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Патент USA US2408437

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Oct. 1, I946._
w. P.‘ MASON '
Filed Oct. 24, 1942
4; Sheets-Sheet l
Oct. 1, 1946.
' zmsgss
Filed Oct. 24, 1942 Y
4 Sheets—$heet 2
Oct. 1, 1946.
Filed Oct. 24, 1942
- 4 sheets-sheet 4
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Patented Oct. 1, 1946
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)
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
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.
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
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.
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
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
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
of the
type described
the individual
components of a wide band of compressional
If the beam of waves, as a whole, en
92 and 93.
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;
‘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; -
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
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
3; cos 6
This determines the direction ofv the ?rstcrder
lobe of the diffracted beam. 'Bygasimilar reason
ing the relationships
F=cos 0:
3T>\=cos 0,1
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,
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 .
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
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 ’
The two outer quarters constituting the remainder
of the structure consist of re?ecting or absorbing
bars 26’, 28’ each having a width of
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
principle of
J'or-1obés'441~a~nd'4’5 may be compared
novel grating‘!
of theprodubes
graph of
mayFig‘.b 7‘ in
which directivity characteristics or responses at a
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 _
'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
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
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
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
the ray ClP indicates the direction of the‘ ?rst
order diffraction. In Fig. '7 the solid line shows
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
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
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.
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
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
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
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
frequency waves comprising parallel alternate
8. A supersonic wave source comprising a super
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.
faces being substantially one-‘Quarter wave
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
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
frequency diffracted therefrom.
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