US2227580A

US2227580A - Acoustic device

Info

Publication number: US2227580A
Application number: US216684A
Authority: US
Inventors: William R Harry; Robert N Marshall
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1938-06-30
Filing date: 1938-06-30
Publication date: 1941-01-07
Anticipated expiration: 1958-01-07
Also published as: GB530483A; FR856854A

Description

Jan. 7, 1941. w, R, HARRY ETAL 2,227,580

ACOUSTIC DEVICE Filed June 50, 1958 2 Sheets-Sheet 1 INVENTORS W. R. HARRY By R.N. MARSHALL Qhaemjrim ATTORNEY Jan. 7, w R HARRY E AL ACOUSTIC DEVICE Filed June 30, 193B 2 Sheets-Sheet 2 RIBBON 54 TRANSMITTER a UNIT v72 ER u/wr 7/ m R. HARRY W' ff RMMARSHALL Qua-m 6. Mud.

ATTORNEY atentecl Jan. 7, 1941 STATS Application lune 3d, 193d, derial lilo. flddtt l3 blalins.

'lfhis invention relates to acoustic devices and more particularly to acoustic transmitters having a higher responselevel to sound from a lim ited range of directions than to sound from outi side of this range.

Une object of this invention is to translate sound waves into electrical energy with high emciency and fidelity throhout a wide range of frequencies.

Another object of this invention is to obtain, for an acoustic transmitter, a substantially unidirectional response characteristic over a very wide range of frequencies.

Another object of this invention is to reduce or substantially minimise diffraction effects in acoustic transmitters.

a further object of this invention is to overcome objectionable phase shift effects in an acoustic transmitter including a pair of microphone units of different types.

Still another object of this invention is to reducethe sire of composite acoustic transmitters and to produce a compact easily fabricable struc ture.

in one illustrative embodiment of this inven tion, an acoustic transmitter comprises a pres-- sure gradient microphone unit, commonly known as a velocity microphone, and a pressure micro phone unit, which may be of the moving coil type. the pressure gradient unit, wherein both sides of the diaphragm are exposed to the sound held, is bidirectional in character from the standpoint of response. The pressure unit, only one surface of the diaphragm of which is erposed to the sound field, is substantially non-directional throughout an errtendcd portion of the low frequency range, example, up to frequencies of the order of three thousand cycles, and is fairly unidirectional at higher frequencies. Means including a charmber or cavity are provided at the rear of the diaphragm of the pressure unit to produce an impedance, primarily resistive, to obtain a uniform response over a wide frequency range, the cham-- her or cavity being of such character that its up stiffness reactance is substantially negligible as compared with the resistance, throughout the frequency range of interest.

in accordance with one feature of this invention, the pressure gradient and pressure unit are so coordinated that in cooperation they define an acoustic transmitter having a uniform response characteristic throughout a desired limtill till

ited range of sound wave directions. More specifically the two units are so coordinated that a submy stantially unidirectional response effect obtains,

fill. lid-ll the response being a maximum to sound waves emanating from a source directly in front of the composite transmitter and zero to waves emanat ing from a source directly behind the transmitter. The response to sources at intermediate points is such that the curve indicative of the response of the transmitter, determined by a plot of response against angle, is a cardioid. In order that the desired characteristics obtain, the relative phase and magnitude of the voltages produced by the two units in accordance with sound waves are in a particular relation throughout the frequency range to be translated.

in accordance with another feature of the invention, the exterior surfaces of the transmitter elements are made generally convex so that diffraction edects on sound waves approaching either diaphragm are minimized.

in accordance with a further feature of this invention means are provided for producing e ualiaation of the combined responses of the two microphone units throughout the range of fro uencies to be translated whereby a uniform substantialiy unidirectional. response throughout this range is obtained.

in accordance with still another feature of this invention, the equalizing means are ineluded within the impedance chamber or cavity to the rear of the diaplriragm of the pressure unit whereby a relatively simple, compact structure of small size is achieved.

The invention and the foregoing and other features thereof will he understood more'clearly and fully from the following detailed description with reference to the accompanying drawings in which:

Fig. l is a side view of a transmitter illustrative of one embodiment of this invention, portions being shown in section and portions of the outer shield or casing being broken away;

Fig. 2 is a front view of the transmitter with the front portion of the shield or casing removed to show the internal structure;

Fig. 3 is a perspective view of the transmitter units included in the transmitter shown in Figs. l and 2;

Fig. a is a circuit diagram illustrating the electrical association of the various elements of the transmitter; and

Fig. 5 is a view in section, along line 5--t of Fig. i, illustrating details of the microphone units.

