US3164686A

US3164686A - Electrodynamic transducer

Info

Publication number: US3164686A
Application number: US841280A
Authority: US
Inventors: George C Tibbetts
Original assignee: Tibbetts Industries Inc
Current assignee: Tibbetts Industries Inc
Priority date: 1959-09-21
Filing date: 1959-09-21
Publication date: 1965-01-05
Anticipated expiration: 1982-01-05

Description

Jan. 5, 1965 G. c. TlBBETTS ELECTRODYNAMIC TRANSDUCER 2 Sheets-Sheet 1 -Filed Sept. 21, 1959 zzz j'w was United States Patent Ofitice 3,164,686 -Patented Jan. 5, 1965.

3,164,686 ELECTRQDYNAMIC TRANSDUCER George C. Tibbetts, Camden, Maine, assignor to Tibbetts Industries, Inc., Camden, Maine, a corporation of Maine Filed Sept. 21, 1959, Ser. No. 841,280 Claims. (Cl. 179115.5)

One of the greatest problems in the high fidelity reproduction of sound is the occurrence of mechanical resonances within the various devices used to make conversions between electrical and mechanical energy, sound waves being a form of mechanical energy. These resonances occur when a structure of appreciable rigidity, mass, and extent is vibrated by a locally applied force. In the case of cone-type direct radiator loudspeakers the need for an appreciable mass is occasioned by the rigid structure needed to apply the motions of a comparatively small voice coil to the comparatively large area from which acoustic radiation must take place if appreciable acoustic power is to be delivered at the lower frequencies.

One attempt to alleviate these problems has been the development of electrostatic loudspeakers utilizing a diaphragm of very low mass. However, these speakers have generally been limited to the high frequency ranges because the diaphragm excursion of such a speaker is limited unless the polarizing voltage is raised to the neighborhood of several kilovolts and, for smaller excursions, extremely large diaphragms would be required to reproduce low frequency sound waves.

A further disadvantage of the electrostatic type loudspeakers is that they present a capacitive and high impedance load to the power source which is very difficult to match.

Accordingly objects of the present invention are to provide a transducer with a wide and uniform frequency response which utilizes adiaphragm of very low rigidity and relatively low mass, and further to provide a transducer inherently capable of very low nonlinear distortion.

Further objects of the present invention are to provide such a transducer which is of simple and very reliable construction not requiring close or precision gaps or other spacings, which does not require a polarizing voltage supply, and which lends itself to simple and inexpensive mass production.

The present invention contemplates utilizing a generally planar diaphragm of very low rigidity which has an electric circuit covering most or all of its area. This circuit is disposed in a magnetic field which is edgewise the diaphragm and which islocally oriented so as to produce aiding forces over the entire diaphragm as'a current is passed through said circuit.

The advantage of this general form of construction is that the driving force is applied to the diaphragm over a large part of its area just as is the air load. Consequently the diaphragm is not required to possess any great structural rigidity with its attendant parasitic mass.

In a preferred embodiment the circuit is constituted by metal foil laminated to a thin plastic film which constitutes the diaphragm. A preferred method of construction is to laminate the metal foil to the plastic film in solid sheets and then to remove the unwanted metal by preferential etching leaving the desired circuit.

'In order to make the air gap in the magnetic circuit as small as possible it is desirable that the conducting path be narrow in comparison with theover-all dimensions of the diaphragm. Therefore, it is proposed that the path reverse its direction periodically so as to still cover most or all of the area of the diaphragm. Accordingly, the magnetic field must reverse its direction with the same spatial periodicity so that the forces produced by the instantaneous current flow will be aiding over the entire current path, that is, so that the magnetic field will cross the current path in the same sense throughout its length.

In one aspect the diaphragm is essentially rectangular and the conduction path covers it in parallel strips, each strip being joined at its one end to one adjacent strip and at its other end to the other adjacent strip, adjacent strips being spaced from one another by a narrow gap. Correspondingly, magnetic poles may lie along the gap between adjacent strips-and in relatively close relationship to the diaphragm. Adjacent poles are then of opposite polarity so that there exists a magnetic field, passing edgewise through the diaphragm and across the current path, which reverses direction at every gap between adjacent conducting strips whereby a current through the circuit will produce aiding forces over the entire diaphragm.

In a more specific aspect the poles may be those of bars having essentially permanent dipole magnetization and lying along every otherstrip in relatively close spaced relationship to the diaphragm, the bars being polarized edgewise of'the diaphragm and normal to their length, and each bar being magnetized similarly.

