US3497638A - Explosion-proof acoustic device - Google Patents

Description

Feb. 24, 1970 A. B. COHEN 3,497,638

EXPLOS ION-PROOF ACOUST I C, DEVI C E Filed March 20, 1967 3 Sheets-Sheet 1 305 ISB 208 I28 FIG 2 ABRAHAM B. COHEN. INVENTOR Feb. 24, 1970 A. B. COHEN 3,497,638

EXPLOS ION-PROOF ACOUSTI C DEVI CE Filed March 20, 1967 3 Sheets-Sheet 2 1111111111 IIIIIIIIIIIIIIIIIIIIIIIIII II 1/ In FIG 3 I40 I80 39 4/" I00 38 20D I50 I D FIG 4 ABRAHAM B. COHEN. INVENTOR AGENT Feb. 24, 1970 A. B. COHEN 3,

EXLOSION-PROOF ACOUSTIC DEVICE Filed March 20, 1967 3 Sheets-Sheet 3 ABRAHAM B. COHEN. INVENTOR BY AGENT United States Patent 3,497,638 EXPLOSION-PROOF ACOUSTIC DEVICE Abraham B. Cohen, Oklahoma City, Okla., assignor to LTV Ling Altec, Inc., Oklahoma City, Okla., a corporation of Delaware Filed Mar. 20, 1967, Ser. No. 624,265 Int. Cl. H04r 7/02, 1/28, 11/02 U.S. Cl. 179115 3 Claims ABSTRACT OF THE DISCLOSURE It has long been recognized that the use of electrically operated devices such as loudspeakers and related electroacoustic devices must be specially designed before they can be safely used in an environment which is susceptible to explosion. Thus, only devices denominated as explosion proof have been safely useable in such locations as munition factories, refineries, flour mills, and in such military vehicles as ships and submarines, tanks, etc. The explosive nature of the atmospheres and the fluids, powders, etc., commonly found in such locations dictates that either explosions be completely precluded or else that they be controlled. This can usually be accomplished by completely eliminating the possibility of a spark which might cause an explosion, or, in the second case, by effectively sealing or isolating the vicinity in which a spark might occur, such that if an explosion does occur, it is constrained and not allowed to spread throughout a large area.

Strictly speaking, the attainment of the first mentioned protective measure is so difficult as to be considered impractical, since electro-acoustic transducers which respond to or provide electrical signals must always have electrical lead-in wires. Accordingly, the possibility of'a short in such lead-in wires means that a potential explosion cannot with absolute certainty be precluded. Examples are relatively abundant which demonstrate the second class of protective measures, including, for example, thick, shatter-proof glass covers which shield bare and otherwise hazardous light bulbs in light fixtures.

Shielding an ordinary light bulb is relatively easy, such that rupture of the glass shell while the filament is at operating temperature will not result in a widespread explosion; thus light from the properly functioning bulb can pass through a relatively thick and protective but transparent shield. With loudspeakers, however, such shielding is not possible; for, obviously, sealing off a loudspeaker diaphragm or vibratile member would prevent the propagation of vibrations, i.e., sound waves, from the vibratile member and render the loudspeaker inoperative. Consequently, the vibratile member must be immersed in or surrounded by the hazardous environment in which explosions are to be guarded against, and the voice coil, lead-in wires, etc., are sealed off as best they can be.

The principal reason that voice coils must be sealed off from the explosive environment is that the lead-in wires at their entrance to the loudspeaker are always fixed, while the ends of the lead-in wires which are connected to the typical movable voice coil must move just as the voice coil moves with the attached diaphragm. Typical lead-in wires which have one portion fixed and one portion moving are therefore made of thin and flexible conductors. As a result thereof, they are sensitive to unusual ice vibration and deterioration through work-hardening after experiencing the thousands of cycles of movement during only a modest amount of operation. Thus, a break in a wire and a consequent spark are more likely to occur around a movable voice coil than in any other part of a typical loudspeaker.

