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Publication numberUS2301744 A
Publication typeGrant
Publication date10 Nov 1942
Filing date31 May 1941
Priority date31 May 1941
Publication numberUS 2301744 A, US 2301744A, US-A-2301744, US2301744 A, US2301744A
InventorsOlson Harry F
Original AssigneeRca Corp
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Electroacoustical signal translating apparatus
US 2301744 A
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Description  (OCR text may contain errors)

Nov; l0, 1-942. H. F. oLsoN 2,301,744

ELECTROACOUSTICAL SIGNAL TRANSLATING APPARATUS Filed May 31, 1941 '7 Sheets--Sheekl l lil-gz@ @5a. lkfcr/a/VHL PEM/WY fyi- C CDA :Snnentor Gttorncg H. F. OLSON Nov. 10, 1942.

ELECTROACOUSTICAL SIGNAL TRANSLATING APPARATUS Filed May 31, 1941 7 Sheets-Sheet 2 Seam LK S 9759/920 May/Wagga N nm s NWN l Zhwentor 19209 017/ G ttorneg Nv. 10, 1942. H. F. oLsoN 2,301,744

ELECTROACOUSTICAL SIGNAL TRANSLATING APPARATUS Filed May 31, 1941 7 Sheets-Sheet 3 Bg M. Gttorneg H. F. OLSON Nov. 10, 1942.

ELECTROACOUSTICAL SIGNAL TRANSLATING APPARATUS 7 Sheets-Sheet 4 Filed May 31, 1941 Nov. 10, 1942. m H, F, OLSON 2,301,744

ELECTROACOUSTICAL SIGNAL TRANSLATING' APPARATUS Filed May 3l, 1941 7 Sheets-Sheet 5 Nov. 10, 1942. H, F, QLSON 2,301,744

ELECTROACOUSTICAL SIGAL TRANSLATING APPARATUS Filed May 31, 1941 7 Sheets-Shee'l'l 6 Illlllllllllllll Smaentor Bg y . Gfforneg H. F. OLSON Nov'. 10, 1942.

ELECTROACOUSTICAL SIGNAL TRANSLATING APPARATUS Filed May 31,l 1941 '7 Sheets-Sheet 7' BSS UBI'

. Patented lova l0, 1942 Harry F. Olson,

Haddon Heights, N. J., assignor to Radio Corporation of America, a corporation of Delaware .application 'May si, 1941, serial No. ssegovs 32 Claims.

This invention relates to electroacoustical signal-translating apparatus, and more particularly to directional microphones.

Directional microphones may be divided into two classes as follows: rst, the wave type ,rmicrophones which depend for directivity upon wave interference; and second, gradient type microphones which depend for directivity upon difference in pressure (or powers of difference in pressure) between two points. Typical microphones of the first class, that is, those in which the directivity depends in some way upon wave interference, are the reflector, lens, and line microphones. In the second class of microphones, the`most common example in use today is that in which the active element is responsive to the velocity, or pressure gradient, component of the sound wave.

To obtain any semblance of directivity in the first class of microphones mentioned above, the dimensions of the microphone must be comparable to the wave length Thus, while such microphones give fairly good efficiency, they are open to the objection that they 'must be big and rather cumbersome in order to be responsive to acoustical Waves in the lower range of the audio spectrum. Obviously, this imposes a serious limitation pon such microphones in many elds of use, as in motion picture work, for example;

The directional characteristics of a simple,

symmetrical pressure gradient responsive microphone are bidirectional. I have discovered that the directional characteristics of combinations of` gradient microphones .of diierent orders, or combinations of gradient elements and appropriate -delay systems, are unidirectional. Employing this discovery, I have found that it is possible to provide unidirectional microphones which not only have a highly directional response over a wide range, but are also relatively small insize while still retaining high directivity at the low frequencies, -andthe primary object of my present invention is to provide a highly directional microphone which is free from the above-noted objections present in prior art microphones which have highlydirectional response characteristics.

Still anotherand important object of my present invention is to provide a highly directional microphone as aforesaid the directional response of which is independent of the frequency.

It is also an object of my present invention to provide a novel highly directional microphone as above set forth which is relatively simple. in construction commensurate with the results attained thereby, and which is highly eicient in use.

In accordance with my present invention, I employ at least one pair, and in some cases a plurality of pairs, of microphone units spaced apart a distance of the order of one-half the wave length of the highest frequency in the range to which the system is responsive and small compared to the wave length of the y'lowest frequency in the range to which the system is responsive, so that each of the individual microphone units will be actuated by the Vacoustical waves from a signal source in slightly out-of-phase relation. The outputs of the individual microphone units may then be combined effectively in opposed f More particularly, itis an object of my present inventionto provide a novel directional microphone having linear dimensions which are small compared to theywave length at the lower end of the response range.

Another object of-my present invention is to provide improved highly directional microphones having either bidirectional or unidirectional characteristics.

phase relation electrically directly, or a suitable delay may be introduced in the channel including one of the microphone units to provide an ultimate output which is a measure of the directivity of the signal source' with respect to the microphone axis. Various arrangements of unidirectional microphones or velocity microphones, as the case may be, are possible to provide various degrees of directivity, and these will be more fully set forth hereinafter.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. 'Ihe invention itself, however, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description of several embodimentsgthereof, when read in connection with the accompanying drawings, in which Figures la, 1b and 1c are diagrammatic illustrations of various orders of the gradient type microphones which may be employed in accordance with the present invention,

Figures 2a, 2b, 2c, 2d and 2e are curves showing directional patterns for gradient microphones of five different orders of the type illustrated in f igures 1a, 1b and 1cl Figures 3a, 3b and 3c are curves showing frequency response characteristics for the micro` phones illustrated in Figures 1a, 1b and 1c,

Figures 4a, 4b and'4c are diagrammatic illustrations similar to Figures 1a, 1b and 1c, but including equalizers for obtaining particular frequency response characteristics,

Figures a, 5b and 5c are curves showing equalizer characteristics for the equalizers required with the microphones illustratedyi'n Figures 4a, 4b andlc to provide the vfrequency response characteristics for these various microphones shown by the curv-es of Figures Sai-6a', Gli-6b', and 6c-6c,

Figures 6a, 6b and 6o are response curves show- `lng the frequency response characteristics of the microphones illustrated in Figures 4a, 4b and 4c at large distances only, and Figures 6a', 6b' and 6c' are similar curves showing the frequency re- IFigure 10 is a diagrammatic View showing ane' other form of my invention employing a pair of velocity, or pressure gradient, responsive microphones, I