A more uniform magnetic field surrounding the conducting path may be obtained if a matching array of mag netic poles is placed on the opposite side of the diaphragm with poles of like polarity facing across the diaphragm. In'this way the nonlinear distortion due to spatial variation of the magnetic field as the diaphragm vibrates may be minimized. r

In a preferred embodiment the magnetic poles are the ends of magnets having their direction of polarization perpendicular to the plane of the diaphragm. The other ends of these magnets may rest against a structure of magnetically soft material so as to complete the magnetic circuit with minimum reluctance.

In another embodiment the invention contemplates that the poles be the ends of compact rectangular bar magnets lying with their direction of polarization parallel to the plane of the diaphragm whereby the opposite poles lie along adjacent gaps between conducting strips, the magnets and correspondingly rectangular air spaces being arranged in the form ofa checkerboard array. In this case, magnets in one row, the row direction being normal to the direction of polarization, are magnetized oppositely from those in an adjacent row.

When this device is used to produce acoustic energy, it is obvious that the diaphragm will radiate from both of itssurfaces. Especially at low frequencies where the wave length is large as compared with the dimensions of the transducer, the soundwaves may travel around the edge of the diaphragm and cause destructive interference. In

, order to avoid this effect it is contemplated that thisdevice be used with a suitable enclosure for isolating the back radiation or for delaying it so that it appears in phase with the forward radiation. Such enclosures are well understood in the art and form no part of the present invention.

It is also contemplated that combinations of units ac-' cording to the present invention may be assembled in; order to handle more power, or in order to cover a greater range of frequency or, for a given range of frequencies, to minimize Doppler type frequency modulation distortion. 1 In the latter case the units may diifer in detail such as size and diaphragm construction so as to make each suited to reproduce a particular band of frequencies. Accordingly the electrical energy delivered to the com- I bination may be divided according to frequency before a going to the transducers.

Such cross-over networks are also well understood in the art and form no part of the present invention.

In the alternative, a single diaphragm may be divided into electrically isolated regions with each region being driven by separate sources such as frequency dividing anegese networks or amplifiers. In this way the requirements as to driving area and mass considerations may be controlled so as to be suitable for various frequency ranges without requiring more than one diaphragm and mounting structure.

For the purpose of illustration several embodiments of the present invention are shown in the accompanying drawings wherein:

FIG. 1 is a partially broken away isometric view of a transducer;

FIG. 2 is a section substantially on the line 22 of FIG. 1;

FIG. 3 is a section substantially on the line 33 of FIG. 2; FIG. 4 is a plan view of a modification;

FIG. 5 is a section on the line 5-5 of FIG. 4;

FIG. 6 is a plan view of a further modification;

FIG. 7 is a section on the line 77 of FIG. 6;

FIG. 8 is a plan view of a diaphragm;

FIG. 9 is a plan view of a modification of the diaphragm;

FIG. 10 is a view on the line 1010 of FIG. 9 also showing the position of the magnets; and

FIG. 11 is a plan view of a further diaphragm modification.

Referring now to FIGS. 1, 2 and 3, the diaphragm 1 is constructed of a plastic film such as .0005 inch thick Mylar and is clamped at its edges as a membrane by gaskets 2. Laminated to the diaphragm 1 is a foil 3 of an electrically conducting material such as .001 inch thick aluminum. The foil 3 is disposed in parallel strips 4 each of which is connected at one end to one of the adjoining strips and at the other end to the other adjoining strip thereby defining a zig-zag pattern covering most of the free surface of the diaphragm 1. Disposed adjacent the gaps 5 between adjoining strips 4 are elongate magnetic pole pieces 7. These pole pieces 7 derive their polarity from elongate permanent magnets 8 whose direction of polarization is perpendicular to the plane of the diaphragm. The magnets are arranged so that adjoining pole pieces 7 are of opposite polarity, and so that pole pieces 7 of like polarity are opposed across the diaphragm 1. A perforate backing plate 9 of a magnetically soft material such as iron supports the magnets 8 and completes the magnetic circuit while permitting the free passage of sound waves through the apertures 11, the structure is substantially acoustically transparent.

Preferably the pole pieces 7, the magnets 8, and the backing plate 9 in combination are shaped so as to form an aerodynamically smooth structure. The pole pieces 7 may be omitted and replaced by extensions of the magnets 8.

In the embodiment shown the backing plate 9 comprises as an integral structure, a grid of intersecting

bars

17 and 18. The bars 17 covering the magnets 8 may be of generally semi-elliptical cross-section so as to form a smooth cross-section with the magnets 8 and the pole pieces 7. The transverse bars 18 may be, of themselves, of elliptical cross-section.

