US 3873784 A
An acoustic transducer of planar shape. An electric conductor means of a coil, preferably flat spiral or flat helical configuration is operably attached to a diaphragm and positioned in the magnetic field of at least one magnetized member adjacent one of its pole faces. Magnetically conductive pole pieces preferably are positioned adjacent the magnetized member to form a dense elongated region of preferably substantially uniform magnetic flux density in said magnetic field where the electric conductor means is preferably located. Preferably said region in said magnetic field is contained in pairs of spaced apart elongated gaps formed by said pole pieces and one said magnetized member. A second diaphragm is preferably positioned adjacent the opposite pole face of one said magnetized member and preferably driven by a separate electric conductor means and the volume contained between said diaphragms is enclosed.
Claims available in
Description (OCR text may contain errors)
nited States Patent 11 1 Doschek [451 Mar. 25, 1975 1 1 ACOUSTIC TRANSDUCER  Inventor: Antony Z. Doschek, Pittsburgh, Pa.
 Assignee: Audio Arts, 1nc., Pittsburgh, Pa.
 Filed: Mar. 29, 1973  Appl. No: 346,171
Related US. Application Data  Continuation of Ser. No. 887,557, Dec. 23, 1969,
52 UIS.C1. 179/11s.5 PV  Int. Cl. H041" 9/00  Field of Search 179/l15.5 R, 115.5 PV, 179/1 15.5 DV,115.5 VC,116, 16 A; 181/31 B  References Cited UNITED STATES PATENTS 1,713,006 5/1929 Round 179/1 15.5 R 1,808,149 6/1931 Smith 179/1 15.5 PV 1,862,582 6/1932 Schlenker 181/31 R 3,015,366 1/1962 Bishop 181/31 B 3,141,071 7/1964 Rich 179/1 15.5 PV 3,153,120 10/1964 Brown 181/31 B 3,268,672 8/1966 Roesel, Jr. et a1. 179/1 15.5 R 3,283.086 1l/1966 Evans 179/1 15.5'DV 3,478,167 11/1969 Sorkin 179/16 A 3,491,204 l/l970 Sherno 179/16 A 3,609,253 9/1971 Ashworth 179/1 15.5 R
FOREIGN PATENTS OR APPLICATIONS 1,035,995 7/1966 United Kingdom 179/l15.5 V(' Primary L'.\'umin'cr-William C. Cooper Attorney, Agent, or Firm-Arland T. Stein; Thomas C. Wettach  ABSTRACT An acoustic transducer of planar shape. An electric conductor means of a coil, preferably fiat spiral or flat helical configuration is operably attached to a diaphragm and positioned in the magnetic field of at least one magnetized member adjacent one of its pole faces. Magnetically conductive pole pieces preferably are positioned adjacent the magnetized member to form a dense elongated region of preferably substantially uniform magnetic flux density in said magnetic field where the electric conductor means is preferably located. Preferably said region in said magnetic field is contained in pairs of spaced apart elongated gaps formed by said pole pieces and one said magnetized member. A second diaphragm is preferably positioned adjacent the opposite poleface of one said magnetized member and preferably driven by a separate electric conductor means and the volume contained between said diaphragms is enclosed.
58 Claims, 43 Drawing Figures PMENTEUHARZSISYS 73,7 4
SHEET 03 0F 15 4 M w/ga FATENTEU M2 5197s SHEET CE 0F 15' IOO m 2 NW a I 1 o w L 4 FEJENTED I975 3 873 78 sum 1 3 OF 15 Fig.38.
Y 229 22s I 2'9 mwcoumm 335 009 09 on SHEET lSUF 15 mv c uogsgAgp 18d 1 ACOUSTIC TRANSDUCER This application is a continuation of then co-pending, now abandonded, application Ser. No. 887,557, filed Dec. 23, 1969, now abandoned.
This invention relates to an acoustic transducer of a planar shape. It is particularly useful in reproducing speech, music and other sounds with a quality associated with hi-fidelity sound reproduction in an inexpensive, dependable and widely adaptable way.