As a means of isolating the voice coil from an explosive atmosphere, it has been suggested to locate the voice coil in a first portion of a loudspeaker housing and place the diaphragm external of that first portion; a suitable connection or linkage is then provided in some way between the moving voice coil and the diaphragm, such that movements of the voice coil are transmitted to the diaphragm. Such a construction carries no guaranty of protection, however, since explosions which may occur within the first portion of a loudspeaker because of a short in the voice coil, etc., are omni-directional phenomena and will not necessarily vent themselves in a hoped-for direction; instead, explosions vent themselves in the path of least resistance, like water seeking its level. Since the connecting linkage between the voice coil and the diaphragm must have at least some portion which is movable and/or flexible and therefore is made of lightweight material, it is the connecting linkage which comprises the weakest part of such a loudspeaker. Consequently, it is the connecting linkage which likely will be shattered or crushed first if an explosion occurs within the loudspeaker housing. With the structure of the connecting linkage damaged, there is no assurance that the explosion will be contained within the housing and will not spread to the medium surrounding the housing.

Accordingly, it is a major object of this invention to eliminate the need for a connecting linkage between the voice coil and the diaphragm of a loudspeaker.

Another object is to provide an explosion-proof acoustic transducer having a fixed voice coil.

A further object is to provide a loudspeaker diaphragm which can be easily replaced in the event it is damaged or becomes inoperative.

Yet another object is to provide a means for isolating a voice coil in an acoustic transducer from ambient air.

A still further object is to provide an improved and simplified explosion-proof and blast-proof loudspeaker.

Still another object is to provide an improved loudspeaker having a rigid means for isolating the voice coil from the diaphragm.

Other objects and advantages will be apparent from the specification and claims and from the accompanying drawing illustrative of the invention.

In the drawing,

FIGURE 1 is a cross-sectional view of a direct-radiator loudspeaker made according to the invention;

FIGURE 2 is a cross-sectional view of another embodiment of the invention;

FIGURE 3 is a cross-sectional view of still a further embodiment of the invention;

FIGURE 4 is a cross-sectional view of another embodiment of the invention; and

FIGURE 5 is a cross-sectional view of a horn-loaded loudspeaker which incorporates still another form of the inventlon.

With initial reference to FIG. 1, an acoustic device 10 comprises a magnetic circuit 11 including a magnet 12, a magnetic return piece 13, and a magnetically susceptible disk 14. For convenience the return piece 13 is assembled by joining a soft iron center pole 15 to an iron disk 16 in a conventional manner such as by welding. The magnet 12 is illustrated as a ring magnet, but this illustration is not meant to be limiting; for it will be recognized later that any of the magnets normally employed in loudspeakers can be employed in connection with the invention. The magnet 12 has a first surface 17, and the return piece 13 has a similar first surface 18 juxtaposed with the first surface 17. The disk 14 is spaced from the respective first surfaces 17, 18 of the magnet 12 and the return piece 13 by an air gap 19, the gap being small enough so that the disk is suitably positioned for completing the magnetic circuit.

A fixed, signal-receiving voice coil 20 is mounted for inducing movements of the magnetically susceptible disk 14 in accordance with signals received by the coil. The leads to and from the coil (not shown) are conventional leads except that, since they remain fixed during operation of the device 10, they can be made of stiffer and stronger wire than would otherwise be possible if they had to accommodate millions of cycles of vibrations during the service life of the device. Thus, the material for the lead wires can be selected by one skilled in the art primarily on the basis of conductivity alone rather than fiexural strength and conductivity.

A diaphragm 21 of non-magnetic material is suspended in a conventional manner by a compliant member or the like such that it can vibrate during operation of the invention. As shown, a housing 22 has a forwardly extending wall 23 which has a suitable surface 24 for securing the periphery of the diaphragm 21. It will be understood that the wall 23 and surface 24 comprise merely one of numerous possible constructions which could equally well provide a means for suspending the diaphragm 21. Attached to and adapted for movement with the magnetically susceptible disk 14 is a central portion 25 of the diaphragm 21, such that the diaphragm and the disk wi ll move together in correspondence with signals received by the voice coil. An optional feature which is illustrated is substantially rigid front and rear forarninated shields 26, 27 which are spaced from the diaphragm 21 by suitable small distance such that they do not interfere with normal vibrations, but neither do they permit the diaphragm to experience large, destructive displacements as a result of blast or shock waves impinging on the dia phragm.

