US2179361A - Condenser microphone - Google Patents

Description

Nav. 7, 1939- H. .1- vom BRAUNMHL 2-:r AL 2.179.361

CONDENSER MICROPHONE Filed March SO, 1956 2 Sheets-Sheet l Nov. 7, 1939.

H. J. VON BRAUNMHL ET AL CONDENSER MICROPHONE Filed March 50, 1956 2 Sheets-Sheec 2 Patented Nov. 7, 1939 UNITED STATES PATENT OFFICE CONDENSER BIICROPHONE Hans Joachim von Brsunmhl, Berlin-Buhleben, und Walter Weber, Berlln- Charlottenburg,

Germany 7 Claims.

This invention relates to'improvements in or relating tp microphones, und mute particularly to electrtatic er condenser microphotxes.

The hast known microphones, particularly the condenser microphones previously introduced into practice, are so-called sound pressure recelvers, since only one slde of the membrane or diaphragm is exposed to the sound fleld. Since the sound pressure is a scalar quantity all sound pres- 10 sure receivers possess the property that thelr sensitivity is not dependent on the direction of incidence of the sound but is the same for sound waves coxning from difierent directlons, at least for the low and mlddle frequencies. 'I'his can be shown by thefact that 1f the sensitlvity 015 the microphone er the efiect of the sound pressure onthe microphone, relative 120 the direction 013 incidence of the sound, is platted for a single plane, the curve obtained is a circle a.t the centre of which the membrane o! the microphone is located. This means, therefore, that the pressure microphone shows a characteristlc, which represents the sensitlvlty according to the dlrection of the sound (known in short as the directional response characteristlc) which is in the form of a circle.

Such pressure microphones are certainly advantageous for many purposes, e. g., for picking up conversations of a plurality 015 speakers guthered around the microphone. For many other purposes, however, where tl1e selective picklng up a single sound source from a particular direction is desired with the suppresslon as far as possible of sound waves from other directions, a microphone having the best possible directional response characteristic, i. e., having a decided maximum sensitivity for one particular direction, is required. It is certainly possible, by employing complicated auxiliary devices of karge dimenslons, e. g., reflectors, to impart t o pressure microphones also a certain directional action. The employment of such devices is, however, possible only to a limited extent.

A microphone which is sensitive for a partieular direction of sound can, however, be constiucted if, instead of uslng the scalar property of the sound pressure, the vectorial quantity of the difference in pressure for two neighbouring points ofthe sound field is used for actuating the microphone. In this case the quantity of the efiect cf the sound on the membrane er a corresponding member of the microphone is represented by the sound pressure gradient. 'Ihis principle has been rea1i seg i for an eleetrodmmi mlcrophone by the ribbon microphone exposed on both sides to the action of the sound.

The object of the present invention is, however, to provide a pressure dlfference microphone operating 011 the capacltative principle. This can 5 be efiected according 130 the invention by arranging the diaphragm or membrane to be struck by ti 2 sound waves not only on one side but on both s es.

According to the present invention, therefore, 10 there is provided a condenser microphone in which the membrane acting as the movable p1ate or electrode of the condenser is exposed on both sides to the sound by the provision of perforatlons in the fixed plate. Further, the distance 15 between the membrane and the perforated platze is arranged to be small in proportion to the size of the perforations a11d the spacing thereof, in

order 120 obta.in high frictional damping of the membrane for a purpose which will be herein- 20 after descrlbed.

Condenser microphox1es of this kind have the property of converting into electrlcal oscillations only those sound waves which come from a particular directions, whilst sound waves from cer- 25 ta.in other directions are transmitted electrically to a, considerably lesser extent or not at all.