While the backing plate is desirable for purposes of increasing the magnetic efficiency, it is obvious to one skilled in the art that the plate might be omitted and the magnets made self-supporting.

In order to minimize nonlinear distortion from mechanical causes during a substantial vibration excursion, the diaphragm 1 and the foil 3 may be bulged locally into dimples 12 which by increasing the effective thickness of the diaphragm, without appreciably increasing the mass, increase the range of quasi-linear deflection of the diaphragm composite as a flexural device. As shown by the broken lines in FIG. 1, adjacent dimples 12 may be directed in opposite directions.

On the sides parallel to the magnets 8 the frame 13 is constituted by an extension of the backing plate 9. This extension 14 is spaced from the end magnet 8 so that lit lies along a magnetic equipotential, that is at a distance equal to half the spacing between adjacent magnets 8.

On the sides transverse the magnets 8 the frame 13 is constituted by a sidewall 15 of a non-ferromagnetic material so as to cause minimum distortion of the magnetic field.

While in the embodiments shown there are magnetic structures on both sides of the diaphragm in order to produce a more uniform magnetic field over the diaphragms excursion, it is to be understood that a less expensive embodiment workable especially at the higher frequencies may be obtained by completely omitting the magnetic structure on one side.

In the embodiment depicted in FIGS. 4 and 5 the elongate magnets 20 have been oriented with the direction of polarization parallel to the diaphragm 21 and perpendicular to the strips 22 of conducting material. The pole faces of the magnets 20 lie adjacent the gaps between conducting strips 22 and the magnets are spaced by a distance equal to their width, whereby the magnetic field configuration resembles that in the embodiment depicted in FIGS. 1, 2 and 3 and there is approximately fifty percent open area, rendering the structure substantially acoustically transparent.

In the embodiment depicted in FIGS. 4 and 5 the magnetic field is terminated without appreciable distortion, that is, approximately as if the array extended indefinitely, by including magnets 23 of half width and a magnetically permeable side plate 24 having its inside surface located at the magnetic equipotential.

In this embodiment it is apparent that the diaphragm 21 cannot be bulged in alternately opposite senses along both dimensions Without increasing the danger that it will strike a magnet when vibrating. However, the dia phragm 21 may be dimpled in a quilted pattern in the same sense in any row along a magnet 20 and in the opposite sense in an adjacent row, the diaphragm always bulging away from the nearest magnet.

In the embodiment shown in FIGS. 6 and 7 the magnets 30 are arranged in a checker-board pattern, again with the direction of polarization parallel the diaphragm 33 and with the poles of the magnets 30 lying adjacent the gaps between conducting strips 31. The magnets in each row are spaced by distance equal to their width and each abuts a space between magnets in the adjacent row, whereby there is approximately fifty percent open area, rendering the structure substantially acoustically transparent. The magnets 30 are supported by bars 32 parallel to the rows which preferably are of a nonferro' magnetic material.

If magnets 30 are to be placed on both sides of the diaphragm 33, the two arrays may be arranged so that each magnet 30 is opposite an opening 35 between magnets in the opposing array.

Again the field may be effectively terminated by the inclusion of magnets 36 of half width and a magnetically permeable side wall 37 at the magnetic equipotential.

In this embodiment, as in the first, the diaphragm may be locally stiffened by bulging or dirnpling if there is one dimple 33 for each magnet 30 and that dimple bulges away from the magnet.

FIG. 8 shows a diaphragm constructed as a laminate of plastic film and metallic foil. Such a diaphragm may be constructed by laminating continuous sheets of plastic film and foil and then creating the gaps 43 by preferential etching. The edge strips 45 of foil may be left to strengthen that part of the diaphragm although it will conduct no current. Means for connection 46 are provided to connect the zigzag foil conductor to an external circuit.

A similar diaphragm might also be constructed by other printed circuit techniques or by other means of metal deposition.

1 In the modification of the diaphragm depicted in FIGS.

and 11 (shown flat for purposes of simplicity) there are conducting strips 51 on both sides of the plastic film 50. Each strip is spaced from the adjoining strips on the same side of the film 50 by a distance slightly less than the width of a strip and the strips on opposite sides are staggered so that they overlap slightly. In this way all portions of the diaphragm are supported by foil without appreciably changing the mass of the diaphragm. The width of these strips will be approximately half the distance between poles 52 so that an aiding current-field relationship may be maintained. As each of the embodiments utilizes an essentially similar magnetic field configuration, this diaphragm may be used with any embodiment shown. A primary advantage of this form of diaphragm is that an essentially constant current density can be maintained out to the equipotential terminating surfaces 'of the magnetic structure, as shown by dotted lines in FIGS. 9 and 10, and hence that the diaphragm is driven by distributed electrodynamic forces over the entire area provided with a substantial magnetic field, without waste of this area.

