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`Nov. 10, 1942.
`ELECTROACOUSTICAL SIGNAL TRANSLATING APPARATUS
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`Nov. 10, 1942.
`
`H. F. OLSON
`
`2,301,744
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`ELECTROACOUSTICAL SIGNAL TRANSLATING APPARATUS
`
`Filed May 31, 1941
`
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`Nov. 10, 1942.
`ELECTROACOUSTICAL SIGNAL TRANSLATING APPARATUS
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`Nov. 10, 1942.
`ELECTROACOUSTICAL SIGNAL TRANSLATING APPARATUS
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`2,301,744
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`ELECTROACOUSTICAL SIGNAL TRANSLATING APPARATUS
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`Filed May 31, 1941
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`Filed May 31, 1941
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`2,301,744
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`H. F. OLSON
`Nov. 10, 1942.
`ELECTROACOUSTICAL SIGNAL TRANSLATING APPARATUS
`
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`Nov. 10, 1942.
`
`H. F. OLSON
`
`2,301,744
`
`ELECTROACOUSTICAL SIGNAL TRANSLATING APPARATUS
`
`Filed May 31, 1941
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`’
`
`‘Patented Nov. 10, 1942
`
`2,301,744
`
`UNITED STATES PATENT OFFICE
`
`_
`
`a
`
`10
`
`15
`
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`2,301,744
`ELECTROACOUSTICAL SIGNAL TRANSLAT-
`ING APPARATUS
`Harry F. Olson, Haddon Heights,
`N. J., assignor to
`a corporation of
`Radic Corporation of America,
`Delaware
`Application May 31, 1941, Serial No. 396,078
`32 Claims.
`(CL. 279—1)
`Still another and important object of my pres-
`This invention relates to electroacoustical sig-
`ent invention is to provide a highly directional
`nal-translating apparatus, and more particularly
`microphoneas aforesaid the directional response .
`to directional microphones.
`of which is independent of the frequency.
`Directional microphones may be divided into
`It is also an object of my present invention to
`two classes as follows: first, the wave type .mi-
`provide a novel highly directional microphone as
`crophones which depend. for directivity upon
`above set forth which is relatively simple in
`wave interference; and second, gradient type mi-
`construction commensurate with the results at-
`erophones which dependfor directivity upon dif-
`tained thereby, and which is highly efficient in
`ference in pressure (or powers of difference in
`use.
`pressure) between two points. Typical micro-~
`In accordance with my present invention, I
`phones of the first class, that is, those in which
`employ at least one pair, and in some cases & plu-
`the directivity depends in some way upon wave
`rality of pairs, of microphone units spaced apart
`interference, are the reflector, lens, and line mi-
`a distance of the order of one-half the wave
`crophones.
`In the second class of microphones,
`length of the highest frequency in the range to
`the most common example in use today is that
`which the system is responsive and small com-
`in which the active element is responsive to the
`pared to the wave length of the ‘lowest frequency
`velocity, or presstire gradient, component of the
`in the range to which the system is responsive,
`sound wave.
`so that each of the individual microphone units
`To obtain any semblance of directivity in the
`will be actuated by the acoustical waves from &
`first class of microphones mentioned above, the
`signal source. in slightly out-of-phase relation.
`dimensions of the microphone must be compa-
`The outputs of the individual microphone units
`rable to the wave length. Thus, while such mi-
`may then be combined effectively in opposed
`crophones give fairly good efficiency,
`they are
`phase relation electrically directly, or a suitable
`open to the objection that they must be big and
`delay may be introduced in the channel including
`rather cumbersome in order to be responsive to
`one of the microphoneunits to provide an ultimate
`acoustical waves in the lower range of the audio
`output which is a measureof the directivity of
`spectrum. Obviously, this imposes a serious lim-
`the signal source’ with respect to the, microphone
`itation tipon such microphones in many fields of
`axis. Various arrangements of unidirectional
`use, as in motion picture work, for example.
`microphones or velocity microphones, as the case
`The directional characteristics of a simple,
`may be, are possible to provide various degrees
`symmetrical pressure gradient responsive micro-
`of directivity, and these will be more fully set
`phone are bidirectional.
