US2751441A - Unidirectional microphone - Google Patents

Description

June 19, 1956 H. F. OLSON ET AL 2,751,441

UNIDIRECTIONAL MICROPHONE Filed March 2, 1953 ATTORNEY Patented June 19, 1956 UNDIRECTONAL MICROPHONE Harry F. Olson, Princeton, and John Preston, Metediconlr, N. J., assignors to Radio Corporation of America, a corporation of Delaware Appiication Marcia 2, 1953, Serial No. 339,808

6 Claims. (Cl. 179-1155) This invention relates to sound translating apparatus, and, more particularly, although not necessarily exclusively, to a universal ribbon-type unidirectional microphone.

The acoustical resistance which is used in the conventional unidirectional microphone consists of a folded tube or pipe which is sniffed with tufts of hair felt. The folded pipe is thus made to be a labyrinth. Such a labyrinth is costly to manufacture because the holes which provide the labyrinth must be accurately drilled very close together. Then the holes must be joined by cutting slots between the holes so that when the top and bottom cover plates of the microphone are placed over the ends, there is a continuous folded pipe. The pipe is then stuied With tufts of felt. The felt must be finely divided. Furthermore, the amount of felt required to obtain the correct value of acoustical resistance is extremely critical. During the process of manufacture, the felt is weighed. The stuffing is then divided so that the exact proportion is placed in each hole in the labyrinth.

It is, therefore, an object of this invention to provide simplified elements for a unidirectional microphone employing a ribbon as the vibrating element.

It is a further object of the invention to provide a universal low-cost microphone which is small, of lightweight construction and unobtrusive.

It is a still further object of the invention to provide a ribbon microphone with a single resistance substituted for the conventional damped labyrinth.

Still another object of the present invention is to provide a directional microphone with the most universal pattern-the cardioid.

In accordance with the present invention, a lumped acoustical resistance is placed behind the ribbon element of the microphone. Enough space is left so that the two apertures in the ribbon clamps can also feed into the space between the acoustical resistance and the ribbon. The ribbon clamps seal oli the volume behind the ribbon except for the apertures in the clamps required to give the desired directional pattern.

The novel features that are considered characteristic of the invention are set forth with particularity in the appended claims, the invention itself, however, both as to its organization and method of operation as well as additional objects and advantages thereof, will be best understood from the following description of several embodiments thereof, when read in connection with the accompanying drawing, in which:

Fig. 1 is a view in sectional elevation of a microphone embodying the present invention; v

Fig. 2 is a sectional view in plan of the microphone of Fig. l, the section being taken along the line 2 2 of Fig. l;

Fig. 3 is a view in section of an earlier type of labyrinth unidirectional microphone;

Fig. 4 is a sectional view taken along the line 4--4 of Fig, 3; and

Fig. 5 is the electrical analogue of the acoustical circuit involved in the microphone illustrated by Fig. l.

Referring more particularly to the drawing, there is shown in Fig. l a sectional View of a microphone embodying the present invention. The main body or case 10 of the microphone consists of a partially hollow metallic tube or cylinder comprising an upper cavity 12, a central or middle cavity 14 and a lower cavity 16, respectively, and including the midsection 18. The midsection 18 is used as a seat and seal for the sound translating unit 24. The sound translating unit 24 comprises two pole pieces 26 and 2S, two permanent magnets 30 and 32, an aluminum ribbon 34, located in the air gap 36 formed by the pole pieces, and a bottom plate 38. The magnets 30 and 32 supply the flux to the air gap 36. The upper cavity 12 lies between the pole pieces. A ribbon clamp 40 is attached to the exposed sides of the magnets 30 and 32 (Fig. 2) to seal the volume in back of the ribbon 34, except for the apertures 42 in the clamps, which apertures are necessary to give the desired directional pattern. A lumped acoustical resistance, in the form of three layers of felt 44, `is placed in back of the ribbon 34 and is or may be arcuately disposed below the apertures 42.` Each felt layer in the illustrative example is one-sixteenth inch thick. The remaining volume in the upper cavity 12 in back of the ribbon 34 is packed with suitable acoustic damping material 46 such as loosely distributed hair felt,

or the lil-1e. The felt 44 is cemented or otherwise secured to the area 48 formed by the juncture of the bottom surface of each of the pole pieces 26 and 28 and the two magnets 30 and 32. The assembled sound translating unit 24 is seated upon and rigidly secured to the upper portion of the midsection 18 and is attached to the midsection through the medium of the bottom plate 38 of a proper diameter and having a large central hole 50 therein to accommodate, among other things, the ribbon leads.

Fastening means such as bolts 58 and 60 pass through holes 22 and engage with the plate 38 to perform the securing function. One of the ribbon clamps is insulated from its associated magnet, as by means of a thin fiber strip 52 (Fig 2). The aluminum ribbon 34 is clamped across the air gap formed by the pole pieces and is secured at each side thereof by means of two machine screws 54 and a top plate 56 at the top of each ribbon clamp, shown in dotted outline in Fig. l. The large middle cavity or aperture 14 through the center of the midsection 18 and the hole 50 in the bottom plate 38 permits the associated ribbon leads, only one lead 62 of which is shown, to be brought from the ribbon clamps to the transformer 64. The transformer 64 is provided with a shield comprising clamp member 72 and bottom member 73. 'Iwo screws 66 and 68 and two fiber washers 63 and 67 secure the transformer 64 to the midsection 18.

The aperture 14 through the midsection 13 gives access to the bottom of the microphone case 10 which increases the volume in back of the ribbon 34 and thus improves the low frequency response. A tapped hole 70 on the transformer clamp 72 provides means for holding the bottom cover 74 of the microphone in place by means of the screw 76.

