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Oct. 1, I946._ w. P.‘ MASON ' 2,408,436 'MULTIPLEX COMPRESSIONAL WAVE SYSTEM Filed Oct. 24, 1942 4; Sheets-Sheet l 43.62, RN . lNl/ENTOR W I? MASON ' Arrow/Ex » Oct. 1, 1946. ' zmsgss W. P. MASON MULTIPLEX COMPRESSIONAL WAVE SYSTEM Filed Oct. 24, 1942 Y 4 Sheets—$heet 2 I 435M. h.bulNkb INVENTORY M.’ P MASON BV z‘ ATTORNEY‘ Oct. 1, 1946. 2,408,436 w. P. MASON MULTIPLEXCOMPRESSIONAL WAVE SYSTEM Filed Oct. 24, 1942 - 4 sheets-sheet 4 FIG. 7 850' lea ‘ L'GUAL AREA GRATING m m s u z u w. m a w u o>4n63:>.mo0e‘838 SQUARE LAW GRAT/NG ’ / 42 / /. flu\ .4»\\ m?E F \ 8_ w-_. ,0ml \ WW /.‘R0 5 .m . \ \ a// m5T Nf\Im \ \ \ \ \ / \ \ \ / 2 4/ / / ./W \ w. ./ WM // . /Mr M. .v. 2,408,436 Patented Oct. 1, 1946 UNETED STATES PATENT OEFIQE 2,408,436 MULTIPLEX COMPRES SIONAL WAVE SYSTEM Warren P. Mason, West Orange, N. 3., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application October 24, 1942, Serial No. 463,279 11 Claims. (Cl. 178-44) 1 This invention relates to multiplex channel transmission systems and more particularly to multiplex compressional wave systems employing diffraction gratings in compressional wave ?lters for separating the energies of the various chan nels. An object of the invention is to improve the selectivity attainable in wave ?lters at relatively high frequencies. Another object is to increase the frequency limits of effective compressional wave ?lters. 2 the case of the re?ecting type are, in the optimum condition for maximum effect, equal to the areas not utilized. In other words, in the transmission grating maximum e?ects are obtained when the slots have faces which are equal in width to the faces of the intervening bars. In the re?ection grating maximum e?ects are obtained when the widths of the re?ecting surfaces of the bars are each equal to the widths of the intervening ab sorbing strips. Such diffraction gratings cause wave energy of a given frequency to reinforce in several directions, the angles of which with ref erence to the plane of the energy exit face of the A further object of the invention is to decrease the number of elements of a diffraction grating transmission grating may be simply expressed by: required to give a de?nite discrimination. Still another object of the invention is to in 15 (l) cos-1 %’ cos-1 2} etc. crease the major lobe directivity of a diffraction grating ?lter with reference to subsidiary or where A is the wave-length of the energy in the minor lobes. medium beyond the grating and d is the distance Diffraction gratings are well known especially between centers of slots, that is, the separation of in the ?eld of optics. ‘They depend upon the fact the virtual sources. Y that a plane wave front may‘ be broken up into The effect of the series of dispersion angles for narrow striations which act as new sources of each frequency is to produce a ?rst order dif waves that reinforce each other in de?nite direc fraction lobe and higher order diffraction lobes tions, depending upon the spacing of the sources for each individual Wave-length. Each order of and the wave-length of the energy. Accordingly, diffraction tends to present a major lobe and sev as the wave-length varies the direction of re eral adjacent minor lobes. It follows that there inforcement for a given grating also varies with are a number of angles at which any one of the frequency with the result that incident waves of frequencies appears so that there is a possibility a wide band of frequencies are broken up by the grating into components dispersed at different 3.0 of overlap of a major lobe of a wave of one fre quency with a minor lobe of another frequency. angles. It is, therefore, possible by means of a If compressional wave responsive devices, which diffraction grating to separate a wave of one fre quency or of one band of frequencies from one of di?erent frequency characteristics. Two types of di?raction gratings have been used, In the re?ection grating parallel bars hav ing re?ecting surfaces and separated by non-re are non-selective as to frequency, are to be used, it is obviously desirable to reduce the minor lobes as much as possible so as to increase the ampli tude differentiation between these unwanted lobes and the desired major lobes. Applicant has dis covered that by varying areas of the grating ele ?ecting absorbing areas return the wave energies ments in such manner that a larger percentage ' to reinforce each other along lines on the same of the wave energy is derived from a central ele 40 side of the grating as the source. The other .type ment and a smaller percentage from a marginal of diffraction grating, the transmission grating, element, the directivity of the grating for a par comprises parallel energy-absorbing or re?ecting ticular frequency may be improved in the sense bars and intervening slots through which the that the ratio of the response at the major lobe