US2680787A - Uniaxial microphone - Google Patents

Description

June 8, 1954 H. F. oLsoN ET AL- UNIAxIAL MICROPHONE Filed Nov. 30,'1951 2 Sheets-Sheet l s l ISIS l s Y R ,N @www E N .L E E EDEL v .EB mFPE. YNN mhh HDD Hdd ATTRNEY June 8, 1954 H. F. OLSON x-:T AL

UNIAXIAL MICROPHONE 2 Sheets-Sheet 2 Filed Nov. 30, 1951 200 460 idd IM@ unz 0 E+. H Y T 1l E E M NnUE l .R WERE. m N PE M mmm Y Hdd B Patented June 8, 1954 UNAXIAL MICROPHONE Harry F. (ilson, Princeton, John Preston, Metedeconk, and John C. Elea-soy, Mercerville, N. J., assignors to Radio Corporation of America, a corporation of Delaware Application November 30, 1951, Serial No. 259,138

ll Claims. (Cl. 179-1155) The present invention relates to microphones, and more particularly to ribbon type unidirectional microphones.

W ith the advent oi television, efforts have been Vmade to make microphones smaller, less obtrusive, without reducing the quality of the performance of the unit to a degree substantially below that of the larger, more conventional In the development of a small microphone as set forth, a motor mechanism was chosen which is substantially similar to that employed in the microphone disclosed in the aforesaid patent, 2,566,094. However, the microphone of the present invention does not include a sound transmitting tube on the sound input side of the ribbon as does the pressure responsive micromcrophones. In the class of microphones 13110116. Consequently u relatively lelge flat Sul"- Wherein the sound source is relatively close, such l fue@ iS presented t0 the mpngirlg Sound Waves as hand-held microphones, considerable adin the Vicinity of the ribbon. Such a iinite flat vancernent has been made. A microphone of SuIeCe, broken by en air gap in which the ribbon that type is shown in Olson et al, Patent Num.. is supported, produces a diffraction interference ber 2,566,094. That microphone is characterin the risher frequency sound waves. The inized, however, in that it is a pressure responsive l5 terelele, thus produced, dStOltS the TeSlOOllSe unit and, hence, has a non-directional response 0f the mClOphOne t0 the SOuIld Waves implsns characteristic. on the ribbon.

For sound pick-up froma more remoto position, It is, therefore, a further object of the present as from a boom, wherein the microphone is at invention to provide a microphone as Set forth a considerable distance from the sound source, 2O Whelel means are provided t0 prevent the deit has been found desirable to use a microphone velopment of diiraction interference in the sound having a unidirectional characteristic. The Waves imlonsng on the ribbon. microphones which have heretofore been used for ln accomplishing these and other objects, there boom operation have been rather large because has been provided, in accordance with the present of the structure thought necessary to produce invention, an improved pressure gradient I'eSpOIla pressure gradient responsive type microphone Sve, ribbon type microphone- The mCrODhOIle having the desired characteristics. is characterized in that the motor element is Since the hand-held type microphones are priarranged With the ribbon supported in an air marily used to pick up a speaking or singing gap in One end 0f e megnet Structure. Thus, Voice, the structure may be quite simple. Howa relatively Wide flat surface would be presented, ever, the remote or boom microphone may be in the vicinity of the ribbon, te the impngng used to pick up dramatic productions, including sound waves. Anti-diffraction lobes are secured numerous sound effects. Among the various to the fiat surface of the end of the magnetic sound effects, many are accompanied by the genstructure on each side of the air gap to prevent eration of an initial high-amplitude, low frethe diiiraction of the sound Waves by the surquency pressure wave. Such a pressure wave faces adjacent to the air gap. In addition, a accompanies a blast from a firearm. If the plurality of baie screens are interposed between microphone is relatively close to the source of the sound source and the ribbon to absorb or the pressure Wave, the ribbon of the microphone dissipate an initial high amplitude low frequency may be permanently damaged. pressure Wave, such as would accompany the r- It is accordingly an object of the present ining of a .45 caliber pistol, to protect the ribbon vention to provide an improved pressure gradient against damage by such a pressure wave. responsive ribbon typo microphone o horootorizod A better understanding of the present invention in that a smaller sized unit 1s obtained Without may be had from the following detailed descrip.. Sacmcmg mamy 9i Performance' 45 tion when read in connection with the accom- It 1s .another oblect of the present invention panymg drawings in which bggvg;earsgleldrgsln Figure l is an elevational view partly in crossvided for protecting the ribbon from damage by .sect-'1011? fn? a mlcrophone embodymg the present the initial pressure Wave such as would accom- ,o mvn lon pany the firing orare sauber pistolet close range. Fltiufe 2 1S a V1eW tak en 310D? th? 1111 2-2 Still another object of the present invention of Flgure 1 and lookmg m the duecton 0f the is to provide a microphone as set forth wherein appended arrows means are provided for dissipating the energy Figures 3 and 4 are VeWS ShOWIlg the labyrinth of the initial pressure Wave before it impinges connector,

on the ribbon.

