 1 27 and a backplate 1 26 comprising a plurality of equal diameter apertures 128 perforated through the backplate 1 26. The effective air gap thickness is varied, by varying the density of the apertures 126, directly in proportion to the aforesaid relationships (1) and (2).

In another embodiment (not shown) of the present invention in which the effective air gap varies along the length of the electret transducer, the metal layer of the electret foil is placed directly in contact with the metal layer of the backplate. A second foil is suspended above the electret foil 121, 123 or 127, supported by insulating elements, such as shown in Fig. 6 by elements 62 and 64. In three separate realizations of this embodiment the effective air gap thickness is varied by varying the diameter, density, or depth of holes in the backplate, as shown in Figs. 11, 1 2 and 1 3. Alternatively, the holes in Figs. 11, 1 2 and 1 3 are replaced by grooves (not shown) whose breadth, density, or depth varies according to relationships (1) and (2).These grooves run parallel to the backplate width.

Referring to Fig. 14, there is shown an electret transducer comprising a backplate 1 30 of uniform thickness and an electret foil 1 32 of thickness varying directly in proportion to the aforesaid relationships (1) and (2). Electret foil 1 32 comprises a polymer layer 1 36 and superimposed thereon, a metal layer 142. The polymer layer 1 36 has a flat surface 1 34 superimposed directly on a naturally rough surface 1 38 of a backplate 1 30. There is coated on surface 1 38 of the backplate 1 30 a thin metal layer.A neutral (or ground) lead 1 33 from the metal layer 1 42 and a positive lead 1 31 from the metal layer on surface 1 38 of backplate 1 30 are terminated at connector 140.

The sensitivity, \mix). given by the aforesaid relationship (3) of the electret transducer in Fig.

14 at any point thereon is directly proportional to the thickness of the electret foil 1 32 at that point. The response, as calculated theoretically from the aforesaid relationship (4) and as measured, are substantially as shown by the response characteristics in Fig. 5, hereinabove.

Fig. 1 5 shows another embodiment of the present invention in which the thickness of the electret foil 154 varies along the length of the transducer according to relationships (1) and (2).

The metal layer 1 58 of the electret foil 1 54 is in direct contact with the metal surface 1 52 of the backplate 1 50. The thickness of the backplate 1 50 is determined by the relationship 1 - w(x) (8) 1 - K (9) A second foil 1 68 is suspended above the electret foil 154, supported by insulating elements 1 62 and 1 64. The distance between the second foil 1 68 and the polymer surface of the electret foil 1 56 is substantially constant along the length and width of the electret transducer.A neutral (or ground) electrical lead 1 53 is attached to the metal surface of the second foil 170, and a positive lead 1 55 is attached to the metal layers 1 58 and 1 52 which are in contact. The electrical leads 1 53 and 1 55 terminate at connector 1 66.

The sensitivity, i.e., +(x), of the electret transducer of Fig. 1 5 expressed by the aforesaid relationship (3) at any point along the length of the electret transducer, is directly proportional to the thickness of the electret foil 1 54 at that point. The response characteristic is substantially as shown in Fig. 5.

Another embodiment of an electret transducer is realized by varying the electrostatic charge, using known methods, on the polymer layer 1 4 of the electret foil 10 in Fig. 4 hereinabove directly in proportion to the aforesaid relationships (1) and (2). The width of the metal layer, on the rough surface 1 6 of backplate 18, however, remains constant along the entire length of the backplate 1 8. Thus, instead of varying the width of the metal layer 16, the electrostatic charge on the polymer layer 14 may be varied. Alternatively, the electret foil 10 is placed directly in contact with the backplate 1 8 as shown in Fig. 6, and a second foil 66 placed over the electret foil.The sensitivity, +(x), given by the aforesaid relationship (3), of the electret transducer at any point thereon is directly proportional to the electrostatic charge at that point. The response of the aforesaid electret transducer is substantially similar to the response characteristic shown in Fig. 5, hereinabove.

As stated hereinabove, it is known how to electrostatically charge an electret foil. One such method is disclosed in an article entitled, "Research in Polymer Electrets" by Messrs. G M.

Sessler and J. E. West, published by the Society of Photographic Scientists and Engineers at the Second International Conference on Electrophotography, pages 162-166 (1974).