wave energies pass to reinforce each other along lines on the opposite side of the grating. These 45 to that obtainable in other directions is increased. Moreover, the absolute energy of the major lobe lines at which the reinforcement occurs lie at is not substantially diminished so that the gain ' angles ‘with respect to the plane of the grating in discrimination is attained principally by re face which depend upon the wave frequency. duction of minor lobes. Various relationships of In both types of grating, the distance between the centers of the elements whether, re?ecting 50 the grating areas may be employed but in each the area of the central element should be limited, elements or transmission elements or absorption as has already been suggested, to a magnitude elements is uniform. Assuming elements of equal not greaterthan the area of the contiguous void lengths, the areas of wave energy utilized, that is, elements. According to one relationship which the areas of the slots in the case of the transmis sion type .or the areas ‘of the re?ecting bars in 55 has been found advantageous the widths of the 2,408,436 a L, 4 . elements may vary in accordance with the square law so that the widths of the individual elements increase passing from the outer element toward the center in accordance with the square of their distances from the outer margin of the grating. may be ascertained to indicate the directions of the second and third order lobes, respectively. It will be apparent therefore that the wave front of the ?rst order diffraction will be directed as is indicated by the ray S3 connecting points or and P the wave fronts of succeeding orders of diiirac tion will be oriented in more upward directions in Fig. l, in accordance with the magnitudes of be Diffraction employed to gratings differently of the diffract type described the individual components of a wide band of compressional waves. If the beam of waves, as a whole, en 92 and 93. 10 compasses a considerable cross section area over lap of the di?erently refracted beams may be markedly reduced by the use of a converging lens. The individual frequency or the subband ,fre quency beams so separated and fccu'ssed at dif_ ferent positions may then be impressed upon com pressional wave responsive devices placed at the respective foci. This, therefore, a?ords a means of effectively separating the different frequency The analysis presented is for the case in which the diffraction grating has a plane facial surface perpendicular to the incident rays. In that in stance the directly re?ected rays 84 and S5 are returned along the paths of incident rays S1 and S2. It will be apparent, therefore, that the angu lar separation between the directly reflected rays which include components of all frequencies and the ?rst order diifracted beam for any particular components of a compressional wave band con frequency or narrow banel of frequencies is meas— sisting of extremely short waves. ured by 90° —0. That separation may be made as great as desired by suitably relating d and‘ >\. If it be desired for any reason to. orient the grating at a di?ferent angle than the perpendicu lar position to incident ‘rays the directly re?ected The invention m'ay-bebest understood bylref erence to the following detailed speci?cation taken in connection with the accompanying~draw~ ings‘in‘ which: Fig. l: is a diagram to assist in explaining the principles of the invention; ~ ' Fig; 2 illustrates diagrammatically the circuit and‘ apparatus of multiplex carrier wave system employing compressional waves; ' e ' - ‘Fig. 3v illustrates on an enlarged scale the de sign- of: a re?ection type diffraction grating‘ em ‘ployed in the system of’ Fig. 2; - r r Fig. 4 illustrates a modi?cation of the system of Fig; 21in which a transmission type diffraction grating is employed; Fig. 5 illustrates’ on an enlarged scale details of the diffraction grating of Fig. 4; Fig; dillustrates a'modi?cation‘ of the grating ofFig_;5;~ < r > r Fig. -'7 is a graph of directional response of a compressional wave ?lter system showing'the ii.. provement attained by the use of» onespccies of rays corresponding to S4 and S5 will of course return along parallel paths determined by the - well-known principle that the angle <I> of reflec tion is equal to the angle of incidence. The dif fracted beams will be sent out from the ‘diffrac tion grating at still different angles determined by theprinciplepresented in. the analysis of Fig. 1 that the energies from the various new centers are in phase coincidence in a particular direction. The direction of the ?rst order of diffraction will accordingly be separated from that of. the directly re?ected beam by an angle [81 so that with ref— erence to the plane of the grating face the angle of the beam of the ?rst order of diffraction will be [QT-(@3801, that of the second order dif fraction beam [90°—(s§i?2) ],_etc. Referring