In order that the invention may be well understood it will now be described by way of eXample only wilzn reference to the accompanying draw 30 ings, in which Figure 1 shows diagrammatically the direc tional response characteristic of the pressure difference microphone which may also be termed a pressure gradient microphone, compared with 35 that cf the pressure microphone,

Figure -2 is a curve illustrating the dependenee on frequency of the response of the pressure difference microphone,

Figures 3a and 3b illustrate in side sectional 40 elevation and rear p1an view respectively a. microphone according to the invention,

Flgure 4 illustrates a. microphone according another embodiment cf the invention,

Figures 5, 6 and 7 illustrate diagrammatically the membrane displacements of a microphone according the invention under the action of the sound pressure anal sound pressure difference for diflerent directions of sound incidence. 0

Figure 8 shows the directional response characteristic of a microphone according to the invention, and

Figures 9 and 10 illustrate in froni; and side View respectively a microphone provided with mea.ns 1'or increasing the sound pressure difference.

Referring first to Figure 1, the circle D represe'znts the directional response characteristic of the pressure microphone and shows that the sound, which acts on a microphone located at the cei1tre of the circle, infiuences the membrane to an equal extent from all directions. II, however, the microphone 1s 01' the above-mentioned construction such that the membrane is influenced on both sides by the sound, the directional response characteristic obtained consists of two circles G, which touch at; the line in whlch the microphone can be regarded as located. The directional response characteristic of the pressure gradient microphone has, therefore, the form of flgure 8. This characteristic of the pressure gradient microphone shows that the membrance is most strongly influenced by sound incident; in a. direction at; right angles to its plane., i. e., at right angles to the horizontal 1ine of the diagram, that is to say from the front or rear, whi1st the membrane is not influenced by sound ineident in a direction parallel to the membrane.

On grounds of symmetry it may be advantageous t0 arrange 011 the second side of the membrane a plate similar to the fixed condenser plate. With an unsymmetrical arrangement the. circle G (Figure 1) would be of unequal size.

Such a. microphone is diagrammatically illustrated in Figures 3 and 3". The fixed condenser platze E has a plurality of openings n, and at the other side cf the membrane M is located a plate E similar in shape and arrangement to the fixed plate E. This platze either does not take part electrically in the operation of the microphone, or is connected in a particular manner known in connection with difi'erential microphones. The function o1 the parts K and L will be referred to later.

In Order that a. capacitive microphone may possess a horizontal frequency curve, it is necessary that the amplitude of the membrane be constant over the audible range. Since the membrane of the microphone is exposed to the sound fleld on both sides, the difierence in sound pressures at the tront and rear sides is proportional to the drive of the membrane. This sound pressure difierence has, however, a rising frequency characteristic up to that frequency at which the half wave length corresponds the difierence in path between the front and rear sides of the microphone button.

During the movement of the membrane the quantity of air located between the membrane and the fixed electrode is displaced. The force necessary for this displacement increases with increasing frequency. An increasing force having this dependence on frequency is available, however, in the operative pressure diflerence, so that the membrane executes oscillations of the same amplitude over the whole range of frequencies.

Since, according to the invention, the distance between i;he membrane and the perforated plate is small in proportion to the diameter of the perforatlons and the spacing thereoi, with the effect that the membrane would sufier a. high damping by friction of air, therefore, a completely horizontal frequency curve over the' whole range of sounds would only be obtained if Lhe Iriction control widely exceeds the mass control and the elastic control of the membrane. Since this is not technioally desirable on grounds of sensitivity, and the emplomnent:v 01 a n. t im great friction 1s adi antageous, there 1s obtained in practical constructions a dependence on trequency having the characteristic of a highly damped resonance curve, a.s shown in Figure 2.

The frequency is plotted as abscissae F and the amplitude 01 the membrane as ordinates A. The resonance curve is shown by a full line. Its flat course with a weak maximum at the point W: shows the high degree of damping. The damped natura] Irequency of the membrane 1s arranged to lie in the middle range cf Irequencies by a proper selection 01 its mass and its elasticity. This curve can, by the following measures, be improved so that the Irequency curve, shown m dash-lines and having for the most part a horizontal cou rse, is obtained.

