US2566093A

US2566093A - Means for improving the sensitivity and the response characteristics of velocity microphones

Info

Publication number: US2566093A
Application number: US36196A
Authority: US
Inventors: Harry F Olson; Preston John
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1948-06-30
Filing date: 1948-06-30
Publication date: 1951-08-28
Anticipated expiration: 1968-08-28

Description

1951 H. F. OLSON ET AL 2,566,093

MEANS FOR IMPROVING THE SENSITIVITY AND THE. RESPONSE CHARACTERISTICS OF VELOCITY MICROPHONES Filed June 50, 1948 up I INVENTORS Herr 7E Ulwon 6 0 12 Pr fan ATTORNEY Patented Aug. 28, 1951 MEANS FOR. IMPROVING THE SENSITIVITY AND THE RESPONSE CHARACTERISTICS ,OF VELOCITY MICROPHONES Harry F. Olson, Princeton, and John Preston,

Metedeconk, N. J assignors to Radio Corporation of America, a corporation of Delaware Application June 30, 1948, Serial No. 36,196

UNITED STATES PATENT OFFICE 12 Claims.

This invention relates to electro-acoustical translating apparatus, and more particularly to a microphone of the dynamic type.

Sensitivity and wide frequency range have always been two incompatible factors in electroacoustical translating apparatus, such as microphones, especially the type of apparatus used in radio broadcasting. With an extension of the frequency band of radio broadcasting, as in the case of frequency modulation and high quality amplitude modulation, noise in the system becomes a very important problem.

The-velocity type microphone, to which our present invention is directed, is almost universally employed in broadcast practice, since the sensitivity is as great as, or greater than other types of microphones in use at the present time.

Since the velocity type microphone is highly of the problem is to make the microphone more w sensitive. While weight is not a factor, if it is kept within reason, restrictions on size must be considered. We have provided an improved ve-- locity microphone, which not only overcomes many of the problems encountered by similar microphones in use at the present time, but which also has greater sensitivity and improved response characteristics. Thus, the primary object of our present invention is to provide an improved velocity type microphone the sensitivity and the response characteristics of which are greatly improved over those of similar, prior art microphones, with no substantial increase in the size thereof.

It is also an object of our invention to provide an improved velocity type microphone which will eliminate or reduce non-uniform response with respect to frequency.

It is another object of our invention to provide an improved velocity type microphone which will average out diffraction and resonance effects and provide a smoother response frequency characteristic.

It is a further object of our invention to provide a high sensitivity velocity microphone in which the difference in sound wave pressure on the front and back of the ribbon diaphragm will continue to increase linearly with the frequency and thereby prevent the falling off of response in the high frequency range.

It is also an object of our invention to provide a high sensitivity velocity type microphone which is simple in constructionand at the same time highly efficient in use.

In accordance with our present invention, we have provided an improved velocity type microphone in which we make use of a pair of pole pieces each of which has an intermediate portion of reduced cross section to shorten or reduce the ribbon diaphragm. An unsymmetrical magnetic field structure is connected to the ends of the pole pieces in order to average out diffraction and resonance effects characteristic of symmetrical field structures. In addition thereto, the microphone is also equipped with a high frequency equalizer of the izind disclosed in. our copending applioationSer. No. 29,844, filed May 28, 1948, for maintaining suitable response throughout the high frequency range.

The novel features of our invention, both as to its organization and method of operation, will best be understood from the following detailed description when read in connection with the accompanying drawing, in which Fig. 1 is a front elevation, partly cut away of a velocity type microphone constructed in accordance with our present invention,

Fig. 2 is a front elevation of the microphone field structure shown in Fig. 1, with the high frequency equalizer plates removed,

Fig. 3 is a side view, in section, of the field structure illustrated in Fig. 2, equipped with high frequency equalizer plates, and taken on the line a-a of Fig. 2,

Fig. 4 is a cross section of the pole pieces and ribbon diaphragm of our improved microphone, taken on the line 4-4 of Fig. 2,

Fig. 5 is a front elevation of a modification of a velocity microphone field structure constructed in accordance with our invention, with the high frequency equalizer plates removed, and

Fig. 6 is a side view, in section, of the field structure illustrated in Fig. 5 equipped with high frequency equalizer plates, and taken on the line 6-6 of Fig. 5.

theacoustic path'from the front to the back of Referring, now, in detail to the drawing in which the same reference numerals refer to like elements, there is shown in Fig. l, a Velocity type microphone I constructed in accordance with our invention, The microphone l is pivotally mounted on a support 3 of conventional design, and comprises, essentially, a ribbon diaphragm 5, a pair of pole pieces 1, 9, and a pair of magnets consisting of top and bottom units ll, [3.

