US2512467A - Single element, unidirectional, dynamic microphone - Google Patents

Description

June20, 1950 H. F. OLSON ET AL 2,512,467

SINGLE ELEMENT, UNIDIRECTIONAL, DYNAMIC MICROPHONE Filed July 51, 1946 3 Sheets-Sheet 1 attorney June 20, 1950 H. F. OLSON ET AL 2,512,467

smcua ELEMENT, UNIDIRECTIONAL, DYNAMIC MICROPHONE Filed July 31, 1946 5 Sheets-Sheet 3 8 on 00 on v ewfon attorneg Patented June 20, 1950 UNITED STATES PATENT-OFFICE SINGLE ELEMENT, UNIDIRECTIONAL, DYNAllflC MICROPHONE Harry F. Olson, Princeton, and John Preston,

- Hopewell, N. J assignors to Radio Corporation of America, a corporation of Delaware Application July 31, 1946, Serial No. 687,419

pinging thereon. The ribbon is terminated at its rear face by a long tube or pipe filled with suitable acoustic resistance material. Immediately behind the ribbon, the aforementioned pipe is provided with an opening which may be closed off more or less to provide either a non-directional characteristic when this opening is entirely closed, a lei-directional characteristic when this opening is entirely open, or a unidirectional characteristic when this opening is only partially closed off. Where it is desired that the microphone shall have only a unidirectional characteristic, an opening of fixed, suitable size is provided in the pipe and no provision need then be made for varying the size of the opening.

As constructed heretofore, unidirectional microphones of this type have suffered from the disadvantage that the response is not uniform throughout the entire frequency range over which the microphone is responsive. This is due largely to the acoustic resistance provided by the pipe behind the ribbon.

The primary object of our present invention is to provide an improved, single element, dynamic microphone which will be free from this disadvantage.

More particularly, it is an object of our present invention to provide an improved microphone of the type set forth which will have a more uniform response, particularly in the low frequency region, than similar microphones of the prior art.

Another object of our present invention is to. provide an improvedform of single element, dynamic microphone asaforesaid into which similar microphones of the prior art can be converted readily.

It is also an object of our present invention to provide an improvedmicrophone as aforesaid which is simple in construction and highly efiicient in use.

In accordance with our present invention, We place over the opening in the pipe behind the vibratory ribbon or other similar conductor a screen covering, preferably of fine mesh, which provides an acoustic resistance for this pipe opening. The acoustic resistance of this screen, whichis comparable to the acoustic reactance of the pipe opening, serves to compensate for the acoustic resistance in the ribbon branch of the microphone and thus provides an improved directional characteristic-as will be described more fully hereina r Ii 1 i 4 Claims. (Cl. 179115.5)

to sound translating ap- I of the ribbon!) are spaced somewhat from the,

The novel features of our invention, as well as additional objects and advantages thereof, will be understood better from the following description of two embodiments thereof, when read in connection with the accompanying drawings in which I Figure 1 is a front elevation of a portion, of a microphone constructed in accordance with one form of our present invention,

Figure 2 is a perspective view of the structure shown in Figure l, as seen from the rear of the microphone,

Figure 3 is a sectional view thereof taken onv the line 3-3 of Fig. 1,

Figure 4 is an electrical wiring diagram of a circuit in which the microphone of our present invention may be employed,

Figure 5 is the electrical analogue of the acoustical circuit involved in the microphone illus--' trated by Figs. 1, 2 and 3,

Figure 6 is a graph showing the directiona in which similar reference characters designatecorresponding parts throughout, there is shown" a pair of pole pieces I connected to a pair of magnets 3 which are supported by a bar 5 of magnetic material, the pole pieces I being spaced from each other to provide an air gap I through which extends a magnetic field supplied by the magnets 3 in known manner. Vibratively mounted in the air gap 1 is an elongated, conductive ribbon 9 of Well known form. The longitudinal or side edges adjacent edges of the pole pieces I to provide slits Ia which afford adequate clearance to permit the ribbon 9 to vibrate freely in response to acoustical waves impinging thereon.

