US2121781A - Sound translating device - Google Patents

Description

June 28, 1938. .,s. BALLANTINE SOUND TRANSLATING DEVICE` 2 Sheets-Sheet l Filed April 1:5, 19:56 .Y

June 28, 1938. s. BALLANTINE A soUND TRANSLATING DEVICE Filed April l5, 1936 2 Sheets-Sheet 2 SENS xumiwmil CODE QOQM., QQO- COW 1 I llaltenfed'Jue I. y' Y i. l

',.UNlTi-:D-sTATi-:s PATENTor-'FICE A I com raszgna A 9 (ci. 11a-114) This inventionerelates to apparatus for trans- Fig. 12 is a vlongitudinal section through a lating mechanical vibrations of the body in the transducer which includes a dierent form of vi-v region of the throat due to the voice into articul y bratory armature; and

late speechy sounds or into electrical currents. Fig. ,13 is a fragmentary circuit diagram of a which yield articulate speech sounds when reprocomplete electrical system which embodies the 5 duced. A -invention.

Apparatus and methods for this general puri 'I'he microphone is adapted to be worn around posehavebeen described in my copending applithe neck like a collar, as shown in Fig. 1 of my cations Ser. Nos. 6,245 and 6,246, filed Feb. 12, copending application Ser. No. 6,246, but other v 1935; Ser. No. 8,392. flied Feb. 26, 1935, and Ser. methods of support may be employed. When 10 No. 54,347,1l1ed December 13, 1935. l i worn as a collar, the .microphone M has the geni An object of the invention is to provide a device eral fOrm 0f a narrow band 0r casing in which A l of novel construction for converting acoustical two mediano-electrical transducers are symmetvibrations of the body due to the voice into elecrically arranged, and an adjustable elastic band 15 tric currents for transmission in an electrical B is sewed to the casing toV form a collar that may 1s communication'system. An object is to provide'A be placed around theV neck in such a way that a microphone, of the character stated, which the microphone units are in contact with the neck operates on the electromagnetic principleand in the region of the larynx. which therefore does not requirethe battery or- The. covering or elongated casing I for the dinarily used with microphones of the carbon transducer unit or units may be of any suitable 20 granule type, material, such as-glove leather, with a snap faslFurther objects are to provide a larynx microtener 2 and fibre stiffener 3 fastened to one end phone which is characterized by 'a simple and of the casing. The side of the microphone which rugged mechanical construction and by an eleccontacts with the neck 'will be designated, for

l 't1-ical oui-,put which compensates, in part atleast, convenience of description., as the front of the 25 for the deficiency of the higher frequency constitmicrophone. The front wall of the casing I is uents in the mechanical vibrations or the larynx. provided with two holes which are covered by a These and other objects and advantages of the thin membranous material l which may be ceinvention will be vapparent from the following mented to the front leather wall, and the actu` 1 specincation whentaken with the accompanying ating buttons 5 of the microphone units extend 3o drawings in which: Y through these holes. 'Not all materials are suit- Fig. 1 is a sectional view of an embodiment of able for the membrane I on account of attenuf the invention; ation of the higher frequency vibrations. Ma-

Figs. 2 and 3 are front and rear views, respecterials which have been used successfully are thin tively, with parts brokenaway, of one of the rubber, animal intestinal linings which may be 35 transducer units; Y grouped under the name of colon leather, and Y Fig.` 4 is a schematic view of the wiring confordinary leather of the order 'ci .005" to .015" nections and shielding of the microphone shown thickness. Thinglove leather has been employed in Fig. 1; f in most of my constructions becauseof its good 40 Figs. 5a and 5b are corresponding mechanical mechanical qualities and because its physical ap-` 40 and electrical impedance diagrams; pearance can be made to match the rest of the Fig. 6 is a curve sheet showing typical output leather covering. This is cemented to the covvoltage-frequency characteristics which may be ering I with rubber cement and extends over Vthe obtainedvwith embodiments of the invention; entire area of the side toward the'neck. In order Fig.7 isa front view of another transducer conto reduce the strain on the .covering I and the `4,5

struction which is particularly adapted for oil thin protective covering 4', a thin metal grommet damping; f '8 is sewed and/or cemented around the edge of IT'ig. 8.is a longitudinal section on line 8 8 of the openings in the front wall I. "Fig 7; l The electromagnetic transducer units may be,