,Figure 11 is `a series of directional response curves showing the response of the individual microphones of the modiiication of my invention shown in Figure 10,l as well as the overall response of the system shown in this figure,

Figures 12 and 13 are diagrammatic views showing a modified form of the system shown in Figure 10, f f

Figure shows a pair of curves representing the directional characteristics of either of the microphone units of Figure 22, and the directional characteristic of the combination thereof as se up in the system of Figure 21, l

Figure 26 is a diagrammatic View showing still another form of my invention in which a pluy rality ofy pairs of microphones are employed for obtaining a very highly directional response,

Figure 27 is a front elevation of two pairs of microphone. units such as are employed in the system of Figure 26,

' 'Figure 28 is a side elevation of the microphone units shown in Figure 27,

Figure 29 is a sectional view taken on the linen XXIX- XXIX of Figure 28,ia7nd A Figure shows a set of curves similar to Figure 25 but applicable to the system of Figure 26. Before referring lto the accompanying drawings in greater detail and considering the .various modifications of my invention shown therein, it is believed well to point out that gradient microphones are "of various orders from zero on up. A gradient .microphoneof the order zero is a microphone in which the electrical response corresponds to the pressure in the actuating sound wave. Such a microphone may be illustrated, for example, by the symbol.y I in Figures 1a and 4a.

Figure 14 shows a pair of directional response curves one of which shows the directional characteristic of each of the individual microphones of the modications of my invention shown in Figures 12 and 13, and the other of which shows the directional characteristic of the combination of these two microphones with the delay networks included in these modications,

Figure 15 is a curv'eshowing the phase frequency characteristic of the lter or phase shifter of Figure 12,

Figure 16 shows a pair of curves representing the outputof a system such as that shown in IFigure 12 when employing two different types of velocity microphones, l

Figure!` 17 is a curve showing-the phase shift in the network of Figure 13,

Figure 18 is "a curve showing the phase shift in the two microphones of Figures 12 and 13 due to the distance between them,

` Figure 19 is a curve showing the lenergy response of the modifications of my invention v.shown in Figures 10, 12 and 13 as a function of the'rati-o of the delay introduced in one of the transmission channels to the delay resulting from the spacing of the individual microphone units,

Figure 20 is a curve showing the directionall The pressure in a sound wave may be written =cgi1 sin Met-r) (1) TLH -p- T sin k(ct r) (2) where c==velocity of sound,

C=211/)\, `=wave length, y pI-:density of the medium in which the sound wave travels, i A=amplitude of the velocity potential, r=distance of the microphone from, the

sound source,-

t=time, and .A

vz m=amplitude of the pressure. To illustrate the characteristics of gradient microphones, it will be assumed that the elements of the gradient microphones are made up of units and that the voltage output of these units is proportional to the sound pressure and in phase with the actuating sound pressure.

At a iixed point, in a sound wave in which the pressure amplitude isindependent of the frequency, the pressure available for actuating the vibrating system of, the' microphone is independent of the frequency, as shown by the curve in Figure 3a. In considering the gradient microphon'es here, it will be assumed that the dimensions of vthe lsystems are small compared to the wave length. Therefore, the pressure which actuates the microphone is independent of the direction of the incident sound. Under these conditions, the pressure microphone is nondirectional, as shownby the curve of.Figure 2a.- The energyfresponse of this microphonato random sounds is unity.

Under the above conditions, a single unit constitutes a gradient microphone of order zero or, as it is commonly termed, a pressure microphone. The voltage response of this microphone is independent `vof the frequency as shown by the curves in Figures 6a and 6a. Furthermore, the shape of the response frequency characteristic is independent of the distance from the sound source (see Figure 6a').

A gradient microphone of order one is a m- Ap=kcpAD[kr cos Met-122+ sin Mci-1)] cos 0 (3) or Apzpmplkr cos Mot-'22+ sin Mat-4)] cos 0 (4) where D--distance between the two points,

6=angle between the direction of the incident' sound and the line joining the two points,

and the remaining terms have the same meanings as above in Equations l and 2.

Ata fixed point in a sound wave located a dis-V tance of several wave lengths from the source, the difference in pressure, for a constant sound pressire in the sound wave, is proportional to the frequency; that is, the pressure available for actuating the vibrating system is proportional to the frequency (see Figure 3b). The directional response of the microphone is proportional to the cosine of the angle between the direction of the incident sound and the line joining the two points (Figure 2b) The energy response of this microphone to random sounds is one-third that of a nondirectional microphone.

A gradient microphone of. order one may be made up of two oppositely phased units. The output of two units of opposite phase, as shown in Figure 1b, is proportional to the frequency. Therefore, an equalizing system must be intro-k duced in which the response is inversely proportional to the frequency if a flat response is desired. Such an equalizing system is represented by the block 3 in Figure 4b and has the characteristic shown in Figure 5b. In the case of the velocity microphone, which is the outstanding example of a first Vorder gradient microphone,Y

this is very simply accomplished by using a masscontrolled element. The response frequency characteristic with a compensating element or equalizing system is, independent of the frequency when the distance between the microphone and the sound `source is several wave lengths (see Figure 6b). Unlike the single-unit, zero-order-gradient, or pressure, microphone, the voltage response of the first order gradient unit does not correspond to the sound pressure when the distance between the sound source and the microphone is small compared to the wave length but is accentuated as the frequency' or distance is decreased. 'Ihe response for various distances, as a function of the frequency, is shown in Figure 6b.

A gradient microphone of order two is a microphone in which the response corresponds to the pressure gradient of the pressure gradient. The difference of the two two-point-systems may be written where D1=distance between the points in a pair of points, Dz=distance between the two pairs,

difference in pressure between and the remaining termsagain have the same means as used above.

At a fixed distance in a sound wave located at a distance of a large number of wave lengths,

the difference of the difference in pressure for a constant sound pressure in the sound wave is proportional to the square of the frequency (see Figure 3c) The directional response of the microphone is proportional to the square of the cosine of the angle between the direction of incident sound and the line joining the system of points (see Figure 2c). The energy response of this microphone to random sounds is one-fifth that of a nondirectional microphone.

