US3378100A

US3378100A - Sound attenuator

Info

Publication number: US3378100A
Application number: US497061A
Authority: US
Inventors: William H Welty
Original assignee: Air Filter Corp
Current assignee: Air Filter Corp
Priority date: 1965-10-18
Filing date: 1965-10-18
Publication date: 1968-04-16
Anticipated expiration: 1985-04-16

Description

INVENTOR ATTORNEYS April 16, 1968 w. H. WELTY SOUND ATTENUATOR 5 SheedS-Sheet 1 Filed Oct. 18, 1965 W/LL/AM H. WELT) BY MWMWM 77w Mfmwi April 16, 1968 w. H. WELTY SOUND ATTENUATOR 5 Sheets-Sheet 2 Filed Oct. 18, 1965 INVENTOR WILLIAM H. WELT) WW ATTOgNEYS April 16, 1968 w. H. WELTY 3,378,100

SOUND ATTENUATOR Filed Oct. 18, 1965 3 Sheets-Sheet l5 5 INVENTOR WILL/AM H. WELT) BY W%m7/M,

ATTORNEY5 United States Patent 3,378,100 SOUND ATTENUATOR William H. Welty, Brookfield, Wis., assignor to Air Filter Corporation, Milwaukee, Wis., 'a corporation of Wisconsin Filed Oct. 18, 1965, Ser. No. 497,061 15 Claims. (Cl. 181--56) ABSTRACT OF THE DISCLOSURE A sound attenuating device having a plurality of sound absorbing bafiies arranged in staggered, spaced apart relation in a plurality of rows extending transversely of air flowing through the device. Each baffle is formed by a metal frame having a relatively rigid panel sandwiched between two rectangularly shaped panels of acoustically absorbent material. The acoustically absorbent panels are mounted in back-to-back relation on the frame and are each cambered away from the frame panel to define a multiplicity of smoothly curved air flow orifices.

This invention relates to sound attenuators and is particularly concerned with silencing structures for absorbing sound accompanying the flow of a fluid medium such as air in a confined space.

The movement of air through confining ducts, conduits, grilles, or other structures, as is required, for instance, in air conditioning, ventilating, and heating systems, is frequently accompanied by objectionable noises which may initiate from the operation of equipment or from other sources. Prior to this invention, it has been the practice to isolate a room or other space from these noises by providing acoustical silencers or so-called sound traps of varying constructions for absorbing the sounds being transmitted with the air stream. Conventional silencers employed for this purpose generally are ineffective in low frequencies to reduce objectionable noise to an acceptable level and/or offer undesirably high resistance to air flow. Increasing the noise suppression efficiency of conventional sound absorbing devices usually is accompanied by a corresponding objectionable increase in resistance to air flow to cause an excessive pressure drop, for example, in an air duct or conduit system.

It is a primary object of this invention to overcome thef oregoing shortcomings of conventional sound absorbing structures by providing for a novel sound attenuating device which obtains maximum attenuation with minimum resistance to fluid flow.

In fulfilling the foregoing object, the sound attenuating device of this invention is provided with a plurality of specially formed sound absorbing baffles arranged in staggered spaced apart relation in a plurality of rows extending transversely of the air stream and having sound opaque surfaces gradually cambered or lofted in a direction facing air flow to define a multiplicity of smoothly curved air flow orifices which are shaped in the form of bellmouths. This construction, in addition to affording an exceptionally high reduction in the noise level, minimizes friction and turbulence resisting air flow to correspondingly minimize the fluid pressure drop across the sound attenuating structure.

Another important object of this invention is to provide a novel sound attenuating device which is compact in construction and which is non-directional to both sound and air flow, providing the same highly etlicient noise reduction performance and the same low resistance regardless of the direction of fluid flow.

As a result, the sound attenuating device of this invention may readily be located between transfer grilles providing fluid communication, for example, between a room and a corridor or another room to cut down noise transfer and cross-talk. By constructing the sound attenuator of this invention as a compact unit, it may easily be located in ducts behind supply or return grilles or in plenum chambers to minimize the noise entering an airconditioned, heated, or ventilated space.

Still another object of this invention is to provide a novel, compact sound attenuating unit which is simple and inexpensive to manufacture and which is easy to install.