Referring to Fig. 16, there is shown an electrostatic charge distribution on the polymer surface 14 of electret foil 10 of Fig. 1. The electrostatic charge is distributed evenly along the entire width of the electret foil surface 14. The charge density, however, varies along the length of the electret foil according to the aforesaid relationships (1) and (2). Furthermore, the sensitivity of the electret transducer, at any point thereon, is directly proportional to the electrostatic charge on the electret foil at that point. And, stated by relationship (4) hereinabove, the response characteristic, shown above in Fig. 5, is dependent on the sensitivity of the electret transducer.

Referring to Fig. 17, there is shown a polymer surface such as polymer surface 1 4 of the electret foil 10, in Fig. 1 above, electrostatically charged by an alternate method. The' electrostatic charge is distributed uniformly along a selected width, as in Fig. 1 6 above, with the charge varying along the length of the polymer surface 14. The width, however, of the electrostatically charged polymer surface 14 varies, along the length of the aforesaid relationships (1) and (2). The width of the electrostatically charged area, however, is not coextensive with the width of the polymer. The response characteristic obtained is substantially the same as before.

The electrostatic charge distribution described with reference to Figs. 1 6 and 1 7 above relate to surface charges. In each case, the same charge distribution can be realized by depositing the electrostatic charge to different depths of the polymer layer of electret foil 10 in Fig. 1. The electrostatic charge a at any point is given by the relationship,

where, o = charge density, = = permittivity of polymer, = = permittivity of surrounding air, d, = depth of electrostatic charge, d = thickness of polymer layer, and V = electrostatic voltage.

Referring to Fig. 18, there is shown yet another method of charging the aforesaid polymer surface of the electret foil. As in Fig. 6, a negative electrostatic charge within a selected area varies along the length of the polymer layer in accordance with the aforesaid relationships (1) and (2). Unlike Fig. 17, however, in Fig. 18, the uncharged spaces on the polymer surface are now positively charged. The effect of charging the polymer surface with both negative and positive charges is to provide a clearly defined edge between the two charged area. Thus, the sensitivity at any point on the transducer is more precise, being directly dependent on the electrostatic charge at that point. The response is highly directional, and as shown earlier in Fig.

5, comprises a main lobe and a plurality of sidelobes below a preselected threshold.

In three of the aforesaid embodiments wherein the metal width is varied, the actual air gap is varied, or the effective air gap is varied, the electret foil may be replaced by a foil with a direct current (d.c.) bias applied thereto. That is, instead of an electrostatic charge being deposited on the foil, a d.c. bias is provided continuously thereto from an external d.c. source.

Furthermore, two separate foils may be used: a foil comprising a metal layer and a polymer layer, or a foil made entirely of metal. Where a foil comprising a polymer layer and a metal layer is used, however, the metal layer must be placed adjacent to the backplate.

Additionally, the foil should not be placed directly on the backplate. Instead, the foil should be suspended over the backplate, such as by the use of insulating stops as elements 62, 64 in Fig. 6.

The leads from the metal layer of the foil and the metal layer of the backplate may be interchanged in termination at the connector. That is, the polarity of the leads is irrelevant.

Referring to Fig. 19 there is shown an isometric view a disassembled acoustic transducer 210 embodying the present invention. Electret foil 220 comprises a metal layer 222 directly in contact with a polymer layer 224. The bottom surface 226 of polymer lever 224 is substantially flat and has induced therein a uniform electrostatic charge. The metal layer 222 is connected via lead 223 through connector 228 to a utilization means (not shown).

The backplate 230 has a rough surface 232 so that when electret foil 220 is placed directly on surface 232, the air pockets between the flat polymer surface 226 and the rough backpate permit vibration of the electret foil 220.

Backplate 230 has deposited on rough surface 232 a metallic electrode 234. Metallic electrode 234 is connected via lead 235 to connector 228.

Referring more particularly to the metallic electrode 234, there are shown a plurality of discrete areas, or blobs, or islands 241, 243, 245.. .249 at distances D1, D2, D3. . .Dj, respectively, from a center 237 of the metal electrode 234. The islands 241, 243, 245.. .249 are interconnected by thin strips, or isthmuses 242, 244, 246...248, respectively.

Likewise, islands 251, 253, 255...259 are located at distances D,, D2, D3. . . D1, respectively, on the opposite side of center 237 of the metallic electrode 234. Islands 251, 253, 255...259 are interconnected by isthmuses 252, 254, 256. . .258, respectively.

