US3413424A - Electro-acoustic transducer - Google Patents

Description

Nov. 26, 1968 E. v. CARLSON ELECTRO-ACOUSTIC TRANSDUCER 2 Sheets-Sheet 1 Filed Sept. 1961 R.- y yzez Fa/eow Nov. 26, 1968 E. v. CARLSON ELECTED-ACOUSTIC TRANSDUCER 2 Sheets-Sheet 2 Filed Sept. 6, 1961 '7 INVENTOR. %9- 52m fiwm United States Patent 3,413,424 ELECTRO-ACOUSTIC TRANSDUCER Elmer V. Carlson, Prospect Heights, Ill., assignor to Industrial Research Products, Inc., Franklin Park, Ill., a corporation of Delaware Filed Sept. 6, 1961, Ser. No. 136,220 7 Claims. (Cl. 179-115) ABSTRACT OF THE DISCLOSURE An electro-acoustic transducer in a flux conductive envelope which is in contact with the armature and forms part of the magnetic circuit of the transducer.

This invention relates to improvements in miniature electroacoustic transducers of the type wherein a portion of an armature is vibratable in the air gap of a magnetic circuit. The principal subject matter of this invention,

namely, a magnet stack whose flux circuit is completed by a flux-conductive envelope or case, is disclosed in applicants copending application, Ser. No. 27,006 filed May 5, 1960, now Patent No. 3,111,563, issued Nov. 9, 1963, for an electromechanical transducer.

In these vibratable armature-type transducers, the armature is positioned inside a coil of wire arranged to link a magnetic flux that flows along the armature in one direction or the other. One portion of the armature is in an air or working gap between two flux conductive surfaces having opposed polarities established by one or more magnets. When the portion of the armature in the working gap is at zero potential, there is no flow of flux along the armature. When this zero potential of that portion of the armature in the working gap ceases, as by flux induced by the coil in a receiver or by a diaphragm or stylus moving the armature toward one of the faces of the working gap in a microphone, flux flows along the armature. Flux flowing along the armature in either direction is frequently called the fluctuating or alternating flux to contrast with the steady unidirectional flux at the working gap, although the two are in fact components of the total flux of the system.

One way of maintaining that portion of the armature in the working gap at zero potential is to position two magnets on opposite sides of the working gap, to connect the opposite poles of the two magnets to each other by a flux conductor, and to fix one end of the armature at or near the midpoint of this flux conductor.

An object of this invention is to adopt the idea of utilizing a magnetic case as both a shield and a part of the magnetic circuit of the transducer motor presented in copending application, Ser. No. 27,006, to a transducer having the flux circuit described in the preceding paragraph.

Another object of this invention is to provide an improved path for the fluctuating or alternating flux that flows along the armature at points outside the armature. This fluctuating flux must complete its circuit through the shells and probably through the magnets themselves. This weak flux encounters much resistance in moving through either one of the central pole pieces so that the sensitivity of the device would be low. A feature of this invention is the provision of pole pieces at the faces of the gaps, each pole piece in contact with a permanent magnet, and additionally of an extension on each pole piece which is turned toward the fluxconductive case so as to enable the weak, alternating magnetic flux to be shunted around the magnets.

Another object of this invention is to adjust the position of the vibratable portion of an armature in the working gap firstly, without touching the armature; sec

Patented Nov. 26, 1968 ondly, without bending it; thirdly, after the diaphragm is in position; and fourthly, while the case is sealed. Applicant pivots one end of the armature at the internal wall of the case with means externally of the case for moving the armature on the pivot. A feature of the present invention is the mounting of one end of an armature between two externally projecting tongues at a joint in the case, but making the juncture of the case with the tongues of a bendable material so that by bending the tongues with respect to the case, the free end of the armature positioned in a working gap within the case may be moved toward one pole or the other of the gap. The armature is not touched. It is not bent. The centering can be done not only after the diaphragm is mounted, but after the case is sealed.