A gradient microphone of order two is made up of a pair of oppositely phased elements, the outputs of which are oppositely phased, as illustrated bythe symbols 2, 2 of Figures 1c and 4c. 1n this case, there must be used two compensating or equalizing systems 3, 3, in each of which the response' is inversely proportional tothe frequency, vin combination with a third equalizer 6 (Figure 4c), the response of which is also inversely proportional to the frequency. A single compensating system 4, the response of which is inversely proportional -to the square of the frequency, may be used alone without'the equalizers 3, 3, if desired. The equalization characteristic required to obtain uniform response with respect to frequency is shown in Figure 5c. The response vfrequency characteristic with a compensating element is independent of theirequency when the distance between the microphone and the sound source is several wave lengths (see Figure 6c). The response as a function 4of various distances is shown in Figure 6c'. From an inspection of Figures 6b' and 6c', it will be seen that the response at the low frequencies is more accentuated with the system of Figure 4c than with the system of Figure 4b.

The characteristics of a gradient microphone of any order may be carried out following the procedures as outlined above. The general expression for any order n of the difference in pressure between two poi ts separated by a distance A1' is Thisexpression shows that the pressure available for driving the microphone is proportional to the nth power of the frequency. The directional characteristics are cosine functions, and the power of the cosine is the'order of the gradient. The energy response to random sounds is (21H- 1) that of a nondirectional microphone. The directionalv characteristics of gradient microphones of the order zero, one, two, three, and four are shown, respectively, in Figures 2a, 2b,4 2c, 2d and 2e.

It has been shown in the preceding discussions that the directivity of a gradient microphone increases with increasing powers of the pressure` gradient. The directional characteristics of these systems are of the bidirectional type.

In many applications, unidirectional characteristics are more desirable. A gradient microphone may be combined with a suitable delay system the frequency for to obtain a unidirectional microphone. Two oppositely phased units one of which is connected directly to a response correcting system while the other is connected to the response .correcting system through an interposed delay system is one example of how this may be done. The output of such a combination is given by es :voltage output constant which depends upon the characteristics of the units and the frequency D1=distance between the units, and

Dz--path length of the delay.

By suitable choice of the constants D1 and D2, all directional patterns between the bidirectional cosine characteristic and the nondirectional characteristic are possible, the transition being from bidirectional, through unidirectional, to nondirectional.

. To providel a system having unidirectional characteristics exhibiting a high order of directivity, two gradient microphones of order i mayy be provided with a suitable delay systemin the put of a system of this sort is given by where en :voltage output constant which depends upon the characteristics of the gradient microphone units of order l and the frequency, D1=distance between the units, and

' Dz=the path length of the delay.

- dient microphones with a suitable delay system in one of the combinations to` provide a unidirectional microphone exhibiting a high order of directivity.

Referring, now, more particularly to the drawings, I have shown, in Figs. '7, 10, 12, 13, 21 and 26, various arrangements of microphone units and systems based upon the principles discussed above for the purpose of providing either unidirectional or bidirectional microphones which exhibit a high order of directivity.

In Fig. '7, there are shown two similar microphone units 5 and 6 spaced apart from each other a distance d and connected either in series or in parallel relation, as may be deemed most desirable.. The distance d should be approximately one-half the wave length of the highest frequency inthe range to which the system is responsive and small compared to the lowest frequency in said range. The outputs of the microphone units 5 and 6 may beamplified by a suitable ampliferl, the output of which may beemployed to operate any suitable output member (not shown), such as a loudspeaker, a recording device, or the like. The individual microphones 5 and -6 are preferably of the type shown in my copending application Serial No.

3l2,053, ;led January 2, 1940, and employ a conductive ribbon-in the air gap of a magnetic 1 output channel of one of them. The voltage outless to provide any of the directional characterstics shown by the curves I, II, III, IV or V of Fig. 8.

Assume that the microphones 5 and 5 are set up to operate .as unidirectional microphones. The directional characteristics of each of the Inicrophones 5 and 5 will be e=K M+m cos 0) cos wt (10) where K= output voltage constant of the combined units.

.Taking a point midway between the microphones 5 and 6 as a-reference point for the signal source, so that acoustical waves therefrom will reach each microphone in the same phase, and assuming that the outputs of the microphones 5 and 6 are connected in opposition elec-` trically, the output of the combination will be eC=K[M-im cos 0] where d=distance between the microphone units. Now let d then where Ki=output voltage constant of the combined units.' For an angle, 0 between the direction of incident sound and a line joining the microphones 5 and 6 through their vertical axes, the distance between the twomicrophoneunits is foreshortened by a factor cos 0. Thevoltage output for an angle 0 is e@=K1(M-im cos @ix cos 0 (13)/ and the directional characteristic is R9=(M+m cos 0) cos 0 (14) The curves I', II', III', IV', and V of-Fig. 9 correspond, 1"espectively,` to the curves I, II, III, IV, .and V of Fig. 8. From an inspection of these two figures, it will be seen that the response of the combination of microphones 5 and 6 arranged as above described is -much more directive than that of the individual microphones 5 and 6.

In 'the system illustrated in Fig. 10, I employ a pair of velocity responsive, or bidirectional, microphones l0 arid I-I spaced apart a distance d equal to approximately one-half the wavelength of the highest frequency in the range to which the system is responsive and small compared toy the lowest frequencyin the same range. The microphone I0. may be coupled directly to a suitable amplier and equalizer l2 of the type dis'- cussed above, while the microphone Il is connected to an amplifier I3 the output of which drives a loudspeaker I4 which retranslates the pulsating electrical signalenergy into acoustical energy. A third microphone,` preferably a pres-l y e15=[:cos wt cos (360) -sin wt sin 360] cos 0 sure microphone having an active element or ribbon I5 vibratively mounted in the air gap of a suitable magnetic field, is spaced from the loudspeaker I4 a distance D, a tube I6 coupling the loudspeakerv I4 to the ribbon element l5 and constituting -an acoustic path therebetween. The tube I6 has a portion I6a lled with tufts of felt I1 and constitutes an acousticalgresist-l ance which terminates the ribbon element I5. Due to the spacing between the loudspeaker I4 and the microphone unit I5, a time lag or acoustical delay is vintroducedinto the channel including the microphone II. The outputs of the` microphones I and II may be combined effectively in oppositeY phase in an electrical senseY through the microphone I5 and, because of the T he ouput of the microphone I I is e15={cos (wt--DBSWN cos 0 Equation 15 may be written sin wt sin {ii-60 cos 0H cos 0 (17) and Equation 16 may be written For D 7i and d 1 e,=[cos rut-sin at 360 cos 0)] cos 6 (19) Vand The difference exo-eis, or'the ouput cf the combination,-ls