`I have discovered that
`forth hereinafter.
`‘
`:
`the directional characteristics of combinations
`The novel features that I consider characteris-
`of gradient microphones of different orders, or
`tic of my invention are set forth with particu-
`combinations of gradient elements and appropri-
`larity in the appended claims. The invention
`ate delay systems, are unidirectional. Employ-
`-itself, however, both as to its organization and
`ing this discovery, I have found thatit is possible
`method of operation, as well as additional objects
`to provide unidirectional microphones which not
`and advantages thereof, will best be understood
`only have a highly directional response over a
`from the following description of several embodi-
`wide range, but are also relatively small in. size
`ments: thereof; when read in connection with the
`while still retaining high directivity at the low
`accompanying drawings, in which
`frequencies, and. the primary object of my pres-
`Figures la, 1b and Ic are diagrammatic illus~
`ent invention is to provide a highly directional
`trations of various orders of the gradient type
`microphone which is free from the above-noted
`microphones which may be employed in accord-
`objections present in prior art microphones which
`ance with the present invention,
`have highly directional response characteristics.
`Figures 2a, 2b, 2c, 2d and 2e are curves show-
`. More particularly, it is an object of my present--
`ing directional patterns for gradient microphones
`- invention to provide a novel directional micro-
`of five different orders of the type illustrated in
`phone having linear dimensions which are small
`. Figures la, 1b and Ie;
`compared to thewave length at the lower end of
`’ Figures 3a, 3b and 3c are curves showing fre-
`the response range.
`i
`quency response characteristics for the micro--
`Another object of my present invention is to
`phones illustrated in Figures la, 1b and Ie,
`provide improved highly directional microphones
`Figures 4a, 4b and 4c are diagrammatic illus-
`having either bidirectional or unidirectional char-
`trations similar to Figures la, 1b and 1c, but in-
`acteristics.
`
`:
`
`40
`
`45
`
`55
`
`
`
`8
`
`

`

`2,801,744
`
`iad
`
`10
`
`20
`
`30
`
`2 c
`
`igure 25 shows a pair of curves representing
`the directional characteristics of either of the
`microphoneunits of Figure 22, and the directional'
`characteristic of the combination thereof as set
`up in the system of Figure 21,
`Figure 26 is a diagrammatic view showing still
`another form of my invention in which a plu- .
`rality of. 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 Figure26,
`‘Figure 28 is a side elevation of the microphone
`units shown in Figure 27,
`Figure 29 is a sectional view taken onthe line
`XXIX—XXIX of Figure 28; and
`-
`Figure 30 shows a set of curves similar to Fig-
`ure 25 but applicable to the system of Figure 26.
`Before referring ‘to 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 micro-~
`phones are of various orders from zero on up.
`A gradient microphone of the order zero is a
`microphone in which theelectrical response cor-
`responds to the pressure in the actuating sound
`wave. Such a microphone ‘may be illustrated,
`for example, by the symbol,
`|
`in Figures la
`and 4a.
`:
`The pressure in a sound wave may be written
`fees sin k(ct—r)
`(1)
`
`luding equalizers for obtaining particular fre- ©
`quency response characteristics,
`Figures 5a, 5b and 5e are curves. showing equal-
`izer characteristics for the equalizers required
`with the microphones illustrated in Figures 4a,
`4b and4c to provide the frequency response char-
`acteristics for these various microphones shown
`by the curves of Figures 6a—6a’, 6b—60’, and
`6c—6e’,
`Figures 6a, 6b and 6c are response curves show-
`ang 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-~
`sponse characteristics at various distances,
`Figure 7 is a diagrammatic view of one form
`of my invention employing a pair of microphones
`which may have more or less unidirectional and
`bidirectional response,
`response
`Figure 8 is a series of directional
`curves showing the type of response: which the
`microphones of Figure 7 may have individually,
`Figure 9 is a.similar series of directional re-
`sPonse curves showing the relatively more direc-
`tional response of the combination of micro-
`phones of Figure 7,
`Figure 10 is a diagrammatic view showing an-
`other form of my invention employing a pair of
`velocity, or pressure gradient, responsive micro-
`phones,
`‘Figure 11 is a series of directional response
`curves showing the response of the individual
`microphones of the modification of my invention
`shown in Figure 10, as well as the overall re-
`sponse of the system shown in this figure,
`_
`ee Pm os
`Figures 12 and 13 are diagrammatic views show-
`p= sin k(et—r)
`ing a modified form of the system shown in Fig-
`where c=Velocity of sound,
`ure 10,
`~
`Figure 14 shows a pair of directional response ~
`k=2a/n,
`40°:
`A=wave length,
`curves one of which shows the directional char-
`p=density of the medium in which the
`acteristic of each of the individual microphones
`sound wave travels,
`of the modifications of my invention shown in -
`A=amplitude of the velocity potential,
`Figures 12. and 13, and the other of which shows
`r=distance of the microphone from, the
`the directional characteristic of the combination -
`sound source, .