A wall 28 comprises at least a portion of a rigid means for segregating the voice coil 20 from the first surfaces 17, 18 of the magnet 12 and the return piece 13, respectively. The effect of the wall 28 is to cooperate in isolating the voice coil 20 from ambient air and from the diaphragm 21, with substantially complete isolation being effected by the wall 28, enclosure wall segment 29, and rear wall 30. All of the walls or wall segments 28, 29, 30 are non-porous and strong enough the withstand any internal explosion which may be caused by ignition of explosive gases that may have leaked into the enclosure 31 formed by the walls. As suggested by the figure, walls 28 and 29 can be conveniently fabricated as one continuous wall, by a casting method, for example. Furthermore, it might possible be desirable in some circumstances to cast walls in one or more steps completely around the voice coil 20 to thereby form a rigid enclosure similar to enclosure 31 but without apparent seams. It is believed, however, that it will usually be desirable to have access to the interior of the device throughout a major portion of the manufacturing process to permit ease of assembly, cleanliness, inspection, etc. Accordingly, a rigid enclosure will normally be provided which comprises at least two wall segments, with provision made to effect a suitable seam between adjoining segments at an appropriate time during manufacture.

Fastening means such as bolts and nuts through lapped wall portions 32, 33 as shown in FIG. 1, comprise the preferred method, of joining two wall segments, whereby a substantially air-tight seam is realized and whereby ambient gases are substantially precluded from entering the enclosure. As will be subsequently explained, however, it is not always gases entering an enclosure 31 but rather the gases that may be leaving the enclosure that are of paramount concern in building what may be denominated an an explosion-proof acoustical device.

To reduce the quantity of gases that might possibly leak into or be expelled from the enclosure 31, the surfaces of the lapped wall portions 32, 33 are finely machined and ground so that very intimate contact between the two surfaces is realized. As a powerful microscope will show, however, when directed at a cross-section of even the most meticulously prepared surface, there always remains a certain degree of irregularity in the surface and a perfectly flat surface is (for the matter considered herein) impracticable of achievement. Since some high spots in the surface of one wall portion 32 will almost inevitably bear against other high spots in the surface of the abutting wall portion 33, there will necessarily be air gaps (even if only infinitesimal ones) between the two surfaces and adjacent abutting high spots; these air gaps when interconnected form minute channels through the joint formed by abutting walls 32, 33 and some gas will inevitably pass therethrough. Accordingly, when the expression substantially air-tight is employed with respect to a seam or joint, it is used thusly in tacit recognition that while the seam may be as good as is practicably obtainable, it is probably not perfectly airtight. To the extent that the lapped joint between wall portions 32, 33 is not air-tight, it is at least long, such that hot gases passing through the joint are cooled in transit.

The permissible gap between mating wall portions 32, 33 is a function of the quantity of gas which is to be safely vented, this quantity in turn being a function of the internal volume of acoustic device 10. For example, if the device 10 has an internal volume of 6 cubic inches or less and the maximum gap in the lapped joint is 0.004 inch, the minimum width of the joint measured along the gap should be at least inch. If the device 10 has an internal volume of approximately 300 cubic inches and the minimum width of the joint is only inch, the maximum gap must be much smaller, viz, 0.0015 inch.

Because of the principles of operation of this invention, at least that portion of the wall 28 between the magnet 12 and the center pole 15 must be non-magnetic, and it is most convenient to make the entire wall of the same material, a satisfactory material being aluminum. A preferred method of placing the magnet 12 and the center pole 15 in proper spatial relationship and effecting a truly air-tight seal between the wall 28 and the magnet and the return piece is to place the magnet and return piece in a suitable mold and cast the wall in place around them. As long as the wall material wets the surfaces of the magnetic pieces 12, 13 when it is molten, a wall 28 is achieved upon hardening of the cast material which insures an air-tight bond between the wall and the contiguous portions of the magnet and the return piece.