to Fig. 2,, T1, T2, 'I's,v T4 illustrate, re spectively, four individual transmitting channels > ~ ' » of a multiplex carrier wave system. As indicated Fig. 8 is a similar graph showing the result ob these channels may each extend over a. frequency tained' with the use of another modi?cation; and 4. range of 3 kilocycles, the four channels as a 9 illustrates- a system- in which aconcave whole lying, withinthe band of, 50 to 65 kilocycles. the invention‘; ‘reflect-ion’ grating- replacesboth the plane grating andthe converging-lens of~theprecedingsystems. . Referring to Fig: 1 let b1 and’ barepresent. t-wo The four transmitters, T1, T2, T3, and. T4 may be connected in series to a transmission linel ter minating at a remote station in an electric wave proximate reflecting bars 01” a di-?’raction grating 50 to sound Wave transducer 4 having a. diaphragm the centers or and c2 of which- are spaceda dis tance d.~ <Two rays 81 and S2‘ rom a plane wave front source of ‘wave-length \ are incident» simul or sound radiating eiement 5. from which ema nates a beam of high frequency. compressional Waves corresponding. in frequency and. in their taneously- upon the two bars at their? central modulation to the electric wave received over the points. Assuming that or be considered as a new transmission line I. The beam of compressional waves produced by the diaphragm. 5 and includ ing components of the various frequencies of the point source, its energy at one cycle later‘will be spread» out over a circle- whose- center- is or and Whose‘ radius is A.’ Accordingly, a line from 02 passing tangent to the circleras' at P will indicate a— locus of equalphaseelrects from the twolcen ters cz-and c1. Denoting by r‘i-the angle'which theray crP makes with reference to the plane of the grating b1, 192 one obtains the well~known're lationship - A 3; cos 6 - ' > (‘2) This determines the direction ofv the ?rstcrder lobe of the diffracted beam. 'Bygasimilar reason ing the relationships _ 1 2X . F=cos 0: (3) 3T>\=cos 0,1 (4) and electric waves transmitted over. the line I‘ is per mitted to fall upon a re?ection type diffraction 60 grating 6 positioned in the path of the beam. The diffraction grating 6, the structure of which will be. explained subsequently, serves to di?’ract the mid-frequency components f1, f2, f3, and f4 of the four bands at the dilTerent angles indicated in the drawings. In the path of. the diffracted beams is placed: a converging lens 1' of- plane concave type andwhich may be either cylindrical or spherical depending on whether it is‘ desired to focus the beams along’- lines or at points. The lens ‘I may consist of any’ suitable homogeneous solid material and preferably of some plastic ma terial such as Tenite II (cellulose acetate butyre ate) isobutyl methacrylate, or vinyl chloride. At thew-‘respective foci‘of the fourgbands are.posi— tioned compressional wave responsive devices R1, 2,408,436. 5 R2,, R3. and R4. The devices R1, R2, R3 and R4 are, illustrated diagrammatically as of the piezo electric type and each may be associated as is shown in the case of R; with suitable translating . 6 is shown in Fig. 3, in an actual grating there should be preferably as many as ?fty re?ecting bars and a structure of two hundred bars is more e?ective. The central re?ecting bar 12 may have and indicating apparatus. Device R4 is, for ex ample, connected by an electric circuit 8 to the input terminals of an ampli?er 9, the output of a width which is connected to a demodulator [0 which in turn supplies speech or other low frequency sig about equal to that of the adjacent absorbing 7 d 2 strips [3 and I4. Succeeding bars should be so nals such as originated at transmitter T4, to the 10 positioned that their centers are at a distance of approximately 11 from the center of the nearest signal indicator H, which may be a telephone bar. Their widths, however, progressively de receiver, a loud-speaker or any suitable signal indicating or recording appliance. It will there crease in accordance with a square law distribu tion so that beginning with the outermost bar I8 fore be apparent that the four individual mes sagestransmitted over the line I as modulated 15 which may have an area A, the area of the-next carrier current may be separated and supplied bar I‘! may be 4A, that of bar l6, 9A, etc. In an alternative arrangement the widths may be to individual terminal circuits by the mechanical wave selecting system comprising sound producer varied in a sinusoidal manner so that the width of the bar l2 will'be the maximum width of any 5, di?raction grating 6, compressional wave lens 1. and compressional wave responsive devices R1, 20 of the bars and that of an imaginary bar at a distance d beyond I 8 will be zero, the distance R2, R3, and R4. between the centers of the imaginary bar and The beams of frequency f1, f2, f3, f4 have been bar 12 corresponding to 90 degrees of the sine