US 3105113 A
Description (Le texte OCR peut contenir des erreurs.)
H. F. oLsoN 3,105,113
2 shets-sneet 1 STEREOPHONIC LOUDSPEAKER SYSTEM Sept, 24, 1963 `Filed. July 15, 1960 Sept. 24, 1963 H. F. OLSON 3,105,113
sT'EREoPHoNIc LoUDsPEAKER SYSTEM 2 Sheefs-Sheet 2 Filed July l5, 1960 v ifmmmgv United States Patent Office 3,10,l13 STEREDPHONHC LUDSPEAKER SYSTEM Harry F. Olson, Princeton, NJ., assigner to Radio Corporation of America, a corporation of Delaware Filed .luly l5, 196i?, Ser. No. 43,04@ 6 Claims. (Cl. 179-1) The present invention relates to stereophonic soundreproducing systems, and more particularly to loudspeaker systems for two-channel stereophonic sound reproduction in radio receivers and the like.
Two-channel -stereophonic sound reproducing systems provide for translating two stereophonically-related audiofrequency sound signals, such as may be derived from two .spaced microphones at an original sound sou-rce or the recordings or radio transmission thereof, through separate signal channels and individual `sound-reproducing loudspeaker means for each channel. It is important `for sound reproduction -in proper auditory perspective, to provide relatively wide spacing between the sound sources as reproduced by the loudspeaker means.
Stereophonic sound reproduction by means of magnetic tape and disc records 'has become an important medium in the home entertainment field. Compatible systems for the transmission of stereophonic sound in Iboth AM and FM radio broadcasting have been developed, and transmission .of stereophonic sound on these two media commercially may be initiated as soon as standards are established. Thus there may be four stereophonic sound systems for commercial use in the home entertainment field to provide effective stereophonic sound reproduction, namely, the disc phonograph, the magnetic-tape reproducer, the AM radio andthe FM radio.
The transmission of stereophonic sound by means of AM and FM radio presents the need for table-model stereophonic radio receivers that may be housed in a single cabinet. This requires loudspeakers for two-channel stereophonic sound reproduction, in a single cabinet or cabinet structure of relatively small dimensions. Under these conditions the separation of the loudspeakers or loudspeaker units, and the virtual sound sources created thereby, must likewise be relatively small. As mentioned above, in stereophonic sound reproduction the spacing between the loudspeakers is important in producing a desired degree of auditory perspective or stereophonic effect in a listening area. Therefore, it is desirable to increase the separation between the sound sources with small table-model radio or like cabinet structures for t-wochannel stereophonic sound reproduction, thereby to overcome the physical limitations imposed by the small cabinet dimensions.
It is, therefore, an object of this invention to provide a table-model loudspeaker system .for two-channel stereophonic sound reproduction which gives effective stereophonic sound reproduction in good auditory perspective l with a single cabinet of relatively small dimensions.
Itis also an object of this invention to provide an acoustical system for table-model and like small radio receivers and like apparatus, in which the separation between the virtual sound forming part thereof and which may be mounted in a relatively-small unitary cabinet structure for the apparatus.
Itis a further object of this invention to provide a twochannel table-model stereophonic loudspeaker housed in a single cabinet and including an acoustic system which gives augmented separation lfor the virtual sound sources without increasing the lateral dimensions of the cabinet, and improved sound response therefrom in a forward direction.
Means have been proposed heretofore for employing spaced loudspeaker means in a single small cabinet structure, as shown, for example, in the patent to Crosley et al.,
i-il Patented Sept. 24, 1963 2,062,390. The Crosley structure causes the sound to be distributed laterally but not directly forwardly. Although such structures increase the effective separation of the vertical sound sources from the two speakers, they do not provide good quality sound reproduction at the customary listening location in front of the cabinet due to a lack of high frequency response in the frontal direction.
Accordingly, it is a `further object of this invention to provide an improved and unitized stereophonic loudspeaker system embodying a table-model radio receiver or like cabinet structure of relatively small size which will give good quality sound reproduction in good auditory perspective from sound sources that appear to be spaced farther apart than the length of the receiver cabinet.