A further advantage is that, for a given thickness of foil and with the two sides connected in series, the input impedance of the device will be nearly quadrupled which will generally aid in coupling of the device to an external circuit.

FIG. 11 shows a single diaphragm divided by gaps 53 into electrically

isolated regions

54, 55 and 56. Edge strips 57 of foil may be left for strength. Means for connection 58 are provided to connect each region to respective external circuits, in the form of separate driving sources such as frequency dividing networks or amplifiers.

It should be understood that the present disclosure is for the purpose of illustration only and that this invention includes all modifications and equivalents which fall within the scope of the appended claims.

I claim:

1. A transducer comprising a support, a flexible diaphragm held at its edges by the support, a thin flexible conductor adhering to said diaphragm for conducting current edgewise the diaphragm along a predetermined path, and magnetic means including magnetic poles mounted on said support adjacent the diaphragm with said poles lying in a plane spaced from and parallel to the diaphragm for producing a magnetic field edgewise the diaphragm which extends across said path always in the same direction, relative the path throughout the length of the path, the conducting path reversing direction periodically, the conducting path being arranged in two series of parallel bands, one series on each side of the diaphragm, each band being spaced from the adjoining bands on the same side of the diaphragm by a distance slightly less than the width of a band and each band being connected at one end to one of said adjoining bands and at the other end to the other of said adjoining bands, and bands on the opposite sides of the diaphragm being staggered so that there is a slight overlap.

2. An electrodynamic transducer comprising a membranous diaphragm having conducting means disposed upon the surface thereof, said conducting means being arranged in two series of parallel bands, one series on each side of the diaphragm, each band being spaced from the adjoining bands on the same side of the diaphragm by a distance slightly less than its width and connected at one end to one of said adjoining bands and at the other end to the other adjoining band, bands on the opposite sides of the diaphragm being staggered with a slight overlap, and a plurality of magnetic pole pieces lying in a plane parallel and closely adjacent the plane of the diaphragm, each pole piece lying adjacent the division between bands carrying current in opposite directions and adjacent poles being of opposite polarity.

3. An electrodynamic transducer comprising a diaphragm having disposed thereupon an electrical circuit,

said circuit being arranged so as to form a series of parallel lines in which each line is joined at one end to one of the adjacent lines and is joined at the other end to the other adjacent line, a suspension for said diaphragm, a plurality of permanent magnets lying in a plane parallel and closely adjacent the plane of the diaphragm, said magnets being arranged in parallel rows with all of the magnets in one row being oriented alike transverse the length of the row and spaced from one another by a distance substantially equal to their width, the poles of said magnets being located adjacent the spaces between adjacent lines on the diaphragm, the magnets in adjacent rows being oppositely oriented and staggered so that each pole adjoins a space between magnets in the adjoining row.

4. An electrodynamic transducer accordingto claim 3 wherein there is a second plurality of magnets generally similar to the first but located on the other side of the diaphragm and disposed so that each magnet is opposite a space between magnets in the opposing plurality of magnets and so that like poles always lie opposed across the diaphragm.

5. A transducer according to claim 4 wherein the diaphragm is bulged locally so as to linearize flexurally the diaphragm, said bulges always extending in a direction away from any magnet.

6. A transducer comprising a support, a flexible diaphragm held at its edges by the support, the diaphragm being linearized flexurally by dimples bulged up from the substance of the diaphragm, a thin flexible conductor adhering to said diaphragm for conducting current edgewise the diaphragm along a predetermined path, and magnetic means including magnetic poles mounted on said support adjacent the diaphragm with said poles lying in a plane spaced from and parallel to the diaphragm for producing a magnetic field edgewise the diaphragm which extends across said path always in the same direction relative the path throughout the length of the path.

7. A transducer comprising a support, a flexible diaphragm held at its edges by the support, the diaphragm being linearized flexurally by dimples bulged up from the substance of the diaphragm in a quilted pattern, a thin flexible conductor adhering to said diaphragm for conducting current edgewise the diaphragm along a predetermined path, and magnetic means including magnetic poles mounted on said support adjacent the diaphragm with said poles lying in a plane spaced from and parallel to the diaphragm for producing a magnetic field edgewise the diaphragm which extends across said path always in the same direction relative the path throughout the length of the path.