It is a physical fact that an electric current moving through an electric conductor means in a magnetic field normal to the direction of the lines of magnetic flux produces a force normal to the directions of the electric current and the magnetic flux lines (in accordance with the well-known right-hand rule"). The electromagnetic acoustic transducer applies this principle to convert variations in electric current into corresponding variations in mechanical force and in turn physical displacement variations of a diaphragm, to produce acoustic waves with corresponding variations in frequency and intensity. I. The intcnsityof an acoustic wave is the time average rate at which energy is transported by the acoustic wave per unit area across a surface usually perpendicular. but possibly oblique, to the direction of propagation.
mon conical loudspeaker.
2v The quality of sound reproduction is determined by the accuracy with which the frequencies of sound and their respective intensities can be recreated.
Acoustic transducers of a planar shape and driven by electrostatic forces are broadly old and well known. They have been used side by side in multiples to provide a planar loudspeaker of very large diaphragm area. Such planar acoustic transducers have had inherent operating advantages over the common full range conical loudspeaker: (i) They eliminate perceptible Doppler effect distortion of middle and high frequencies because of the relatively small displacement amplitude of a very large diaphragm at low frequencies. (ii) They do not exhibit the characteristic rim resonance" of conical loudspeakers and therefore provide a smoother transition in reproduction from low-frequency to midfrequency sounds. (iii) They provide low frequency reproduction of greater intensity because of the movement of a relatively very large diaphragm through a comparatively small displacement amplitude to reproduce such low frequencies. But such planar transducers utilizing electrostatic driving means are very expensive to build and sensitive to acoustical overloads and inefficient. Such transducers were not to my knowledge heretofore commercially made with electromagnetic driving means moving the diaphragm.
l have presented one way of overcoming these disadvantages and difficulties in my copending application Ser. No. 784,669, filed Dec. 18, 1969, now US. Pat. No. 3,651,283. The present invention overcomes these disadvantages and difficulties in another way. It makes the use of an acoustic transducer of a planar shape driven by electromagnetic driving forces commercially feasible with sound reproducing apparatus of hi-fidelity grade. Moreover, it provides a versatility and decora- 2 tiveness in the design, application and installation of planar acoustic transducers heretofore unseen in the art, and it makes planar acoustic transducers commercially competitive with conical loudspeakers.
I provide an acoustic transducer of planar shape having at least one electromagnetic driving means and at least one diaphragm operably attached thereto. The electromagnetic driving means is comprised of at least one magnetized member and at least one electric driver means. Each magnetized member creates a magnetic field and has at least one substantially planar pole face. Each electric driver means is comprised of at least one electric conductor means and possibly in addition, a conductor forming means to support the electric conductor means and operably attach it to the diaphragm. Each electric conductor means is substantially in coil form, preferably in flat configuration, and is spaced from one said planar pole face. Each electric conductor means is positioned in the magnetic field of said magnetized member and in some embodiments preferably forms a plane substantially parallel to at least one said planar pole face. At least one essentially nonmagnetic diaphragm, preferably of planar shape, is operably attached to at least one said electric conductor means or conductor forming means, and preferably forms a plane substantially parallel to, and spaced from, at least one said planar pole face.
I prefer that each acoustic transducer unit contain only one magnetized member, although a plurality of magnetized members can be used (see FIGS. 7,83,12,13 and 14). Each magnetized member may be an electromagnet or a permanent magnet. I prefer. however, that each magnetized member be a permanent magnet because of the avoidance of magnetizing coils and additional electric power requirement and because its magnetic field is substantially stable and fixed, i.e., the magnetic field will not fluctuate to any substantial degree during operation of the transducer. Further, the fabrication of permanent magnets is less costly than the fabrication of electromagnets of comparable field strength.