In accordance with the invention, a loudspeaker system for two-channel stereophonic sound reproduction ernbodies the feature that the sound sources extend beyond the ends of the cabinet structure, in laterally spaced relation, so that an effectively Wider spread between the actual loudspeaker sound sources is obtained. The effectively vvider spread is combined vwith forward sound projection which is accomplished by means of acoustic wave guides Within the ends of the cabinet structure. Each of these wave guides includes a series of sound-wave-directing vertical vanes or partitions, in parallel relation, at an angle to the end of the cabinet structure and to the plane of the face of a direct-radiator type loudspeaker unit, which plane is generally vertical and at an acute angle to the end of the cabinet struct-ure. The loudspeaker units preferably may be of the relatively-small conical-diaphragrn type having a central voice-coil and a flexibly mounted, or flexible, cone rim or edge carried by the usual frame or spider which also carries the field structure.
This system is thus particularly adapted for applications requiring reproduction of two-channel stereophonic sound from table-mode radio receivers having small cabinets and like housing means. In any case, the loudspeaker units lor mechanisms are spaced apart in stereophonic relation in separate left and right end sections of the enclosure or housing means, facing semi-forwardly against the wave guides. rIhre enclosure or housing means is a single cabinet, generally rectangular and elongated when practical, of table-model di-mensions. The loudspeaker units mounted in the spaced ends or sides of the cab-inet increases the effective distance between the virtual sound sources of the loudspeakers by a distance of substantially twice that of the diameter of the loudspeaker cone, which may, depending on cabinet length be twice that which is possible with the ordinary spaced mounting of the loudspeaker units in the front of the cabinet. The acoustic wave guides direct the sound waves or radiations from the end-mounted loudspeakers in a forward direction, so as to insure suliicient high frequency response in front of the cabinet to provide good quality sound reproduction.
These units are connected for operation stereophonically in response to applied two-channel stereophonic sound signal-s from the radio receiver, or other suitable source which is included in the enclosure or housing means. The loudspeaker system therefore has two signal input circuits, one connected with the right-hand loudspeaker unit and the other connected with the left-hand loudspeaker unit, and is particularly adapted for use in table-model AM and FM stereophonic radio receivers.
It may, therefore, be considered to be a specic object of this invention, to provide a single-unit stereophonic loudspeaker system which operates to reproduce stereophonic sound from a two-channel stereophonic radio signal source, in a forward lor frontal direction and with good auditory perspective, and which may simply and effectively be housed and battled in a unitary cabinet structure or like enclosure means of limited dimensions required for table-model radio apparatus, whereby it is system embodying the invention, together with diagrammatic vrepresentations of sound patterns illustrating a principle. of operationy thereof;
FIGURES 2r and 3 are end and frontal views, respectively, of the Vsound system shown in FIGURE 1, FIG- URE 3 being broken away and showing only one end portion'of'th'e structure ofFIGURE v1 tosimplify the drawing; and, Y
FIGURE 4 is a diagrammatic plan view, in crosssection as taken on section line 4 4 yof FlGURE 3 and on an enlarged scale, showing further details of construction and. certain principles of" operation of the system ofFIGURE 1.
Referringto the drawings, inwhich, throughout the various''gures, like reference characters are used to refer to like elements, a small cabinet S of rectangular box-like construction. is shown asy of thetype adapted for-.a table model radio receiver and the like. As. will be seen from an inspection of the top, end, and front views of FIG- URES 1, 2, and 3 respectively, the cabinet is provided with a central interior space or section 6 for housing stereophonic radio and like apparatus which may be provided to drive twov stereo-channel'loudspeaker units 7 Vand located respectively in left and right-hand end chambers 9 and 19 Vin the cabinet. The end'charnbers are providedrwith partitions 11 extending across the interior of the cabinet, as indicated, to separate the loudspeakers from the apparatus where there is any problem of 'acoustic feedback encountered. Each end or side wall of the cabinet is provided 'with a relatively large rectangular sound-conducting opening communicating lwith the adjacent 'end chamber, as shown more particularly at l2 in FIGURE 2, `toward which the respective speaker units areangularly faced as indicated in FIGURE 1. The end or side walls, `as in the present example, are generally substantially parallel and vertical, andthe cabinet structure is rectangular.