The ratio of the response for the angle 0 to the response at 6:0 is

v Ri I+D 22) It will be seen fromtheforegoing that the combination of microphones I0 and I I with the delay D in the transmission line connected to the microphone II results in a combined response for the system ,which is more directive than for either of the microphones I0 or II alone. This cos 9 -result is represented by the curves IX, X, XI, XII

i may be employed and thus a networks 20 and 2i produce, electrically, the delay that is introduced acoustically in the system of Fig. due to the distance D between the loudspeaker I4 and the microphone unit l5. In the case of the system shown in Fig. 12, the phase shift of the network is practically proportional to the frequency up to 180, as shown by the curve in Fig. 15. Of course, more filter sections greater phase shift may be obtained. The same is true of the lter shown in Fig. 13. It will be apparent that the sam'e directional characteristics can be obtained with the systems of Figs.,12 and 13 as with the system of Fig. 10, and the same theory applies. The curves XIV, XV of Fig. 14 show, respectively, thel directionalcharacteristics of the indi-` vidual microphones I0 and II of the systems oi Figs. 12 and 13, and the combination of these two microphones with the networks. An increased output may be obtained with the systems of Figs. 12 and 13 by employing a gradient microphone of the type shown, for example, infrny copending application Serial No. 376,861, filed January 31, 1941, wherein a slik baille is employed around the ribbon element of the microphone. The curve A of Fig. 16 shows the output that may be obtained with the conventional velocity vmicrophones, and the curve B shows the increased output that may be obtained when using microphones with the silk bailles. The curves of Figs. 17 and 18 show, respectively, the phase shift in degrees in the network of Fig. 13 and the phase shift in degrees due to the spacing of the microphones I0 and II a distance d.

The energy response, E. R., of the systems shown in Figs. 7, 10, 12 and 13 is given by The energy response as a function of the ratio D/d is shown by the curve of Fig. 19. The minimum energy response is 0.125, or in the ratio of 1 to 8, whereas the energy response of the velocity microphone or of the cardioid unidirectional microphone is 1 to 3. The curve of Fig. 20 shows more particularly the percent response which may be obtained with the systems thus far described.

In the system shown in Fig. 21, I employ a pair of microphone units S and T of the type responsive to the ,pressure gradient component of an acoustical wave and each therefore having a 'bidirectional characteristic. The units S and T include, respectively, the conductive ribbon elements 25 and 26 which are shown in spaced relation to .each other in a common plane in Fig. 21 merely for the sake f clearness and to denote two dif'- ferent electrical units. Actually, however, these two electrical units may be embodied in one physical unit with the ribbon element 25 vibratively supported in the air gap 21 between a pair of pole pieces 28, 28, and the ribbon element 26 similarly supported behind the ribbon 25 in an air gap 29 between a pair of pole pieces 30, 30, as clearly shown in Figs. 22 to 24. A pair of magnets 3i, 3i secured to the ends ofthe pole pieces 28 and 30, and therefore common to all these pole pieces, supply the necessary flux. 'I'he ribbon elements 25 and 26 are disposed in parallel planes spaced from each other axially of the microphone a distance o`f\approximately one-half the wavelength of the highest frequency in the range to which the system is responsive and small compared to the wavelength of the lowest frequency to which the system is responsive in the same range. i

The output of the ribbon element 25 is connected by a transmission line which includes an Aamplifier 32, a loudspeaker 33 and a tube 34 to one side of'a conductive ribbon element 35 of a,A

third pressure gradient responsive microphone unit U. The tube 34 serves as an acoustical path between the loudspeaker 33, which retranslates into acoustical energy the electrical pulsations derived from the ribbon element 25, and the ribbon 35 and conducts the retranslated acoustical signals to one side of the ribbon element 35, the portion 34a, of the tube 35 being lled withfelt tufts 36 and constituting lan acoustical resistance for terminating one side of the ribbon element 35. The output of the ribbon element 26 is similarly connected to the. opposite side of the ribbon element 35 by a transmission line which includes an amplifier 3l coupled to a loudspeaker 38 and a tube' 39 having a bend 39a therein, the portion 39h of the tube 39 being also filled with tufts ofl felt 35 for -providing an acoustical resistance for terminating the other side of the ribbon 35. Dueto the bend 39a in the tube 39, it Will be obvious that a delay is introduced in the transmission line coupled to the ribbon element 26 and that, therefore, a pressure gradient will be set up between the two opposite sides of the ribbon element 35, the output of which may be utilized as the ultimate response from a source of acoustical signal energy Q which is at an angle with respect to the axis of the physical microphone unit shown in Figs. 22 to 24.

The system shown in Fig. 21 is essentially the same as that shown in Fig. 10 save that, instead of employing a pressure operated microphone in the line (such as the microphone i of Fig. 10) and connecting the outputs in-opposition elecftrically, the system of Fig. 21 employs the pressure gradient microphone unit U which measures thed'ierence in pressure in the'two tubes 34 and 39. The theory involved is, howeven'essentially the same in both systems, and the directional patterns that can be obtained with theV system of Fig. 21 are the same as those obtainable with the system of Fig. 10. `Assuming that each .ofthe electrical microphone units S and T .has

the same sensitivity and that each has a directional pattern as shown in curve XVI of Fig. 25, the ultimate directionalpattern of the system, or combination of units S, T and U, will be that shown in curve XVII of Fig. 25.

The modification of my invention shown in Fig. 26 is merely an extension of that shown in Fig. 21 and consists, essentially, of two systems of the type sh`own in Fig. 21 with their respective outputs combined in a third similar system. The

' system of Fig. 26 employs, as the pickup devices,

four pressure gradient responsive .electrical microphone units S, T, V and W, the units S and T operating jointly as one pair, and the units V and Vi@ operating jointly as a second pair; Each of the units S, T, V and W may be constituted as a physical entity as well, but I prefer to combine them all in one physical unit as shown in Figs. 27 to 29 wherein the magnets 3|, 3| 'arel `again common to all ofthe pole pieces. `In addition to the ribbon element 25 and 26, which act jointly as one paii, this physical unit also in-y cludes the ribbon elements 4| and 42 which fgrm a part of the electrical microphone units V and W, respectively, and also act jointly as a second I pair. The spacing of the ribbon elements 25, 26, 4| and 42 from eachother is approximately the same but, in any case, the ribbons 4|. and 42 should be spaced apart a distance which is of the order ofone-half the wave length of the highest y"frequency in the range to which the system is responsive and small compared to the lowest frequency to which the system is responsivein the same range, similar to the spacing between the ribbon elements 25 and 26.