For the high frequencies the arrangement of a, flat air cavity in front of the membrane brings about an increase in the driving force. By suitable dimensioning of the air cavity the position and the degree of this increase can be suited to the falling characteristic of the frequency curve (right band part of the curve, Figur-e 2).

For the lower range of sounds the fall 015 the irequency curve (left band part of the curve, Figure 2) can be compensated by coupling one or more closed air cavities with parts of the mem brane. Figure 3 shows such a construction by way of example. A completely closed cavity L is coupled. by means of a tube K with the space between the membrane M and. the fixed electrode E. By suitable shaping and dimensioning this coupled closed air cavity the strength and progress with frequency of the desired compensatlng increase can be dimensioned for the fall at lower frequencies.

I1 a microphone constructed in accordancc with the invention is of such dimensions that its diameter is not greater than the wave length u! the highest frequency 1:0 be transmitted, there is obtained a microphone which is extremely satisfactory in operation, since the satisfactory properties of directional response are further supplemented by a good frequency curve. The upper limit of the diameter lies a.t about 3 cms. corresponding to a wave length of 10.000 Hertz (cycles per second). F01 the lower limit the consideration arises in connection with the construction that with reduction 015 the diameter the sensitiv ity of the microphone is reduced. It follows from this that the most satisfactory dimension of the microphone described is such that even having regard to the lower limit of sensitivity the diameter is approximately 3 cms.

The basic form cf the pressure gradient microphone, which as above described possesses a fixed perforated platze, can be altered by mounting a second membrane on the rear side cf this fixed plate. The action obtained thereby will be explained more detailed in the following.

For a large number 015 purposes of employment a condenser microphone would be required which, whilst retaining its otherwise good transmitting properties accepts sounds Irom a particular direction selectively and is less sensitive for the remaining directions of incidence. This property can be achieved by the arrangement constructed as described in the following and illustrated by way of example in Figure 4 in cross-section.

011 both sides o f a. metallic platze b provided with bares a extending through it, which plate constitutes the fixed plate of the condenser, are mounted two membranes c and d having similar mechanical properties. One mexnbrane c ls electrically conducting and is used as the counterpla.te or electrode for the perforated plate or electrode b; the other membrane d is electrlcally operative. Under influence of the sound waves such a system of two membranes executes two principal movements. 011 the one band the sound pressure causes a. movement with respect to one another of the two membranes wlth compression of the enclosed volume of air. On the other band there occurs, owlng to the diflerence in the sound pressures actlng on the two sides a. parallel displacement of both membranes, the enclosed volume cf air belng carried along with them. The manner 013 opera.tlon for sound ineidence from various dlrections is shown in Figures to 7. In the case of lnoidence of sound normally to the electrlcally operative membrane (Flgure 5) the sound pressure represented by the arrow S, causes a membrane movement of the sense and magnitude of the arrows S1, S2, and the sound pressure difference causes a movement of the sense and magnitude of the arrows S1, S2. The resultant movement of the electrioally operative membrane c is proportional to the sum of the two sound pressures acting on lt, that is Sri-S1.

It is represented in Figure 5 in direction and magnitude by the arrow P. In the case of lateral sound incidence represented in Figure 6 by the arrows S, the arrows S1, S: again represent the membrane movement under the influence of the sound pressure. Since they are equal but oppositely directed, there is no sound pressure difference (represented by 0). The arrow P represents the small movement in this case cf the membrane c. In the case of incidence of the sound from the rear on to the electrically inoperative membrane d (arrow S in Figure 7), the arrows S1, S2 as in the previous cases show the membrane movement under the influence of the pressure and the arrows S1, 82 represenl: the membrane movement under the influence of the pressure difference. The resultant movement of the operative membrane c is in thls case zero, as S1 and s1 are oppositely directed. Movement of the ele :trically inoperative membrane d only occurs. Such a microphone possesses on this a.ccount a. directional response characteristic in the form of the cardioid shown in Flgure 8. In this case the microphone is to be regarded as being on the horizontal line parallel with the membrane, so that the vertical line in Flgure 8 is perpendicular to the electr ically operative membrane.