As shown more clearly in FIGURE 4, each loudspeaker unit is mounted-at an acute angle to the adjacent endfofthe cabinet andiisfof the direct-radiator type having aV conical-type diaphragm 14 the front face of which stands in a verticalplane Y which is located, inthe present example, at substantially 60 to the plane of` the end wall .13 of the cabinet `or the opening l2 therein. The axis-.or centerline 16 of the speaker unit forms an acute angle, with'respect to the opening 12 and the end of the cabinet. In the presentexample this is substantially with respect to the end `ofthe cabinet and the-loudspeaker unit,therefore, is positioned to direct sound from the conical diaphragm l14 substantially forwardly or frontaliyof the cabinet in a general direction as indicated by the center line '16.
The loudspeaker units, as indicated in FIGURE 4,
f provide. ilexible rim` mounting for the conical diaphragm Vusein small radio cabinets. In the-present example these units maybe consideredto be typical 8 inch direct-radiator loudspeaker mechanisms,-in which case the diameter ofthe face of each speaker diaphragm or cone `is subsources represented bythe distance Dsz.
stantially7 inches. In relation to this siz'e speaker, the cabinet size as herein shown, may be considered to be substantially 9 inches high, 10 inches deep and 30 inches wide on the outside. Y Asshown in FIGURE 2, the openings 12 occupy a major portion of each side :or end of the cabinet. Y
A typical unitary table-model stereophonic loudspeaker system of ordinary construction would have the two speaker units 7 and 8 facing directly frontally at the ends of the cabinet substantially on center lines indicated at 20 and 2l in lFGURE l. Assuming 8 inch diameter speakers and end wall thickness of 1 inch overall, the center lines 2t) and 2l would each be about 5 inches from the ends of thek cabinet for widest or maximum separation. This would make the distance DS1 equal to approximately -20 inches. Theietfective separation between the two loudspeakers and between the virtual sound sources would thus be limited. In other words', both the physical and subjective location of the sound" sources would be the same with this type of construction, and it will -be seenthat the separation between the sound sources would be small in a table model cabinet of this type.
A.V unitaryr two-channel stereophonic loudspeaker system with the direct-radiator loudspeaker mechanisms or units mounted in the ends of the cabinet as4 shown herein provides an' effective separation of the virtualv sound Y From theA diagramv shown in FIGURE'l it will be seen that mounting the loudspeaker mechanisms inthe ends of they cabinet. Assuming that Dg is approximately 4 inches in the present example, the additional separation of the soundsources is thus 16=inches plus the thickness of the cabinet walls at the ends, which may be considered as additional two inches, soy that the ZO-inch spacing represented bythe distance DS1 is substantially doubled for the distance D52. Thisrepresents a considerable 'irnprovement in the spacing of the sound sources without extending the width of the cabinet.
. The construction shown in the Crosley patent referred to, wherein the speaker units face the ends of the cabinet, does not provide as high .a quality of sound reproduction'as may be desired due to a lack of high frequency response fonwardly or frontally of the cabinet, which is assumed to be the customary listening location with a stereophonic sound reproduced,A system;
' However the stereophonic loudspeaker system ofthe presen-t invention provides for spreading the virtual sound sources as `shown while restoring the high-frequency responsein the forward direction. As indicated in FIG- URES 1, 2 and 3 and as shown more clearly in FIGURE 4, acoustic wave guides ZSand 2n' lare provided respectively in the left and righty hand compartments to direct the sound waves fromthe loudspeaker uni-ts in a definite forward direction inaccordancewith Vthe' arrangement shown in ldetail in 'FGURE 4.
Considering the constructionshown in FIGURE 4,`the acousticy rwave guider 26, representative of bothv wave guides, comprises a pluralityv of' substantially parallel vanes or guides 27-133, inclusive,.standing vertically and extending between the plane Y'of the speaker diaphragm and the opening 12 or end wall 15 of the cabinet. A sufficient number of vanes are provided, in` this case seven, to define and divide Vthe sound conducting area of the wave guide, between the diaphragm and the end opening 12 of the cabinet, into ducts which are relatively thin and less than one wave length in width or thickness, and of gradually decreasing length from front to rear of the cabinet. In other words the vanes or guides are spaced less than one wave length apart, at the highest useful frequency of transmission. They are in substantially parallel vertical planes extending between the plane of the diaphragm and the end opening in the cabinet and operate to convey sound waves from the speaker unit and direct them forwardly in an area centered talon-g a line graphically represented at 35 passing through the vir-tual sound source 23. This line may thus be taken as representative of the general direction of the sound frontally. Since it passes through the virtual sound source 26, it is shown at a distance from the end of the cabinet substantially equal to the radius of the loudspeaker unit diaphragnl The geometry of the wave guide, cabinet and speaker relation at each end of the cabinet, as depicted in FIG- URE 4 may now be considered. The outer ends of the vanes or guides 27 33 project into the openings tat the ends of the cabinet, such as the opening i2, and at their inner ends terminate at the forward or frontal plane of the speaker diaphragm, such as the plane Y. This is adjacent to a baille board 37 of rectangular shape extending from top to bottom of the interior of the cabinet, as indicated in FIGURE 2. The baffle board 37 has an opening 38 conforming to the size of the speaker diaphragm `for transmitting the ovenall sound through the wave guide. As was indicated in FGURE 2, the wave guide vanes extend from top to bottom of the end openings such as l2 to provide complete control of the direction of sound propagation from each end of the cabinet.