As in the case of the modication vof Fig. 21,

cludes an amplier 44, a loudspeaker and a.

tube .46 which, like the tube 34, has no bend therein and terminates in a portion 46a, lled with tufts of felt. The output of the ribbon element 42 is connected to the opposite side of the conductive ribbon 4 3- by a transmission line which includes an amplifier 4l, a loudspeaker l5 and.

a tube t8 having a bend 49a therein for intro ducing an acoustical delay similar to the bend 39a of the tube 39 and terminating in a portion 49h which is also lled with tufts of felt. Thus, the two transmission lines connected to the ribbons il and l2 will feedvacoustical energy to the opposite sides of the ribbon 53 in out-of-phase relation due to the bend 49a whereby to set up a pressure gradient between the two sides thereof and thereby actuate the ribbon Q3.

The microphone units U and Y operate jointly, and their respective outputs may be applied to the opposite sides of a seventh pressure gradient responsive microphone unit Z having a conductive ribbon 50. For this purpose, the output of the ribbon element 35 is connected to one side of the ribbon 5|) by a fifth transmission line which includes an amplier 5i, a loudspeaker 52 and a tube 53 which has no bend therein and which terminates in a portion 53a iilled with tufts of felt or the like. Similarly, the output of the ribbon element 43 is connected through an ampliner 54, a loudspeaker 55 and a tube 56 having a bend 56a therein to the opposite side of the ribbon element 50, the portion 565 of the tube 56 being also lled with tufts of felt. Since the acoustical path provided by the tube 56 between the loudspeaker 55 and one side of the ribbon element is longer than that provided by the tube 53 between the loudspeaker 52 land the opposite side of the conductive ribbon 50, it is apparent that a pressure gradient will be set up between the opposite sides of the ribbon 50 to actuate the. latter, and the output -from the unit Z may, therefore, be employed as ,the ultimate, highly directional response of the entire system.

Assuming that each of the first pair of units Se and T has the same sensitivity, that each of the secondpair of units'V and W has the same sensitivity, and thateach of the third pair of units U andY'also has the same sensitivity, if the directional,characteristics of the respective units S, T, V and W are 'as shown by the'curve XVIII of Fig. 30, the directional characteristic of each of the units U and Y wi1l`be as shown by the curve XIX of Fig. 30. It willbe noted that, whereas the u nits S, T, V and W have the familiar bidirectional cosine characteristic, each of the units U Iand Y has a unidirectional char# acteristic. `The unit Z has an even more unidirectional characteristic, as shown by the pat- ,tern XX of Fig. 30.

The voltage output of the system shown in Fig. 26, assuming equal sensitivity for each of the units S, T, V and W is sin (wzkseo) 25) where D=length of the bend 39a, or 49a,

d=distance between the units S, T or V, W,

g=distance Vbetween the center of the combination S, 'I' and the center of the combination In the case where 71:0, that is, with no bend'in the tube S,

e=K1 (D-l-d cos 0) cos2 0 (27) Thus, from Equations 26'and 27, it will be seen thatD the system of Fig. 26 is more directional than the system of Fig. 21 by a factor cos 0. As pointed out above, this is demonstrated by the curves XIX and XX of Fig. 20.

From the foregoing description, it will be vapparent that I have provided a variety of both unidirectional and ybidirectional microphones which have a highly directional response over a wide range and the pickup units of which are small, thereby permitting easy portability. Thus, in the systems of Figs. 7, 10, 12, 13, 21 and 26, only the microphone unitswhich pick up the sound directly from the original source need be placed in the range of the sound,l the remainder of these systems including the amplifiers, loudspeakers, delay devices, etc. being located at suitable remote locations. It follows, therefore, that the pickup devices proper are embodied in small, compact units which can be handled with great ease.

Although I have shown and described a number of modications of my invention, it will be apparent to those skilled in the art that many other modifications thereof, as well as changes in those described, are possible. For example, in the modifications of Figs. '7, .1012`and 13, the electrical microphone units 5, 6 and Ill, H may be embodied in a single physical unit, such as shown in Figs. 22-24, if desired. Many other similar changes are also possible. I therefore desire that my invention shall not be limited exceptinsofar as is made necessary bythe prior art and by the spirit of the appended claims.

I claim as my invention:

, l. In an electro-acoustical signal translating system, means for'picking, up at a plurality of spaced points acoustical waves .in slightly out of phase relation, means for converting lthe acoustical energy picked up at each of said points into 'electrical energy, an output member, and means employing saidv electrical energy for applying to said output member in opposed relationship the l outputs from each of said rst named means, said last named means including equalizing means for maintaining a constantA electrical output to acoustical input ratio in said system regardless of the frequency'of said signal.

2. In an electro-acoustical signal translating system, a pair of acousticallyresponsive pickup devices spaced from each other a distance which is small compared to the wavelengths of the lower acoustical frequencies in the range to which said microphone is responsive whereby said devices are adapted to pick up vacoustical waves in slightly out of phase relation, means for converting the acoustical energy picked up -by each of said devices into corresponding electrical pulsations, an output member, and means employing said electrical energy for applying to said output member in opposed relationship the outputs from each of said devices, said last named means including equalizing means for maintaining a constant electrical output to acoustical input ratio in said system regardless of the frequency of said signal,

3. In an electro-acoustical signal translating system, the combination of a plurality of. microphone units including acoustically responsive ele- -ments arranged in physically spaced relation to each other whereby they are adapted to be successively actuated by acoustical waves in slightly out of phase relation, means associated with each of said elements forgenerating voltages corresponding to "signal forces applied to said elements by the acoustical waves, the diflerence'in voltages generated by each of said units being a function of the distance between said devices, and said units having their outputs eiectively connected in opposite phase relation electrically, and equalizing means associated with said units for maintaining a constant electrical output to acoustical input ratioin said system regardless of 'the frequency of said signal. t

4. In an electro-acoustical signal translating system, the combination of a plurality of microphone units including acoustically. responsive elements arranged in physically spaced relation to each other whereby they are adapted to be successively actuated by acoustical waves in slightly out of phase4 relation, the spacing 'between said elements being small comparedvto the wavelength of the loweracoustical frequencies in the range to which said system is responsive, means associated with eachof said elements orgenerating voltages corresponding to signal forces applied to said elements by the acoustical waves, the difference in voltage generated by each of said units being a function of the distance between said devices, and said units having their outputs effectively connected in opposite phase i relation electrically, and equalizing means associated with said-units for maintaining a constant electrical outputto acoustical input ratio in said t system regardless of thefrequency of said signal.