The condition for a decrea.sing or vanishing sensltivity for sounds from rearward directions is the equality of the membrane displacements under the influence of the sound pressure and under the influence o1 the sound pressure diflerence. The displacement of the membrane caused by the sound pressure diflerence is determined by the properties controlling the movement of the membrane and the magnitude of the sound pressure difierence. The latter ls determined by the length of path between the front and real sides of the membrane, i. e. simply by the diameter of the microphone.

For the production of a. straight line frequency curve the movement of the membrane mu.st be principally determined by friction. Similarly for the procluction of a constant dlsplacement of the friction controlled system independent of the frequency, the driving force must increase with the frequency, i. e. the pressure diflerence must ipcrease as fax as the upper limiting frequency of the transmission range. This requirement is fulfilled if the length of path between the front and rear sides of the membrane for the upper llmlting frequency is smaller than a half wave length. Aocordingly the dlameter is preferably arranged to be about 3 cm., as above explained.

The electrode b possesses, accordlng to Figure 4, besides the hores a extending through lt a number of shorter bores e not extending through it. Their number, size and arrangement influences the membrane displacement caused by the action of the sound.

The requirement referred to above, that the cophase membrane movement shall be friction controlled, has the result that the sensitlvlty of the microphone is reduced in proportion. It is however, possible to compensate for the falling- 01T in sensitivity in the case of reduced friction for the low frequencies due to the elastic properties, by increasing the pressure difference for these frequencies. This can, for example, be done by surrounding the microphone button, as shown in Figures 9 and 10, with an annular sheet f of a materlal such as possesses a sound permaability dependent on frequency in the sense that the outer ring or annules oifers a greater resistance to the passage of sound at low frequencies tha.n at middle and high frequencies. An arrangement such as is illustrated in Figures 9 and 10 acts for the low frequencies as if the diameter of the microphone had been increased. A suitable material for the sheet f is ganze.

The described microphone arrangement may also be so constructed that it can be used as a directional or non-directional microphone according to the manner in which it is connected. For this purpose it ls only necessary to make both membranes electrically conducting. For a directional m crophone one membrane remains unconnected, and for a non-directional microphone both membranes are used connected in parallel.

Having now particularly described and ascertained the natura of nur said invention and in what manner the same is to be performed, we declare that what we claim is:

1. In a condenser microphone a fixed condenser plate having transverse perforations extending from side to side and also having recesses extending from each platte side only partway to the other side, a diaphragm constituting the movable condenser plate and being spaced from said fixed plate by a. distance small compared with the diameter and spaclng of said perforations and recesses and of a value which would render the frictional air damping on said dlaphragm hlgh as compared with its massand elasticity damping and a. second diaphragm disposed on the other side of said fixed platte at a distance similar to the spacing of said first mentioned diaphragm.

2. A condenser microphone having a flxed elentrode and a movable diaphragm electrocle flxed only at its periphery and spaced from sald fixed electrode, said flxed electr0de having a relatively small number of wide-spaced transverse perforations through which the side of the diaphragm electrode facing the fixed electrode is exposed to the effect of the sound waves, tl1e distanoe between the diaphragm and said fixed electrode being small with respect to the diameter and the spaclng of said perforations and of a value which would render the frictional air damping on the diaphragm high as compared with its massand elasticity damping and a plane sheet of material surrounding said fixed electrode in its plane, said material having a sound transmission Which decreases with increasing frequency.