Furthermore, islands 241, 243, 245...249 and 251, 253, 255...259 are located symmetrically on opposite sides of center 237 of the metallic electrode 234. The distances D1, D2, .... . D1 bear a nonlinear relationship to each other as disclosed in British patent application No. 2066620A.

That application proposes a plurality of acoustic transducers arranged in an array according to a predetermined relationship. In the present invention, however, a single transducer is used having a single backplate. The metal islands on the backplate correspond to the transducers in prior proposal. Thus, when acoustic waves impinge on the metal layer 222 of the electret foil 220, the electret foil 220 vibrates causing the air pockets between the electret foil 220 and the rough surface 232 of backplate 230 to correspondingly contract and expand. In response to the air contraction and expansion, the islands 241, 243, 245. .249 and 251, 253, 255...259 convert the acoustic energy to electrical signals, sum the signals and transmit the signals over lead 235 to the connector 228.That is, the summing of the signals take place within the acoustic transducer 21 0.

Because the metal electrode 234 is continuous, a template (not shown) may be placed on surface 232 of the backplate 230 and the metal evaporated thereon. Alternatively, the entire surface 232 of backplate 230 may be coated with the metal layer 232 and the pattern of metallic electrode 234 obtained by lazer trimming.

The shape of the islands 241, 243, 245...249 and 251, 253, 255...259 are irrelevant.

The areas of the aforesaid islands, however, are important in determining the sensitivity. In order to insure uniform sensitivity, all the islands have substantially the same area. Alternatively, if the aforesaid islands have different areas, the corresponding distances of the aforesaid distances D1, D2, .... Dj of the islands from the center metal electrode 34 must be varied.

In another embodiment of the present invention (not shown), the metallic electrode 234 and the metal layer 222 may be interchanged.

The response pattern in both of the aforesaid embodiments comprise a main lobe and a plurality of sidelobes at or below a predetermined threshold level. The response pattern is disclosed in greater detail in the aforesaid application.

When the backplate 230, metallic coating 234, and electret foil 220 are fabricated from pliable material, the entire acoustic transducer 210 may be rolled into a compact package for shipping.

In the assembled state, electret foil 220 is placed directly in contact with backplate 230 so that the flat polymer surface 226 and the metallic electrode 234 are in direct contact.

The acoustic transducer 210 may be used as a microphone or a loudspeaker. When used as a microphone for teleconferencing, backplate surface 239 of acoustic transducer 210 may be placed on a supporting member (not shown) and the end 261 of the transducer 210 mounted on a pedestal (not shown). Alternatively, ends 261 and 263 may be suspended from a ceiling, or the assembly placed on a wall.

Claims (65)

1. An acoustic transducer comprising a supported foil, senstivity of the transducer so varying across the surface of the foil as to produce a directional response pattern comprising a main lobe and a plurality of smaller side lobes.

2. An acoustic transducer as claimed in claim 1, wherein the foil comprises a metal coated electret.

3. An acoustic transducer as claimed in claim 1 or 2, wherein the foil is supported by a backplate.

4. An acoustic transducer according to claim 2 or 3 wherein the back plate is conductive over an area having a width, w, defined by the relationship

where J, = Bessel function of the first kind, j=(- 1)1/2 p = 1n [r+(r2- 1)1/2], r = ratio of amplitude of said main lobe to a threshold level equal to or larger than said side lobes, (= normalized length of any point on said backplate from the center of said backplate, and L = normalized length of said backplate beyond which said width of said conductive area is a constant, K.

5. An acoustic transducer according to claim 4, wherein the width of the conductive area is substantially the same at equal distances from and on opposite sides of a center line, parallel to the shorter side, of said backplate, and the distance of the two edges of the conductive area on opposite sides of a second center line, parallel to the longer side, of said backplate are substantially the same.

6. An acoustic transducer according to claim 3, 4 or 5, wherein said foil is placed directly in contact with said backplate, the surface of said backplate being rough.

7. An acoustic transducer according to any preceding claim, wherein said foil comprises a first metallic layer and a second electret polymer layer, said first layer and said second layer being rectangular in shape.

8. An acoustic transducer according to claim 7, wherein the electret foil is electrostatically charged to a predetermined value.

9. An acoustic transducer according to any preceding claim, wherein a first lead from said foil and a second lead from said metallic coating on said backplate are terminated at a connector for transmitting signals therebetween.

10. An acoustic transducer as claimed in any of claims 3 to 9, wherein the backplate comprises an insulated circuit board and a selectively coated metallic layer on one side of said insulated circuit board.