5. In an electro-acoustical signal translating system, the combination of a plurality of microphone units including acoustically responsive elementsv arranged in physically spaced relation toA each other whereby they are adapted to be successively actuated by acoustical waves in slightly out of phase relation, said units all having substantially the same sensitivity, means associated with each of said elements for generating voltages corresponding to signal ,forces applied to said elements by the acoustical waves,

the difference in voltages generated yby each of said units being a function of the distance between said devices, and said units having their outputs eiectively connected in opposite phase relation electrically, and equalizing means associated with said units for maintaining a constant electrical output to acoustical input ratio in said system regardless of the frequency of said signal.

6. In an electro-acoustical signal translating system, the combination of a plurality of microphone units including acoustically responsive elements arranged in physically spacedrelation to each other whereby they are adapted to be successively actuated by acoustical waves in slightly out of Aphase relation, said units each having the same directional characteristic, means associated with each of said elements for generating volt'- ages corresponding to signal forces applied t-o said elements bythe acoustical waves, the difference in voltages generated by each of said units being a function of the distance between said devices, and said units having their outputs eiectively connected in opposite phase relation' electrically, and equalizing means associated with said units for maintaining a constant electrical output to acoustical input ratio in said system regardless of the frequency of said signal.

7. The invention set forth in claim 6 characterized in that each of said units has'a substantially unidirectional response characteristic, and characterized further in/that said system has a unidirectional response characteristic which is more directive than the individual response c/haracteristics of any one of said units. y,

8. The invention set forth in claim 6 characteriZed in that saidmicrophone units are all of thetype wherein the acoustically responsive elements are responsive to the pressure gradient component of a` sound wave whereby each 'of said units has a tai-directional response characteristic, and characterized further in that said system has a loi-directional responseI characteristic which is more directive than the individual directional response characteristics of any one of said units.

9. In an electro-acoustical signal translating system, the combination of a plurality of micro- Cil ing means for maintaining a constant electrical output to acoustical input ratio in said system regardless of the frequency of said signal.

1 1. In an electro-acoustical signal translating system, the combination of a pair of microphone units including acousticallyresponsive elements arranged in physically spaced relation to each other whereby they are adapted to be successively actuated by acoustical waves in slightly out of phase relation, means associated with, each of said elements for generating voltages corresponding to signal forces applied to saidfelements by the acoustical waves, a pair of transmissionl lines one of which is associated with one of said units and the other of which is associated with the other of said units, means'in one of said transslightly out of phase relation, Ymeans associated l with each of said element-s for generating voltages corresponding to signall foro/es applied to said elements by the acoustical-waves, a plurality of transmission'lines of vwhich each one is associated with one"of said units, and means foricombiningl the outputs of said transmission lines in opposed relation, said last named means including Ae'qualiz'ing means for maintaining a constant electrica-l output to acoustical input ratio in said system regardless of the frequency of said signal.

y10. In an electro-acoustical signal translating system, the combination of-a plurality of microphonel units including acoustically responsive elements arranged in physically spaced relation to yeach other whereby they are 'adapted to be successively actuated by acousticalwaves in slightly out of phase relatin, the spacing'of said units being of the order of one-half the wavelength of the highest acoustical `frequency in the range to which said system is responsive and being small compared to the wavelength oithe lowest acoustical frequency to which said- 'system is responsive, means associated with each of said elements for generating voltages corresponding to signal forces applied to said elements bythe vacoustical waves, a plurality of transmission lines of which each one is associated with one of lsaid units, and means for combining the f outputs of said transmission lines in opposed relation, said last named means including equalizmission lines for delaying the output of the signal energy transmitted thereby relative to that transmitted by the other of said transmission lines, and means combining the outputs of said transmission lines.

12. In 'an electro-acoustical signal translating system, the combination of a pair of microphone units including acoustically responsive elements arranged in physically spaced relation to each other whereby they are adapted to be successively actuated by acoustical waves in slightly out of phase relation, means associated with each of said elements for generating voltages corresponding to signal forces applied to said elements by the acoustical waves, a pair of transmission lines one of which is associated with `one of said units and the other of which is associated with the other of said units,`means in one of said' transmission lines for delaying the output of the signal energy transmitted thereby relative to that transmitted by the other Lof said transmission lines, and means coupling the outputs of' said 'transmission lines in opposite phase relation electrically.

13.y In an electro-acoustical signal translating system, the combinaiton of a pair of microphone units each including an element responsive to the pressure gradient components of acoustical waves, said elements being arranged in physical; 1y spaced relation to each other a distance which is small compared to the wavelengths of the lower acoustical frequencies in the range to which said system is responsive and being adapted 'to be actuated successively by acoustical waves, means associated with each of said elements for generating voltages corresponding to signal forces applied to said elements by the acoustical waves, a pair of transmission lines one of which is associated 'with one of said units and the other of which is associated with the other of said units, means in one of said transmission lines for ,delaying the output of the signal energy transmitted thereby relative to that transmitted by the other of said transmission lines, and means acterized inv that said delay means includes a loudspeaker for converting the electrical signal energy deliveredY by the element associated there- With into acoustical energy, a 'third microphone unit, and' means coupling said loudspeaker to said last named microphone unit and constituting an acoustic path between said loudspeaker and wsaid last named microphone unit, said last named ...microphone unit serving to recouvert the acoustical energydeliveredby said loudspeaker into electricalenergy, .and theoutput of vsaid last named microphone unit being combined with the output of said other transmissionv line.

combined with the output of said other transmission line.

16. The invention set forth in claim 13 characterized in that said delay means includes a YYloudspeaker for converting the electrical signal energy delivered by the element associated therewith into acoustical energy, a third microphone unit, said last named microphone unit being of the type responsive to the pressure compo-A nent of an acoustical wave, and a tubular member coupling said loudspeaker to said last named microphone unit and constituting an acoustic path between said loudspeaker and said last named microphone unit, said last named microphone unit serving to recouvert the acoustical energy delivered by said loudspeaker into electrical energy, and the output of said last named microphone unit being combined with the output .of said other transmission line in opposite phase relation.

