Patent 3733445 - INERTIAL REACTION TRANSDUCERS
|United States Patent||Patent Number 3,733,445|
|Sebesta , et al.||Issued May 15, 1973|
INERTIAL REACTION TRANSDUCERS
An acoustic transducer having good speech intelligibility with inherent significant suppression of ambient noise. Incorporates inertial mass reaction in efficient, compact and lightweight arrangement. In one form includes a predetermined weight or block suspended by a vibratory diaphragm and linked to the electro-mechanical or motor section, to impart a specific upper resonant frequency. In another, the weight is mounted directly upon the armature. Effective over a relatively wide speech band. Has a single resonant frequency that is readily controlled and manufactured. Provides inherent roll-off beyond this frequency to extend response, yet substantially reduce noise. Applications include: contact head microphone; in-mask microphone; in-ear receiver.
|Inventors:||George J Sebesta (Hicksville NY)|
Richard W Carlisle (Hicksville NY)
|Assignee:||Dyna Magnetic Devices, Inc. (Hicksville, NY)|
|Filed:||February 17, 1971|
|Current U.S. Class:||381/418|
|Current International Class:||H04R01/14 (20060101)|
|Current CPC Class:||H04R 1/14 (20130101); H04R 1/46 (20130101); H04R 11/00 (20130101)|
|Field of Search:||179/114A,115A,119A,181R,138VL,115.5VT,115R 340/8R,10|
Assistant Examiner:Thomas L Kundert
This patent application is a Continuation in replacement of our earlier application Ser. NO. 650,776 filed July 3, 1967. The latter case was abandoned in favor of the present one.
What we claim is:
1. A compact inertial transducer comprising a housing; an electromagnetic assembly within said housing, said assembly being attached to the housing and including permanent magnet means, an armature having a vibratory arm the free end of which extends close to said magnet means and forms a magnetic gap therewith, a pole piece structure containing said magnet means, and a winding in electromagnetic coaction with said armature; a member of relatively minor dimensions and predetermined mass; a flexible strip of elongated form mounting said member apart from said armature arm fully within said housing and in inertial vibratory relation with respect to said housing, the opposite ends of said strip being affixed to said housing and thereby compliantly coupling the member to the housing and its attached assembly, the sides of said strip being unattached, and means mechanically interconnecting said armature arm and member to vibrate in substantial phase relation; the mass of said member and the compliance of said strip and arm being proportioned to provide a significant response in the audible frequency range by the transducer through inertial reaction.
2. An inertial transducer as claimed in claim 1, further including a frame supported in the housing, said flexible strip being secured to said frame in a vibratory mode and mounting said member centrally thereof.
3. An inertial transducer as claimed in claim 1, in which said member is maintained substantially parallel to the vibratory arm, and a drive pin arranged perpendicularly to said member and connecting it to said armature arm substantially centrally of the assembly.
4. An inertial transducer as claimed in claim 3, in which the said armature is in U-form, the end of one of the armature arms being attached to said pole piece structure, the other armature arm being the said vibratory one that coacts with said magnet means, thereby establishing a low reluctance magnetic flux path between said pole piece structure and the free end of said other armature arm.
5. An inertial transducer as claimed in claim 4, in which the ratio of the mass of said member plus that of the vibratory arm to the mass of the remainder of the transducer is of the order of 0.10.
This invention relates to transducers that embody a significant vibratory mass in inertial reaction, and more particularly relates to novel acoustic microphones and receivers of electromechanical construction, with compact dimensions, good speech intelligibility and significant noise suppression.
Speech communication microphones and receivers that have heretofore utilized the principle of inertial reaction, suspended their entire electroacoustic translation section within the housing in vibratory relation thereto. The bulk of such transducer's mass was thus concentrated apart from and suspended with respect to the housing. The prior art inertial transducers were relatively bulky for effective transfer of acoustic energy by contact, whether as a microphone or receiver. Their size and the complications incident to suspension of their electroacoustic sections resulted in spurious resonances, which in turn made them sensitive to undesirable noise components.