11. An acoustic transducer as claimed in claim 2, including a backplate having a conductive surface a first foil comprising a polymer layer and a metal layer, said first foil being fastened to said backplate so that the metal layer of said first foil faces the conductive surface of said backplate, and a second foil comprising a polymer layer and a metal layer, the width of said metal layer of said second foil varying along the length of said second foil, said second foil being suspended above said first foil.

1 2. An acoustic transducer as claimed in claim 2, including a backplate having a conductive surface, a first electret foil, comprising a polymer layer and a metal layer, said metal layer of said electret foil being fastened to said conductive surface of said backplate, the width of both said metal layers and said conductive surface varying in accordance with a predetermined relationship along the length of said backplate, and a second electret foil comprising a polymer layer and a metal layer, said second foil being suspended above said first electret foil.

1 3. An acoustic transducer as claimed in claim 1 or 2, and including a conductive backplate, said foil being substantially parallel to said backplate along a width of said foil at any point along a length of said foil, the distance between said foil and said backplate varying along said length of said foil to vary the sensitivity of the transducer.

14. An acoustic transducer according to claim 13, wherein said distance, d, between said foil and said backplate at any point on said foil is defied by the relationship

where, J, = Bessel function of the first kind, = ( - 1)1/2 v = 1n[r+(r2- 1)1/2] r = ratio of amplitude of said main lobe to a threshold level equal to or larger than said side lobes, 5= normalized length of any point on said backplate from the center of said backplate, and L = normalized length of said backplate beyond which said distance is a constant, K.

1 5. An acoustic transducer according to claim 14, wherein said distance between said foil and said backplate is realized by a plurality of posts on said backplate supporting said electret foil, the heights of said posts varying in accordance with said distance along the length of said backplate.

16. An acoustic transducer according to claim 14, wherein said distance between said foil and said backplate is realized by a plurality of ridges on said backplate supporting said electret foil, the heights of said ridges varying in accordance with said distance along the length of said backplate, the upper most surface of said ridges being parallel to the surface of said backplate.

1 7. An acoustic transducer according to claim 15, or 1 6 wherein said backplate is of uniform thickness along the length of said backplate.

1 8. An acoustic transducer according to claim 15, or 1 6 wherein the thickness of said backplate varies according to said predetermined formula.

1 9. An acoustic transducer according to claim 18, wherein the volume of air in holes in said backplate is uniform along the length and width of said backplate.

20. An acoustic transducer according to claim 18, wherein the volume of air in grooves in said backplate which are parallel to said backplate width is uniform along the length of said backplate.

21. An acoustic transducer according to any of claims 1 3 to 20, wherein said foil comprises a metal layer and an electret polymer layer, said metal layer and said polymer layer being rectangular in shape.

22. An acoustic transducer according to any of claims 13 to 21, wherein said foil is electrostatically charged to a predetermined value.

23. An acoustic transducer according to any of claims 1 3 to 22, wherein a first lead from said foil and a second lead from said conductive backplate are terminated at a connector for transmitting signals therebetween.

24. An acoustic transducer as claimed in claim 1 or 2 including a backplate having a conductive surface, a first foil fastened to said backplate so that a metal layer on said first foil faces the conductive surface of said backplate, and a second foil comprising a polymer layer and a metal layer, said second foil being suspended above said first foil, whereby the distance between said second foil and said first foil varies along the said length of said foil.

25. An acoustic transducer as claimed in claim 1, including a conductive backplate, the effective air gap between said foil and said backplate varying to vary the sensitivity of said acoustic transducer.

26. An acoustic transducer according to claim 25, wherein said effective air gap is realized by a plurality of holes traversing the thickness of said backplate said holes having the same diameter and the density of said holes varying along the length of said backplate according to the following relationship

where, J, = Bessel function of the first kind, j=(~ 1)1/2 v= 1n[r+(r2- 1)1/2] r = ratio of the amplitude of said main lobe to a threshold level equal to or larger than said side lobes (= normalized length of any point on said backplate from the center of said backplate, and L= normalized length of said backplate beyond which said groove density is a constant, K.