dealt with on the basis of the mid-band fre function which determines the bar areas. Referring to Fig. 4 transmitters T1, T2, T3 and T4 which may correspond in every respect to those. of Fig. 2 are connected to the line i which ter minates at a remote point in the electric wave to sound transducer which may be a loud-speaker 4 the mid-band frequency ray to substantially af 30 having a sound-producing diaphragm 5. Beyond the diaphragm is a transmission type compres feet the operation of the system. sional wave diffraction grating 20 positioned in The transverse dimensions of the sound wave the path of the sound beam in the diaphragm 5. source 5 for effective directive transmission The diffraction grating 20 serves to di?erently should be relatively large compared with a wave diffract the four beams whose mid-frequencies length of the emitted energy. The diffraction are f1, f2, f3 and L; as indicated. A focus'sing grating should therefore encompass a space great lens 2! in every respect similar to ‘the lens ‘I of enough to receive e?iciently substantially all of Fig. 2 may serve to focus the individual beams of the directive beam from the diaphragm. It fol the four receiving elements R1, R2, R3, R4 in the lows that the supersonic beam will have a fairly large cross section in space. This situation is 40 manner already described in connection with the supersonic wave responsive devices of Fig. 1. indicated in Fig.2 in which the marginal rays of As indicated in Fig, 5 the arrangement of the the bands 11, f2, f3, f4 are indicated. The lens 1 elements of the grating 20 corresponds in a gen should be given such dimensions and should be eral way to that of the elements of the grating B so placed with reference to the grating that the as shown in Fig. 3 with the central opening 22 foci at which’ the bands respectively converge of the transmission grating 20 corresponding to may be sufficiently separated to enable the com the central reflecting bar I2 of the re?ecting grat pressional wave responsive devices R1, R2, R3, R4 ing 6 and with the successively adjacent openings to be given practicable dimensions and so insure 23, 24, 25 of the grating of Fig. 5 varying in width that the zone which’ each responsive device oc in the same manner as the re?ecting bars l5, Ni, cupies is reasonably free from energies of the un ii’ of the grating 6. The absorbing strips 26, 21, wanted bands. The orientation of the re?ection 28, etc. of the grating 20 may consist of any ‘grating performs an additional function in en abling the compressional wave receiving appara suitable material such as, for example, Vistanex (polymerized isobutylene of extremely high tus to be placed on the same side of the di?rac molecular weight), or the strips 26. 21, 28, etc. tion grating as is necessary in the case of the re quency of each beam. Since each band encom passes a range of 3000 cycles the limiting fre quency rays will diverge slightly from the central frequency range, but the amount of divergence over distances which are not excessive will not cause too great a departure from the position of flection type without interfering in position with can be made of steel which will re?ect the unde the compressional wave beam emitter 4, 5 or be sired energy away from the direction of the receiv ing directly affected thereby. If necessary a ‘sound ba?le [9 may be interposed to prevent transmission directly from diaphragm 5 to com pressional wave receiving devices R1, R2, R3, R4. In general, however, these compressional wave ing elements R1, R2, R3, R4. devices are preferably made as directive as pos sible with respect to their receptivity so as to re duce the e?ect of waves from extraneous sources. : Fig. 3 shows in more detail the structure of part .of the re?ection type di?raction grating 6. As illustrated, the grating consists of alternate com pressional wave re?ecting bars and intermediate compressional wave absorbing strips. The bars may, for example, consist of highly polished steel. For most ef?cient re?ection, the steel bars should have a thickness of about one-quarter wave length. .Although for convenience in illustration a much fewer number of bars and absorbing strips The individual compressional wave responsive devices R1, R2, R3 and R4 may each be con nected to its individual ampli?er, demodulator and signal indicator as indicated at 29, 39 and 3!. Fig. 6 discloses a modi?ed form of transmis sion diffraction grating in which the central half of the structure consists of re?ecting or absorbing bars 21' having a width‘of ’ 2 The two outer quarters constituting the remainder of the structure consist of re?ecting or absorbing bars 26’, 28’ each having a width of g? _ "Ml-h “. 