Figures 5 and 6 are graphs indicating the frequency response cliaacteristic, respectively, with and without the use of the anti-diffraction lobes,

Figure 7 is a fragmentary view partly in section similar to Figure 1 but incorporating a modication of the invention, and,

Figure 8 is a diagram of an electrical circuit analogous to the acoustical circuit of the microphone of the present invention.

Referring now to the drawings in more particularity, there is shown in Figures 1, 2 and? an outer casing 2 within which a microphone is housed. The microphone motor assembly includes a pair of magnetic members Il and 6 which are secured, at one end, to a magnetically permeable base member 3. A pair of soft iron pole pieces It and I2 are secured to the magnetic members at the end opposite from the base member 8. The.mag.- netic members are approximately semi-cylindrical in shape and are mounted upon the base meinber with their flat surfaces facing each other 1n spaced apart relation. Similarly the pole pieces ID and I2 are fiat and approximately semi-cylindrical. These pole pieces are secured to one end of the magnetic members respectively and are also arranged with their straight side in spaced apart relation facing each other. Thus, the pole pieces denne an air gap. A conductive vibratile member or ribbon ld is supported for vibratory movement in the air gap. A motor unit of this type is selected because it presents several advantages, among them being simplicity and magnetic eiiiciency. Simplicity is accomplished by the use of a small number of easily machined parts. High magnetic efficiency is obtained due to the small flux leakage inherent in magnetic designs of this t e.

ydjacent to the end of the motor assembly opposite from the air gap, there is an acoustical labyrinth iii substantially as defined in Olson Patent Number 2,271,988. In the space between the magnetic members l and 5 there is positioned a conduit or connector I6 which connects one side of the air gap through an aperture I8 in the base member 8 to the acoustical labyrinth I5. The construction of the connector IB is substantially the same as that described in the aforesaid Patent 2,566,094, with the exception that there is provided one or more orifices I'I (two being shown) near the end of the conductor adjacent to the air gap as shown in Figures 3 and 4.

Bridging the space between the magnetic members along the peripheral edges there is positioned a perforated metal screen 2D. A layer of closely woven fabric such as silk 22 is secured to the back of the metal screen. A similar piece of fabric covers the orifices Il in the connector I6. The space between the connector and the metal screens is filled with an acoustical damping material such as felt or iiber glass 24.

Secured to each of the pole pieces IB and I2 on the flat surfaces thereof opposite from the inagnetic members, there is an anti-diffraction lobe 26. These lobes may be made from any suitable material. However, a non-magnetic material is preferred. In cross-section, the lobes are substantially semi-circular in shape and are positioned on the pole pieces closely adjacent to the air gap. Between Vthese lobes there is positioned a plurality of successively spaced baffles 28-3 which includes a perforated metal support with a backing of a closely woven fabric. The spacing of the baffles is such that sounds impinging on the ribbon I4 must first pass'through one of the bafies and then the other baffle before arriving at the ribbon. The end closure of the outer` casing is formed of a perforated metal support screen 32 and having a fabric backing 34. However, here the dimensions of the perforations and the fabric of the backing are such that they are substantially transparent to the sound waves.

In Figures 1 and 2 the bafes 28-30 bridge the space between the anti-diffraction lobes. The perforated metal screen constituting the end closure of the casing has portions 36 which extend longitudinally of the casing. These extended portions are arranged to be aligned with the orifices I'I in the connector I6. The fabric backing also extends along the extended portions. The perforated metal screens 20 are substantially parallel to the extended portions of the end closure and are spaced therefrom. In Figure 7 the construction of the baffle is somewhat modified in that the bafes are in the form of boxes 28a, 33a, one within the other, which are substantially pyramidal in shape. The inner box 28a is spaced from the outer box 30a as are the bafiles 28-30 of Figures 1 and 2.