27. An acoustic transducer according to claim 25, wherein said effective air gap is realized by a plurality of grooves in said backplate, said grooves being parallel to one another and parallel to the width of said backplate said grooves having the same cross section and the density of said grooves varying along the length of said backplate according to the following relationship

where, J, = Bessel function of the first kind, j = ( 1)1/2 V= 1nEr+(r2- 1)1/2], r = ratio of the amplitude of said main lobe to a threshold level equal to or larger than said side lobes (= normalized length of any point on said backplate from the centre of said backplate, and L= normalized length of said backplate beyond which said groove density is a constant, K.

28. An acoustic transducer according to claim 25, wherein said backplate comprises a plurality of holes traversing the thickness of said backplate, said holes being uniformly dispersed in said backplate and the diameter of said holes varying along the length of said backplate according to the following relationship

where, J, = Bessel function of the first kind, = ( - 1)1/2 p = 1n Er+(r2- 1)1/2] r = ratio of the amplitude of said main lobe to a threshold level equal to or larger than the sidelobes, ( = normalized length of any point on said backplate from the center of said backplate, and L= normalized length of said backplate beyond which said volume of air in said holes is the constant, K.

29. An acoustic transducer according to claim 25, wherein said backplate comprises a plurality of grooves in said backplate, said grooves being parallel to one another and parallel to the width of said backplate, said grooves being uniformly dispersed in said backplate and the cross section diameter of said grooves varying along the length of said backplate according to the following relationship

where J, = Bessel function of the first kind, j=(~ 1)1/3 p = 1n [r+(r2- 1)1/2 r = ratio of the amplitude of said main lobe to a threshold level equal to or larger than the side lobes, t = normalized length of any point on said backplate from the center of said backplate, and L = normalized length of said backplate beyond which said cross section of air in said grooves is the constant, K.

30. The acoustic transducer according to claim 25, wherein said backplate comprises a metalized circuit board and a structural element for supporting said metallized circuit board, said metallized circuit board comprising a plurality of uniformly spaced holes being of identical diameter, and said holes extending into said structural supporting element so that the depth of said holes and the volume of air in said holes varies along the length of said backplate according to the following relationship

where, J, = Bessel function of the first kind, j=( 1)1/2 p= In (r+(r2- 1)1/2], r = ratio of the amplitude of said main lobe to a threshold level equal to or larger than the sidelobes, (= normalized length of any point on said backplate from the center of said backplate, and L = normalized length of said backplate beyond which said volume of air in said holes is a constant, K.

31. An acoustic transducer according to claim 25, wherein said backplate comprises a metalized circuit board and a structural element for supporting said metalized circuit board, said metalized circuit board comprising a plurality of uniformly spaced grooves being of identical cross section, and said grooves extending into said structural supporting element so that the depth of said grooves and the volume of air in said grooves vary along the length of said backplate according to the following relationship

where, J, = Bessel function of the first kind, = ( ~ 1)1/2 p = in [r + (r2 - 1)1/2] r = ratio of the amplitude of said main lobe to a threshold level equal to or larger than the side lobes (= normalized length of any point on said backplate from the center of said backplate, and L = normalized length of said backplate beyond which said volume of air in said grooves is a constant, K.

32. An acoustic transducer according to any of claims 25 to 31 wherein said foil comprises a metal layer and a polymer layer, said metal layer and said polymer layer being rectangular in shape.

33. An acoustic transducer according to any of claims 25 to 32 wherein said foil includes an electret electrostatically charged to a predetermined value.

34. An acoustic transducer according to claim 33, wherein a first lead from said electret foil and a second lead from said metallic coating on said backplate are terminated at a connector for transmitting signals therebetween.

35. An acoustic transducer as claimed in any of claims 25 to 34, including first foil fastened to said backplate so that a metal layer of said first foil faces a conductive surface of said backplate, and a second foil comprising a polymer layer and a metal layer, said second foil being suspended above said first foil whereby the effective air gap varies in accordance with a predetermined relationship so that the sensitivity of said acoustic transducer varies in proportion to said effective air gap.

36. An acoustic transducer as claimed in claim 2, including a backplate the electrostatic charge on said electret foil varying to vary the sensitivity.

37. An acoustic transducer according to claim 36, wherein said electrostatic charge varies according to the following relationship

where, J, = Bessel function of the firsr kind, j=( 1)1/2] r = ratio of amplitude of said main lobe to a threshold level equal to or larger than said sidelobes, (= normalized length of any point on said backplate from the center of said backplate, and L= normalized length of said backplate beyond which said electrostatic charge is a constant, K.