4 24081436 8 highly This distribution directive diffraction of di'?ractingl' effe‘ " area-“s yields The effect W-h’iclrthév unequal‘ a-r fractidnigrating» ?lter" in“ which the areas‘ of the iridiv’i'dualelements' are vaneam accordancewith sine law.’v This graph is pio'aed's‘o that th'e'rnaf 4 principle of J'or-1obés'441~a~nd'4’5 may be compared upon appliifa-nt’s consideration novel grating‘! of theprodubes graph of mayFig‘.b 7‘ in which directivity characteristics or responses at a U! particular frequency are plotted against the phase 273 states less thanthe major lobes.‘ This an angle between energies emanating from progn mate elements of the) diffraction grating.‘ The solid line curve marked “equal area grating-if indi catesthe relative energies at variousangles from the condition of phase vcoincidence to-thatof 360 degreessepalration. In order to make clear the reason for-this we may‘advert once more to .the diagram of Fig’. l. e p _ 1 'The ray ClP combines with'the' energy emanat ing‘from 02 because the two are in phase agree merit. For rays, of all other directions between 01F and Ss-there will be lack; of phase agreement and a’ red-ueedlresultanti This situation'is por~ trayedl in Fig. '2 inwhich the resultant intensi ties in various‘ directions are plotted-in- termsv of phase difference between the energies emanating lob'e's' 3-1 and» 3'4, ‘respectively of Fig; 7. The minor ‘lobes mane 4*‘! have peak intensities of the order of rangeine’nt- therefore does not yield quite‘ as high 10 a’ discrimination between the major lobe’ and the ?rst minor’ lobe tut» itZ does have'the ‘advantage that in the central portion of the range‘ as; will be observedby referring to‘ the“~ envelope 4_8 the intensity falls to'a magnitude 60 decibels below that of the major lobe. Since it is possible‘ as‘lfi'as already been‘ explained by a suitable’ design of'the filter‘ to orient the diffracted beams‘ to‘ the most favorablevangular position this sinusoidal distri btrticn' affords a means of securing a very high discrimination between the desired energy of the ?rst order diffraction and undesired‘ energy at a position or angle substantially removed there from; I Fig. 9 discloses'a‘ system in which there?ection from crand oz.‘ The directly-reversed raye S4 V grating is given a’ concave conformation men: and S5 are in phase agreement. Hence the phase able it toirepla'ce‘ both the plane‘ diffraction grat diiierence of these reversed rays is zero. The ing of the preceding figures and the'conve'rging phase at point P of the ray all? is one full wave~ le'n's employed therewith. It is well known that length ahead of the ray just starting from 02 in the: case of light if a point source be positioned at the instant that the ray c1P has reached point P. Hence the phase difference is 360 degrees and 30 on a; circle having a radius of curvature R the ray ClP indicates the direction of the‘ ?rst order diffraction. In Fig. '7 the solid line shows 2 . distribution of energy for a‘ grating having equal having a center at the centraipoint of a spheri area diffracting bars. At the zero degree phase calgra‘ting' facing the-circleland‘h'avii’ig" a radius positionv which exists‘ between the reflected rays 35 of curvature R, the various frequency compt S4 and S5 the intensity of the re?ected wave is nents will‘ be separated from ‘each; other by- dif indicated by the major lobel3l. It will be noted fraction and‘ will" fall at di?'erent- points on the thatthis lobe falls rapidly to a low magnitude but circle'at lwhich'th’e point source lies; (‘See Wood's is closely followed by the minor lobes 32 and 33 ‘Optics, New and‘ Revised Edition; 1928; which are of successively smaller magnitude.’ At 40 Physical pages; 231 to" 236, inclusive.) Thisv pl‘il'lciplle~ is the phase angle 360 degrees there is a major lobe employee; in the system‘ of Fig.9 in which Ti‘, 'I'z_, 34 for the ?rst order of diffraction. Correspond ing minor lobes 35 and 35 slightly precede the T3‘ and( T4‘ represent transmitting? sources similar to those'of the systems-liirevidusly'described; ‘The major lobe. Throughout the intervening angular transiri-ittersw a'r'e connectédto- a common- line‘ I which terminates in a compressional‘ wave emit range between lobes 33 and 35 the envelope 3‘! ‘of the lobe peaks falls to a minimum value approximately 33 decibels below the magnitude of lobes 3| and-34. tingi element’ 56 ‘which: may be‘ of piezoelectric type; A series‘of energy absorbingielements' R1, R2,?"R3, R4 tuned‘ respectively to the frequencies ‘of The broken line graph portrays the- perfornn ance of a diffraction ?lter having ?fty elements - with the phase areas of the individual elements varied in accordance with the square law. The major lobes» 38 and‘ 39 at zero’ degree and 350 degree phase angles, respectively, are equal in intensity to the major lobes 3i and 3450f the solid line graph. The first minor lobeti? adjacent the zero phase position and the ?rst-minor lobe M adjacent the 369 degree division are greatly re; duced, their peaks lying some 27 decibels below the peak ofthe major lobe. Moreover, as indi cated by the envelope :32 the