US2897475A - Movable actuating coil transducer array - Google Patents

Description

July 28, 1959 w. T. HARRIS MovABLE ACTUATINQCOIL TRANSDUQER ARRAY Filed April 13. 1954 FIG.

INVENTOR w/a/e r Aff/.s

,\ ATTORNEYS Unite Wilbur T. Harris, Southbury, Conn., assignor to The Harris Transducer Corporation, Woodbury, Conn., a corporation of Connecticut Application April '13, 1954, Serial No. 422,787

11 Claims. (Cl. .M0-9)V My invention relates to improved electroacoustic transducer means and' in particular to electromagnetic transducers, as of the character described in greater detail in my copending patent application, Serial No. 241,470, filed August 11, 1951.

It is an object of the invention to provide an improved transducer of the character indicated.

It is another object to provide an improved means for coupling a transducer of the character indicated to the medium into which the transducer is to radiate or is otherwise to respond.

It is a further object to provide means for vastly improving the radiating eiliciency of transducers of the character indicated.

Another object is to provide a transducer with improved high eliciency and broadband characteristics.

A further object is to provide an improved transducer utilizing an acoustically hard structure over a substantial part of the radiating face thereof, whereby the prospect of damage due to mishandling may be minimized.

Other objects and various Ifurther features of novelty and invention will be pointed out or will occur to those skilled in the art from a reading of the following specification in conjunction with the accompanying drawings. In said drawings, which show, for illustrative purposes only, preferred forms of the invention:

Fig. 1 is a fragmentary sectional view of a transducer incorporating features of the invention;

Fig. 1A is a simplified fragmentary plan view on a reduced scale and partly broken-away, to illustrate the array configuration in Fig. l; and

Figs. 2 and 3 are views similar to Fig. l, but illustrating modifications.

Briefly stated, my invention contemplates the creation of a highly eilicient broadband transducer from an acoustically hard structure, which presents to the radiating medium, such as water, an acoustically hard, essentially continuous plate; I term this plate a baille because of its property to reilect acoustic signals. At sp-acings substantially less than a wavelength, preferably less than one-tenth Wavelength at the desired response frequency, relatively small openings are provided in the baille, and electrically driven force elements are supported for reciprocation in these openings. In the forms to be described, the force elements are essentially electrodynamic coils, which, if formed of sufficiently wide laminated-strip mateal, may themselves effectively constitute the radiating or driving areas. Alternatively, various area-transforming piston configurations may be applied to the coils in order to assure greatest eiciency of force application Within the areas of baille openings.

In one general form to be described, the baflle openings are circular and provided in a two-dimensional array, there being a separate electrodynamic force coil for such opening. In another general form, the openings are parallel slots, spaced as indicated with respect to the wavelength, and elongated racetrack-shaped force coils are provided, with opposed stretches in adjacent slots.

States Patent ECC Referring to Figs. 1 and 1A of the drawings, my invention is shown in application to an electrodynamic transducer, in which the baille plate 10 is a separate member of extensive area and provided with a plurality of openings 11-12-13 therein, said openings being spaced by an amount S, representing something substantially less than a wavelength at the desired response frequency; for example, the spacing S may be less than one-tenth of a wavelength. Adjacent the non-radiating side of the baille 10, I provide for each of the openings 11-,12-13 a sep'- arate magnetic-core structure 14-15--16, defining, symmetrically with respect to each of such openings and vadjacent the non-radiating side of baille 10, an air gap such as the annular air gap 17 in the case of magnet 15. The air gap 17 is defined between the enlarged head or pole piece 18 of an upstanding central member secured, as by bolt 19, in the base of a generally cup-shaped magnetic core; the core 15-18 may be permanently magnetized, but I show that polarizing llux is established in the gap 17 by coil means 20. It will be understood that the coils 20 [for each of the magnet structures 14-15-16 may be energized in series or in parallel, depending upon their impedance with respect to that of the power supply.

Supported for reciprocation in each of the air gaps, as at 17, I provide electrodynamic-coil means, and for the case shown in section in Fig. 1, the coil 21 is a helical development of laminated conductive strips suitably consolidated into unit-handling relation and supported on the core structure 20, as by readily yieldable acoustically absorbent means 22 (such as air-filled rubber). In korder to provide area transformation for eilcient application of electrodynamically generated forces in the relatively restricted area of opening 12 at the radiating face of baille 10, I have shown a piston 23 carried by each of the coils, the connection being through suitable incompressible insulating means 23. The piston 23 is preferably of lightweight construction, as of magnesium or aluminum, and is excavated as much as tolerable, as suggested in the drawings. The piston 23 may be in effect a truncated cone, -with the smaller or truncated area substantially in the plane of the radiating face of baille 10. Additional locating support for the coil may be provided by a relatively thin sheet-metal diaphragm 24, peripherally secured at the opening 12 and centrally secured to the truncated face of piston 23. For sealing purposes, and to present a clean appearance, a boot or outer layer 25 of acoustically transparent material may be bonded to the diaphragms 24 and baille 10.