When the magnetized members are permanent magnets the dimensions and shape of the magnetized member are critical with respect to obtaining maximum magnetic efficiency from the magnetized members, but are not limiting except as hereinafter preferred. They will vary with the material used to make magnetized members and the shapes and relations of the other elements of a particular embodiment to the magnetized members. The magnetized members may, for example, be rectilinear or curvilinear in cross-section. I prefer that the magnetized members have a large pole face area to thickness ratio. A large ratio of pole face area to magnet thickness provides for a thinner embodiment of the planar transducer, as well as a. larger area of the diaphragm that can be driven directly by the electric conductor means attached thereto. The ratio cannot however be enlarged without limit because the density of the magnetic field produced will be diminished as the thickness is reduced beyond a point, or as the pole face area is increased beyond a point. The optimum ratio will vary with the material used to make the magnetized members and the shapes and relations of the other elements of the particular embodiment to the magnetized members. Such permanent magnets can, for example, be made of an alloy of iron, nickel, aluminum, or cobalt. I prefer, however, that sintered ferrite material be used because I have found that it can provide a permanent magnet of a given magnetic strength that has a larger pole face area to thickness ratio than most metallic alloys.
1 contemplate that the planar pole faces of the magnetized members be as nearly flat as practicable. The shape of said pole faces may however be varied to produce a stronger and more uniform magnetic field in the region where the electric conductor means is located. Said planar pole face may, for example, be curvilinear in shape. (see FIG. 17). The periphery of the pole faces may be in any advantageous configuration, e.g. circles, elipses, rectangles, irregular curves. l prefer that the configuration of the periphery be a rectangle or the like where the periphery is large for a given surface area. In this way, the reproductive quality of the transducer is improved and the efficiency increased.
The positioning of the electric conductor means relative to the magnetized member is critical but not limiting. The essential feature is that the electric conductor means be positioned in the magnetic field of the magnetizedmember. In addition, the electric conductor means should be positioned in said magnetic field in such a way that the regions of the magnetic field in which the electric conductor means are positioned are substantially uniform and in turn, the forces exerted on the diaphragm are substantially symmetrical with regard to movement of the electric conductor means from a fixed point in opposing directions. In this way, the diaphragm will not pitch or yaw to any significant degree and thereby produce audibly spurious acoustic waves. The symmetry of the positioning can be accomplished by varying the magnetic field, or by varying the number of turns or spacing between turns of the electric conductor means. lf the electric conductor means is positioned in said magnetic field in such a way that the regions in which it is located are not substantially uniform, the support means for the diaphragm should be adjusted, or some other compensating means used so that the diaphragm does not pitch or yaw.
Preferably, the electric conductor means is positioned and moved within regions of said magnetic field that are of substantially constant magnetic flux density. In this way, electric current conducted through said electric conductor means causes responsive and substantially uniform linear forces to be exerted on the diaphragm at any given frequency. 1 have found that (where pole pieces are not used, see infra, p. the electric conductor mmeans should be positioned close to one planar pole face of the magnetized means, yet spaced from it a distance necessary to allow clearance between the planar pole face and the electric conductor means during operation of the transducer.
The electric conductor means can be made by mechanically winding or sewing a coil configuration directly onto a diaphragm, or can be made by mechanically winding a spiral and/or helical coil configuration on a conductor forming means. I prefer that the configuration of the coil be flat. In addition, I prefer that the coil be one continuous winding; it can however be multiple (i.e., discontinuous) and spaced spiral or helical configurations of varied design that are woven into, sewn to, or otherwise fastened to the diaphragm or the conductor forming means.
The electric conductor means is preferably made of material which is highly electrically conductive (i.e., low resistance) but essentially non-magnetizable, such as copper, aluminum and silver wire. Further, I prefer that the electric conductor means be made of conductive pathway patterns such as those formed by printed circuit, printed wiring or pressed wiring methods; use of such manufacturing methods permits the production of low cost and high quality electricl conductor means.
In addition, I contemplate that interlaid with and insulated from the electric conductor means can be magnetically conductive material such as iron or iron alloy. They should be interlaid generally in the plane traversed by the magnetic flux lines through the electric conductor means. By this arrangement, the magnetic lines of flux of the magnetic field created by the magnetized member will tend to concentrate in the region where the electric conductor means is moving and thereby increase the efficiency of the acoustic transducer.