A 5o Fig. 9 is a central transverse sectioni and V,preferably are, fof substantially identical 50 Fig. 10 is a longitudinal section through construction.A The motion transmitting button transducer which includes adjustableY elements 5 is of substantiallyrigid material, such as bakefor controlling the air gap; i lite, balsa wood,vwhite pine, etc., andis fastened v Hg.11isasectionalviewofatransducerwhich. Vtothe middle of the vibratoty elementl. ,This

includes another form'of magnet structure; vibratory element isa resilient strip ,of` mag-55 are so chosen that the armature is mechanically resonant at the upper end of the range of speech Y netic material, such as silicon steel, which may have metallic pieces 8, 8' soldered at the ends thereof to assist in dening the eifective length of the vibratory strip or amature, thus rendering the resonant frequency substantially independent of the vicissitudes of` assembly, screw pressure, etc. The end pieces are not essential but, when used, are usually somewhat thicker than the vibratory strip and partake of its vibrations only to a minor extent. The magnetic strip'1 bridges across the ends of a at E-shaped magnetic yoke that, as shown in Figs. 1 and 2, comprises end pole pieces 9, 9' on a backing strip I which carries a central and somewhat shorter pole piece I0'. The pole pieces 9, 9 are permanent magnets of suitable material such as cobalt steel, aluminum-iron steel or the like which are magnetized, as indicated by the polar desigv nations, N, S, to present like polesto the vibratory armature. The back piece or strip I0 is of ,iron having a lowvariational reluctance, such as pure annealed or Swedish iron. A small air gap of the order of from 0.001 to 0.010 inch is provided between the pole piece I0' and the vibratory strip 1, and a multi-turn coil of insulated wire Il surrounds this pole and has its leads brought out through terminal pins I2 that pass through insulating bushings I2' which are held in the back piece I0 of `the magnetic yoke The vibrations. of the throat are picked up by the button 5 and communicated to the amature 1 and, due to the mass of the transducer unit which prevents it from vibrating as a Whole, the forces imparted to the armature 1 result in a vibration of the amature with respectto the magnetic eld system. The air gap is therefore periodically altered in conformity with the vibrations and the variations influx through the central pole I0' inducevan electromotive force in the'coil Ii which may be led to the. external electrical circuits, ampliiiers, line, etc. Due to the polarity of the permanent magnets, their. flux adds up in the central pole and the two halves of the symmetrical magnetic circuit may be regardedas in parallel. Y

The thickness of the resilient strip 1, and its length between the edges of the end pieces 8, 8',

frequencies to be reproduced.V Good results have Y at its ends varies as t/l2, where t is the .thickness and Zvthe'length, and is' independent of the width to the first order. However, no rigorous rule can be given for the calculation of the resonant frequency on account ofA thepresence of the buttonv ,5 whichtends to lower the frequency by adding mass and to increase it by virtue of the added resistance `to bending caused by the' cementing vof the lower planar surface of the button to the armature. The above rule does furnisha good practical guide, however, andthe exactdiinen` sions can be determined experimentally. Typical dimensions which Ihave employed with lsilicon 1 steel for a resonant 'frequency in the neighborhood of 3500 cycles are l==0.8 inch, t=0.014 inch,

I andwidth varying from 0.25 to 0.6 inch. Al-

though the resonant frequency is not greatly affected by variations in the width of the arma.-

ture, theV width does affect the mechanical impedance as Vviewed from the'button 5 so that 2,121,7s1 l c by varying this dimension the impedance of the transducer can be regulated inrelation to that of the larynx with which it is in contact. lWith a length and thickness as given above', widths of from 0.2 to 0.7 inch are suitable for the average larynx. 1 l

The air gap may be adjusted by screws I8 which pass through the resilient strip 1 and into the back piece I0, but such an adjustment Y steel or other material which will stand repeated bendings. `The unit itself is protected from external elds by a close tting metallic c asing I1 which has `a `slightly domed bottom wall to provide space for the terminal connections and the ends of springs I5. The connecting cords branch from the main cable at the middle of the microphone strap, see Fig. 4, the shielding lead or braided conductor I8 being connected to the neg- ,ative terminal pin I2 of one unit and to the shield springs I5 of both units by a T-shaped metallic piece I9 that fits around the bifurcation of the cable and prevents relative movement loi the cable and the branch connections. The central conductor 20 of the cable is .connected to the positive terminal of the other unit by one lead Y I4, and the unlike terminals of the two units t The transducer units may be protected against v the weather and sea spray when used in open cockpit airplanes and seaplanes by placing the units in closely tting bags of thin rubber havin g a thickness of the order of 0.002 to 0.005 inch.

v u The open ends of the bags are cemented together beenv obtained by making the armature resovnant at frequencies above'1000 cycles and, in

scribed in my copending applications Ser. Nos. ,i

v 6,246 and 54,347. A mechanical impedance dla'- gram of the microphone is shown in Fig. 5a, and an approximate electrical circuit lequivalent of the mechanical system is shown in Fig. 5b. -In these diagrams, vo represents the impressed velocity due to the vibrations of the larynx, Zu represents the mechanical impedance of the throat structure (cartilage, muscle, etc.), and m1 represents the mass of the button 5 and the eiiective mass of the armature 1. C1 represents the compliance of the armature, that is the reciprocal of the deflection per unit force applied between the button and the transducer unit, C: represents the compliance due tothe elastic band which supports the unit at the neck, and ma represents the mass of the unit, the housing and the covering. AThe resistors r1, rn, ra represent the eiiects of disthe neck, Zo.

sipation in the several associated elements. The compliance C: is probably so high as to be negligible, that is Ca can probably be short-circuited (omitted) from consideration for all practical purposes.