In accordance with the present invention the basic electromagnetic or "motor" section is secured with the housing in stable and efficient mechanical and magnetic array. An armature is incorporated with this section, and in turn is mechanically connected with a block of predetermined mass. In one embodiment this mass is held in vibratory suspension with respect to the housing, being secured to a diaphragm of selected stiffness. In another form the block is mounted directly on the armature, for vibration therewith. The auxiliary mass introduces a specific single resonance peak in the upper transducer response. Such peak frequency may be selected over a substantial frequency position; in the 2,000 to 3,500 Hz range in the exemplary units. The mass-block mounted or coupled to the transducer armature effects this practical result. The desired resonant frequency is readily produced in mass production, and remains stable and clean of extraneous resonance frequencies.
The block-diaphragm embodiment is a subassembly, readily inserted or removed from the transducer; the mounted-block form also being readily incorporated individually in production. The frequency response of the final assembly can thus be selectively produced for specific application or specification; as will be set forth hereinafter. The basic electromechanical or motor section is standardized for economy. The resultant transducer hereof is rugged, compact, with good sensitivity and efficiency. Its frequency response covers the usable speech range up to and beyond the preset resonant point, and rolls-off with retention of intelligibility and reduction of the higher noise frequencies, as above 3,500 Hz in the embodiments hereof. The inherent shape of the response curve compensates for the propagation characteristics of one's head relative to the vocal chords in contact microphone use, to provide optimum overall performance with minimum external compensation. An optimum ratio of speech to noise is thereby maintained.
As a contact microphone, the invention transducer is significantly advantageous in contact with the top of one's head, as within a helmet for pilots, police, and firemen. It relieves the use of one's hand, and eliminates the boom in front of the face. It is of course practical for use on other areas of a person, as the forehead. Further, we have found that the inertial reaction transducer hereof can be used as a microphone actuated by voice directly through air vibrations, for good speech output. Its location ahead of the mouth, as in an oxygen mask, provides unhindered, safe and effective communication. Its lightweight and compactness permit wide application as microphone or receiver. Thus its tiny size potential makes it practical as an inertial reaction tip receiver within the ear canal, in a self-contained ear insertion hearing aid.
The above and further features, objects and advantages of this invention will become apparent from the following description of exemplary embodiments thereof, illustrated in the accompanying drawings, in which:
FIG. 1 illustrates the use of the invention transducer within a pilot's helmet, as a head contact microphone.
FIG. 1-A is a diagrammatic view of the microphone mounting of FIG. 1.
FIG. 2 is an enlarged plan view of an exemplary transducer hereof, with the cover partially cut away.
FIG. 3 is a vertical cross-sectional view through the transducer, taken along line 3--3 of FIG. 2.
FIG. 4 is an exploded view, in perspective, showing the components of the exemplary transducer of FIGS. 2 and 3.
FIG. 5 is a perspective illustration of the magnet and pole piece section of the transducer seen from the plane indicated by the lines 5--5 in FIG. 4, in the direction of the arrows.
FIG. 6 is a longitudinal cross-sectional view of another embodiment of the transducers hereof.
FIG. 7 is a cross-sectional diagram of the strain-relief arrangement for external leads of the transducers.
FIG. 8 is a characteristic frequency curve of the transducer of the present invention.
The inertial reaction transducer hereof may be used effectively as a contact microphone, or as an acoustic microphone or receiver, as aforesaid. In FIG. 1 it is used as a head contact microphone in a pilot's helmet 15. The helmet is cut-away to show the central microphone assembly 16 from which extends electrical cable 17 that is connected to the microphone (not shown). The assembly 16 fits within an opening 18 in the middle pad 20 of the helmet. A flexible strap 21 retains the microphone assembly 16 in operative contact with the head. Strap 21 passes across the inner face of pad 20, its outer ends being anchored to the helmet.