The diaphragm is fastened to said electric conductor means and itself supported so that the electromagnetic driving means displaces the diaphragm when current passes through the electric conductor means. The diaphragm is of an essentially non-magnetizable material such as paper, resins, rubbers, metal foil, plasticized paper, metallic paper or treated or untreated textiles. The diaphragm should be relatively thin and of small mass so that it is very sensitive to displacement by minute electromagnetic driving forces. The surface portions of the diaphragm may be of any suitable shape and may even conform to the decor of the listening room or of sound reproduction system housing in which the acoustic transducer is used.
Preferably, the diaphragm is of a membrane-like shape and is driven over a substantially large portion of its surface area. Such membranous diaphragms are of small mass and low cost and are easily fabricated. 1 prefer that such diaphragms be made of material of long term stability and high flexibility but low elasticity, such as varnished cambric, and be placed under tension over a supporting frame. Such membranous diaphragms permit more accurate response to audio signals than do relatively heavy and relatively stiff speaker cones and do not exhibit the rim resonance" of speaker cones, thereby improving the transition between sounds of differing frequency. They also can be made to generate less harmonic distortion of the sound reproduction than do stiff speaker cones.
The membranous diaphragm can be driven over a substantially large portion of its area by reason of the configuration of the electric conductor means and/or the configuration of conductor forming means and the way they are attached to the diaphragm. Alternatively or supplementally this can be. accomplished by stiffening the diaphragm by painting or spraying the diaphragm over a substantially large portion of its area with materials such as certain epoxy and polyester liquid resins that solidify in place on curing, or by overlaying a portion of the membranous diaphragm with light weight plastic or textile patches. Alternatively the diaphragm can be comprised of two membranes pneumatically sealed around the edges and/or at intermediate points and separated by internal strut members. Preferably the diaphragm is inflated under pressure to a shallow lenticular form either with or without internal strut members. In this way, the membranous diaphragm can be made sufficiently stiff to respond to the forces exerted by the electric conductor means over substantially its entire surface area and still be light and com erted by the electric conductor means. These conditions are compromised when the diaphragm is sprayed or painted with stiffening material or overlaid with stiffening members. In addition, I have found that inflation of the diaphragm increases the efficiency of the transducer.
The diaphragm may be supported by either rigid or flexible means. If rigid support means are used, the diaphragm should be under tension or have adequate compliance to restore itself to its original position when it is displaced by the electromagnetic driving means. If flexible support means are used, the support means should have compliance for returning the diaphragm to its original position when it is displaced by the electromagnetic driving means. I prefer that such support means be structural materials that are essentially non-magnetizable such as wood, molded and/or reinforced plastics, aluminum, brass, etc. Furthermore, I have found that the diaphragm (along with the electric conductor means) can be supported by essentially non-magnetizable, compliant material positioned on any planar pole face of said magnetized member.
The electric conductor means, conductor forming means (if any), and diaphragm is sometimes called the mobile assembly. The mobile assembly as a whole is preferably of small mass, especially where response to a high frequency signal is desired.
I prefer that pole pieces be positioned adjacent the periphery of the magnetized member to direct and intensify the magnetic field by providing a low reluctance path for the magnetic flux from one pole face of the magnetized member to the conjugate pole face of the i magnetized member. The pole pieces may be made of any magnetically permeable material. Preferably the pole pieces are made of material having high magnetic permeability and low magnetic retentivity such as soft" iron and low carbon steel. The pole pieces may be laminated to reduce eddy-current effects. The pole pieces can be of any suitable geometric shape that is appropriate to direct and intensify the magnetic flux of said magnetic field in the region where the electric conductor means is located, and in addition, make said magnetic field preferably substantially symmetrical and preferably uniform in the region where the electric conductor means is located. One form of pole piece can be used that is of a substantially U-shaped and adjoins the opposite pole face from the planar pole face. The pole pieces may be independently supported or may be supported by the magnetized member through spacer members of essentially non-magnetizable material, i.e., any material that is substantially inert to a magnetic field such as plastics and other materials showing high magnetic reluctance.