Between the vanes or guides there kare acoustic sound paths which tare designated by the arrowed directional lines D1-D6 extending from the plane Y to the center line 55, along which line are designated the distances D0 indicating the incremental sound path lengths between the dilferent acoustical guide channels. Along the line 55, the iinal distance to any chosen listening point il is indicated by the distance DX. The distance DX may be any distance frontally of the listenin-g point from the sound output area of the wave guide. The following relation exists with regard to the acoustical path lengths:
Thus i-t will be seen that the acoustic path lengths from the plane Y through the wave guide to the point it are all the same. If the phase of the sound waves over the plane Y is the same, the sound waves from the six paths will arrive at the point il in the salme phase. rlille sound output of .the loudspeaker `8, and likewise of the loudspeaker "i, is therefore directed forwardly, as is desir-able for better sound coverage of the listening tarea frontally of the cabinet.
Assuming lthat an upper frequency of 5000 cycles may represent a reasonably good high-frequency response from the loudspeaker system for table-model radio receivers and the like, it Iwill be seen that the wave-length of the sound at this frequency will be two and six tenths inches, whereas the spacing between the wave-guide elements or vanes is slightly over 11/2 inches in the present example. Therefore the vanes or guide walls preferably are spaced apart less than one wave-length lat the assumed highest useful operating yfrequency for the loudspeaker system.
Also it may be observed that the end of the cabinet or the plane of the opening l2, the plane of the loudspeaker baille 37 or the plane Y, -and the planes of the waveguide elements 27-33, are all generally vertical when the cabinet setting is in the normal manner, such as setting on a horizontal surface like a table or shelf. Thus the sound from the speaker units is changed in direction by the Wave guides and directed outwardly and forwardly from the ends of the cabinet and in horizontal direction. The planes of the wave-guide vanes extend between and intersect -the planes of the openings in the cabinet ends and the planes of the loudspeaker diaphragms. The planes of the loudspeaker diaphragrns, that is the plane Y in FIG- URE 4, are likewise at angles of 60 to the ends of the cabinet as is each of the guide vanes. The area within the wave-guide, in the present example, is therefore an equilateral triangle, the sides of which substantially equal the diameter of the loudspeaker unit with Iwhich it is associated.
While the wave-guide vanes are shown as being straight, it should be understood that they may be curved or variously shaped and positioned at different angles with respect to the cabinet Iand end openings whereby they provide similar geometrical relations for the path lengths of the sound waves as provided in the example shown. In any case the wave-guide vanes are at such angles to the plane Y that the sound waves from the loudspeakers are deflected :and directed through the ducts or channels of the wave guides in a frontal and right and left direction as the case may be, lat each end of the cabinet to provide stereophonic sound distribution in depth frontall-y and with good yauditory perspective.
The directivity patterns of typical 8 inch direct-radiator loudspeaker mechanisms and acoustic Wave guide systems as above described, are shown in FIGURE 1. Only three of the uniform sound amplitude curves are directly shown. These are indicated for 500 cycles at 40, for 2000i cycles at 41, and for 4000 cycles at 42. In some cases the sound output will vary from the patterns shown and in many cases may extend slightly to the right and left, at different angles forwardly toward the front, depending upon the location and background for the cabinet 5. The performance from the standpoint of directivity is superior to that of a system in which the speaker units face fully into the ends of the cabinet, and is substantially the same as that for front-mounted mechanisms as referred to in the description in FIGURE l. The increase in the effective distance between the virtual sound sources represented by the distance D52 and the points 22 and 23; with respect to the distance DS1, is substantially twice the diameter of the direct-radiator loudspeaker mechanisms in the cabinet shown. This is a significant increase in separation which is of particular importance in small unitary sterophonic loudspeakers.