`of these two microphones with the delay net-
`t=time, and
`works included in these modifications,
`‘—pm==amplitudé-of thepressure.
`Figure 15 is a curve showing the phase fre-
`To illustrate the characteristics of gradient mi-
`quency characteristic of the filter or phase shift-
`er of Figure 12,
`crophones, it will be assumed that the elements
`_ Figure 16 shows a pair of curves representing
`of the gradient microphones are made up of .
`the output of. a system such as that shown in
`units and that the voltage output of these units
`‘Figure 12 when employing two different types of
`is proportional
`to the sound pressure and in
`velocity microphones,
`phase with the actuating sound pressure.
`Figure.17 is a curve showing the phase shift
`At a fixed point, in a sound wave in which the
`in the network of Figure 13,
`pressure amplitude is.independent of the fre-
`Figure 18 is'a curve showing the phase ‘shift
`quency, the pressure available for actuating the
`in the two microphones of Figures 12 and 13 due
`vibrating system of thé microphone is inde-
`to the distance between them,
`pendent of the frequency, as shown by the curve
`‘igure 19 is a curve showing the energy re-
`in Figure 3a.
`In considering the gradient mi-
`sponse of
`the modifications. of my’ invention
`crophones ‘here, it will be assumed that the di-
`-Shown in Figures 10, 12 and 13 as a function of
`mensions of the systems are small compared to
`the ratio of the delay introduced in one of the
`the wave length. Therefore, the pressure which
`transmission channels to the delay resulting from
`actuates the. microphone is independent of the
`the spacing of the individual microphone units,
`direction of the incident sound.. Under these
`Figure 20 is a curve showing the directional
`conditions, the pressure microphone is nondirec-
`characteristic of such system,
`tional, as shown. by the curve of. Figure 2a.. The
`Figure 21 is a diagrammatic view showing an-
`energy: response of. this microphone: to random
`other form of my invention,
`sounds is unity.-
`Under the above conditions, a single unit con-
`Figure 22 is a front elevation of a pair of
`microphone units such as are employed in the
`stitutes a gradient microphone of order zero or,
`system of Figure 21,
`©
`as it is commonly termed, a pressure microphone.
`Figure 23 is a side elevation of the microphone
`The voltage response of this microphoneis inde-
`|
`units shown in Figure 22,
`' pendent of the frequency as shown by the curves
`Figure 24 is a sectional view taken on the line
`U5 in Figures 6a and 6a’, Furthermore, the shape
`of the Tesponse frequency characteristic is inde-
`XXIV—EXIV of Figure 23,
`
`(2)
`
`_ or
`
`55
`
`60
`
`70
`
`9
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`20
`
`30
`
`2,301,744
`and the remaining terms again have the same
`pendent of the distance from the sound source
`means as used above.
`,
`(see Figure 6a’).
`At @ fixed distance in a sound wave located at
`A gradient microphone of order one is a mi- -
`a distance of a large number of wave jengths,
`crophone in which the electrical response corre-
`the difference of the difference in pressure for a
`sponds to the pressure gradient between two
`constant sound pressure in the sound wave is
`points. Such a microphoneis shown symbolical-
`proportional to the square of the frequency (see
`ly at 2 in Figures 1b and 4b.