Another object of this invention is to establish a rigid relationship between the bendable flanges of the thin walled case to the working gap. One of the features of this invention is a magnet stack which in combination with straight flanges and side walls hold the parts in rigid relationship during the setting of the armature. The case being made of somewhat ductile material so that the flanges may be bent, must be reinforced so that bending of the flanges has no eflect on the position of the magnet stack with respect to the held end of the armature. This magnet stack is provided with structural members at right angles to the plane of the working gap so that the case of the transducer may be clamped at the opposite ends of the magnet stack, and when so held, the flanges may be worked with precision.

Another object of this invention is to provide a transducer whose dimension along a line at right angles to the plane of the armature is minimal. Heretofore, the diaphragm has been spaced laterally of the armature so that the dimension of the case must be increased to provide some vibration space between the diaphragm and the motor and space for the front cavity. Two features of this invention are the mounting of a diaphragm in a space longitudinally adjacent the working end of the armature and positioning the plane of the diaphragm substantially in the plane of the armature.

Another object of this invention is to more rapidly, accurately relate the components of the transducer to each other during assembly. A feature of this invention is the building of the operating components of the transducer as separate entities whose external walls Will exactly seat against the inside walls of a complementarily designed case. The front cavity is a complete cavity in itself. The magnet stack is a complete assembly with the pole faces accurately spaced. The coil and leads are a rectangular parallelepiped, and the case is formed of a ductile material of U-shaped configuration so that the various components may be slipped in and the whole clamped together.

These and such other objects as may hereinafter appear are attained in the embodiment of the invention disclosed in the accompanying drawings, wherein:

FIGURE 1 is a longitudinal, vertical section taken along the line 11 of applicants transducer set forth in FIGURE 2;

FIGURE 2 is a section along the line 2-2 of FIG- URE 1;

FIGURE 3 is a section taken along the line 33 of FIGURE 1;

FIGURE 4 is an end view of the assembled transducer;

FIGURE 5 is an exploded view of the component parts of the transducer when ready for assembly;

FIGURE 6 is a perspective view of the pole piece having a projecting tab or ear;

FIGURE 7 is a view in section of a combined armature-diaphragm with Thuras tube incorporated therein in proportionate relationship with the corresponding parts in FIGURE 1; and

FIGURE 8 is a plan view of the diaphragm shown in FIGURE 7.

Continuing to refer to the drawings, applicants transducer consists of a case 16 which is made of flux-conductive material selected for its ability to carry both an intense steady flux and a superimposed fluctuating or alternating flux. The case is somewhat pear shaped, see FIGURE 5, and consists of a diaphragm and magnet or main chamber 25 and a coil chamber 27. The main chamber 25 has end wall 18, top wall 26 and bottom wall 32. It is joined to the coil chamber 27 by shoulders 28 and 30 leading to top wall 38 and bottom wall 40. The end 29 of the case 16 has two flanges 34 and 36 between which is positioned one end of an armature 48 which together form an adjustable tongue. The flanges and armature are joined by welding or brazing. Two sides of the case, 94 and 96, are attached to the ends, top and bottom. The case is sealed. Ports 90 and 92 to pass conductor leads are provided at the flanges.

A coil 42 is disposed around the armature 48 and may be held in position either by encapsulating the coil in a form which seats exactly between the walls 38, 40, 94 and 96, or by mounting the coil on the armature. Under this construction, the coil tunnel clears the surface of the armature 48 by a distance suflicient to permit free movement of the armature within selected limits. In some applications, the coil may be mounted on the armature in which case clearance is provided between the outer walls of the coil and the inside walls of the case.

Mounted on that end of the main chamber 25 which is adjacent to the coil 42 is a magnet-pole piece stack 49 consisting of bar magnets 56 and 58. To improve the path for the alternating or fluctuating flux, applicant provides pole pieces 52 and 54 spaced by lugs 61 and 63 mounted on members 60 and 62 respectively, said members 60 and 62 being of material of low flux conductivity. The bar magnets are elongated normal to the axis of magnetization, but their shape depends upon the particular configuration of the case. The magnets are disposed so that their maximum potential difference is disposed across the gap 64 between the pole pieces 52 and 54. The pole pieces 52 and 54 have extensions 80 and 82 respectively which extend toward the adjacent top 26 and bottom 32 of the case.