A further means for insuring isolation of the voice coil 20 from ambient gases and from the diaphragm 21 is represented in FIG. 2 in which a non-magnetic, solid means 34 completely encloses the coil 20B and all except the first surfaces 17B, 18B of the magnet 12B and the return piece 13B. The solid means 34 which preferably comprises a compound analogous to a non-flammable potting compound, serves to fill the space enclosed by the rigid enclosure 31B to the exclusion of all gases. A satisfactory solid material 34 for this use is aluminum oxide filled epoxy. The thickness of the walls 28B, 29B, 30B in this embodiment are represented as being approximately the same thickness as the corresponding walls in FIG. 1, with thickness being indicative in the drawing (though it is not always so in practice) of strength. In view of the added protection afforded the device 10B by excluding all potentially hazardous gases from the interior of the device, the walls 28B, 29B, 30B could safely be made appreciably thinner than those which might be advisable for a device such as the one shown in FIG. 1. Further more, the walls 28, 29, 30 in the device 10 in FIG. 1.

can be made of thinner or weaker material than would be the case if the walls had to be strong enough to withstand the forces of an explosion without benefit of any venting, such venting being described in greater detail at a subsequent paragraph.

It Will be recognized, however, that in some endeavors no precaution is deemed to be too trivial; it is not that the chance of failure is so high, but that the risk of loss is so great. For example, in a submarine the use of both blast-resistant Walls and a gas-excluding solid means in an acoustic transducer is perhaps justified, even though either feature alone would usually provide the necessary protection.

To complement the protection afforded by the construction thus far described, an explosion-proof coupling 35 may be integrally mounted in the wall 29B for admitting electrical leads to the voice coil 20B. Such a coupling 35 may be mounted in the device by the use of conventional threads, or it may be cast in place much like the magnet 12B may be secured in the wall 28B. When employed, the coupling 35 is logically connected to a metal-sheathed conduit or the like (not shown) which affords an optimum degree of protection to the signal-carrying leads connected to the coil 20B.

FIG. 3 illustrates a further embodiment in which protection against explosion within the device =C is achieved primarily by the fixed coil 20C and solid means 34C. A relatively thin shell 36 is employed to provide at least some structure for mounting the diaphragm 21C and a mounting location for installing the device 10C on a bulkhead, in a cabinet, etc.

A disk 14C having a thickness which is greater near its center than near its periphery is illustrated in FIG. 3, it being understood that such a disk could be employed equally well in any embodiment of the invention. In contrast to disks of uniform thickness, the disk 14C is made thick near its center and thin near its edge as a means of providing a disk of minimum weight while permitting an optimum amount of magnetic flux to pass through the magnetic circuit 110. Since the same amount of flux which passes through the magnetic surface 17C and through the center pole surface 18C must also pass radially through the disk 14C, i.e., between its center and its periphery, and since that portion of the disk surface area that is in register with surface 17C is much greater than that portion of the disk surface area in register with surface 18C, it is desirable to compensate for this inequality of surface areas by an opposite imbalance of thicknesses. Computation of an appropriate thickness may be easily made by setting T A =T A where A and A represent the surface areas of disk segments through which magnetic flux passes, said areas being respectively adjacent the pole piece surface 18C and the magnet surface 17C, and where T and T represent the average thicknesses of the same disk segments. While the thickness of the disk 14C is illustrated as being appreciably thicker in the center than, say, the uniform disk 14 of FIG. 1, the weight reduction effected by thinning the disk toward its periphery more than offsets a weight increase near its center. Considerations of fringe flux behavior, and such practical items as manufacturing economy, etc., can be incorporated as desired to vary the geometry of the disk 14C from a strict T A =T A relationship to merely an approximation of that relationship.