`Figure 3c). The directional response of the mi-
`between two points
`The difference in pressure
`crophone is proportional to the square of the
`in space may be written
`cosine of the angle between the direction of in-
`cident sound and theline 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 out-
`puts of which are oppositely phased, as illus-
`trated by the symbols 2, 2 of Figures le and 4e.
`tn this case, there must be used two compensating
`or equalizing systems 3, 3, in each of which the
`responseis inversely proportional to the frequen-
`cy, in combination with a third equalizer 4 (Fig-
`ure 4c), the response of which is also inversely
`proportional to the frequency. A single compen-
`sating system 4, the response of which is inverse-
`ly proportional to the square of the frequency,
`may be used alone without the equalizers 3, 5, if
`desired. The equalization characteristic required
`to obtain uniform response with respect to fre-
`‘quency is shown in Figure 5c. The response fre-
`quency characteristic with a compensating ele-
`ment is independent of the frequency when the
`distance between the microphone and the sound
`source is several wave lengths (see. Figure 6c).
`The response as a function of the frequency for
`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 ex-
`pression for any order “n’”of the difference in
`pressure between two points separated by a dis-
`tance Ar is
`:
`arpaBarr=o,(fede)) (ar cos 6)"
`* (6)
`5p
`6
`ér™
`ér*
`the pressure available
`This.expression shows that
`proportional
`to
`for driving the microphone is
`the nth power of the frequency. The directional
`characteristics are cosine functions, and the
`powerof the cosine is the order of the gradient.
`The energy response to random soundsis
`_i
`(2n+1)
`that of a nondirectional microphone. The direc-
`tional characteristics of gradient microphones of
`the order zero, one,
`two, three, and four are
`shown,respectively, in Figures 2d, 2b, 2c, 2d and
`2e.
`It has been shown in the preceding discus-
`sions that the directivity of a. gradient micro-
`phone increases with increasing powers of the
`pressuregradient. The directional characteris-
`tics of these systems are of the bidirectional type.
`
`(4)
`ap=paD|™ cos beltsin Kei)| cos 8
`where D=distance between the two points,
`g=angle between the direction of the in-
`cidentsound and the line joining the
`two points,
`.
`and the remaining terms have the same mean-
`ings as above.in Equations 1 and 2.
`' Ata fixed point in a sound wave located a dis-
`tance of several wave Jengths from the source,
`the difference in pressure, for a constant sound
`pressure 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 microphoneis 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 intto-
`duced in which the response is inversely propor-
`tional to the frequency if a flat response is de-
`sired. Such an equalizing system is represented
`by the block 3 in Figure 4b and has the charac-
`teristic shown in Figure 5b..
`In the case of the
`velocity microphone, which is the outstanding
`example of a first order gradient microphone,.
`this is very simply accomplished by using 2 mass-
`controlled element. The
`response
`frequetcy
`characteristic with a compensating element or
`equalizing system is, independent of
`the fre-
`quency when the distance between the micro-
`phone 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 mi-
`crophone is small compared to the wave length
`but is accentuated as the frequency’ or distance
`is decreased. The response for various distances,
`as a function of the frequency, is shown in Fig-
`ure 60’.
`-
`.
`A gradient microphone of order two is a Mmicro-
`phone in which the response correspondsto the
`pressure gradient of the pressure gradient. The
`difference of the difference in pressure between —
`two two-point systems may be written
`[= sin k(ct—r)+2kr 008 k(ct—1r)+2 sin Kel)| cok 6 6)
`where Di=distance between the points in a pair
`of points,
`:
`.
`De=distance between the two pairs,
`
`40
`
`50
`
`60
`
`A(Ap) paDiD:
`
`In many applications, unidirectional characteris-
`tics are more desirable. A gradient microphone
`75 may be combined with a suitable delay system
`
`10
`
`
`
`10
`
`

`

`5
`
`(3)
`
`4 t
`
`2,301,744
`o obtain a unidirectional microphone. Two op-
`positely phased units one of which is connected
`directly to a response correcting system while the
`other is connected to the response correcting sys-
`tem through an interposed delay system is one
`example of how. this may be done. The output
`of such a combination is given by
`€1=é€a(D2+-Di
`cos 6)
`
`field, the ribbon being terminated by a pipe hay-
`ing an opening which may be closed off more or
`less to provide any of the directional character-.