FIG. 1-A illustrates the microphone 25 imbedded in the lower central region of a cylindrical elastomer pad 22, that is held within assembly 16. The exposed surface of the microphone 25 is in firm contact with the head, indicated at 23. The small weight of transducer 25 results in a good match of mechanical impedance between the microphone and the head, whether or not the wearer has hair interposed therebetween. This provides substantially no attenuation of sound vibrations from the head 23 to transducer 25. The intrinsic sensitivity of the transducer 25 hereof translates minute head vibrations due to speech, for practical amplification in known manner. The transducer may be pressed against the head 23 with a force of the order of 6 ounces by strap 21. Such strap may be of rubber mesh or equivalent.
Essentially, transducer 25 comprises motor element 30 mechanically linked with vibratory block-mass assembly 45, each of which is independently supported within housing 40,41. Motor element 30 is similar to that of the electro-acoustic transducer shown and described in our copending patent application "Sub-Miniature Sound Transducers", Ser. No. 454,377 filed May 10, 1965, now U.S. Pat. No. 3,432,622 and assigned to the same assignee. However, for the broad purposes of the present invention, the motor unit 30 may take over equivalent forms known to those skilled in the art.
Motor unit 30 is a self-contained electromagnetic system that transduces vibrations of its armature arm 31 into corresponding electrical signals at terminals 42,42 of winding 33, in the microphone mode; or electrical signals that are impressed on coil 33 into corresponding vibrations of arm 31, in the receiver mode. Arm 31 extends from a folded U-shaped armature, the opposite arm 32 of which is secured to the top portion 34 of pole piece 35. Pole piece 35 is an enclosed folded member, as seen in FIG. 5, and contains therein spaced permanent magnet blocks 36,36. The free tip of armature arm 31 extends centrally in the space between the magnets, forming air gaps therewith.
The described and illustrated configuration of armature 31,32 enables it to readily flex through a major portion of its length. Its material is of relatively high permeability, such as an iron-nickel alloy containing approximately 75% nickel for microphone applications. This provides optimum ac permeance and consequent good generated voltage. A small bar 37 is welded to the armature across the outer section of its fold. The bar 37 is adjacent a rectangular opening of the casing 40. By inserting a small tool the air gaps between the tip of arm 31 and the permanent magnets 36,36 are readily and delicately adjusted. This is accomplished by slight displacements of the bar 37, which in turn sets the armature. Stable gap adjustments of the order of one ten-thousandth of an inch are feasible thereby, for maximum efficiency and output, and minimum dc flux saturation in the armature.
The pole piece 35 is bent-up and folded out of a flat form. The pole piece is made of high-permeability material. The exemplary component 35 is of 0.016 inch thick metal, and of unitary construction. The rear section of the pole piece is formed into a generally rectangular enclosure in band-form.
The permanent magnets 36,36 are of rectangular cross-section, and good permeance and dc coercive force. We have found that Alnico VIII and Lodex-32 magnets are suitable, in view of the small bulk required and their minimal ac reluctance characteristic. The magnets 36,36 are secured to the pole piece 35 by a polyester cement as epoxy, or by solder. The pole piece 35 has two parallel lateral extensions 38,38 along the sides of the assembly. These extensions serve as low reluctance ac and dc flux paths for the armature 31,32 and coil 33 magnetic array.
The coil 33 is supported in the open region between the lateral legs 38,38. Coil 33 is suitably cemented to the interior of the legs 38,38 as with epoxy spots. The armature 31,32, pole piece 35 and coil 33 are thus precisely, stably, and ruggedly assembled as a unitary mechanical and magnetic device. The pole piece 35 is fitted into case 40, touching its side walls and bottom for mechanical stability and magnetic coaction therewith. The side walls of the pole piece are made flush or contiguous with the adjacent side walls of the can or casing 40 and suitably cemented to the case in such relation. The cover 41 abuts edge to edge with case 40, and is suitably secured in place. Cover 41 may of course overlap the case with a snug fit. The casing and cover are preferably of high-permeability material.
The vibratory weight assembly 45 is independently mounted in container or can 40. In the illustrated embodiment two lateral brackets 43,43 are welded to the end walls of the can, see FIGS. 2 and 3. The upper shelf of each bracket 43 is at a predetermined height above the base of the can, and supports assembly 45 parallel to armature 31,32 as illustrated. Its position in the transducer is to afford compactness, yet avoid contact between the unit 45 and cover 41 during its vibratory excursions.