The loudspeaker system may be operated from any suitable source of stereophonic sound signals indicated by the block unit 45 in FIGURE l. This may be considered to be a stereophonic FM or AM radio receiver mounted in the cabinet in .the center section 6 and having two stereophonic output circuits 46 and 47 for the left and right channels respectively, which are connected through suitable audio-frequency amplifiers 48` and 49, respectively, with the input circuit leads 18 of the loudspeaker units. Like the receiver elements, the ampliliers 4S and 49 may be mounted in the center section of the cabinet. The application of stereophonic -sound signals to the speaker system from the signal source 45 provides stereophonic forwardly projected sound patterns in front of the cabinet and in auditory perspective, from the two virtual sound sources 22 and 23. With a relatively small cabinet structure such as that indicated in the present example, the spacing of the virtual -sound sources is increased from approximately 20 inches to nearly twice that distance by the loudspeaker system as shown and described.
The unitary loudspeaker system of the present invention is thus embodied in a single cabinet that may be of table- -model dimensions. The direct-radiator loudspeaker mechanisms are mounted in the ends of the cabinet, which increases the effective distance between the virtual sound sources of the loudspeakers by a distance of twice the diameter of the loudspeaker, as compared to mounting the loudspeakers in the front of the cabinet. The acous- Vmounted loudspeakers in a forward direction.
Having described the invention, what is claimed is:
1. A stereophonic loudspeaker system for .the reproduction of sound in auditory perspective, comprising, in cornbination, a cabinet having two end walls with a soundconducting opening in each of said walls yand a front wall, a pair ofrloudspeaker units mounted in said cabinet each angularly-facing frontally and outwardly toward an adjacent end wall opening, means providing a sound-directing wave guide acoustically connecting each of said loudspeaker units with the adjacent end wall opening for conducting sound waves therefrom in auditory perspective outwardly and forwardly with respect to the ends of said cabinet, said sound directing wave guide comprising la plurality of vanes of differing lengths extending from a loudspeaker unit `to its adjacent end wall, said vanes increasing in length from the back towards the front of said cabinet and means providing a stereophonic,sound-signal source connected with said loudspeaker units in twochannel stereophonic operating relation.
2. A stereophonic loudspeaker system for the reproduction of sound in auditory perspective, comprising, in combination, a table-model radio cabinet having two end walls with a relatively large sound-conducting opening in each of said walls, a pair `of direct-radiator type loudspeaker unitsmounted one in each end of said cabinet and angularly facing frontally and outwardly toward the adjacent end-wall l'opening in said cabinet, aV pair of sound-directing waveguides mounted in the end portions of the cabinet and acoustically connecting the face of each of said loudspeaker units with the adjacent end-wall opening of the cabinet, said-wave guides each comprising a plurality of V substantially vertical vanes in spaced relation to each other, said vanesrbeing spaced less-than one wave-length lapart at the highest operating frequency -for said system and providing sound-conducting paths of substantially equal length from they face of each loudspeaker unit forwardly of the cabinet fromeach end thereof and a virtual sound source established by the associated loudspeaker unit, and radioV receiver means providing a lstereophonic sound-signal source connected with said loudspeaker units in two-channel stereophonic operating relation.
3. A stereophonic table-model radio loudspeaker systern, comprising, in combination, a radio cabinet of tablemodel dimensions and rectangular configuration having two substantially parallel and spaced end walls withV a sound conducting-opening in each ofv said walls and occupying a major portionthereof, a direct-radiator conicaldiaphragm type loudspeaker unit mounted one in each end of said cabinet and angularly facing frontally and outwardly toward the adjacent end wall opening, said units thereby operating to establish virtual sound sources outside the end walls of the cabinet and an effectively widerspread between the actual speaker unit sound sources, sound directing wave-guide means in each end of said cabinet acoustically connecting the face of each of said loudspeaker units with the adjacent end wall opening, said wave-guide means comprising a baille element havingan opening for the face of the loudspeaker unit and a plurality of substantially verticaltvanes in parallel spaced relation -to each other, said vanes being of different lengths spaced less than one wavelength apart at the highest useful frequency of operation of the system and extending between ,the baffle element opening and the adjacentend-wall opening to provide sound paths of substantially equall length from each loudspeaker diaphragm forwardly of said virtual sound sources to a predetermined listening point, 4and radio receiver means providing a stereophonic sound signalsource connecting said loudspeaker units in stereophonicbperating relation.