8. A transducer comprising a support, a flexible diaphragm held at its edges by the support, a thin flexible conductor adhering to said diaphragm for conducting current edgewise the diaphragm along a predetermined path, said conducting path reversing direction periodically in space, and magnetic means including magnetic poles mounted on said support adjacent the diaphragm with said poles lying in a plane spaced from and parallel to the diaphragm for producing a periodically reversing magnetic field edgewise the diaphragm such that the magnetic flux extends across said path always in the same direction relative the path throughout the length of the path, the diaphragm being divided into electrically isolated regions, the conducting path within each region including means for connection with an external circuit.

9. A transducer comprising a support, a flexible diaphragm held at its edges by the support, a thin flexible conductor adhering to said diaphragm for conducting current edgewise the diaphragm along a predetermined path, and substantially acoustically transparent, magnetic means including magnetic poles mounted on said support adjacent the diaphragm with said poles lying in a plane spaced from and parallel to the diaphragm for producing 7 a magnetic field edgewise the diaphragm which extends across said path always in the same direction relative the path throughout the length of the path, the conducting path reversing itself periodically and being arranged in two series of parallel bands, one series on each side of the diaphragm, wherein each band is spaced from the adjoining bands on the same side of the diaphragm by a distance slightly less than the Width of a band and each band is connected at one end to one of said adjoining bands and at the other end to the other of said adjoining bands, and wherein bands on the opposite sides of the diaphragm are staggered so that there is a slight overlap.

10. A transducer comprising a support, a flexible diaphragm held at its edges by the support, a thin flexible conductor adhering to said diaphragm for conducting current edgewise the diaphragm along a predetermined path, said conducting path reversing direction periodically in space, and substantially acoustically transparent magnetic means including magnetic poles mounted on said support adjacent the diaphragm with said poles lying in a plane spaced from and parallel to the diaphragmfor producing a periodically reversing magnetic field edgewise the diaphragm such that the magnetic flux extends across said path always in the same direction relative the path throughout the length of the path, the magnetic field being terminated at a magnetic equipotential by a magnetically permeable member at the edges of the diaphragm paralleling the conducting path.

- References Cited by the Examiner UNITED STATES PATENTS 484,339 7/24 Lorenz et al. 179.115.5 1,537,671 5/25 Hewlett 179115.5 1,601,656 9/26 Thayer 179181 1,604,532 10/26 Riegger 179115.5 1,676,668 7/28 Round 1'79--181 1,749,635 3/30 Gerlach 179181 1,793,483 2/31 Hewitt 179181 1,955,390 4/34 Schifii 179115.5 2,003,908 6/35 Smith 179-181 2,041,163 5/36 Burke 179l81 2,044,608 6/36 Harford 179181 2,293,372 8/42 Vasilach 179115.5 2,404,798 7/ 46 Harry et al. 179115.5 2,535,757 12/50 Root 179-115 2,978,671 4/61 Harris 179-1155 3,013,905 12/61 Gamzon et al. 179--115.5

FOREIGN PATENTS 227,162 1/ 25 Great Britain.

329,328 5/30 Great Britain.

484,339 7/ 24 Germany.

OTHER REFERENCES Publication: Elements of Loudspeaker Practice, by N. W. McLachlan, London, Oxford University Press, 1935.

ROBERT H. ROSE, Primary Examiner.

L. MILLER ANDRUS, WILLIAM C. COOPER,

Examiners.

Claims

7. A TRANSDUCER COMPRISING A SUPPORT, A FLEXIBLE DIAPHRAGM HELD AT ITS EDGES BY THE SUPPORT, THE DIAPHRAGM BEING LINEARIZED FLEXURALLY BY DIMPLES BULGED UP FROM THE SUBSTANCE OF THE DIAPHRAGM IN A QUILTED PATTERN, A THIN FLEXIBLE CONDUCTOR ADHERING TO SAID DIAPHRAGM FOR CONDUCTING CURRENT EDGEWISE THE DIAPHRAGM ALONG A PREDETERMINED PATH, AND MAGNETIC MEANS INCLUDING MAGNETIC POLES MOUNTED ON SAID SUPPORT ADJACENT THE DIAPHRAGM WITH SAID POLES LYING IN A PLANE SPACED FROM AND PARALLEL TO THE DIAPHRAGM FOR PRODUCING A MAGNETIC FIELD EDGEWISE THE DIAPHGRAM WHICH EXTENDS ACROSS SAID PATH ALWAYS IN THE SAME DIRECTION RELATIVE THE PATH THROUGHOUT THE LENGTH OF THE PATH.