`istics shown by the curves I, II, III, IV or V of
`* Pig. 8.
`Assume that the microphones 5 and & are set
`up to operate .as unidirectional microphones. The
`directional characteristics of each of the micro-
`phones 5 and & will be
`
`where
`
`10°
`
`és =voltage output constant which depends upon
`the characteristics of the units and the
`frequency
`Di=distance between the units, and
`De=path length of the delay.
`By suitable choice of the constants Di and Dz,
`all directional patterns between the bidirectional
`cosine characteristic and the nondirectional char-
`acteristic are possible, the transition being from
`bidirectional,
`through unidirectional, to nondi-
`rectiona).
`.
`. To provide a system having unidirectional
`characteristics exhibiting a high order of direc-
`tivity, two gradient microphones of order { may -
`be provided with a suitable delay system-in the
`‘output channel of one of them. The voltage out- _
`put of a system of this sort is given by
`e2=eb(D2+D1 Cos @) cos é
`
`(8)
`
`where
`ép =voltage output constant which depends upon
`‘
`the characteristics of the gradient. micro-
`phone units of order 4 and the frequency,
`Di=distance between the units, and
`' D2=the‘path length of the delay.
`The maximum discrimination against. random.
`sounds occurs when
`: D=2D,
`For this condition, the energy response to random
`sounds is one-eighth that of a nondirectional mi-
`crophone.
`It is also possible, of course, to pro-
`vide two combination pressure and pressure gra-
`- dient microphones with a suitable delay system
`in one of the combinations to. provide a unidirec-
`tional microphone exhibiting a high order of
`directivity.
`Referring, now, more particularly to the draw-
`ings, I have shown, in Figs. 7, 10, 12, 18, 21 and
`26, various arrangements of microphone units
`_ and systems based upon the principles discussed
`above for the purpose of providing either uni-
`directional or bidirectional microphones which
`_ exhibit a high order of directivity.
`In Fig. 7, there are shown two similar micro-
`phone units 5 and § 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 approxi-
`mately one-half the wave length of the highest
`frequency in the 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 be. amplified by
`a suitable amplifier.7, the output of which may
`be'employed to operate any suitable output mem-
`ber (not shown), such as a loudspeaker, a re-
`cording device, or the like. The individual mi-
`crophones 5. and 6 are preferably of the type
`‘shown in my copending application Serial No.
`312,053, ‘filed January 2, 1940, and employ: a
`conductive. ribbon in the air gap of a magnetic
`
`11
`
`,.+m cos 6)R=Tm
`(9)
`where M and m are constants and ¢ is the angle
`between the incident sound and the horizontal
`microphone axis.
`:
`Assume also that
`the sensitivity. of each of
`the two units 5 and 6 is the same and that the
`directional pattern is also the same. The volt-
`age output of each of the microphones 5 and 6 is
`e=K(M+™ cos 6) cos wt
`(10)
`where K=output voltage constant of the com-
`bined units.
`:
`Taking a point midway between the micro-
`phones 5 and 6 as a reference point for the sig-
`nal source, so that acoustical waves therefrom
`will reach each microphone in the same phase,
`and assuming that the outputs of the micro-
`phones § and 6 are connected in opposition elec-:
`trically, the output of the combination will be
`eo= K[M-+m cos 6]
`
`(13).
`
`{eos (ot-S22) —cos (ot+SO I (11)
`where d=distance between the microphone units.
`Now let d<<a; then
`(12) |
`eo=KiM-+m cos o}4
`where K:i=cutput voltage constant of the com-
`bined units. For an angle @ between the direction
`of incident sound and a line joining the micro-
`phones 5 and 6 through their vertical axes, the
`distance between the two- microphone . units is
`foreshortened by a factor cos 6. The. voltage
`output for an angle @ is
`ée=K,(M-+m cos a4 cos 6
`and the directional characteristic is
`(14)
`Ry= Mam cos 6) cos 6
`M+m
`The curves I’, If’, ITI’, IV’, and V’ of Fig. 9
`correspond, respectively, 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 ar-
`ranged as above described is-much more direc-
`tive than that of
`the individual microphones
`5 and 6.