minimum intensity occurring at the 180 degree phase‘ angle is'more than 50 decibels below the peak‘ of the major lobe. It is accordingly apparent that a very im transmitters T1; T2, T3 and T4- are" arranged; along the’ circufnfefencev5 or a circlelpas‘sing‘ through the element 50\ and-having its‘c‘enter at thepoint 51. A spherical ‘diffraction grating‘ 52 having a radius of curvature equal to twice‘ the distance between element 595 and point 5|‘ is positioned facirigthe array of energy absorbing elements vand with’its centra-limendber at the point-5i. The entire as: semblage‘of- energy radiator ??jenergy absorbing ele'n’ie'iitsRl', R2, Rf; and R4, and the spherical dif fraction grating; 52=is> enclosed within" a“ container -5-3'i?lled with a liquid such as castor oil. The con; taine'r‘ preferably eonsists of acoustic ‘absorbing materialt'o- prevent reflections from its; walls to the interiorelenfientsr It may consistof an outer 5'4 which‘ is?uid'tight’ with'an‘innerscreen portant increase in directional discrimination is 65 shell 55 @betw'eje'ii; which ‘ are retained copper ‘shavings'or attained by the use of‘the‘ square law area of variation principle. This increase in discrimina tion may be employed to produce more effective ?ltering with a diffraction grating structure of a given superficial area and cost oi- it'may be utilized to reduce the number of elements-and the cost of a diffraction grating’ ?lter with reference to the area and cost of a ?lter utilizing equal area elements. . Fig. 8 shows the results obtained with a dif copper‘ foil" saturated with castor oil.“ A‘ baffle‘ 56 is'introduce'd in direct'lindbetween‘ the energy radiator 59’ and theien‘e'rg'yl absorbing devicesto prevent ‘direct transfer of‘compressional'wave en' ergy therebetw‘ee‘n. :In‘ operation compressional waves; set‘ up‘ inv the ca'tor'oil' by‘ théfc'o‘rnprés s'ional ' wave emitter“ 5 0 ‘ impinge umsmhe' ‘concave reneaidrrtyrié unnatural"gratingsztana a're‘fdif frac'ted" iii‘ di?er‘enti dire'cti'on‘sf so’ ‘that, wares‘ ‘of 7,5. the‘ ‘frequency originating" at transmitter "I‘i' im'i 2,408,436 pinge upon the energy absorbing member R1 and a diffraction grating having parallel elements wavesof the frequencies originating at each of » the other transmitters likewise impinge upon a each provided with a face substantially aligned with corresponding faces of the remaining ele single respective energy absorbing member. As in the case of the preceding ?gures each of the energy absorbing members is provided with its individual ampli?er, demodulator and signal in dicator. In this system as in the preceding sys tems the principle of utilizing maximum e?icien cy reflection at the center with decreasing reflec tion toward the outer periphery is employed. The ments to constitute a striated surface, the sepa ration of the central longitudinal axes of prox imate elements being uniform, the widthof the central diifractive elements vbeing _ substantially equal to the width of the spaces intervening and 10 the width ofthe outer elements decreasing from central re?ector may therefore have the form of a concave disc. .10 5. .A ?lter for compressional waves comprising The remaining reflector ele a maximum at the central part of the structure in accordance with the square of the distance therefrom to a minimum at the outermost ele ment. ments comprise annuli of spherical contour which 6. A diffraction grating comprising a plurality have equal spacings between their center lines 15 of parallel elements having one face of each and which decrease systematically in width in aligned with the corresponding faces of the re accordance with the sine law from the central maining elements to constitute a striated surface member to the outer member. By this expedient upon which wave energy may fall, the elements the major lobe for the ?rst order of diffraction is having uniform spacing between the central lon greatly accentuated in amplitude relative to the gitudinal axes of proximate elements, the width remaining lobes. of the aligned faces of the elements nearest the What is claimed is: center of the structure being greater than that l. A diffraction grating for re?ecting waves of the elements more remote from the center and having substantially plane wave fronts to differ width of the faces of the series of elements ent angles according to their frequencies com 25 the decreasing progressively from the center to a prising a grill of parallel re?ecting bars having minimum at the outermost element. ?at reflecting surfaces so disposed with respect 7. A multichannel selective system for super to each other as to constitute elements of a com sonic waves comprising a source of supersonic mon larger surface, the central longitudinal waves of different frequencies, a di?raction grat axes of the bars being equally spaced, the re 30 ing positioned in the path of a beam of waves fleeting surface area of the central