According to one form of our invention, as shown in Figs. 1, 2, 3 and 4, the microphone i is provided with a pair of elongated pole pieces I, 9 disposed in spaced, parallel relation to provide an air gap ,[5 between adjacent flat faces ll, IQ of the pole pieces. The pole pieces may be of any desired shape. As shown in Fig. 4, the pole pieces illustrated are substantially circular in cross section with the exception that they are provided with the fiat faces I], I9 disposed on adjacent sides of the pole pieces I, 9. Each of the pole pieces I, 9 has an intermediate portion, the cross sectional area of which decreases toward the center thereof, forming longitudinal concave surfaces 25, 21 around the pole pieces except on their adjacent sides, or flat faces ll, I9. The cut-out intermediate portion of the pole pieces I, 9 serves to reduce the acoustic path from the unsymmetrical field structure.

. 3 front to the back of the ribbon, whereby good response is maintained throughout the high frequency range.

The ribbon diaphragm 5 is suspended in the air gap i5 between the pole pieces 7, 9, and is held in place by two insulated clamps '29, 31 carried by the field structure. The ribbon 5 is also maintained in spaced relation to the pole pieces so that it will be free to vibrate in response to differences in sound wave pressures on the front and back of the ribbon.

The top and bottom unit il, 13 comprise two substantially 'U-s'haped magnetic structures which are attached opposite ends 33, 35, 3'5, 39 of the pole pieces l fiffor supplying magnetic fiux across the air gap l din a manner well known in the art. Referring to Fig. 3 off thefdrawing, it will be seen that the bottom unit i3 'is larger than the top unit 'I l, thus forming an unsymmetrical field structureior the microphone. For simplicity of construction a-nd to facilitate manufacture, the units i i, is contain combinations of magnetizing bodies or permanent bar magnets each of equal size made from material'having ahigh degree of retentlvi ty, such as Alnlico V (composed of aluminum 8 percent, nickel i'ipercent, cobalt 24 percent, copper 3 percent, and the remainder iron). As shownin Figs? and "'3 of the drawing, the top unit it has tiwob ar magnets d3 disposed in spaced, parallel relat'ionyw'ith the ends :35, 47 of these bar magnets connected by a soft iron plate element The opposite ends 5%, "53 of the bar magnets are respedtively co'nnected to outer pole rnezrib'ersfidtl which, in turn, are connected to the top ends "33,37 of thepole pieces E, ii. The bottom un'it l 3'has two pairs of bar mag provided with a high frequency acoustic equalizer or" the type described in our above-identified copending application. The equalizer comprises two perforated plates or screens 85, 83 which are disposed in spaced,parallel, overlapping relation to the ribbon diaphragm The equalizer plates 81, 83, may be supported in any suitable manner to insulate them from the magnetic structure (for example, by attaching them to the insulated ribbon clamps 29, Each of the plates has a plurality of perforations 85. The size of the perforations or openings 85, as well asthe spacing between the plates 81, as "and the ribbon -5 are determined according to the frequency range to be compensated for,'and'inay be varied according to the condition to befc'orrect'ed. The plates -8i, 83 form resonance chambers on opposite sides of the ribbon '5 and operate as reflectors for sound Waves transmitted through the perforations whereby a standing Wave system is produced between the plates which will maintain the necessarydifference in sound wave pressure on the front and back of the ribbon and prevent the falling off of response in the high frequency range. 7

Referring to Figs. 5 and 6; there is shown a modified form of field structure 9 i for a velocity type microphone constructed in accordance with our invention. The microphone itself comprises the same essaitial'elementsas that-shown in'Figs. 1 through 3 and differs primarily in respect to the field structure. The modified field structure t! comprises, essentially, a pair of pole pieces-9%,

nets 53, $5, each of which is the same as the bar to the'lower ends 35,1339 ol the polepieces l, 9. g

The plate elements'an dthe outer pole members should preferably be-inade from analloy or combination of alloys Which'is capable if carrying a high flux density before becoming saturated.