The structure may be completed by employment of heavily ilanged pieces 2.6, constituting a housing to which baille 10 is bolted, as at 27. The baille 10 may be coextensive with the outwardly extending flanges 28 of member 26; but in any case I prefer to avoid loss of efciency due to end eiects, by providing further acoustically hard baille radiating surface laterally outside the array of force elements, to a lateral extent substantially exceeding the spacing S between force elements.

For pressure-equalizing purposes, so as to render the transducer less depth-dependent, the side-ilange pieces 26 may project substantially rearwardly of the backing plate 29 for the magnet assemblies, so as to support a back housing plate 30 spaced rearwardly of the plate 29 and ldefining therebetween a cavity or chamber 31. Within the chamber 31, I show a bag 3?. of flexible material having an external opening at 33, so as to be free-ilooding. Air-bleed passages 34 (in the backing plate'29) and 35 (in the magnet structure) provide a means for assuring ambient pressure in the region in which the electrodynamic coil is suspended, as will be understood.

In use, if the coil elements for the respective force elements are driven in phase, the described small-spot array radiates like a large diaphragm driven at larger torce but 3 smaller amplitude; to the medium this diaphragmappears to have the size of the baille. The forces and amplitudes are subject to a transformation ratio approximating the total-to-active arearatio, as long as oneassllmes thespacing Sto be a relatively smallV fraction of a wave.- length. Since the alternating acoustic pressure cannever be appreciably diierent at two `points which arepseparated only a very small fraction of a wavelength, theY driven spots are not likely to produce cavitation effects when driven at high intensity. The impedance transformation greatly improves the electroacoustic conversion efficiency.

In Fig.'2, I show application of theprinciples of my invention to a parallel-gap array, as of the vtype disclosed in the above-mentioned copending application. This type of construction comprises essentially a magnetcore structure 40 having elongated parallel ygaps41-42, spaced (as indicated above) by the amount S, representing a small fraction of a wavelength. The magnet-COI@ material may itself be at least the central part Of the acoustically hard baille structure, so that no additional baille need be provided, except at lateral flanges 28 of `the housing 26; the air gaps 41-42 are, therefore, strictly in the baille surface. Separate coils 43--441-45 may be excited with polarizing current so as Vto assure the desired distribution of polarity at theW respective gaps l1-42, and in each two adjacenty of said gaps I-'may support opposed stretches 46-47 of a single racetrack-shaped coil, which may be a helical development of laminated conductive strip material. The front face of such coil 46-47 may be substantially in` the` plane of the radiating face of the baille, that is, the face48 of core structure 40; and I have shown thin, yieldable, metal diaphragms 49 peripherally secured to the baile face 48 at the gaps, and centrally secured to the respective. stretches of the coil means 46-47. As in the caseof Fig. l, a boot of acoustically transparent sealing material 50 may surround the structure. Also, when. adapted for Apressure-equalization, apertures, as at 51, may communicate between the space behind the magnet Vcore 40 .and the region in which coil means 46-47is supported; the pressure-equalizing means may be ofthe typedescribed at 32 in Fig. l and is therefore not shown in Fig. 2.

In Fig. 3, I show a modification of the structure of Fig. 2, in which I employ area-transforming piston elements 55 carried by the respective coil means 46-47, and preferably of magnesium, aluminum, or other lightweight material. The function served by the piston ele- 'ments 55 is eifectively to enlarge the force-transmitting area; elements 55, therefore, may be of trapezoidal crosss'ection and may project slightly beyond the bafe surface 48. It is convenient to provide a thin-metal Ydiaphragm 56 for support of the coils, as by securing the same both to the coils and to the unexcited structure in a common plane. For this reason, I have applied a layer 57 of acoustically transparent material over the baille area, except at the gaps 4142, and at these gaps the material'57 may be ilared outwardly, as shown, to accommodate the sectional contour of piston elements S. The 'structure of Fig. 3 is otherwise as described for Fig. 2, and therefore the same reference numerals are `fused.