The voltage generated by the microphone is proportional to the velocity of the armature with respect to the other elements of the unit, and is represented in Fig.'5b as v1, the current flowing in the C1 branch. 'Ihe form of the vfrequency response characteristic with constant impressed velocity depends upon the driving impedance of Ifa piston vvibrating at constant velocity be used for driving purposes, a curve of the type shown in Fig. 6 is obtained for the described unit, or pair of units. This curve is chiefly characterized by a resonant rise in output near 300 cycles and another near 3500 cycles. The first is due -principally to resonance between the armature compliance Ci and the mass ma of the unit. The upper -resonance point at 3500 cycles arises principally from resonance between the mass of the armature m1 and the compliance C1. The exact shape of the characteristic curve may be controlled by adjustment" of the mechanical.

impedances. The low frequency response, for example below 1000 cycles, may be controlled by the lower resonance. This, in turn, can be controlled by the mass me of the u nit and the armature compliance C1. The latter can be modified without varying the upper resonance byvarying the-Width of the armature. The lower resonance peak can thus be moved up and down in frequency to secure an augmented response at any desired frequency, with a sharp decrease in response below that frequency, as shown by the solid line curve A of Fig. 6. This particular curve represents the performance of a microphone having the following characteristics; armature 'i of silicon steel, 0.8 x- 0.5 x 0.014 inch; button, white pine, mass= 0.15 gram; mass of unit=15 grams.

'I'he upper resonance can be controlled, as dea scribed above, by varying the length and thickness of the armatup It is not always desirable to have a high sharpV peak, such as is present in` curve A at about 3500 cycles, and this can bev avoided by providing damping in addition to that arising from the internal viscosity of the metal and the cement used to fasten the button to the amature. One of the simplest and most eil'ective methods of damping is to insert a drop or two oi oil in the air gap between the Apole piece lli' and the armature 1. Dotted line curve A' of Fig. 6 illustrates the effect of a drop of oil, of

viscosity 30 in the standard commercial scale for automotive lubricating oils, inserted in the air gap. The oil is permanently held inplace in the air gap by capillary attraction.

Another transducerconstruction which is par.- ticularly adapted for oil damping is shown in Figs. 7 to 9.- The general construction isas previously described and parts which are, or may be, sub- .stantiaily identical with the elements of Figs. 1,v

2 and 3 are identified by corresponding vreference numerals. A cover plate 2| is arranged parallel to the armature 'l and spaced therefrom by a few thousandths of aninch by shims 22, 22', the plate having a central hole through Awhich the button 5 projects. Damping material 23 is placed in the space between the plate 2| and armature and this is preferably oil ybut may be cotton, cloth soaked inV oil, rubber, felt, glyptal, etc. When oil is used, it is held in `place by capillary attraction. The cover plate 2| isxpreferably metallic, about if inch'thick, and it presents the advantage annif/s1 the space below the .button section,

even when used without oil or other damping material of protectingv the armature and acting as an acoustical shield to prevent vrind and external acoustical 4disturbances from reaching the I armature. A y5 In the embodiment shown in Fig. 10, van annular piece 24 of rubber or other resilient` material is placed between the armature land the cover piece 2|, `and the central pole piece I0 is drilledshv and threaded to receive a cylindrical plug 25 of 10 resilient material, such as rubber or glyptal, and an adjusting screw 26. .Pressure applied through the plug 26 tends to bend the armature upward, and this is resisted by the ring 24. By altering the dimensions of the ring 24 and the 15 position of the screw 26, some control of the length of the air gap is obtained. The upper ringV 24 may be omitted if the armature 1 is given an initial adjustment to lie against the pole piece l0', the stiifness of the armature itself being then` 20 relied upon to oppose the thrust of the plug 25 as the screw 26 is adjusted to obtain the desired air gap. Damping may be obtained, as previously described, by materials placed in the space between armature and cover piece 2|, or by the 25 viscosity of the resilient materials 24. 25.

Another method o f damping is to construct the armature in laminar form, using a strip of damping material between two resilient strips of 4metal. The strips may be pressed or cemented'30 into an integral whole which may be mounted in the same way as a solid iron armature.