A single layer of silk (not shown) is glued over the tips of the pole pieces 26 and 28 to keep dust and chips out of the air gap 36. A metallic screen 78 may be disposed over the ribbon assembly. A layer of silk is or may be fastened inside the perforated top cover or grill 80. A wire screen 82 is placed over each of the apertures 42 in the ribbon clamps 40. A fifty mesh copper screen was used in the illustrative embodiment shown in Fig. l.

A cable from the transformer is brought out through a the relative disposition of the lumped acoustical resistance provided by the felt 44 behind the ribbon 34.

There is shown in Fig. 3 a labyrinth unidirectional microphone 96 of an yearlier type. The relatively complicated labyrinth structure 92 with its alternate drilled out portions 94 and slotted interconnecting portions (not shown) is clearly illustrated. The "connecting pipe 95 from the back side of the ribbon element 93 to the center portion of the labyrinth 92 is indicated in the drawing.

Fig. 4Y of the drawing Vshows the alternate drilled out sections 94 and slotted portions 100 as they appear from above along the line 4--4 of Fig, 3. Each of the interconnected pipes and passages must be hand stuffed with the exact proportion of acoustical resistance in the form of felt or ozite tufts 102. Y

Fig. 5 Vis the electrical analogue of the acoustical circuit involved in the microphone in the present invention and is thought to be representative of the conditions prevailing vin better understanding of the principles involved herein.

In that network,

MA and rAA are the inertance and acoustical resistance respectively of the air load upon the ribbon 34,

v MR and CAR are the inertance and acoustical capaci- Vtance respectively of the ribbon 34,

Mz and m2 are the inertance and acoustical resistance of the aperture 42,

rAs is the acousticalr'esistance of the resistive element 44 behind the ribbon 34,

Y CAi, m4 and rAs are, respectively, the acoustical capacitance and the series and shunt acoustical resistances of the iirst or upper cavity 12,

Ma and ras are the inertance and acoustical resistance of the aperture 50 connecting the upper and middle cavities 12 and 14, respectively,

CA2, rAs, `and r'ne are the acoustical capacitance and the series and shunt acoustical resistances, respectively, of the middle cavity 14,

CAa is the acoustical capacitance of the lower cavity 16,v

p1 Vis the sound pressure Vat the front of the ribbon 34,

p2 is the sound pressure at the apertures 42 in the ribbon clamps 40.

The measured response frequency characteristics of an experimental microphone using the present invention has been shown to be ilat within i3 db from 170 to 11,000 cycles per second. Listening tests have indicated that it gives adequate performance for most applications. The weight of amicrophone constructed in accordance with the invention is only ounces.

VThere has thus been described a universal low-cost microphone which has been Vdeveloped lfor such varied uses as public 4address systems, home recording and any application where an expendable, cheap microphone is desired. The invention involves a ribbon microphone with a single resistance Vsubstituted for the conventional damped labyrinth to give a unidirectional characteristic. It is economical to manufacture because the expensive operation of very accurately stuffing the labyrinth is eliminated.

What is claimed is:

1. In a microphone, the combination comprising means defining a magnetic eld, a vibratory conductor -mounted for vibration in said eld, a pair of parallelly disposed magnetic members, means including said magnetic members deiining a cavity adjacent said vibratory conductor, and a layer of fibrous material disposed adjacent said conductor to damp said conductor andsaid cavity.

and means providing a high order of directivity for saidV microphone. I

3. in a unidirectional ribbon microphone including means defining a magnetic iield, a ribbon element mounted for vibration in said iield, a pair of parallcliydisposed magnetic members, means including said magnetic mentbers defining a cavity adjacentsaid ribbon element, an

acoustical resistance further comprising three arcuately disposed layers of felt disposed adjacent said ribbon to said ribbon and said cavity, and means providing a high order of directivity for said microphone.

4. In a unidireccional ribbon microphone including means defining a magnetic field, a ribbon element mountcd for vibration in said eld, a pair of paralleliy disposed magnetic members, means including said magnetic members defining a cavity adjacent said ribbon element, the improvement therein, or. layers of brous material disposed adjacent said ribbon to form an acoustical resistance termination for said ribbon, and a cavity terminating said acoustical resistance, said cavity being llcd with acoustical damping material.- l

5. A unidirectional ribbon type microphone comprising a magnetic field structure generally cyiindrical in shape and including a spaced pair of substantially semi-cylindrical magnetic members and a pair of iiat substantially semi-circular pole pieces positioned onY one end of said magnetic members, said pole pieces defining an elongated air gap, a ribbon clamp disposed at each side of said spaced magnetic members, each ribbon clamp spanning the space between the adjacent sides of said members,

said members and said clamps enclosing a cavity, an

6. A unidirectional ribbon type microphone comprising a magnetic .field structure generally cylindrical iu shape and including7 a spaced pair of substantially semicylindrical magnetic members and a pair of dat substantially semi-circular pole pieces positioned on one end of said magnetic members, said ,pole pieces defining an elongated air gap, a ribbon clamp disposed at each side of` said spaced magnetic members, cach ribbon clamp spanning the space between the adiacent sides of said, members, said 'members and said clamps enclosing a cavity, an aperture in cach ribbon clamp, a conductive vibratile member supported in 'said air gap by said ribbon clamps, an arcuate member `providing a lumped'acoustical resistance adjacent said conductive vibratile member, said arcuate member lying tothe side of each aperture in each ribbon clamp vopposite said vibratile member, and Va til-ling of acoustical absorbing material in the space between said magnetic members to provide .a terminating acoustical resistance located to the side of said arcuate member opposite said vibratile member.

References Cited in the file of this patent UNITED kSTI-KEES JPKITENTSv 2,566,094 Olson et al Aug. 28, 1.951

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