The weight assembly 45 is basically constructed of three parts: diaphragm 46; block 47; and frame 48. The frame 48 seats on end brackets 43,43; its longer sides fitting against the side walls of can 40 for mechanical stability. The diaphragm 46 is essentially of rectangular form with a central square region 50 to which block 47 is secured with solder or epoxy cement. The tips 51,51 of the diaphragm are bent-over, span frame 48, and seat in end recesses 52,52 thereof. Tips 51,51 are soldered to the frame edges to form a firm assembly therewith. The central regions 53,53 of both arms of diaphragm 46 have shallow corrugations that facilitate control of its flexibility. A suitable material for diaphragm 46 is a beryllium alloy, as "Berylco 25", 0.004 inch thick. The weight-block 47 of the exemplary transducer 25 is of lead 0.120 inch .times. 0.120 .times. 0.016 inch, with a central aperture 0.030 inch in diameter. Diaphragm 46 has a corresponding central aperture 54.
FIGS. 2 and 3 are drawn substantially to the scale of the size of the respective assemblies, component parts, and their compact arrangement in exemplary transducer 25. The block-mass 47 is arranged to freely vibrate, up-and-down as seen in FIG. 3, without mechanical contact with the frame 48, the cover 41, or the armature 32, the order of 40 milligrams. The effective mass of the weight-block 40 plus the central vibratory portion 50 of the diaphragm 45 and the armature arm 31 is of the order of 60 milligrams, for a resonance herein in the 2,500-3,000 Hz range. The weight of the exemplary transducer 25 is 0.67 gram. The ratio of the vibratory mass 31, 47, 50 to the motor 30-housing 40,41 mass is thus about 0.11. Such relation, being much less than unity, provides a steady roll-off of high frequency response, as described hereinafter in connection with FIG. 8. The ratios thereof herein contemplated, namely that of the mass of the weight-block together with the elements directly vibratory therewith (of the diaphragm where used, and of the armature arm) to the mass of the remainder of the transducer, are in the range of 0.05 to 0.35 to afford the effective-transducer results herein stated. The resonant or peak frequency varies according to this ratio, being higher as this ratio is lower.
An important feature of the present invention, as aforesaid, is to provide an inertial reaction transducer with desirable characteristic frequency response; without spurious resonances beyond the desired peak frequency (f.sub.o); with ready control of the position of the resonant frequency (f.sub.o) in the response of any given transducer; and with ready manufacture of the desired transducer response. The use of the separately mounted separate mass 47, in resilient array on a diaphragm 46, of coordinated predetermined mass-compliance relation, which mass-diaphragm is mechanically linked to the vibratory armature 31 of an electromagnetic motor 30 in a common housing, combine to provide the aforesaid advantageous results as a lightweight compact unit 25. The mass-diaphragm 46,47 is connected to the armature arm 31 of the motor 30 with a connecting rod or drive pin 55.
The rod 55 is secured with epoxy cement to arm 31 at an aperture thereof, as seen in FIG. 3; and to the block 47 at its central aperture which is located directly above. Rod or pin 55 is thus arranged substantially perpendicularly to arm 31 and to block 47, for faithful mechanical interaction. When transducer 25 is used as a contact microphone, in the manner of FIGS. 1 and 1-A described hereinabove, the housing 40,41-and-connected motor assembly 30 are vibrated by the head. The weight 47 remains relatively stationary and thereby actuates the magnetic system. The weight 47 being directly linked to the armature arm 31 holds the latter stationary as well relative to the remainder of the motor assembly 30 that is vibrated by the head.
The magnetic translation of these relative movements between the arm 31 and the magnetic system including magnets 36,36 generates a signal voltage in correspondence therewith in coil 33. This signal appears at terminals 42,42. The relatively small size and weight of transducer 25 greatly contributes to the resultant good sound reproduction on the head, with low noise pickup. A miniaturized amplifier is used to raise the output of the microphone 25 to any desired level.