17. The invention set forth in claim 13 characterized in that said delay means comprises an electrical network for shifting the phase of the signalenergy transmitted by said one transmission line relaitve to that transmitted by said other transmission line. l

18. In an electro-acoustical signal transmitting system, the combination of a pair of microphone units each including anelement responsive to the pressure gradient component of an acoustical wave, said elements being arranged in physically spaced relation to 'each other a distance which is small compared to the wavelengths of the lower acoustical frequencies in the range to 'which said system is responsive and being adapted to be actuated successivelyby acoustical waves, means associated with each of said ele-- ments for generating voltages corresponding to vsignal forces applied to said elements by the acoustical waves, a third microphone unit including a vibratile element responsive to the pressure` gradient components of-acoustical waves and means associated therewith for converting the vibrations thereof into voltages corresponding to acoustical signal Vforces applied thereto, and a pair of transmission lines one of which is associated with one of said -iirst named elements and the other of which is associated with ,the

other of said rst named elements, said transmission lines each including a loudspeaker for converting the electrical signal energy delivered by its associated element into acoustical energy tile element, `and said last named tubular member having an acoustic delay therein whereby the acoustical waves transmitted thereby to said vibratile element are out ofv phase with those transmitted by said iirst named tubular member to said vibratile element to thereby set up a pressure gradient between the opposite sides of said vibratile element for actuating said last named element.'

19. The invention set forth in claim 18 characterized in that said acoustic delay is constituted by a bend in said second named tubular member.

20. In an electro-acoustical signal translatin system, the combination of at least seven microphone units each including an element responsive to the pressure gradient componentw of a sound wave and means associated with each of said elements for generating signal voltages corresponding to signal forces applied to said eiements by acoustical waves, the iirst and second of said Aelements cooperating jointly as a iirst pair, the third and fourth of said elements cooperating jointly as a second pair, and the fth and sixth of said elements cooperating jointly as a third pair, the respective elements of each oi said iirst arid second pair being spaced physically from each other a distance of the order of onehalf the wavelength of the highest acoustical frequency in the range to which said system is responsive, and said'distance being small compared to the wavelength of the lowest acoustical frequency in the range to which said system is responsive, at least six transmission lines one each associated with each of said six named elements and each of said six elements constituting v a source of signal energy for its associated transmission line, said transmission lines each including a loudspeaker for converting electrical energy derived from its associated source element into acoustical energy and a tubular member, the tubular member of the transmission line associated with said first element coupling its associated loudspeaker to one side of said iifth element, the tubular member of the transmission line associated] with `said second element coupling its associated loudspeaker to the opposite side'of said fth element and having an acoustic ldelay therein whereby the acoustical waves transmitted thereby to said fth element are out of phase with those transmitted to said fifth element by said ilrst named tubular member to thereby set up a pressure gradient between the two sides of said fth element for actuating said fthlement, the tubular member of the trans-v mission line associated with said third element coupling its associated loudspeaker to one side of said sixth element, the tubular member of and a tubular member. the tubular member of one of said transmission lines coupling its associated loudspeaker with one side of said vibratile element and constituting an acoustic path between said last named loudspeaker and said vi'` the transmission line associated with said fourth element coupling its associated loudspeaker to the opposite side ofsaid sixth element and having an acoustic delay therein whereby the acoustical waves transmitted thereby to said sixth element are out ofv phase with those transmitted to said sixth element by said third named tubular member to thereby set up a pressure gradient between the two sides of said sixth element for actuating said sixth element, the outputs of said fth and sixth elements being connected, respectively, to the inputs of said fth and sixth transmission lines, the tubular member of said fth transmission line coupling its associated loudspeaker to one side of the seventh of said elements, and the tubular member of said sixth transmission line coupling its associated loudspeaker tothe named tubular member also having `an acoustic delay therein whereby the acoustical Waves transelement by the tubular member of said ith named transmission line to thereby set up a pressure gradient between the two sides of said seventh element for actuating said seventh element.

2l. The invention set forth in claim 20 characterized in that the microphone units including said rst, second, third and fourth elements/,all have substantially the same sensitivity, and characterized further in that the microphone units including said fth and sixth elements bothY have' substantially the same' sensitivity.

22. in a microphone, the combination or" means providing a magnetic leld and including a purality of air gaps spaced from each other axially along the microphone, and a plurality of electrically conductive, velocity responsive elements yone each vibratively supported in each. of said air gaps. 4

23. In a microphone, the combination of means providing a magnetic 'field and including a plurality of air gaps in parallel relation to each other microphone, and a plurality of electrically ccn- `ductive elements one each vibratively-supported in each of said air gaps.

2li. l'n a microphone, the combination of means providing a magnetic iield and including a plurality of air gaps spaced lfrom each other axially along the microphone, and a plurality of electricaily conductive, velocity responsive ribbons vibratively supported in parallel relation to each other one each in each of said air gaps.

25. lin a microphone having a directional chary acteristic of relatively high order of directivity, the combination of means providing a magnetic eld and including a plurality o air gaps spaced from each other axially along the microphone, and a plurality or" electrically conductive elements vibratively supported one each in each oi' said air gaps, said elements being arranged to act jointly in pairs, and the spacingbetween the two elements of any pair being of the order f one-half the wavelength of the highest acoustical r said microphone I and a plurality of elctrically conductive elements gvibratively supported one each in each of said air gaps, said elments being arranged to act jointly in pairs, and the spacing between the two elements of any pair being small compared to the Wavelength of the lowest acoustical frequency in the range to which said microphone is responsive.

27. 'rhemethod of obtaining a relatively highly directional response` to acoustical signals which Y comprises picking up the acoustical signals at a plurality of spaced points and in a dilerent phase relationat each of said points, translating the acoustical signals picked up at each of said points into corresponding electrical pulses, utilizing the electrical pulses derived from each of said points in a separate channel to actuate an output memopposite Vside of said seventh element, said last stant electrical output to acoustical input ratio regardless of the frequency of said signals.