38. An acoustic transducer according to claim 36 to 37, wherein one surface of said backplate has a metallic coating.

39. An acoustic transducer according to claim 38, wherein said electret foil comprises a metal layer and a polymer layer, said polymer layer facing said metallic surface of said backplate.

40. An acoustic transducer according to claim 39, wherein said electrostatic charge varies along the length of said polymer layer.

41. An acoustic transducer according to claim 40, wherein the electrostatic charge at any point along the length of said polymer layer is uniform across the entire width of said polymer layer at said point.

42. An acoustic transducer according to claim 40, wherein the electrostatic charge at any point along the length of said polymer layer is nonuniform across the entire width of said polymer layer at said point.

43. An acoustic transducer according to claim 42, wherein the electrostatic charge at any point along the length of said polymer layer is substantially the same at equal distances from and on opposite sides of a center line, parallel to the shorter side of said polymer layer.

44. An acoustic transducer according to any of claims 36 to 43, including a second foil comprising a polymer layer and a metal layer said second foil being suspended above said electret foil.

45. An acoustic transducer as claimed in claim 2, including a backplate, and superimposed directly thereon an electret foil comprising a metal layer and a polymer layer, a first area of said polymer layer being electrostatically charged with a negative charge, the width of said first area varying along the length of said polymer layer, and the rest of said polymer layer being electrostatically charged with a positive charge.

46. An acoustic transducer as claimed in claim 2, including a conductive backplate, the electret foil having a thickness varying along the length of said foil to vary the sensitivity, one surface of said electret foil being substantially flat and substantially parallel to said backplate.

47. An acoustic transducer according to claim 46, wherein said thickness, d, of said electret foil is define by the relationship

where, J, = Bessel function of the first kind, j=( 1)1/2 d= 1n[r+(r2- 1)1/2], r = ratio of amplitude of said main lobe to a threshold level equal to larger than said sidelobes, (= normalized length of any point on said backplate from the center of said backplate, and L = normalized length of said backplate beyond which said thickness of said foil is a constant K.

48. An acoustic transducer according to claim 46 to 47, wherein said electret foil comprises a metal layer and a polymer layer, the thickness of said polymer layer varying, and the thickness of said metal layer being substantially uniform.

49. An acoustic transducer according to claim 48, wherein one surface of said polymer layer is substantially flat and said flat surface is substantially parallel to said backplate.

50. An acoustic transducer according to claim 49, wherein the distance between said flat surface of said polymer layer and said backplate is substantially uniform along the length and width of said backplate.

51. An acoustic transducer according to claim 40 or 50 wherein said flat polymer surface faces a conductive surface of said backplate.

52. An acoustic transducer according to any of claims 46 to 51, wherein said electret foil is electrostatically charged to a predetermined value.

53. An acoustic transducer according to any of claims 46 to 52, wherein a first lead from said electret foil and a second lead from said metallic coating on said backplate are terminated at a connector for transmitting signals therebetween.

54. An acoustic transducer as claimed in any of claims 46 to 53 including a second foil comprising a polymer layer and a metal layer, said second foil being suspended above said electret foil.

55. An acoustic transducer according to claim 54, wherein the distance between said flat surface of said electret foil and said second foil is substantially uniform along the length and width of said second foil.

56. An acoustic transducer, as claimed in claim 1, wherein sensitivity of the transducer across the surface of the foil is confined substantially to discrete areas, so spaced as to produce a directional response pattern.

57. An acoustic transducer as claimed in claim 56, wherein the foil is superimposed directly on a backplate which includes an electrode, the electrode comprising a plurality of discrete areas interconnected by a plurality of thin strips.

58. An acoustic transducer according to claim 57, wherein said discrete areas are located symmetrically on opposite sides of the center of said electrode.

59. An acoustic transducer according to claim 58 wherein said discrete areas have substantially the same area.

60. An acoustic transducer according to claim 59 wherein the relationship among said discrete area is nonlinear.

61. An acoustic transducer according to any of claims 57 to 60 wherein said backplate has a rough surface thereby providing air pockets between said rough surface and said foil for vibration of said electret foil.

62. A acoustic transducer according to any of claims 57 to 61, wherein said foil conprises a metal layer and a polymer layer.

63. An acoustic transducer according to claim 62, wherein said polymer layer has induced therein a uniform electrostatic charge.

64. An acoustic transducer according to claim 62 or 63 wherein said metal layer and said electrode are connected to a connector.

65. Any embodiment of an acoustic transducer described hereinbefore with reference to the accompanying drawings.