Any ferromagnetic material would be satisfactory in manufacture of the disk 14C, but powdered ferrites are especially suitable since they are readily molded to obtain the desired tapered cross section.

A further embodiment of the invention is illustrated in FIG. 4 in which a solid means 34D serves to preclude any potentially explosive gases from accumulating in a cavity, pocket, etc., within the device 10D where they might possibly be ignited by an accidental spark. It will be recognized that the construction of the device =10D is quite similar to those described above, with a fixed voice coil 20D so placed about a pole piece 15D that it is capable of affecting the magnetic flux through the pole piece and thereby affecting the attraction of the magnetically susceptible disk 14D toward the first surface 18D. The magnetic circuit in FIG. 4 includes the magnet 12D, a magnetically conductive top plate 37, the magnetically susceptible disk 14D, the center pole 15D, an iron disk 16D, and a magnetically conductive bottom plate 38. An annular member 39 made of a non-magnetic material, such as aluminum, separates the center pole 15D from the innermost surface of the top plate 37, and additionally serves to isolate the coil 20D from any potentially explosive environment.

A still further embodiment of the invention is illustrated in FIG. 5 which represents a horn-loaded loudspeaker 10E, showing how it can be made explosion-proof just as readily as a direct-radiator loudspeaker. As before, the coil 20E is shielded from contact with any environmental fluids by a solid material 34B and the center pole 15E. The driver 40 comprises a magnetic circuit including a magnet 12E, a return piece 13E, and a magnetically susceptible disk 14E. The disk 14B is attached to the center of a diaphragm 21E, said diaphragm lying near the throat 41 of the horn 42. The diaphragm 21B is so mounted with respect to the exposed surfaces 17E, 18E of the magnet 12B and return piece 13E that the disk 14B is spaced from the surfaces by a suitable air gap 19E.

Referring again to FIG. 1, operation of the acoustic device 10 as a loudspeaker is effected by connecting the lead wires to a suitable source of electrical signals. The signals passing through the voice coil 20 cause an alternating flux to be generated which corresponds directly to the electrical signals in the coil. This signal-correlated alternating flux is imposed in the quiescent flux passing through the magnetic circuit 11, alternately adding to and subtracting from the flux caused by the magnet 12 and thus alternately increasing and decreasing the attraction of the magnetically susceptible disk 14 toward the magnet and the return piece 13. The entire diaphragm 21 moves with the magnetically susceptible disk 14 and, accordingly, vibrates in accordance with signal pulsations in the coil 20, causing sounds of high fidelity to emanate from the device 10.

During operation of the same device '10 as a microphone, a reverse operation is realized in which sound waves striking the diaphragm 21 cause it to vibrate, whereby the diaphragm and the attached disk 14 by their movements alternately enlarge and diminish the air gap 19; this in turn causes variations in the magnetic flux passing through the circuit 11 and the coil 20. These variations in flux induce a varying current in the coil 20 which, of course, is known as the signal current.

Since there is no connecting linkage required between the vibrating diaphragm 21 and the fixed signal coil 20, the diaphragm may be immersed in or surrounded by any hazardous gases or liquids while the fixed coil is safely protected and isolated within the housing 22 of the device 10. Thus, the device 10 in its basic construction is as nearly explosion proof as practicable, but other features are added to further preclude risk to personnel and property.

If the device 10 is surrounded by explosive gases for months or years, there is always a possibility that some of these gases will migrate into the cavity formed by the walls 28, 29, 30. This possibility alone is of no great import, however, since there is no arcing which is part of normal operation of the device 10. If a spark were somehow to occur, however, with a housing 22 filled with explosive gases, an explosion would result which must be kept localized or the device could not properly be classified as explosion-proof. Usually, explosions in other devices are contained by the strength of the housing alone, as for example, by the strength in the cylinder walls of an internal combustion engine. Protection against the spread of an explosion in the subject device 10 is realized not by wall strength alone but by allowing at least some of the hot explosive gases to be vented, i.e., to controllably leave the device, via the relatively long, minute channels between the lapped surfaces 32, 33. The relative sizes of the average channel cross-section and the average channel length guarantees that there will be an appreciable amount of heat transfer to the lapped surfaces 32, 33 during the time that the hot, explosive gases are in transit to the medium surrounding the device 10. Hence, initially hot gases which result from an interior explosion and which pass through the lapped joint by the shortest route from the interior to the exterior of the enclosure will not exit the joint with sufficient residual heat to initiate a subsequent explosion of the medium surrounding the transducer 10 upon contact of the gases with said medium.