28. The method of obtaining a relatively highly ,directional response to acoustical signals which .picked up at each of said points into corresponding electrical pulsations, utilizing'the electrical and spaced from each other axially along the ber in opposed relation, and maintaining a conpulsations derived from each of said points in a separate channel to actuate an output member in V'opposed relation, and maintaining/al, constant electrical output to'acoustical input ratio regardless of the frequency of said signals.

29. The method of obtaining a relatively highly directional response to acoustical signals 'which comprises picking up the acoustical signals at a plurality of spaced points and in a diiierent phase relation at each of said points, translating the acoustical signals picked up at eachoi said points .into corresponding electrical pulsations, combining said pulsations intoA a single output channel n0 the frequency ui.

'directional response to acoustical signals which comprisespicking up the acoustical signals at a plurality of spaced points and in. av different phase relation at each of said points, translating the acoustical signals picked up at each oi. said points intc corresponding electrical pulsations,

.feeding the electrical pulsations derived from each of said points along a separate channel, in-

troducing ra delay in one oi' said channels, and

iinally utilizing the outputs oi' said channels in opposed relation to actuate an output member.

`31. The method ol obtaining a relatively highly directional response to acoustical signals from a signaly source which comprises picking up the acoustical signals at a pair of spaced points and in a dierent phase relation at each ci said points, translating the acoustical signals picked up at each of said points into corresponding electrical pulsations, feeding the electrical pulsations derived from each of said points along a separate channel, retranslating said electrical pulsations in each oi' said channels into acoustical pulsations, conducting the acoustical pulsations in one of said channels to one side of a vibratile pressure gradient'responsive device, simultaneously conducting the acoustical pulsations in the other of said 'channels to the other side of said device While introducing a delay in said last named channel whereby to set up a pressure gradient between the two sides of said device for actuating said device, and utilizing the output of said device as the response from said source. y

32. The method of obtaining a relatively highly directional response to acoustical signals from a signal source which comprises picking up the acoustical signals at at least two pairs or spaced points and in a slightly different phase relation at each of said points, translating the acoustical along a separate channel, retra-nslating the electrical pulsations in each of said channels into acoustical pulsations, conducting the acoustical asomes pulsations in the channel associated with one of a nrst pair of said points to one side of a vibratile pressure gradient responsive device, simultaneously conducting the acoustical pulsations in the channel associated with the other point of said first pair of points to the other side of said rst pressure gradient responsive device while introducing a delay in said last named channel whereby to set up a pressure gradient between the two sides of said first pressure gradient responsive device for actuating the latter, simultaneously conducting the acoustical pulsations in the channel associated with one of a second pair of said points to one side of a vibratile second pressure gradient responsive device, simultaneously conducting the acoustical pulsations in the channel associated with the other point of said second pair of points to the other side of said second pressure gradient responsive device while introducing a delay in said last named channel whereby to set up a pressure gradient between the two sides of said second pressure gradient responsive device for actuating the latter device, translating the vibrations of each of said iirst and second devices into corresponding electrical pulsations, feeding the electrical pulsations derived from said first and second devices along a ilfth and sixth channel, respectively, retranslating said last named electrical pulsations into acoustical pulsations in each of said i'lfth and sixth channels, conducting the retranslated acoustical pulsations in said fth channel to one side oi' a vibratile third pressure gradient responsive device, simultaneously conducting the retranslated acoustical pulsations in said sixth channel to the other side of said third pressure gradient responsive device while introducing a delay in said sixth channel whereby to set up a pressure gradient between the two sides of said third pressure gradient responsive device for actuating the latter, and finally utilizing the output of said third device as the response from said source.

' HARRY F, OLSON.

Certificate of Correction *Patent No. 2,301,744.

November l0, 1942.

HARRY F. LSON It is yhereby certied that errors appear in the printed specification of the above numbered patent requiring correction as vfollows: Page 3, second column, line- 2,

for means'read meanings; page 4, secon 'read Re; page 5, second column, line 24, for

23 and 24; page, firstcolumn, elemepts; page 7, rst column, lines 5 to l0, Equation 25 or between Equations instead of as shown in the patentd column, line l0, in the equation, for R0 slik read silk; line 40, insert the word line 7l, for element read should read as shown below e =K(D +d cos 6) cos 6 [sin (wt+g):360 cos 0) -sin (wt-XGQH and second column, line 45, for length` for "elctr ically read electrically; columnl1ne 3, before pressure read lengths; page l0, rst column, line 58,

line 60, for elments read elements; page ll,rst insert first; and that the said Letters Patent should be .read With these corrections therein that the same may conform to the record of the casein the Patent Oilce.

Signed and sealed this 8th day of June, A. D. 1943.

HENRY VAN ARSDALE,

Acting Commissioner of Patents.

/ Certicate of Correction Patent No. 2,301,744. November. 10, 1942.

HARRY F. OLSON It ishereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows: Page 3, second column, line 2, for "means read meanings; page 4, second column, line 10, in the equation, for R0 read R0; page 5, second column, line 24, for slik read silk; line 40, insert the Word or between Equations 23 and 24; page, first column, line 71, for elemen` read elements; page 7, rst column, lines 5 to 10, Equation 25 should read as shown below instead of as shown in the patent-F and second column, line 45, for lengthfread lengths; page 10, rst column, line 58, for "elctr 1cally read electrically; line 60, for elments read elements; page 11,'rst co1umn lme 3, before pressure insert vrst; and that the said Letters Patent should be .read with these corrections therein that the same may conform to the record of the case in the Patent Olce.

Signed and sealed this 8th day of June, A. D. 1943.

[amt] HENRY VAN ARSDALE,

Acting Commissioner of Patente.

Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US2417927 *22 Mar 194325 Mar 1947Automatic Elect LabSound direction finder
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US7116792 *5 Jul 20003 Oct 2006Gn Resound North America CorporationDirectional microphone system
US87983048 Dec 20105 Aug 2014Knowles Electronics, LlcAcoustic valve mechanisms
DE1274192B *15 May 19651 Aug 1968Philips NvMikrophonkombination mit einer Verstaerkerschaltung, die aus zwei Mikrophongruppen besteht
DE2936082A1 *6 Sep 197920 Mar 1980Polaroid CorpRichtempfangssystem mit vorbestimmtem richtungsansprechen
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Classifications
U.S. Classification381/87, 381/97, 381/356, 367/153, 381/103, 367/182
International ClassificationH04R3/00, H04R1/40
Cooperative ClassificationH04R1/406, H04R3/005
European ClassificationH04R1/40C, H04R3/00B