The magnetic systems so far described have comprised" a pair of short bar magnets 8, 9

mounted on the ends of a back piece having an 35 integral central pole piece Ill'. As shown in Fig. 11, the back piece Ill may be a singlepermanent magnet having an inserted pole piece I0", with short bars of soft iron'9 at the ends of the bar magnet. The magnet. lilEl is magnetized in, a Jig 40 Acoercive force with tungsten steel in place of cobalt and like steels.y The separate soft iron end 50 pieces 9* can bejreplaced by integral extensions of the bar magnet HP.

Fig. 12 illustrates another structure which `offers certain advantages. In this construction, the button and armature are formed in one piece, 55 the button being pressed out as a boss on the y armature strip 1. This avoids the possibility of the button becoming loosened. Also by utilizing a greater number of turns can be .accommodated on the oo coil Il', thus increasing the voltage output. A method of adjusting the air-gap is also shown which may be applied to the other constructions .as well. This is accomplished by sliding the central pole vpiece |0 through the back plate IIId 65 A complete electrical system incorporating the V microphone is shown schematically in Fig. 13.

Only one unit is shown as the microphone M, and this works into an am'pliiier 21 of the type which is described in detail lin copending application Ser.

No. 54.347; the circuit diagramfappearlng as Fig. I"

, tions of the body due to the voice, said microphone K comprising a permanent magnet, a yoke and pole 4 of that application and characteristic as Fig. 5. The overall frequency response of the microphone and amplier, when the microphone unit is clamped and driven by a constant velocity piston, is shown in Fig'. 9 of the earlier application. The amplifier 28 Vmay work into a radio transmitter or, as shown. into a transformer 28 and headphones 20.

I am aware that microphones operating on the electromagnetic principle were proposed as early as 1884, but the constructions herein described have novel mechanical and'electrical characteristics which particularly adapt this type of microphone for use as a throat microphone. It is to be understood that the invention is not limited to the exact constructions herein illustrated its frequency response and described as many variations which will oc- 1 art fall within cur to those familiar with this the spirit of the invention as set forth in the following claims.

I claim:

1. An electromagnetic transducer comprising an E-shaped ileld magnet structure magnetized to have end poles of one polarity and an inter# mediate pole of the opposite polarity; an armature consisting of a resilient strip of magnetic material extending between said like end poles and over but spaced from the intermediate pole, a coil on the intermediate pole, and means consisting of a fluid medium between said resilient strip and a rigid member for damping the vibrations of said resilient strip.

2. An electromagnetic transducer as claimed in claim 1, wherein said fluid medium of the damping means is in the, gap between said resilient strip and the intermediate pole.

3. An electromagnetic transducer as claimed in claim 1, wherein the rigid member of said damping means comprises a relatively rigid cover plate extending in spaced relation over said resiiient strip.

4. A throat microphone of the electromagnetic type adapted to be actuated by mechanical vibrapiece, a coil surrounding said pole piece, a resilient magnetic strip positionedto complete the magnetic circuit except for asmall air-gap between said strip and said pole piece, said resilient magnetic strip being mechanically resonant above 1500 cycles. and means for transmitting vibrations of the throat to said resilient strip.

V5. A throat microphone of the electromagnetic type comprising a permanent magnet, a yoke and pole piece, a coil surrounding Asaid pole piece, a resilient magnetic strip positioned to complete the magnetic circuit except for a. small air-gap between said strip Aand said pole piece, said re siiient magnetic strip being mechanically resonant between 2500 and 4000 cycles, and means for transmitting vibrations of the throat to said resilient strip.- v

6. A throat microphone of the electromagnetic type comprising a permanent magnet, a yoke and pole piece, a coil surrounding said pole piece, a resilient magnetic strip .positioned to complete the magnetic circuit except for a small air-gap between said strip and said polelpiece, said resiiient magnetic strip being mechanically resonant above 1500 cycles, means for critically damping the vibrations of said resilient strip, and means for transmitting vibrations of the throat to said resilient strip.

l 7. In a contact microphone adapted to translate vibrations of the throat dueto the voice and of the ,type comprising a vibration element having compliance and mass. and Vsupporting means.

having mass; the method of reducing frequency distortion due to the mechanical vibratory char-r acteristics of the throat which comprises tuning the natural vibration frequency due to the cooperation of the mass of said supporting means and the compliance of said vibrating element to a frequency between and 1000 cycles, and simultaneously tuning the natural frequency due to the cooperation of the mass of said vibrating element and its compliance to a frequency above 1500 cycles.

8. A throat microphone comprising a resilient vibration member to which the vibrations of the throat are transmitted, and means for supporting said vibration member on the throat, the comrpliance of said resilient vibration member being resonant with the mass of said supporting means at a frequency ofthe order of between 100 and 1000 cycles, and said resilient vibration member being resonant at a frequency above 1500 cycles.

9. A throat microphone as claimed in claim 8 in combination with means damping said vibration member at said resonant frequency above 1500 cycles.

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