A microphone thus constituted may be termed a uniaxial microphone because the axis of maximum sensitivity corresponds to the axis of the assembly. The orifices I'I in the labyrinth connector I6 permit variations in sound pressure to impinge on the back side of the ribbon I4. However, the structure of the microphone is such that the sound waves impinging on the back side of the ribbon are delayed and shifted in phase with respect to the sound waves falling on front of the ribbon. So long as the sound source is disposed along the axis of the microphone. on the front side of the ribbon, a maximum phase difference between the sound Waves falling on the front and back of the ribbon occurs, resulting in a maximum output. However, deviations from the axis are accompanied by a corresponding reduction in the phase difference and a lower output, with maximum cancellation occurring when the sound source is disposed along the axis of the microphone and on the back side of the ribbon. If the microphone as described were to be employed without the use of the anti-diffraction lobes, the flat surfaces of the pole pieces would produce a diffraction in the sound waves which would in turn cause an irregularity in the response characteristic of the microphone substantially as shown in the chart of Figure 5. There it may be seen that as, the frequency of the incident sound waves increases, an irregularity in the response of the microphone is produced in the upper portion of the audible frequency range. It has been determined that this irregularity is a result of the diffraction of the sound waves as they impinge on the flat surfacesr of the pole pieces. When the anti-diffraction lobes are positioned on the pole pieces, the response characteristic is substantially as shown in Figure 6. There it may be seen the irregularities have been smoothed out and substantially eliminated. In addition, the contour of the lobes is such that they effectively constitute a small horn, thereby increasing the sensitivity of the microphone.

When the microphone is used to pick up such sounds as the discharge of firearms, the ribbon may be permanently injured by the initiallow frequency, high amplitude pressure wave which accompanies the blast of such rearms. This initial pressure wave, of course, varies in time duration with the type of instrument producing the wave. Forv example, the time might var-y from a 20th of a second for a .454 caliber pistol to a liOth of a second for a .22 caliber pistol. These Ydispensed with, without low frequency pressure waves are not reproduced through the complex chain of elements which constitute the sound channel of recording and broadcasting equipment. However, it is the low frequency component in the blast pulse that produces the large deflection in the ribbon and -stresses it beyond the elastic limit and thereby introduces a permanent deformation in the ribbon. Since the initial pressure wave forms no part of the reproduced sound it may readily be materially affecting the desired sound effect. In accordance with the present invention this initial pressure wave is dissipated before it impinges upon the vibratory member. The system of successively spaced bafes provides means for so dissipating the energy of the blast. Consider such a sound wave as emanating from a point beyond the perforated end closure of the microphone. The pressure wave will first encounter baiTle 3] or 30a where it is attenuated by the acoustical resistance and the inertance thereof. From thence it falls upon the inner baie 23 or 28a where, again, it is attenuated. By this time the pressure wave has been attenuated to the extent that it will no longer have a deleterious effect upon the ribbon. Similarly the sound wave impinges upon the opposite side of the ribbon after having passed the perforated metal screen 20, the acoustical damping material 24, and the orifices Il, thus attenuating the energy of the pressure wave impinging upon the reverse side of the ribbon. Although the acoustical impedance of the baffle structure is substantially constant for all frequencies, effectively, it constitutes a low frequency acoustical impedance. This is true because the impedance of the baie structure must be compared with the acoustical impedance of the ribbon itself. For low frequencies, the acoustical impedance of the ribbon is very small in comparison with the impedance of the baffles. Hence, for low frequencies the dominant impedance is that of the baffles. On the other hand, for higher frequencies, the acoustical impedance of the ribbon dominates that of the baffles.

The acoustical network shown in Figure 8 is representative of the conditions prevailing in the microphone herein set forth and may prove helpful to a better understanding of the principles involved herein.

In that network,

P1 is the sound pressure on the front of the microphone,

MA1 and Rm are the inertance and acoustical resistance respectively of the air load on the front of the microphone,

MB1 and Rm are the inertance and acoustical resistance respectively of the first baille 30, while MBi and Rsl are inertance and acoustical resistance respectively of the inner baffle 28,

C1 is the acoustical capacitance of the volume between the baflies 28 and 30 or 28a and 30a, C'1 is the acoustical capacitance of the volume between the inner baffle and the ribbon,

Ms and Rs are inertance and acoustical resistance of the slit between the edge of the ribbon I4 and the pole pieces I0 and l2,

MR, RR and CR are respectively inertance, acoustical resistance and acoustical capacitance of the ribbon,

ZE is the acoustical impedance due to the electrical circuit associated with the ribbon,