`,
`In the system illustrated in Fig. 10, I employ
`@ pair of velocity responsive, or bidirectional,
`microphones {0 arid | spaced apart a distance a
`equal to approximately one-half the wavelength
`of the highest frequency in the range to which
`the system is responsive and smal! compared to.
`the lowest frequency. in the same range. The
`microphone 10.may be coupled directly to a suit-
`able amplifier and equalizer 12 of the type dis-
`cussed above, while the microphone ft!
`is con-
`nected to an amplifier 13 the output of which
`drives a loudspeaker 44 which retranslates the
`pulsating electrical sigrial. energy into acoustical
`energy. A third microphone. preferably a pres-
`
`20
`
`30
`
`35
`
`40
`
`on vu
`
`60
`
`70
`
`15
`
`11
`
`

`

`5
`
`2,801,744
`networks 20 and 24 produce, electrically, the de-
`sure microphone having an active element or
`lay that is introduced acoustically in the system
`ribbon {5 vibratively mounted in the air gap of
`of Fig. 10 due to the distance D between the loud-
`@ suitable magnetic field,
`is spaced from the
`speaker 14 and the microphone unit 15.
`In the
`loudspeaker 14 a distance D, a tube 16 coupling
`case of the system shown in Fig, 12, the phase
`the loudspeaker. {4 to the ribbon element 15 and
`shift of the network 20 is practically proportional
`constituting .an acoustic path therebetween.
`to the frequency up to 180°, as shown by the
`The tube {6 has a portion {6a filled with tufts
`curve in Fig. 15. Of course, more filter sections
`of felt i7 and constitutes an acoustical,resist~-_
`- may be employed and thus a greater phase shift
`ance which terminates the ribbon element 15.
`may be obtained. The sameis true of the filter
`10
`Due to the spacing between the loudspeaker 14
`shown in Fig: 13.
`It will be apparent that the
`and the microphoneunit 15, a time lag or acous-
`same directional characteristics can be obtained
`tical delay is introduced into the channel in-
`with the systems of Figs..12 and 13 as with the
`cluding the microphone !!. The outputs of the
`system of Fig. 10, and the same theory applies.
`microphones {0 and {{ may be combined effec-
`The curves XIV, XV of Fig. 14 show, respec-_
`tively in opposite phase in an electrical sense.
`“tively, thedirectional-characteristics of the indi-
`‘through the microphone {5 and, because of the
`vidual microphones {0 and 1{ of the systems of
`delay introduced in the distance D, the resulting
`Figs. 12 and 13, and the combination of these
`system will have a highly. directional response,
`two microphones with the networks. An in-
`Let it be assumed that each of the microphones
`creased output may be obtained with the systems
`{9 and 11 has the samesensitivity and that each
`of Figs. 12 and 13 by employing a gradient micro-
`has the same directional characteristic as shown,
`phoneof the type shown,for example, in-‘my co-
`for example, by the curves VI and Vilof Fig. 11.
`~ pending application Serial No. 376,861, filed Jan-
`Let the microphone {! be the reference point.
`uary 31, 1941, wherein a slik baffle is employed
`The output of the microphone !0.is
`,
`around the ribbon element of the microphone.
`e={00s [art (4360) cos o|| eos a
`The curve A of Fig. 16 shows the output that
`may be obtained with the conventional velocity
`microphones, and the curve B shows the in-
`The ouput of the microphone {fis
`creased output thet may be obtained when using
`eis008 («:—2a60°)} cos 6
`microphones with the silk baffles.
`‘The curves of
`Figs, 17 and 18 show, respectively, the phase shift
`in degrees in the network of Fig. 13 and the
`Equation 15 may be written
`phase shift in degrees due to the spacing of the
`eo=[c08.0t cos {2860 cos of
`:
`microphones [0 and {1 a distance d.
`The energy response, E. R., of
`the systems.
`35
`shown in Figs. 7, 10, 12 and 13 is given by
`.