The main cavity 25 is occupied principally by a diaphragm 68 and front sound chamber 72. This sound chamber cavity 72 is a complete receptacle in itself consisting of a tub 70 having outer dimensions adapted to seat firmly in the lower portion of the cavity 25. The open side of the tub 70 is closed :by a vibratable dia phragm 68 to the surface of which is afiixed a substantially rigid plate 66. Two openings in the sound chamber are shown, but only one will be used for a given application. If an opening at the end is desired, a port 20 is formed in the end wall .18 of the case 16 and a tube 24 may be inserted therein. To provide access to the outer sound chambers 72, an opening 7 6 is provided in the wall of the tub 70. If a lateral opening is desired, aligned ports 71 may be placed in tub 70 and case bottom 32.

The armature 48 projects to the left of the magnet stack 49 by a distance sufiicient to reach the diaphragm plate 66 and its end is attached thereto as for example, by a spot of adhesive 69. The end of the armature could be carried further to the left and centered over the rectangular plate 66. The plate 66 and the armature could be in a common plane joined by adhesive between the edge of the plate 66 and the end of the armature 48. Under certain conditions, the plate 66 and armature can be integral.

Mounting the diaphragm substantially in the plane of the armature eliminates the connecting member usually found in those transducers and the diaphragm provides limited support for the free end of the armature. When a transducer is dropped and the shock is along a vector at an angle to a flat surface of the armature, the momentum of an armature end may place a bend or set at, for example, the edge 83 of the upper edge of the pole piece 54. Applicants construction shows an armature rigidly supported at one end and flexibly supported at the other end with an intermediate portion vibratable in a working gap.

In operation, the polarizing or steady fiux follows the dot-dash line 84 when the armature 48 is positioned in the magnetically neutral point of the gap 64. When the armature is deflected upwardly or downwardly, the fluctuating flux will flow through the armature. This change in magnetic flow will complete its circuit partially through the pole piece extension or ear surrounding space and partially through the magnet 56, along the dot-dash line 85, or through pole piece extension 82 surrounding space and magnet 58, along dot-dash line 87.

The transducer shown may be used as either a microphone or a receiver. Where used as a microphone, it may be equipped with a Thuras or inertance tube mounted on the diaphragm and formed in the plate 66. This Thuras tube is similar to one type shown in Knowles Patent No. 2,966,558, and consists of two tubes 77 and 79, both formed as a groove in plate 66. Referring to FIGURE 2, tube 79 opens into the top of the plate 66 at 91, see also FIGURE 1, and through a hole 81 in the diaphragm into the front or outer sound chamber 72. The other tube 77 opens similarly on opposite sides of the diaphragm. The tubes are dynamically balanced on the diaphragm so that they do not interfere with vibration.

While the foregoing transducer could be fabricated in various ways, applicant has devised subassemblies which promote exact predetermined relationship between the parts. Referring to FIGURE 5, the top, bottom and ends are part of a single stamping which includes mating grooves to form the ports and 92 for the terminal posts for the coil leads. The coil 42 is encapsulated or enshrouded sufficiently to define its external dimensions and its vertical thickness is such that when the coil is positioned between the walls 38 and 40 and the armature is positioned in the tunnel 50 of the coil 42 with the arms of T between the flanges 34 and 36, all in a jig, the flanges may be welded, soldered or brazed to the armature and the walls 38 and 40 will hold the armature with mechanical clearance to the walls of the tunnel 50.