P2 is the sound pressure on the back side of the vibrating system,

MAZ and RA2 are respectively the inertance .and acoustical resistance of the air load of the back side of the vibrating system,

M132 and Rs2 are respectively inertance and acoustical resistance of the baffles 20 on the side of the microphone,

C2 is the acoustical capacitance of the volume behind the baffles 20,

M2 and R2 are the inertance and acoustical resistance respectively of the screen covering the orifices I1 in the labyrinth connector I 8,

M3 and R3 are the inertance of acoustical resistance respectively of the orifices in the labyrinth connector,

RP is the acoustical resistance of the labyrinth In the event that the acoustical damping material is used in the space between the baies 20 and the labyrinth connector I6, the acoustical network would be modified to the extent of having the acoustical resistance of the damping material connected in shunt across the acoustical capacitance Ccz.

Similarly, acoustical damping material may be inserted between the front baiiies 28 and 30 or 28a and Sila. This would, in effect, modify the acoustical network by the inclusion of the acoustical resistance of the material connected in shunt with the acoustical capacitance C1 of the volume between the bailles.

Thus it may be seen that, by controlling the values of the elements of the network, through the control of the dimensions of the components represented thereby, the very low frequency pressure waves may be effectively filtered out, without materially affecting the microphones response to the desired sound waves.

From the foregoing, it may be seen that there has been provided an improved small-size pressure gradient responsive microphone wherein the ribbon is protected from damage by high amplitude, low frequency pressure waves, and wherein means are provided for preventing the formation of diffraction patterns which would distort the microphones frequency response characteristic.

What is claimed is:

1. A pressure gradient responsive ribbon type microphone comprising a magnetic field structure generally cylindrical in shape having substantially flat circular ends, said field structure defining an elongated air gap in one of said circular ends, a conductive vibratile member supported in and substantially coextensive with said air gap, a plurality of successively spaced bales effectively constituting a low frequency acoustical impedance interposed between said vibratile member and the ambient, and a pair of antidiffraction lobes mounted on said end adjacent to said air gap..

2. The invention as set forth in claim 1 wherein said anti-diifraction lobes are substantially semicircular in cross section.

3. The invention as set forth in claim 2 wherein said lobes are secured to said end on opposite sides of said air gap.

4. The invention as set forth in claim 3 characterized in that at least two of said baffles are disposed between said lobes bridging the space therebetween.

5. The invention as set forth in claim 1 wherein said bailies comprise perforated metal supports and a layer of closely woven fabric secured to one surface of each of said supports.

G. A pressure gradient responsive microphone comprising a magnetic eld structure having an air gap in one end thereof, a conductive vibratile member supported in and substantially coexn tensive with said air gap, said magnetic structure being disposed substantially all on one side of said air gap, a plurality of successively spaced bafiies effectively constituting a low frequency acoustical impedance interposed between said vibratile member and the ambient, and a pair of anti-diifraction lobes secured to said structure adjacent to said air gap.

7. A pressure gradient responsive microphone comprising a magnetic iield structure having an air gap in one end thereof, a conductive vibratile member supported in said air gap, said magnetic structure being disposed substantially all on one side of said air gap, and a pair of anti-diffraction lobes secured to said structure adjacent to said air gap.

8. The invention as set forth in claim "I wherein said magnetic structure is generally cylindrical in shape having substantially flat circular ends, one of said ends being divided thereby defining said air gap.

9. The invention as set forth in claim 8 wherein one of said lobes is secured to each portion of said divided end.

10. The invention as set forth in claim 9 wherein said lobes are substantially semi-circular in cross section.

11. A pressure gradient responsive ribbon type microphone comprising a magnetic field structure generally cylindrical in shape and including a spaced pair of substantially semi-cylindrical magnetic members and a pair of fiat semi-cylindrical pole pieces positioned on one end of said magnetic members, said pole pieces defining an elongated air gap, a conductive vibratile member supported in said air gap, an acoustic labyrinth adjacent the end of said magnetic structure opposite from said air gap, a connector member of substantially constant cross sectional area connected between said air gap and said labyrinth, said connector member having a sound admitting aperture adjacent to the end thereof nearest said air gap, a plurality of successively spaced bafes effectively constituting a low frequency acoustical impedance interposed between said vibratile member and the ambient, and a pair of anti-diffraction lobes respectively mounted on said pole pieces.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,348,356 Olson May 9, 1944 2,566,094 Olson et al Aug. 28, 1951

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