Further objects of this invention will appear as the description proceeds in connection with the appended claims and the annexed drawings wherein:

FIGURE 1 is a sectional View illustrating the sound attenuating device of this invention mounted between opposed air transfer grilles;

FIGURE 2 is a front elevation of the sound attenuating device shown in FIGURE 1;

FIGURE 3 is a left hand end elevation of the sound attenuating device shown in FIGURE 2;

FIGURE 4 is a section taken substantially along lines 4-4 of FIGURE 2;

FIGURE 5 is an end elevation of one of the main sound absorbing, baffling modules shown in FIGURES 2 and 4;

FIGURE 6 is an end elevation of another of the sound absorbing, battling modules illustrated in FIGURES 2 and 4;

FIGURE 7 is a sectional view illustrating the sound attenuating device of FIGURE 2 mounted behind a return air grille in a ventilating, heating, or air conditioning system; and

FIGURE 8 is a sectional view illustrating the sound attenuating device of FIGURE 2 mounted behind a return ceiling grille in an air conditioning, heating, or ven tilating system.

Referring now to the drawings and more particularly to FIGURE 1, the reference numeral 10 generally designates a sound attenuator constructed according to a preferred embodiment of this invention and shown to be mounted in a straight air flow passage 12 between tWo opposed air transfer grilles 14 and 16 of conventional form. Passage 12 may be formed in a wall 18 or other partition separating, for example, two rooms or a room and corridor to provide for the bi-directional transfer of air between the spaces separated by wall 18. Passage 12 may be straight and rectangular in cross-section to accommodate grilles 14 and 16.

As best shown in FIGURES 2 and 3, attenuator 10 comprises a rigid, metallic support frame 20 having flatsided parallel

transverse plates

22 and 24 rigidly joined together by parallel, fiat-

sided side plates

26 and 28 to form a rectangular frame construction. Frame 20 may be fixed in arfy suitable manner in passage 12 with

plates

22, 24, 26, and 28 butting against related wall surfaces of passage 12 so that all of the air flowing in either direction through passage 12 passes through frame 20.

Referring to FIGURES 2 and 4, attenuator 10 is provided with a plurality of sound absorbing baflling units or modules 30 which extend in parallel relation across the entire width of frame 20 between

side plates

26 and 28 and which are fixed to frame 20 in a manner to be described later on. Modules 30 are symmetrical about normally intersecting longitudinal and transverse planes and are arranged in two parallel, spaced apart,

straight rows

32 and 34 extending essentially at right angles to the movement of air flow through passage 12. Row 32 is formed entirely of modules 30 which are uniformly spaced apart and extend transversely of the movement of air in either direction through passage 12. Row 34 contains a plurality of modules 30 and a further pair of sound absorbing, bafiiing units or modules 36. Modules 36 essentially are transverse half sections of modules 30 and are disposed at opposite ends of row 34. As shown, modules 36 extend the entire Width of frame in parallel uniformly spaced apart relation to the adjacently disposed modules and are fixed to frame 20 by means to be described shortly.

The modules in row 34 are uniformly staggered with respect to the modules in row 32. This staggered spacing of

modules

30 and 32 in their respective rows is such that a module 30 in one row is in overlapping relation with adjacently disposed modules in the other row to define a solid rectangular figure. With this arrangement, air flowing along a straight line in either direction will impinge against at least one of the

modules

30, 36 in either row 32 or row 34.

With continued reference to FIGURE 4,

modules

30 and 36 each are formed with opposite facing, gradually and uniformly cambered acoustically

opaque surfaces

38 and 40. Surfaces 38 on the modules 30 in row 32 are lofted or combered toward air flow entering the right side of the attenuator, and surfaces 38 on the

modules

30 and 36 are lofted toward air flow entering the left side of attenuator 10. Surfaces 40 on the modules in

rows

32 and 34 respectively are in opposed relation as shown. Surfaces 38 on adjacently disposed modules in row 32 define a plurality of uniformly spaced apart, equally dimensioned, bellmouthed orifices 42. Similarly, surfaces 38 on the modules in row 34 define a plurality of uniformly spaced apart, bellmouthed orifices 43 having the same dimensions as orifices 42.

Orifices

42 and 43 respectively converge in opposite directions toward throat sections of reduced fluid flow area. Surfaces 40 on adjacently disposed modules in each row define opposed

bellmouthed orifices

42a and 43a which flare outwardly from the throat sections of

orifices

42 and 43 toward the space between

rows

32 and 34. Each aligned pair of

orifices

43 and 43a in row 34 and each aligned pair of

orifices

42 and 42a in row 32 thus forms a venturi passage through which air in passage 12 is adapted to flow.