3. A condenser microphone having a fixed electrode und a movable dtaphragm electrode flxed only atlts pertphery and spaced from said flxed electrode, said flxed electrode havlng a relatlvely small number cf wlde-spaced transverse perfora ttons. the dtstance between t;he dlaphragm and said flxed electrode betng small wlth respect t o t;he dtameter and the spaclng 01 said pertorations und. 01 a value whlch would render the h lctlonal alr damplng an the diaphragm hlg'h 9.s compared wttzh ltzs massa.nd elasticttydamping a second diaphragm disposed on the other side 01 said flxed electrode at a distance slmilar to t:he spacing of said flrst-mentloned dlaphragh and adapted to transmtt: the sound waves through said perforations t;o said flrst dlaphragm, and a plane sheet; 01 materlal surroundlng said flxed electrode in lts plane, said materlal having a sound transmisslon whtch decreases wlth lncreasing frequency,

4. In a condenser microphone a flxed condenser plate having a relatively small number 01 transverse perforatlons untformly spaced over its area, and havlng a diameter not greater than the wave length 015 sound at the highest frequency to be transmltted, and a. movable condenser platze of simllar dtameter and being spaced from one side cf said flxed platze a dlstance small with respect t:o the diameter and spacing 012 said perforations and of a value which would rentier the frlctional alr .damplng on t:he diaphragm hlgh es compared wlth its massand elastlcity damping, a.nd whereby the sound act1s upon bot h sides o1 said dlaphragm, and a plane sheet of matiarial surroundlng said flxed electrode in 1ts plane, said materlal having a sound transmlssion whlch decreases with increasing trequency.

5. In a. condenser microphone a fixed condenser platze having transverse perforattons extending from s1de to side and also having recesses extendlng from euch plate stde only partway to the ot;her slde, a dlaphragm constitutlng t;he movable condenser plate and being spaced froxn said flxed platze by a distance small compared with the diameter and spacing 015 said perforattons a.nd recesses and of a value whlch would render the trictional a.ir damptng on said diaphragm high as compared with it:s massand elasticity damplng,

and a second diaphragm disposed an the other side of said flxed platze at; a distance simllar to the spacing of said flrst mentioned diaphragm, the diameter of said fixed condenser plate and of said dtaphragms belng not great;er than tlxe wave lengt:h o1. sound at the hlghest; trequency to be transmltzted.

6. In a condenser microphone a flxed condenser pla.te havlng transverse perforatlons extendlng Iro'm side to slde and also havlng recesses extend- Ing trom euch platze side only partway to t:he other slde, a. dlaphragm constzituttng the movable condenser plate and belng spaced from said fixed platze by a distance small compared wlth the diameter andspacing of said perforatlons and recesses and o! a. value whlch would render the trict;lonal a1r damplng on said dlaphragm high as compared wtth its massand elasticlty damping, and a second dlaphragm disposed on the other sldeof said flxed platze ab a distance simllar to the spacing cf said first: mentloned diaphragm, the diameter of said flxed condenser platze and of said diaphragms being not greater than the wave lengt:h of sound ab the hlghest; frequency to be transmitted, and a plane sheet of material surroundlng said flxed electrode in its plane, said material having a. sound transmisslon which decreases with increasing Irequency.

'7. A condenser microphone having a fixed electrode and a movable diaphragm electzrode fixed only at; its perip'hery and spaced from said fixed electrode, said fixed electrode having a relatlvely small number of wide-spaced transverse perforations through which the side of the diaphragm electrode faclng the flxed electrode is exposed t:o the efiect cf the sound wavas, the distance between the dlaphragm and said fixed electrode being small w1t:h respect: to the diameter and t;he spaclng 013 said perforatlons and of a va1ue which would render the frictlonal air damping on the diaphragm high as compared wit;h its massand elasticity damping. a second diaphragm disposed on the other side 01 said flxed electrode at; a distance similar t;o the spaclng 0f said first-mentioned diaphragm, and a plane sheet of material surrounding said 'flxed electrode in its plane, said material having a sound transmtssion which decreases wlth increaslng frequency, the diameter of said fixed condenser platze and of said diaphragm being not; greataer than t:he wave length 0f sound at: the highest: frequency t;o be transmitted.

HANS JOACHIM vor: BRAUNMHL. WALTER WEBER.

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