Referring now to the drawings, the transmitter there illustrated comprises a perforated shield or casing having front and back portions H) and l l, respectively, and pivotally mounted on a yoke if till extending from a suitable standard, a portion of which is shown at I3. Mounted within the shield or casing is a support or bracket member |5 having oppositely extending arms I6 and I1 .and an angular portion l8. The arms l8 and I1 have bent orificed end portions for attachment to the yoke |2 by suitable fastening means, such as bolts l9 and 20. The shield portion H is secured to the support or bracket member |5 by a screw or equivalent means 2|. The shield is also supported by the fastening means l9 and 20 in cooperation with the yoke I2 and the bracket arms I6 and I1. Two

bands

22 and 23 of metal or other suitable material may extend around the outside of the screen to strengthen it.

The pressure microphone may be a moving coil unit including a diaphragm 24, a magnet assembly 80 and a driving coil 8| mounted in the front portion 25 of a casing 26. The driving coil 8| is located in an air-gap 82. An acoustic resistance comprising a ring 83, having perforations 84, covered with a fabric 85, is mounted behind the air-gap 82, as may be seen in Fig. 5. The back portion 21 of casing 26 is secured to the front 25 by fastening means, such as screws 28. The moving coil unit is tightly secured against a shoulder in an opening 29 by a screw 30 threaded through the back portion 21 of the casing 26 and bearing against the unit.

The casing 26 is substantially air-tight except for connection to the atmosphere through the low frequency equalizing means. This means is shown as a resilient tube 31, which may be of rubber. One end of tube 31 is open to the outside air through a fitting 38 and the other end communicates with the interior of the casing 26.

The moving coil unit has a high mechanical impedance and, therefore, requires but a small cavity or chamber to the rear of the diaphragm to provide a terminating impedance of the magnitude requisite to assure a uniform response. This impedance, as noted heretofore, is primarily resistive, the cavity being of such size and. character that the stiffness reactance is substantially negligible in comparison with the resistance throughout the desired range of frequencies. The acoustic impedance of the moving coil unit is determined principally by the cavity or chamber mentioned and the tube 31.

A transformer 3| is secured, by screws or similar means (not shown), to a pair of parallel ribs extending from the back wall of the casing portion 21. One of these ribs is shown at 32 in Fig. 1. 34 are held in place by a securing means, such as a clip attached to the transformer and having arms 35 and 36.

The ribbon velocity microphone may comprise a pair of U-shaped

permanent magnets

39 and 43, such as shown in Fig. 3. These magnets, which may be of substantially circular section, may be arranged approximately in parallel relation to each other and at substantially right angles to pole-pieces 4| and 42. The pole-pieces have

parallel portions

43 and 44 forming an airgap for a ribbon diaphragm 49, .and ears or

projections

45, 46, 41 and 48 for attachment to the pole portions of

magnets

39 and 40. The pole pieces may be attached to the magnets by suitable means such as pins 50 or by welding. The ribbon diaphragm 49 is mounted between the pole-piece members by support means 5| and 52, and may be transversely corrugated for only a small portion near its ends. The part between the corrugations is stiffened by virtue of its An inductance coil 33 and a condensertransverse curvature, as shown in Fig. 5. The material from which the ribbon is made may be any suitable conductor material capable of being worked to a very thin section. A preferred ma-. terial is aluminum foil of about .00025 inch thickness. If desired, the ribbon may be transversely corrugated near one end only. Such a ribbon may be substituted for the one previously described and give substantially equivalent performance. Ribbons oi. the shape and thickness described reduce wind disturbances considerably. It is possible to employ successfully a ribbon having a greater stiffness than is ordinarily found in microphone ribbons because of the use of the low frequency equalizing means to be later described.

To maintain the magnets 39 and in spaced relation and to strengthen the structure a plurality of rods 53 .are provided. These may be secured to the magnet in any suitable manner as by welding, riveting and the like.

The casing 26 is formed with a recessed portion 54 which fits around magnet 40 whereby a very compact composite structure is obtained. he diaphragms of the two microphone units are thereby located close together, thus minimizing phase difficulties.

The bracket member |5 has its main portion fastened to the bottom of casing 26 by screws, one of which is shown at 55 in Fig. 1. The angular portion I8 is secured to the back part 21 of casing 26 by a screw 56. A portion of magnet 4|] is clamped between bracket 5 and recessed portion 54 of casing 26. The two microphone units and the bracket member are thereby firmly secured together.