Behind the ribbon 9, there is mounted a closed pipe II which may terminate in a; suitable labyrinth structure or the-like I3 (Fig. 3) filled with tufts of felt or similar damping material IS, the pipe II and labyrinth structure I3 with its damping material I5 acting as an acoustic resistance which terminates the ribbon 9. The ribbon 9 substantially closes the mouth of the pipe II at itsupper end, as clearly shown in Figure 3 and in a manner more fully disclosed, for example, in Reissue Patent No. 19,115 granted to H. F. Olson and J. Weinberger. The pipe H is provided behind the ribbon 9 with an opening ll of such size as to impart to the microphone a substantially unidi rectional characteristic in the manner more fullydisclosed in the above mentioned Olson Patent- In accordance with our present invention, We place over the opening I! a screen 19 having a mesh preferably of about 120 threads per inch,

. The sound pressure acting on the aperture or with a thread diameter of 0.006 inch. It is the provision of this screen I9, which may be made of wire or any other suitable material, which imparts to the microphone an improved response.

The ribbon or conductor 9 may be connected through a first transformer 2|, a transmission line 23 and a second transformer25 to an amplifier 21, as shown in Fig. 4, for amplifying the voltages generated by the ribbon 9 upon vibration thereof through the magnetic field in the air gap 7. From a consideration of the acousti= cal system represented by Figure: 5, it will be seen, by means of the following: analysis, how the addition of the screen 19' improves the response of the microphone:

In the acoustical circuit of Figure 5,. there are shown three branches ZAl, ZAZ and ZA3.. In the branch ZAl,

Ms=inertance of the slits "la,

rAs=acoustical resistance of the'slits 7a,

Me=inertance of the ribbon 9;

rAR=acoustical resistance of the ribbon 9,

CAR=acoustical capacitance of the ribbon 9,

Ms=inertance of'the air load upon the ribbon 9,

rAA=acoustica1 resistance'of'the' air load upon the ribbon 9;

ZAn=acoustical impedance due to the electrical circuit shown in Fig. 4. The value of ZAE is given by the equation ZAE=A2(ZE1+TER)109 (1) where =fiux density in the air' gap I,

Z=length of the ribbon 9;

A=area of the ribbon;

Zar=electrical impedance'of externalloadn ribbon 9; as shown in Fig. 4, and" 1 mn=electrical resistance of the ribbon 9 (see Fig. 4)

ZAr is the acoustical impedanceof the branch composed of the above elements.

In the branch 2A2,

In the branch ZA3,

ZA3=rAP=acoustical resistance of the acoustically damped pipe H, l3, I5.

The sound pressure actingon the opener front side of the ribbon 9 may be written J'( P p1 6 where P =amp1itude of the sound pressureof the sound Wave upon the I ribbon 9;

opening-Flat the screen [9 may be written where 1 =amplitude of the sound pressure upon rear surface of the aperture, and 2=phaseangle of the sound pressure upon the rear surface of the aperture with respect to the same, arbitrary reference point as used in Equation 2.

The reference point for the" phase may be changed so that The phase ang1e=a= is a function of the angle of the incident sound (6) where c= anglebetween the-normal to the-surface of the ribbon and the direction of the incident sound, and

=consta=nt phase angle obtained from the geometry of the microphone.

Specifically the phase-angle in degrees, is the effective path length, in centimeters, from the ribbon 9 to the aperture 01'- opening I! on the back of' the pipe ll multiplied by 360 and divided by the w-avelength of the sound, in centimeters.

The volume current of the ribbon due to the sound pressure n is The volume current of the ribbon due to the sound pressure 102 is I Pizza);

ZAIZAZ+ZAIZA3+ZAZZAB- The resultantvolume currentXisof the ribbon 9 is the differencebetween" the volume currents defined 'by'e'quations 7 and '8', or

v I X =X X The'voltage output of the ribbon is:

e,6l A (10) From Equations 9--and- 10; it will-"b'e-seen that, with theabove system, it 'is theoretically possible,