(For'examplefwe 'have found that an alloy con= taining 50 percent cobalt, and "5!) -,percent iron will carry more flux than pureiro'n is satisfactory for the field structure of our .pres'ent'invention.)

As pointedout above, the use oftwo barmagnets M, 53 in'the topurii't H and'tw'o pairs'of bar.

magnets d3, 85in the bottom'unitl'ii defines an By providin an unsymmetrical fie1dstiucture,tWo advantages result. (l) The field's'tructureforms two cavities '78, 8d of different dimensions be'tween'the bar magnets the plate elements and theends ofthe pole pieces. Under these conditions a fundamental resonance at diiferent frequencies is obtained which averages out the deleterious eiiects of these resonances upon the response. '(2) "Providing an unsymm-etricalfield structure also varies the path length between opposite sides of the'ribbon diaphragm, as a result of which, the necessary difference in pressure, due to the diflerence in phase, is also varied. 'These difierentpath lengths extend an definite cut-off frequency in the re sponse, which, under symmetrical conditions would occur.

Our improved velocity microphone l is also ill, and top and 'bottom'magnet units 98, it i. The pole pieces are spaced apart to provide an air gap in which adiaphra nrdsisreceived. The top magnet unit "9 comprises twoicylindrical-b'ar magnets its, 35 which aredisposed in spaced, parallel relation so-as-to provide-an airspace Mil between them. The bar magnets its, we have two outer pole-elements'or members 1%, i H respectively connecting the endsof the magnets to the upper ends H3, i of the pole pieces-95,97 to provide a substantially U 'shaped magnetic structure. The bottom magnet unit til is similar to the top unit-dd except-that itha's three bar magnets HT, HQ, iii instead -of two as in the top unit. Thus, modifiedunsymmetrical form of field structure is provided in which thetop and bottom'units have unequal magnetic flux densities. The bottom unit his isalso'prcvid'ed with outer pole elements or

members

123, 125 which connect the 22?, are or" the bar-magnets Hi, its, 12! to the lower ends H35, 1330f the pole pieces'aii, fill. 'As in the 'ca'seof-the top unit 93], the bar magnets l'i'l, HQ, 1'2! "are disposed in spaced, parallel relation 'so as to provide air spaces L35, it? between them. The air 'spaces till, 135, 1?? between the bar magnets in the top and bottom units serve to reduce -resonance in the spaces it'd, l'iiiihetween'the bai'rnagnets and the ends of the ribbon and -pol'epieces.

Also, in our modified form dffieldstructure we have provided pole pieces "95ft? withinterm'ediate sections having concave surface'si'3'9, Ml, the same as the concave "surfaces 25, '2? "shown in Figs. 1 through 3, for reducing the "sound wave be parallel to the axis of the pole pieces, and the modified field structure 9i shown in Figs. 5 and 6 shows the axis of the bar magnets to be perpendicular to the axis of the pole pieces, this is merely illustrative. Position of the bar magnets with respect to the pole pieces, their size, and number are not controlling. For the purposes of our invention, it is necessary, however, that an unsymmetrical magnetic field structure be provided in order to average out diffraction and resonance effects inherent in symmetrical structures. While we have illustrated but two forms of unsymmetrical field structure herein, it is obvious that other combinations and arrangements of bar magnets may be utilized to advantage.

It will readily be seen by those persons skilled in the art, that we have provided a new type of magnetic and acoustic field structure for the velocity type microphone which distinctly improves the sensitivity and response thereof. While we have described but two modifications of our invention, it will be recognized that various other changes and modifications will, no doubt, readily suggest themselves. We, therefore, desire that the particular form of our invention described herein be considered as illustrative and not as limiting.