Preferably the pole pieces are positioned adjacent the periphery of the magnetized member to form pairs of spaced apart elongated gaps with one said magnetized member or other pole pieces, said gaps containing regions of relatively high magnetic flux density. The regions of said magnetic field contained in said gaps preferably approach substantially linear and symmetrical magnetic flux density. And the electric conductor means are positioned and, in operation, moved within said regions of substantially linear and symmetrical magnetic flux density in said gaps. By this arrangement, the forces exerted on the diaphragm by the electric driver means at any given frequency are substantially constant and thereby limit distortion of the audio signal in reproduction.
In addition, said gaps containing the magnetic fields should be adjusted within narrow limits. Said gaps should be as narrow as possible to produce magnetic flux of highest density and to approach a substantially constant magnetic flux density in said gaps. Yet, said gaps should be sufficiently broad so that a slight change of the electric conductor means, the pole pieces, or magnetized member does not cause the electric driver means to bind or scrape in said gaps. In this way, electric current conducted through said electric conductor means causes highly responsive and substantially uniform linear forces to be exerted on the electric driver means positioned in said gaps at any given frequency.
l contemplate, in addition, that a substantial portion of said gaps may be filled with resilient, essentially non-magnetizable material, such as highly compliant rubber or foam resin, to support the electric conductor means, to preserve accurate alignment of the parts, and to improve the ruggedness of the electromagnetic driving means during mounting. Such resilient essentially non-magnetizable material should be thermally conductive to aid in dissipating whatever heat is generated from the passage of electric current through the electric conductor means. By this arrangement, electromagnetic driving means can be made separately as intermediate assemblies, and can be disposed geometri cally or preferably randomly over any mounting surface and covered with a single diaphragm or sections of diaphragm to form an acoustic transducer.
I prefer that the electric conductor means be of a substantially flat, coil configuration. Such electric conductor means can be (i) of a spiral configuration and form a plane that is substantially parallel to the flux lines of said magnetic field in said gaps or (ii) of a helical configuration and substantially perpendicular to flux lines of said magnetic field in said gaps.
The pole pieces can be a first pole piece adjoining a pole face of said magnetized member and a second pole piece of substantially U-shape adjoining the opposite pole face of said magnetized member. The spaced apart elongated gaps containing substantially linear and symmetrical regions of relatively high flux density are thereby formed between said first pole piece and said second pole pieces. in the alternative, the pole pieces can be first pole pieces of magnetically conductive material adjoining both faces of the magnetized member and second pole pieces spaced from the periphery of said magnetized member. Said second pole pieces can be independently supported, or supported by the magnetized member through spacer members of essentially non-magnetized material. By the latter means, one or two pairs of spaced apart elongated gaps containing substantially linear and symmetrical regions of relatively high flux density are formed between said first pole piece and said second pole piece, depending on the geometry and positioning of the spacer member relative to said first and second pole pieces. By these arrangements, where the pole pieces adjoin the pole faces, the magnetic flux density of'regions of the magnetic fields in said gaps are intensified and approach closer uniformity.
I further prefer that the electric conductor means drive two diaphragms positioned substantially parallel to each other on opposite sides of the magnetized member. The electric conductor means can be one means 7 driving both diaphragms (see FIGS. 37, 38 and 39); but for operational purposes, I prefer two separate electric conductor means to drive the diaphragms independently. The electric driver means is preferably substantially symmetrical about the center lines of the magnetized member.
The applications for two-diaphragm planar acoustic transducers are varied. The separate electric conductor means may be driven independently, as in the case of stereophonic audio signals, or they may be driven outof-phase by any phase displacement up to 180". In addition, two diaphragms increase the transient response, the linearity of motion with regard to the amplitude of the audio signal, the damping, and the efficiency of such acoustic transducers.
Furthermore, by this arrangement the effects caused by moving either one of the two electric conductor means through a non-linear region of the magnetic field of the magnetized member can be reduced to a negligible factor because the sound produced is the resultant of both diaphragms. As inferred earlier, herein when the electric conductor means carrying a current is caused to move through regions of varying magnetic flux density, the forces exerted on the diaphragm by the electromagnetic driving means varies and in turn results in the reproduction of distorted sounds; In the acoustic transducer having one diaphragm, I therefore emphasized that the magnetic flux density should be substantially uniform in the region of the magnetic field in which the electric conductor means is moved. However, in the acoustic transducer having two diaphragms and driven by two independent electric conductor means, the electric conductor means can be arranged and driven electrically so that one electric conductor means is moving into a region of high magnetic flux density while the other is moving into a region of low magnetic flux density. In this way, a substantial equality of the resultant forces exerted on the diaphragms are preserved, and in turn the distortion of the sounds reproduced limited, if not eliminated; The concern is therefore not with the uniformity of the regions of the magnetic field through which the electric conductor means move, but rather with the symmetry of the regions of the magnetic field through which the two electric conductor means move.