bar being sub emanating from’said source, refractive means stantially equal to the area of each of the con for focussing a plurality of different frequency tiguous slots between it and the adjacent bars components of a diffracted beam at respectively and the remaining bars having re?ecting sur different points and individual compressional 35 faces the widths of which progressively decrease wave responsive devices positioned at each of said from that of the central bar to that of the outer points but having energy receiving surfaces upon most bars in a substantially sinusoidal fashion. which the focussed diffracted beams may im~ 2. A diffraction grating for separating diiferent pinge. frequency waves comprising parallel alternate 8. A supersonic wave source comprising a super 40 transmission and non-transmission elements of sonic wave energy emitting member having a substantially equal lengths, the central axes of the transmission elements being uniformly spaced and the transmission elements decreasing in area from the central element to the outer ele ment in sinusoidal fashion with relation to dis tance from the center of the grating whereby the non-transmission elements increase sinusoidally from the center to the outermost element but in a converse manner. 3. The method of increasing the directive se- ' lectivity of a diffraction grating which comprises placing proximate elements so that the loci of their center points are substantially equidistant one from another, spacing the central diifractive elements from each other by about their own substantially plane surface, the transverse di mensions of which are relatively high compared to the wave-length of the supersonic energy to be transmitted whereby a highly directive beam of supersonic energy may be emitted, a diffrac tion grating in the path of said beam compris ing parallel elements, the central points of which are equally spaced throughout the series of ele ments but the widths of which progressively de crease from the central element to the outer ele ments, a converging lens positioned in the path of a diffracted beam from said grating, and a plurality of compressional wave responsive de vices positioned respectively at the feel at which said lens causes a corresponding plurality of dif widths in order to obtain maximum primary ferent predetermined frequency components of order diffraction therefrom, and varying the the diffracted beam to be focussed. widths of the dilfractive elements to cause them 9. A diffraction grating comprising a plurality to progressively diminish from the center out of parallel bars, said elements having one side wardly approximately in accordance with a 60 of each so positioned as to constitute a striated square law relationship. surface, the thickness of the re?ective bars 4. The method of increasing the directive se thereof in the direction perpendicular to said lectivity of a diffraction grating comprising par surface corresponding to substantially one alel elements of substantially equal lengths, the quarter wave-length in the medium of the re?ec central longitudinal axes of the proximate ele tive bars for waves of a predetermined frequency ments being uniformly spaced, which consists in in combination with a wave-responsive device spacing the central elements by such distance as sensitive to said frequency, and so positioned with to give substantially maximum diffraction ef respect to said grating as to receive a maximum fects and varying the areas of diffractive ele beam of said frequency diffracted therefrom. ments more remote from the center to reduce 10. A diffraction grating comprising a plurality them with reference to the areas of the central of parallel flat bars having their corresponding diifractive elements whereby the discrimination major faces aligned in substantially the same between the major lobe of a particular order of planes, the faces in one of said planes being diffracted energy with reference to a minor lobe 75 highly polished and the distance between said is augmented. 11 2,408,436 faces being substantially one-‘Quarter wave length the medium of ‘the bars for Waves of a predetermined frequency, in combination with a wave-responsive device sensitive to said fre quency, and so positioned with respect to said grating as to receive a maximum beam of said 12 path of a beam of Waves emanating from said source and having a plurality of parallel bars, said bars having one side of each so positioned as to constitute a striated surface, the thickness of the reflective bars thereof in the direction per pendicula-r to said surface corresponding sub stantially to one-quarter of the wave-length of 1,1. A source of wave energy comprising a the emitted frequency in the medium of the re means -for emitting waves of substantially one ?ective bars, frequency, a di?raction grating positioned ‘in the 10 WARREN P. MASON. frequency diffracted therefrom.