4.-A stereophonic loudspeaker system for the reproduction of sound in auditory perspective Ifor-small radio receivers and the like, comprising in combination, a table-model radio cabinetV structure having two end sections providing laterally spaced end walls Yeach with a sound conducting opening therein, a'direct-radiator conical-diaphragm loudspeaker unit mounted in each of saidv end sections and facing the adjacent end wall opening in substantially a vertical plane and at an acute angle thereto, acoustic wave guideaneans comprising a plu-` rality of substantially parallel vanes extending'between the face of each of said loudspeaker units and the adjacent opening in the end wall of the cabinet structure Y for directing sound therefrom in a forward direction in auditory perspective, and said acoustic wave guide pro- Viding a plurality of sound paths of substantially equal length from the face of each speaker unit in said forward direction at each endlof the cabinet structure, thereby t to provide inphase sound signals forwardly of said cabinet structure and in said auditory perspective in response to applied inphase stereophonically related sound signals at said loudspeaker units.
5. A loudspeaker system comprising in combination, a substantially rectangular cabinet structure having end walls with relatively wide lateral spacing and sound con-Y ducting end openings therein, a pair of loudspeaker units mounted in said cabinet structure and positioned for establishing virtual sound sources externally of the cabinet and beyond the ends thereof in laterally spaced relation, whereby an effectively wider spread is provided between said virtual sound sources than between said loudspeaker units, and acoustic wave guide means .withiny the ends of the cabinet structure and between the end openings and each of the speaker units for directing sound waves eectively from said sound sources in a forward'direction and along sound paths of substantially uniform length in response to signals applied to said loudspeaker units, each of said wave guide means including a series of sound-wave-directing Vertical vanes in spaced relation and at an angle to the adjacent end of the cabinet structure, said wave guide means acoustically connecting the face of each of said loudspeaker units with the adjacent sound conducting opening in the end wall of the cabinet Astructure in a series of relatively thin ducts the thickness of each of which is less `than one wave-length of the highest useful operating frequency of the loudspeaker system.
6. A unitized table-model stereophonic loudspeaker system for small stereophonic radio signal receivers having two-channel output circuits for -two stereophonicallyrelated sound signals, comprising in combination, a single table-model radio cabinet structure having two spaced side walls each -with an end opening therein lfor con- "veying sound waves externally of the cabinet structure,
' means providing a central interior section for housing stereophonic radio apparatus, a pair of channel loudspeaker units of the direct-radiator type mounted-in the endsV of the cabinet adjacent to said openings and angularly spaced with respect thereto in substantially vertical planes having acute angles with respect to the planes of the end openings, circuit means connecting said channel loudspeaker units with the respective channel output circuits for applying said signals to said units,V a pair of acoustic wave guides each comprising a plurality of substantially vertical vanes spaced less than one wavelength apart at the highest useful operating frequency of'said system and extending different distances kbetween and in registration with the diaphragm face of each speaker unit and the corresponding opening in the end wall of the cabinet structure, the angle of the vanes being Ysuch that sound waves `from the speaker units are changed in direction thereby and directed in sound paths of substantially equal lengths outwardly and forwardly from the ends of the cabinet structure and in a horizontal direction, the sound waves from the loudspeaker -unitsV being thereby deflected and directed through the ducts or channels of the wave guides ina frontal and right and leftV direction at Ythe respective ends ofthe cabinet structure to provide stereophonic sound distribution in depth frontally and with auditory perspective in response to signals applied to said loudspeaker units from said output cincuits.
References Cited in the le of this patent UNITED STATES PATENTS 21,002,390* Crosley et al. May 21, 1935 21,102,212 Olson Dec. 14, 1937 2,145,318 Schnek Ian. 31, 1939 2,210,477 Beneoke Aug. 6, 1940 1@ 2,272,937 Emde Feb. 10*I 1942 2,463,762 Giannini Mar. 8, 1949 2,580,439 Kock Jan. 1, 1952 2,750,245 Maclntyre June 12, 1956 FOREIGN PATENTS 943,598 Germany May 24, 1956 OTHER REFERENCES tem, by George L. Auspurger, pp. 64-67.
Citations de brevets