Preferably, the space between the two diaphragms is enclosed or otherwise separated from the exterior ambience by enclosure means. This can be done by structural members and/or frame-like supporting members for the diaphragms. Small vent holes should be provided in the enclosure means to permit compensation for changes in atmospheric pressure. Further, it may be desirable in some embodiments to provide larger openings, e.g., slots, in the enclosure means to provide partial relief of the air pressure within the volume contained by the enclosure means and diaphragms to improve the acoustic response of the transducer by introducing such acoustic resistance. I prefer however that the enclosure means substantially seal-off the volume enclosed by the enclosure means and' the diaphragms from the exterior ambience except for the small vent holes. By use of the latter arrangement, I have found that when the electromagnetic driving means is operated so that the two diaphragms are motionally in phase, (i.e., when they physically move in the same direction at the same time powered by the same electrical signal), the bass response is improved. On the other hand, when the diaphragms are operated motionally out-of-phase the bass responses are essentially suppressed and the middle frequency and high frequency responses are more prominent. These results are believed due to the volume of air contained by the enclosure means being relatively small compared to the exterior ambience and its compressibility low compared to the compressibility of the exterior ambience. As a result, the entrapped air acts as a substantially stiff medium mechanically linking the internal surfaces of the diaphragms and therby causes each diaphragm to act as a motional control on the other. This allows for an amplification of the response from the diaphragm at low frequencies and a heavy damping of spurious motion of the diaphragms. In addition, it permits compensation for acoustic distortions and imperfections caused by the variations in assembly tolerances of the various elements of the transducer by slightly shifting the phase of motion of the two diaphragms either electrically or mechanically by slightly weighting one of the two diaphragms.
An acoustic transducer installation comprising a number of my two diaphragm acoustic transducers within an enclosure means can be connected electrically so that some of them have their two diaphragms motionally in-phase and are provided with a simple low-pass filter to reproduce principally low frequen cies, while the remaining acoustic transducers of a given array are electrically connected so that their oppositely disposed diaphragms are motionally out-ofphase. By this arrangement favorable differences of overall sound reproduction can be achieved at various frequencies without the use of expensive and complex electrical crossover networks or separate amplifiers. In addition, I have found to my surprise that the two diaphragms can be driven by the separate channels of a stereophonic recording either in-phase or out-of-phase with negligible change in mid-frequency and highfrequency reproduction, and with no perceptible change in low-frequency reproduction. This is unexpected: The effect of reversing the phase relation of the two diaphragms while reproducing a monophonic recording results in cancellation of low frequency sounds and unpleasant distortions of mid-frequency and highfrequency sounds, as would be expected with the small air volume of relatively low compressibility contained between the diaphragms. The stereophonic arrangement however corresponds in effect to an acoustic monopole a condition theretofore not realized in commercial practice to my knowledge.
I contemplate that the total displacement amplitude of the diaphragm of my acoustic transducer be small (e.g., 0.0005 to 0.2500 inch) so that the Doppler-effect distortion (FM modulation) will be small. In application, my acoustic transducers may be assembled in multiples so that a verylarge area of diaphragm exposed to the ambient air of a listening room may be driven uniformly by the electromagnetic driving means. Therefore, the acoustic transducer need not be mounted in a horn or an acoustically sealed or tuned enclosure (as a conical loudspeaker often needs to be) to radiate wide range sound reproduction into a listening area, even though the average intensity of the sound recreated by a given area of the acoustic transducer is low. In addition, my acoustic transducers can be mounted in a stacked array to adapt the assembly for mounting on a wall; this arrangement reduces the effect of the high