It will be noted, particularly from Figs. 3 and 5, that the exterior contour of the elements of the units is generally convex, with no sharp corners nor edges. 'Although these surfaces in the form illustrated are generally curved, they may comprise a plurality of substantially plane intersecting surfaces provided the angles of intersection are greater than about 120 degrees. This avoidance of sharp comers and edges greatly reduces difiraction effects on sound waves approaching either microphone unit.

A circuit whereby'the parts of the transmitter may be interconnected is shown in Fig. 4. The parts enclosed in broken lines are those of the assembled transmitter having output terminals 60 and 6|. The ribbon microphone unit is connected to the primary 62 of the three-winding transformer 3|. The inductance coil 33 of Fig. 1 is shown in the wiring diagram as inductance 64 and resistance 65 connected across the ribbon unit. In the device 33 the resistance may be incorporated in the winding. The condenser 34 is connected across the tertiary winding 61 of transformer 3|. The resistance 68 is that of the winding 61. The moving coil microphone is connected to terminals 60 and 6| in series with the secondary winding 68. Switches 69, 10 and 1| may be employed for a purpose to be described.

A plurality of taps 12 are shown on the secondary winding 68. These may be employed, if necessary, to obtain a substantially exact output voltage match between the microphone units.

The terminals 60 and 6| of the transmitter are connected by way of a transformer 13 to suitable amplifier means represen-ted by its first tube 14.

The impedance 33 (inductance64 and resistance 65) which is shunted across the ribbon microphone unit is a low frequency damping means coil device are so adjusted that the relative relid phone impedance.

tility of the device.

spouses are substantially equal at low frequencies.

The network: connected to the tertiary winding til of transformer Si is for high frequency equali nation. The effect of this network is to make the ribbon microphone response correspond close ly to that of the moving coil unit up to about three thousand cycles-per second. For frequenciesabove three thousand cycles per second, the networlr acts as a filter to gradually cut out the ribbon unit as the frequency increases. Since the moving coil unit becomes increasingly directive above three thousand cycles per second, the unidirectional characteristic of the transmitter is maintained.

it particular function of the tertiary winding of the transformer 3i is to allow the use of a high frequency filter condenser of small size. The impedance of the ribbon microphone circuit is made such that it matches the pressure micro- For such impedance a filter condenser of the proper reactance would be physically large as compared with other elements of the transmitter. By transforming to a higher impedance a smaller condenser could be used but a second transformation down to the matching impedance would be necessary, thus requiring an eatra transformer. in accordance with this invention, by using a tertiary winding having an impedance that is high relative to that of the secondary and connecting the capa-citaltive ele merits thereto,a small condenser will serve. Thus a great saving in size and weight is achieved. For example, with-out the tertiary winding the condenser ti l (see Fig. l.) which is of. the order of .2 microfara'd with a tertiary-secondary impedance ratio of 20 to i, would have to be of about i microfarads. Such a condenser would be approximately 20 times as large as condenser The switches til, "ill and ill previously mentioned may be employed to increase the versa if switch it is closed and switch 'lli left open the transformer secondary it is short circuited and the ribbon unit is cut out. This leaves the moving coil unit alone in service. If, on the other hand, a bidirectional transmitter is desired, switch it is closed and switches lid and lit left open. The moving coil unit is then cut out and the high frequency equalizing network rendered. inoperative. For normal unidirectional operation switch til is closed and switches it and it are left open.

The transmitter described in the foregoing translates sound waves with e-lfdciency and fidelity and has an average directional discrimination of about 20 decibels between 0 degree and lflll degrees soun d incidence over .the range from fifty to ten thousand cycles per second. lit is both small and light in weight for a device of its character and efficiency.

Although the invention has been disclosed with reference to a specific embodiment it will be un derstood that it is not restricted thereto but is limited in scope by the appended claims only.-

W hat is claimed is:

l. An acoustic transmitter comprising a casing including a reduced portion and containing a moving coil pressure type transmitter unit and a ribbon diaphragm, velocity type transmitter unit including a pair of U-shaped permanent magnets having common elongated pole-piece members between which is mounted a ribbon diaphragm, one of said magnets embracing and closely fitting the reduced portion of said casing, thereby allowing compact assembly of the 'two units.

2. A composite acoustic transmitter comprising a bidirectional microphone and a second microphone, said second microphone comprising a moving coil unit mounted in a casing, said casing providing a substantially closed air chamber behind said unit, and means for equalizing the responses and combining the outputs of the two microphones, said means including a transform er, a condenser, an inductor and an acoustic impedance comprising a low frequency equalizing tube.