It will be appreciated that I have described relatively `simple transducer structures lending themselves to improved efliciency and exhibiting broadband characteristics. These structures are further characterized by extreme ruggedness inasmuch as the exposed radiating surface is substantially entirely acoustically hard and may therefore be a heavy steel plate.

While I have described the yinvention in detail for the preferred forms shown, it will be understood that modications may be made within the scope ofthe invention as defined in the claims which follow.

I claim:

l. vIn a transducer of the character indicated, a ,passive -bafe plate of acousticallyhard material having two relatively small laterally spacedopenings in the radiating face thereof and spaced by an amount representing a small fraction of a Wavelength at the desired response frequency, flexible diaphragm means peripherally secured at each of said openings and covering each of said openings, magnetic-circuit means establishing air gaps adjacent the non-radiating side of said baille and symmetrically disposed with respect to the axes of said openings, and electrodynamic driving means in said-respective gaps and extending in direct force-transmitting relation with the back side of each of said diaphragms.

2. A transducer accor-ding to claim l, in which the lateral extent of said baffle plate outside the space of said openings substantially exceeds the space between said openings.

3. A transducer according to claim 2, and including a boot of acoustically transparent sealing material overstanding the radiating surface of said baffle plate and of said diaphragms.

4. In a transducer of the character' indicated, magneticcore means dening a plurality of spaced elongated gaps, a movable strip of conductive material Vincluding `opposed stretches in the respective elds of two `of said gaps, said magnetic-core means being formed with an essentially continuous surface constituting an accustically hard bathe interrupted only at said gaps, whereby said movable conductive strip may in part ill said gaps to provide a substantially `continuous radiating surface, and means ilexibly supporting said strips in said gaps for reciprocaton on axes generally normal to said surface.

5. A transducer laccording to claim 4, in which said movable strip `of conductive material comprises a lhelical ycoil containing a plurality of turns of laminated strip material, there being a Vplurality of laminations in the field of each `of said gaps.

' 6. In a transducer of the character indicated, magneticcore meanscomprising -a plurality of pole pieces oriented to define in ya radiating surface a plurality of spaced elongated gaps, a moving conductive coil including two opposed ystretches movably supported in two of said gaps, and piston means carried by said coil at the radiating end thereof in clearance relation with said gaps and substantially filling said gaps.

7. A transducer according to claim 6, in which said piston means has an etfective cross-section of ytrapezoidal configuration Withthe base thereof secured to said coil so as -to provide an area-.transforming element.

8. A transducer according to claim 7, and including a 'layer of acoustically transparent material covering the radiating surface except at said openings, said layer being of .a thickness to place the front face thereof in substantially the plane of the radiating face of said piston.

`9. A transducer according to claim 7, and including acoustically absorbent supporting means lfor said coil on the non-radiating side thereof.

l0. In a transducer of the character indicated, a baille plate of Iacoustically hard material having two relatively small laterally spaced openings in the radiating face thereof and spaced by an vamount less than a wavelength at the desired response frequency, flexible diaphragm means peripherally secured at each of said openings .and covering each `of said openings, magnetic-circuit means establishing air gaps adjacent the non-radiating side of said baille and symmetrically disposed with respect to the axes of said openings, electrodynamic driving means in said respective gaps and extending in direct force-transmitting relation with the back side of each `of said diaphragms, a rigid housing ixedly supporting said magnetic circuit means and defining ybetween said magnetic circuit means 'and a part of said housing a pressure-equalizing cavity with an external opening on a side other than the radiating side of said transducer, `a collapsible llexible bag within said cavity and covering said opening, Whereby fluid may enter said opening and till said bag, there being a fluid-communicating opening Abetween one of said gaps and the other side of said collapsible bag, whereby air Within said transducer may be maintained under ambient fluid-pressure conditions.

11. In a transducer of the character indicated, an extensive facoustica'lly hard baffle plate, an array of separate magnet assemblies 'adjacent one side of said plate, said magnet assemblies defining in each instance Iau annular air gap immediately adjacent said bale plate, said bale plate being locally recessed adjacent each said air gap and having for each said recess an opening to the opposite side thereof, piston means comprising .an electrodynamic coil suspended for axial reciprocation in each of said gaps and including a part projecting through said 'baffle plate .at the recess therein, said projecting part comprising a tnuncated conical area-transforming member secured at the base to said coil, a flexible diaphragm periphenally secured to the opening in said bale plate and centnally secured to said area-transforming member for supporting said member with the truncated base substantially in the plane of said other side of said bafe plate.

References Cited in the tile of this patent UNITED STATES PATENTS

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