Each of the modules 30, as best shown in FIGURE 5, comprises a mounting frame 46 and a pair of relatively thick, solid, rectangular, flat-

sided panels

48 and 50 made of any suitable acoustically absorbent material such as, for example, long strand fibre glass preferably having a density of about four pounds per cubic foot. Preferably,

panels

48 and 50 are of uniform thickness and are flexible and deformable.

Panels

48 and 50 are faced with a covering indicated at 51 to define

surfaces

38 and 40. Covering 51 may be made of any suitable material.

Frame 46 comprises a pair of separately formed, fiat

sided sections

52 and 54 respectively mounting

panels

48 and 50 and made of sheet metal or other suitable material. The upper and lower marginal edges of section 52 are sharply bent laterally outwardly at right angles from an intermediate, substantially straight body portion 56 to define a pair of

lips

58 and 60.

Lips

58 and 60 are smoothly curved backwardly toward each other to form oppositely facing rounded corners at opposite ends of body portion 56. As shown,

lips

58 and 60 respectively form with body portion 56 opposed, open ended,

parallel grooves

62 and 64 each having a curved bottom wall surface normally intersecting with a straight side wall surface defined by body portion 56 and smoothly merging with an outwardly curved side wall surface defined by the lip end.

Section 54 is of the same construction as section 52. Accordingly like reference numerals have been used to identify like elements.

Still referring to FIGURE 5,

sections

52 and 54 are rigidly joined together by any suitable means in back-toback relationship with the body portions 56 of

sections

52 and 54 butted along an interface containing in a longitudinal plane which medially intersects module 38.

With reference to FIGURES 3 and 5, body portions 56 of

sections

52 and 54 are wedged apart to define tapered holes 66 at opposite ends of frame 46. Suitable sheet metal or like screws 68 extending through side walls 4 26 and 28 are threaded into holes 66 to removably, rigidly mount frame 46 on frame 20.

As best shown in FIGURE 5, the upper and lower marginal edges of each of the

panels

48 and 50 is slidably received in

grooves

62 and 64.

Lips

58 and 60 snugly clamp and compress the marginal edges of each panel, thus causing each panel to bow uniformly outwardly so that the central body portion of each panel extending between

lips

58 and 60 is spaced forwardly from the body portions 56 of mounting frame 46. By this simplified

clamping arrangement panels

48 and 50 are supported in back-to-back relation on mounting frame 46 and are provided with uniformly lofted or air-foil cross sections which camber in opposite directions.

In the assembly of attenuator 10, modules 30 are first assembled by sliding

panels

48 and 50 endwise into their associated pair of

grooves

62 and 64 on

frame sections

52 and 54. Modules 30 thus assembled are mounted in frame 20 in the manner already described. If it is desired to replace

panels

48 and 50, modules 30 are removed from frame 20, the old panels slid out of

grooves

62 and 64, and new panels inserted in their place.

Referring now to FIGURE 6, the module 36 at each end of row 34 comprises a mounting frame 70 for supporting a pair of flat-sided, solid, rectangular panels 72 and 74 each having uniform thickness and preferably made of the same material as

panels

48 and 50. Frame 70 comprises a pair of separately formed, flat-

sided sections

76 and 78 respectively mounting panels 72 and 74 and made of sheet metal or other suitable material.

With continued reference to FIGURE 6, section 76 is bent at its upper end to provide an outwardly extending portion 80 normally intersecting an upstanding body portion 82. The outer marginal end of portion 80 is bent downwardly to form a flange portion 84 which is in parallel relation to body portion 82. The lower end of body portion 82 is sharply bent outwardly to form a lip 86 which is smoothly curved forwardly and upwardly to provide a rounded corner. Lip 86 and body portion 82 define an open-ended groove 88 similar to

grooves

62 and 64 in modules 30.

The lower end of panel 72 is slidably received in groove 88 and is clamped and compressed between opposed surfaces on lip 86 and body portion 82. The upper end of panel 72 is slidably received in an open-ended groove defined by flange portion 84 and body portion 82. As shown, flange portion 84 is spaced laterally outwardly from lip 86, and the lateral dimension between flange portion 84 and body portion 82 is appreciably greater than the thickness of panel 72. Thus, by clampingly engaging and compressing the lower end of panel 72 between lip 86 and body portion 82, the upper end of panel 72 being resilient and flexible is sprung or bowed outwardly to engage flange portion 84 so that the contour of its outer surface is lofted in the form of a half camber having a crest disposed adjacent flange 84 and being smoothly bowed back to the clamped end at lip 86.