What is claimed is:

1. A magnetic structure for sound translating apparatus comprising a pair of elongated pole pieces disposed in spaced relation to provide an air gap therebetween, each of said pole pieces having a flat face disposed longitudinally thereof on the side adjacent said air gap, and also having an intermediate portion of reduced cross section area forming longitudinal concave surfaces on all sides of said pole piece except on the side containing said fiat face, and a magnetic field structure coupling the ends of said pole pieces to provide a continuous magnetic flux path between the pole pieces external to said air gap, the por tion of said magnetic field structure which couples adjacent ends of said pole pieces at one end thereof being larger than the portion of said magnetic field structure which couples the opposite adjacent ends of said pole pieces.

2. A magnetic structure according to claim 1 characterized in that said magnetic field structure comprises a pair of magnets each having at least one magnetizing body, and pole members coupling said pole piece ends to said magnetizing bodies.

3. A magnetic structure according to claim 2 characterized in that each of said pair of magnets has at least one bar magnet of permanent magnet material, and further characterized in that one of said pair of magnets is composed of more bar magnets than the other of said pair.

4. A magnetic structure according to claim 1 characterized in that said magnetic field structure includes a pair of magnets, each of said magnets having a plurality of spaced bar magnets of permanent magnet material.

5. A magnetic structure according to claim 1 characterized in that said magnetic field structure comprises a pair of magnets having a plurality of permanent bar magnets disposed in spaced, parallel relation, each of said bar magnets having its axis disposed parallel to the axis of said pole pieces, characterized further by the addition of pole members connecting an end of said bar magnets to said pole pieces, and characterized also by the addition of a plate element connecting the opposite ends of said bar magnets of each of said pair of magnets.

6. A magnetic structure according to claim 1 characterized in that said magnetic field structure comprises a pair of magnets, each one of said pair of magnets having at least one permanent bar magnet the axis of which is disposed perpendicular to the axis of said pole pieces, and characterized further by the addition of pole members connecting the ends of each of said bar magnets to said ends of said pole pieces.

7. In sound translating apparatus, the combination consisting of (1) a pair of elongated pole pieces disposed in spaced, parallel relation to provide an air gap therebetween, each of said pole pieces having a flat face disposed longitudinally thereof on the side adjacent said air gap, and also having an intermediate portion of reduced cross section area forming longitudinal concave surfaces on all sides of said pole piece except on the side containing said flat face, (2) a magnetic field structure comprising a pair of magnetizing bodies coupling the ends of said pole pieces to provide a continuous magnetic flux path between said pole pieces external to said air gap, (3) a ribbon diaphragm mounted in said air gap for movement in response to diiferences in sound wave pressure on opposite sides thereof, said magnetic field structure coupling adjacent ends of said pole pieces at one end thereof being larger than said magnetic field structure coupling the opposite adjacent ends of said pole pieces, and (4) a pair of plates having a plurality of perforations therein, said plates being mounted in spaced, parallel, overlapping relation to said ribbon diaphragm and forming resonance chambers on opposite sides thereof to provide a standing wave system for sound waves transmitted through said perforations.

8. Sound translating apparatus according to claim 7, wherein each of said magnetizing bodies comprises at least one bar magnet of permanent magnet material.

9. Sound translating apparatus according to claim 8 wherein each of said magnetizing bodies includes a pair of pole members connecting opposite ends of said magnets to adjacent ends of said pole pieces.

10. Sound translating apparatus according to claim '7, wherein said magnetizing bodies are disposed with their axes parallel to the axis of said pole pieces.

11. Sound translating apparatus according to claim '1", wherein one of said magnetizing bodies comprises at least two bar magnets of permanent magnet material, said bar magnets being mounted in spaced, parallel relation.

12. Sound translating apparatus according to claim 9, wherein said magnetizing bodies :are disposed with their axes perpendicular to the axis of said pole pieces.

HARRY F. OLSON. JOHN PRESTON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,086,834 Williams July 13, 1937 2,113,364 Anderson Apr. 5, 1938 2,227,530 Harry Jan. 7, 1941 2,346,395 Rettinger Apr. 11, 1942 2,348,356 Olson May 9, 1944