3. A composite acoustic transmitter comprising a bidirectional microphone and a second microphone, said second microphone comprising a moving coil unit mounted in a casing, said casing providing a substantially closed air chamber behind said unit, and means for equalizing the responses and combining the outputs of the two microphones, said means including a transformer, a condenser, an inductor and an acoustic im pedance, said equalizing means being housed within the air chamber portion of said casing.

ll composite acoustic transmitter comprising a ribbon velocity microphone unit, a moving coil pressure microphone unit, and a. high frequency corrective networlr for the ribbon microphone, said networlr including a condenser and a transformer for matching the impedances of the two microphones, said transformer including a primary, a secondary and a tertiary winding, said tertiary winding being connected to said networh and having an impedance ratio with said sec-- ondary winding such that the condenser capacity necessary for correction is low, whereby said condenser is physically small relative to the microphone units.

5. A. composite acoustic transmitter assembly comprising a ribbon velocity microphone, a moving coil pressure microphone, a pair of output terminals, an inductance coil, a condenser, a

transformer having a. primary, a secondary and a tertiary winding, said transformer having its primary winding connected to the output of the ribbon microphone, its secondary winding con" nected serially with the moving coil microphone and the output terminals, and its tertiary winding connected to said condenser, said inductance cell being connected in shunt with said primary winding and a casing for said moving coil micro phone, said casing comprising an acoustic impedance and also the housing for the transform er, inductance coil and condenser.

ii. An acoustic transmitter having a unidirec tional sound piclt-up characteristic, said transmitter comprising a velocity microphone unit, a pressure microphone unit, and equalizing means including electrical and acoustic impedances for equalizing the high and low frequency responses of said units, said pressure unit having a diaphragm, a coil attached to said diaphragm, magnetic structure cooperating with said coil and a substantially closed chamber behind said diaphragm and coil, said chamber comprising part of the acoustic portion of the equalizing means and also housing all of the remaining equalizing means.

di l

7. A sound pick-up device comprising a moving to the plane of the ribbon diaphragm, said pressure unit having its casing recessed to interfit with one of said magnets, thereby allowing the respective diaphragms of the two units to be located sufliciently close together to minimize relative phase difficulties.

8. A unidirectional acoustic transmitter comprising a velocity microphone unit and a pressure microphone unit, said velocity unit comprising two ,U-shaped permanent magnets and a thin ribbon diaphragm mounted between parallel polepieces, the pole-pieces each being secured at either end to a pole portion of one of the magnets, said magnets being of generally circular cross-section and extending at right angles to the plane of said ribbon diaphragm, and said pressure microphone comprising a moving coil unit mounted in a casing, the casing having a reduced portion fitting within one of the U-shaped magnets to provide a compact unitary transmitter structure.

9. A unidirectional electroacoustic transmitter in which a pressure and a velocity microphone unit are combined, said transmitter including electrical impedance means connected across the velocity microphone terminals and acoustic impedance means comprising a tube connected to the pressure microphone for substantially equalizing the low frequency response of the two microphones.

10. A unidirectional acoustic transmitter comprising a ribbon velocity microphone and a moving coil pressure microphone combined in a unitary structure, said ribbon microphone having an electrical impedance shunted across its terminals and said moving coil microphone having an acoustic impedance connected thereto, the relative values of said impedances being such as to so equalize the low frequency responses of said microphones that they combine efliciently for unidirectional response.

11. In combination, a moving coil pressure type transmitter unit, a ribbon diaphragm velocity type transmitter unit, a casing for said pressure unit, said casingihavingarecess for receiving a portion of the velocity unit therein, and means for combining the outputs of said units so that the combination has a substantially unidirectional response characteristic.

12. A unidirectional acoustic transmitter comprising a ribbon velocity microphone and a moving coil pressure microphone, said velocity microphone including a pair of parallel pole-pieces defining an elongated air-gap, a ribbon diaphragm mounted in said air-gap, and a pair of substantially parallel U-shaped magnets of circular section secured at substantially right angles to said pole-pieces, said pressure microphone including a moving coil unit, a. casing enclosing said unit and defining an air chamber therebehind, said casing being generally convexly curved exteriorly and having a portion shaped to fit within and partly around one of the U-shaped magnets, means for equalizing the responses of the two microphones comprising electrical and acoustic impedance elements, all of said impedance elements being housed in the pressure microphone casing, and an exteior perforated casing enclosing said transmitter.

13. A velocity microphone comprising a ribbon diaphragm of the order of .00025 inch thickness and having a compliant portion adjacent at least one end and a stiffened portion intermediate its ends, and a low frequency damping means comprising an electrical impedance shunted across said diaphragm.

WILLIAM R. HARRY. ROBERT N. MARSHALL.