Conversely when the transducer is used as a receiver, the sound signals are impressed electrically at terminals 42,42 which in turn correspondingly vibrates armature arm 31, in typical motor response. The receiver construction is similar to that for the microphone 25, with improved results by making the armature 31,32 of nickel-iron alloy with 50% nickel content. Arm 31 directly actuates mass 47 through drive pin 55, and thereby establishes an inertial reaction response that vibrates the motor-housing 30,40, in turn evidenced as an external vibration of the transducer. Faithful translation of speech frequencies is provided by the transducer 25 hereof, in either its microphone or receiver mode. The exemplary unit of 0.67 grams is 0.440 inch .times. 0.220 .times. 0.180 inch; it being understood that different dimensions and weights are intended within the scope of the principles and practice of this invention.
FIG. 6 is a longitudinal cross-sectional view through a modified form of transducer 60 embodying the principles of the present invention. The basic motor unit is similar to that of transducer 25, comprising a vibratory armature arm 61 extending between spaced permanent magnets 63,63 set in pole piece 64,65. The armature is U-shaped, with its upper arm 62 secured to pole piece section 64. The coil 66 surrounds vibratory arm 61 near its bend portion. The housing or can 67,68 of good permeability material closely surrounds the motor assembly.
The distinguishing feature of transducer 60 is the securement of a mass-block 70 to an extension 71 of armature arm 61 in an unobstructed region 72 of the housing. The vibratory-mass relationship of the armature-weight 61,70 combination to the remainder of the transducer 60 is similar in principle and result to that of transducer 25 as described hereinabove. The preselection of the mass (and size) of block 70 for the desired resonance frequency response of the transducer is readily accomplished therein, and secured by solder or epoxy to armature tip 71, in any suitable manner. The optimum setting of arm 61 between magnets 63,63 is accomplished through bar 73 as aforesaid.
The transducer 60 can be made in more compact size and with fewer parts than transducer 25, as will be apparent to those skilled in the art. Their respective performance is the same for equivalent constructional parameters. FIG. 8 is a typical output response curve of transducers as 25 and 60, used as a contact microphone. There is a rising characteristic from 300 to 2,400 Hz peak (f.sub.o) with approximately 9 db rise per octave. The speech intelligibility of such response is found to be very good. It is to be realized that this response is by mere contact of the transducer on the top of the head as shown in FIGS. 1 and 1A, where attenuation of the higher frequencies is rather pronounced. The rising frequency characteristic of FIG. 8, coupled with the steady roll-off from the peak frequency (f.sub.o) to 3,800 Hz results in excellent inherent speech pick-up with very low accompanying ambient noise.
While the aforesaid transducers 25 and 60 were described as head contact microphones or receivers, other applications therefor are practical. An important alternate use thereof has been found as direct speech actuated units, through air-conducted sound vibrations. For optimum operation, the transducer is placed in a pliant mounting in front of the mouth, as in an oxygen mask. The minute vibrations thus imparted to the transducer directly by one's voice sets up the inertial reaction counterpart and generates significant electrical output signals in a manner similar to the above stated manner by contact vibrations.
FIG. 7 illustrates an exemplary lead strain-relief arrangement for the relatively tiny transducers hereof. The internal coil leads 76,77 of transducer 75 are soldered to their terminals 78,79 external thereof. Towards this end a pair of eyelets 80,81 are affixed in opposed apertures in the housing 82. The ends of leads 83,84 are soldered to the respective terminals 78,79 and are positioned on opposite sides of the housing. They are thereupon passed in opposite directions through the eyelets 80,81. A dab of RTV self-curing rubber 85,86 affixes each lead 83,84 to its eyelet, as shown. The rounded openings of the eyelets provides a smooth anchorage for the leads. Their rubber seals 85,86 also serves as a moisture seal for the transducer 75. A smaller third eyelet 87 serves as the housing ground connection. One of the leads, as 83, may be connected to it by solder 88 thereto, or a separate ground lead used if so desired.
Although the present invention has been described with exemplary embodiments and applications, it is to be understood that variations thereof are feasible within the broader spirit and scope of the invention, and that it is not intended to be limited except as set forth in the following claims.
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