The elements of the magnet stack 49 are preassembled with adhesive and the stack slipped in laterally of the shell 16 to the left of the end of the armature, see FIG- URE 1. It is then moved to the right, the working gap 64 being slipped over the armature 48, and under pressure from the top and bottom of the case, is cemented in the position shown in FIGURE 1. The tub with diaphragm mounted therein is pushed into the cavity 75 and the end of the armature 48 is attached to the diaphragm plate 66 by a spot of adhesive 69. Separate sides 94 and 96 may be attached, or a side wall assembly 98 having a protective end wall 100 with aligned openings may be used.

The foregoing sequence of assembly may be varied. Thus the top wall may be spread back and the units 70, 49, 48, 42, positioned on the bottom 32 and then the top 36 brought around.

The unit 98 incorporates both side walls and end wall and has a protective shield 100 with through openings 101 and 103 which register with openings 90 and 92. This may be slipped over the end of the open-sided case 16 and by suitable means such as resistance welding, the entire case may be sealed.

In FIGURES 7 and 8, there is shown a second embodiment of the invention wherein the armature and diaphragm are a single member and the Thuras tube is in that member. The armature is identified by the numeral 120 and its fixed end 122 is held between the flanges 34 and 36 as in the first embodiment. The pole pieces 52 and 54 and magnets 56 and 58 occupy the same relative position as shown in FIGURE 1. The second cross arm 124 of the T, however, constitutes the piston head of the diaphragm and hence replaces the plate 66 of FIGURE 1. The cross member of the armature is vibratable with the diaphragm.

Having thus described his invention, what applicant claims is:

1. An electro-acoustic transducer comprising a steady fiux circuit which includes an enclosing flux-conductive envelope, two permanent magnet means having opposite poles contacting the interior wall of the envelope at spaced points to flux-conductively connect said opposite poles, said magnets being spaced from each other with their other opposite poles confronting one another across a working gap in said steady flux circuit within the envelope; a signal flux circuit which includes an armature having one end mounted in flux-conductive relationship to said envelope and having a vibratable portion in the working gap of the steady flux circuit, a coil around said armature, and acoustic means including a diaphragm forming with the inside walls of the envelope front and back acoustic cavities drivingly connected to a vibratable portion of said armature.

2. The electro-acoustic transducer of claim 1 wherein the fixed end of the armature is mounted on a wall of the flux-conductive envelope.

3. The transducer of claim 1 wherein the vibratable portion of the armature is connected to the acoustic means by a drive pin.

4. An electro-acoustic transducer comprising a ductile, flux-conductive envelope, a diaphragm dividing the interior of the envelope into a front sound cavity and a back motor cavity, a magnet stack having two spaced,

permanent magnets forming a working gap therebetween and having their outer poles in flux-conductive engagement with the interior walls of the envelope, an armature having a vibratable end in said working gap and having the other end fixedly mounted on the inside envelope wall, a coil around the armature, a drive pin connecting the armature to the diaphragm, and adjusting means mounted on the outer side of that portion of the wall to which the armature is mounted for bending the wall and the armature so as to vary the position of the working end of the armature in the working gap.

5. The electro-acoustic transducer of claim 4 wherein the external adjustment means is a tongue protruding outwardly from the wall of the envelope with the mounted end of the armature engaging the tongue so that bending the tongue at right angles to the surface of the armature will alter the position of the vibratable end of the armature in the working gap.

6. The electro-acoust-ic transducer of claim 4 wherein the envelope consists of two facing cups joined along a seam, and wherein each cup has a protruding tongue at the same side with the end of the armature positioned between said tongues and wherein said cups are sealed to each other along the seam and at the tongues.

7. The electro-acoustic transducer of claim 6 wherein the cups are substantially identical so that the armature lies in the plane of the seam.

References Cited UNITED STATES PATENTS 2,927,977 3/ 1960 Knauert 1791 14 2,927,976 3/1960 Pearson 179115 2,966,5 5 8' 12/1960' Knowles 1791 14 2,994,016 7/1961 Tibbetts 179108 3,076,062 1/1963 Fener 179-114 FOREIGN PATENTS 1,052,458 4/ 1959 Germany.

KATHLEEN H. CLAFFY, Primary Examiner.

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