The device 10B in FIG. 2 operates in the sam simple manner as the device described above, since the advantages of excluding hazardous gases are obtained without alteration of any active parts of the electrical system. Thus, the voice coil B is fixed, and it is isolated from the diaphragm 21B by rigid structure; too, it is isolated from ambient air and other fluids not only by rigid structure but also by the solid means 34. The diaphragm 21B and disk 14B are not dependent on any physical connection to the voice coil 20B for operativeness, and thus they can be easily removed as a unit by merely first removing the front blast shield 2613. Having repaired or replaced a damaged diaphragm 21B as required, it is re-installed by simply centering it properly with respect to the wall 23B and re-attaching the front shield 26B. To the extent to which the diaphragm 21B is made waterproof, the entire device 10B will be waterproof, and can even be located in severe humid environments or even immersed in liquids and then operated. The waterproof feature is not dependent on any so-called soft items such as rubber seals, bellows, or O-rings for its success, and thus is unlike many prior art loudspeakers in that it is not subject to potential failure due to the deterioration of such items as a result of exposure to oil vapors, ozone, etc. With proper selection of structural materials (e.g., plastics), the device 10B can even be made immune to acid vapors, etc.

The embodiments illustrated in FIGURES 3, 4, and 5 are structurally similar to the previously described embodiments, and operate in the same manner. Accordingly, no added description of their operation is deemed necessary.

While only the principal embodiments of the invention have been described in detail herein and shown in the accompanying drawing, it will be evident that various modifications are possible in the arrangement and construction of its components without departing from the scope of the invention.

What is claimed is:

1. An acoustic transducer, comprising:

a magnetic circuit including a central pole piece having first and second spaced surfaces and an annular pole piece having first and second spaced suifaces, said annular piece being concentrically spaced about said central pole piece, and one of said pieces constituting a permanent magnet, a magnetically susceptible piece rigidly connecting the second end of said central pole piece with the second end of said annular pole piece, and a magnetically susceptible disk spaced from the first surfaces of the central and annular pole pieces by an air gap, and said disk having a thickness which decreases radially from its center such that the relationship T A :T A is substantially satisfied, wherein T and T are the thicknesses of annular segments of the disk at given distances from the center of the disk, and A and A are the corresponding circumferential areas of the respective annular segments having the aforementioned thicknesses;

a signal-receiving, fixed voice coil mounted for inducing movements of the magnetically susceptible disk in accordance with signals received by the coil;

non-magnetic means for enclosing, in combination with one or more pieces of the magnetic circuit, the voice coil so as to isolate the vocie coil from the ambient air; and

a diaphragm attached to and adapted for movement with the magnetically susceptible disk.

2. The acoustic transducer as claimed in claim 1 wherein the diaphragm is non-magnetic and the magnetically susceptible disk is separated from the first surfaces of the central and annular pole pieces by both an air gap and a portion of the diaphragm.

3. The acoustic transducer claimed in claim 1 wherein the magnetically susceptible disk is formed of one or more powdered ferrites.

References Cited UNITED STATES PATENTS 2,029,282 1/1936 Serge 1791l7 2,492,255 12/1949 Angehrn 179-119 2,848,560 8/1958 Wiegand 179-ll9 2,902,668 9/1959 Savit 1791 15 3,178,512 4/1965 Ashworth 179-115 WILLIAM C. COOPER, Primary Examiner W. A. HELVESTINE, Assistant Examiner US. Cl. X.R.

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