by pro-per choice of constants; to obtsili-n the cardioid characteristic of Fig. 6;- At' 1'8 0 L'thB" theoretical response is-zero: An experimentally determinedresponse characteristic is shown in Fig. 7; It will be seen that the response characteristic at (curverr) is not zero"; It is down about 40 clb in the'midqange.- This approximates the ideal C0I'1diblOn-- b'eC&l1Se= 40 db represents an energy ratio of 10,009 to 1; However: in the low frequency range (40 to 200 cycles the" cancellation is not as good. We have found that the addition of the fine mesh screen l9, which provides the additional acoustical elements M3 and ms, increases the cancellation at 180 in the low frequency region to provide the characteristic shown by the curve B in Fig. 7. This is due to the fact that the addition of the screen I9 introduces an acoustic resistance which compensates for the acoustic resistance in the ribbon branch and thereby produces better cancellation. From the foregoing description, it will undoubtedly be apparent to those skilled in the art that we have provided an improved, single element, dynamic microphone which has a more desirable response than similar microphones of the prior art. It will also be apparent that various changes and modifications may be, made within the spirit of our present invention. For example, in place of a single screen IS, a slide 35, or other suitable support, having several discrete screens 31, 39, 4|, may be adjustably mounted on the pipe I I over the opening H. In Fig. 8 of the drawings, the slide 35 is shown comprising a framework provided with openings 43, 45, 4! over which the screens 3?, 39, 4| are mounted, each of the screens being of difierent mesh to provide a plurality of acoustic resistances of difierent values for the opening I1. The slide 35 is slidably mounted on the pipe II' by brackets 49, 5| attached to the pipe II on either side of the opening I? and arranged in a manner such that any one of the openings 43, 45, 41 may be brought into registration with the opening ll to thereby place screens of different mesh over the opening I! and thus vary the degree of cancellation from the back of the microphone at will.

Figure 9 is an electrical analogue of the modification of our invention illustrated in Figure 8. As in the case of the analogue illustrated in Figure 5, there are also shown in Figure 9 three branches ZA1,Z'A2 and Z53. The branches ZA]. and ZA3 contain identical elements which cooperate and function in the same way as the branches 2A1 and ZA3 in Figure 5. In the branch Z'nz, the acoustical impedance is composed of the following elements:

M'2=inertance of the air in aperture or opening l1 M3=inertance of the screen 31 rAs=acoustical resistance of the screen 31 M"3=inertance of the screen 39 r"A3=acoustical resistance of the screen 39 M3=inertance of the screen 4| r"'Az=acoustical resistance of the screen 4| M4=inertance of the air load upon the screen being used and the opening I! rA4=acoustical resistance of the air load upon the screen being used and the opening ll The analysis and functioning of the branch Z'Az in comparison with that-oi the branch ZAZ is identical except that the values of the inertance and acoustical resistance of the screen will change according to the particular screen selected for use.

Other changes will, no doubt, readily suggest themselves to those skilled in the art. We therefore desire that the above description and the form of our invention shown in the accompanying drawings shall be taken as illustrative and not as limiting.

We claim as our invention:

1. In a sound translating device, the combination of means providing an air gap and a magnetic field in said gap, a conductor mounted in said gap for vibration therein in response to sound wave energy, said conductor and gap constituting a conductor branch of said translating device which has a characteristic acoustic resistance, a pipe mounted behind said conductor in close proximity thereto and including means constituting a terminating acoustic resistance for said conductor, said conductor substantially closing the mouth of said pipe, said pipe having an opening therein behind said conductor and means disposed over said opening constituting an acoustic resistance behind said opening, said last named means comprising a screen formed of threads having such diameter and spacing that said screen has an acoustic resistance which is comparable to the air in said opening and acts to compensate for the acoustic resistance in said conductor branch whereby to lower the loW frequency response or said translating device due to back waves.

2. A sound translating device according to claim 1 wherein said opening is of such dimension as to impart to said device a substantially unidirectional characteristic.

3. In a sound translating device, the combination of means providing an air gap and a magnetic field in said gap, a conductor mounted in said gap for vibration therein in response to sound wave energy, said conductor and gap constituting a conductor branch of said translating device which has a characteristic acoustic resistance, a pipe mounted behind said conductor in close proximity thereto and including means constituting a terminating acoustic resistance for said conductor, said conductor substantially 0T0?- ing the mouth of said pipe, said pipe having an opening therein behind said conductor, means providing for said opening a plurality of acoustic resistance elements, each of said acoustic resistance elements comprising a screen formed of threads having such diameter and spacing that the screen has an acoustic resistance which is comparable to the air in said opening and acts to compensate for the acoustic resistance in said conductor branch whereby to lower the low frequency response of said translating device due to back waves, the thread diameters and spacing of each of said screens being difierent whereby each screen has a different, fixed value of resistance than the other of said screens, and means for selectively placing any one of said elements over said opening.

4. A sound translating device according to claim 3 characterized in that the means for selectively placing any one of said acoustic resistance elements over said opening comprises a member movably mounted on said pipe over said opening, said movable member carrying said plurality of screens in spaced'relation therealong whereby any preselected one of said screens may be brought into registration with said opening by moving said member along said pipe.

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 1,964,606 Thuras June 26, 1934 2,196,342 Ruttenberg Apr. 9, 1940 2,301,638 Olson Nov. 15, 1942

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