`fd _
`,
`
`sin wi sin 7x360 cos 6+|cos @ (17)
`Inf (D cos 6+d cos? 6)? sin 6 dé
`BR
`an(d+D9
`28)
`and Equation 16 may be written
`Di @
`eis=|08 wt cos (3360)sin wt (sin P60)|
`=zts
`=35
`
`15
`
`20
`
`25
`
`(15)
`
`(16)
`
`30
`
`cos 6
`
`40
`
`(18)
`
`For D<<\ and d<<aA
`
`eo=|08 wt—sin w0($360 cos 0)| cos 4 (19)
`
`> and
`
`;
`
`The energy respotise as a function of the ratio
`D/d is shown by the curve of Fig. 19. The min-
`imum energy response is 0.125, or in the ratio of
`1 to 8, whereas the energy response of the veloc-
`ity microphone or ofthe cardioid unidirectional
`microphone is 1 to 3. The curve of Fig. 20 shows
`:
`D
`.
`more particularly the percent response which
`€1s=1 cos wt—sin wt 360 )1cos @ (20)
`
`
`may be obtained with the systems thus far de-
`The difference e10—€15, orthe ouput cf the com~-
`scribed,
`In the system shownin Fig. 21,1 employ a pair
`bination, is
`of microphone units S and T of the type respon-
`sive to the pressure gradient component of an
`acoustical Wave and each therefore having a ‘bi-
`directional characteristic. The units S and T in-
`clude, respectively, the conductive ribbon elements
`25 and 26 which are shown in spaced relation to
`-each other in a commonplane in Fig. 21 merely
`for the sake of clearness and to denote two dif-
`ferent electrical units. Actually, however, these
`two electrical units may be embodied in one
`physical unit with the ribbon element 25 vi-
`pratively supported in the air gap 27 between a
`pair of pole pieces 28, 28, and the riboon element
`26 similarly supported behind the. ribbon 25 in
`an air gap 29 between a pair of pole pieces 30, 30,
`as clearly shown in Figs. 22 to 24. A pair of
`magnets 31, 31 secured to the ends of the pole
`pieces 28 and 30, and therefore common to all
`these pole pieces, supply the necessary flux. The
`ribbon elements 25 and 26 are disposed in parallel
`planes spaced from each other axially of the mi-
`ecrophone a distance of.approximately one-half
`the wavelength of the highest frequency in the
`
`(21)
`eo15=8i0 wi($ cos 0+2))36000s @
`The ratio of the response for the angle ¢ to the *
`response at ¢=0 is
`.
`_D-+d cos 6
`(22)
`:
`Ro=—GTD_
`It will be seen from the foregoing that the com-
`bination of microphones §0 and (4 with the delay
`D in the transmission line connected to the mi-
`crophone {ff results in a combined response for
`the system which is more directive than for
`either of the microphones {@ or I! alone. This
`- result is represented by the curves IX, X, XI, SII
`and XII of Figure 11 for the various relative
`values of d and D applied to each of these curves
`inthe drawing...
`In place of the acoustical delay shown in the
`system of Fig. 10, an electrical network may he
`employed in the transmission line coupled to the
`microphone (1, as shown in Figs. 12 and 13, Fig.
`12 showing a ‘typical T-filter or network 20 and
`Fig. 13 showing a typical lattice network 21. The
`
`cos @
`
`:
`
`60
`
`70
`
`(6)
`
`12
`
`12
`
`

`

`Or
`
`10
`
`15
`
`S r
`
`2,301,744
`ange to which the system is responsive and
`pair. The spacing of the ribbon elements 25, 26,
`small compared to the wavelength of the lowest
`4{ and 42 from each other is approximately the
`frequency to. which the system is responsive in
`same but,
`in any case, the ribbons 4{_ and 42
`the same range.
`:
`should be spaced apart a distance which is of the
`The output of the ribbon element 25 is con-
`order of one-half the wave length of the highest
`siected by a transmission line which includes an
`‘frequency in the range to which the system is re-
`amplifier 32, a loudspeaker 33 and a tube 34 to
`sponsive and small compared to the lowest fre-
`one side of a conductive rib