Frame section 78 is of the same construction as section 76. Accordingly, like reference numerals have been used to identify like portions. Panel 74 is mounted on frame section 78 in the same manner as panel 72 is mounted on section 76.

The body portions 84 of

frame sections

76 and 78 are rigidly fixed together in back-to-back relation by spot welding or any other suitable means.

Sections

76 and 78 are thus .joined along aninterface which medially intersects module 36. Body portions 84 of sections 7 6 and 78 are wedged apart to define opposite opening, tapered holes 96. Suitable sheet metal screws 92 extending through

side walls

26 and 28 are threaded into holes 90 to removably, rigidly secure mounting frame 70 to frame 20. In the assembled position of parts, portions 80 of mounting frame 70 butt against frame 20.

From the foregoing description, it will be appreciated thatair flowing from right to left through passage 12 in FIGURE 1 passes serially through

orifices

42, 42a, 43a, and 43 and will impinge against at least one of the

modules

30, 36 in

rows

32 and 34. The sound energy accompanying air that impinges against modules 30 in row 32 will readily be absorbed by the cambered panels defining surfaces 38 which direct the impinging air to flow smoothly into orifices 42 without producing turbulence. The confined, dead air spaces between panel 48 and frame section 52 and between panel 50 and frame section 54 in each module serve to minimize the transmission of sound produced vibrations through the module itself. Similar dead air spaces are provided in modules 36.

The air channeled through orifices 42 pass smoothly through orifices 42a which allow the air stream to gradually diverge and impinge in a uniformly distributed pattern against surfaces 40 on the modules in row 32 to further reduce the noise level. Surfaces 40, being uniformly cambered, smoothly direct the air into orifices 42a which channels the air stream without turbulence into orifices 43. Orifices 43 gradually allow the air streams to-diverge again for continued flow through passage 12. Thus, the arrangement and construction of modules 30 and

36jin rows

32 and 34 and the related contours of

orifices

42, 42a, 43 and 43a provides for maximum attenuation ofsound with least resistance to air flow and with negligible air turbulence.

Since each of the

modules

30 and 36 is symmetrical and ,since the related dimensions of bellmouthed orifices and modules in

rows

32 and 34 are the same, it will be appreciated that attenuator provides for the same highly effective noise reduction and the same low resistance to air flowing in either direction through passage 12. Air moving from left to right in FIGURE 1 will serially pass through

orifices

43, 43a, 42a, and 42 in the same manner as just described for air flowing in the opposite direction. This aspect of the invention and the compact, slim construction of attenuator 10 thus enables attenuator 10 to easily be located in a straight passage between opposed air transfer grilles as shown in FIGURE 1. In addition to being simple and inexpensive to manufacture, the attenuator device of this invention is easy to install in any desired location, as shown, for example in FIGURES l, 7, and 8. Experiments have shown that the attenuator of this invention is instrumentally effective in all bands and cuts sound energy in half with every three decibel reduction.

As shown in FIGURE 7, attenuator 10 is readily and easily mounted behind air supply grille 100 in a rectangular passage 102 which is formed in a wall 104. A duct 106 is shown connected to passage 102 to convey air through grille 100.

' In FIGURE 8, attenuator 10 is shown to be mounted behind an air return grille 110 in a rectangular opening 112 which is formeciin a suspended ceiling 114. Air enters the space below ceiling 114 through a duct 116 and is drawn through grille 110 and then through attenuator 10 into the confined space above ceiling 114.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to he considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by Letters Patent is:

1. A sound attenuator for absorbing sound energy accompanying flow of gases and comprising a frame delimiting a passage through which said gases are adapted to fiow, a plurality of first baflling units mounted on said frame in uniformly spaced apart relation extending across said passage and arranged in a straight row extending transversely of gas flow through the space delimited by said frame, and a plurality of second bafiling units mounted on said frame and extending across said passage in parallel relation to said first baffling units, said second bafiling units being arranged in a second row extending rearwardly of and in parallel, spaced apart relation to said first row, said first and second baffling units having panels of sound absorbing material extending across said passage and being uniformly cambered in parallel directions facing flow of gas through said passage, the cambers on the panels of adjacently disposed baflling units in each row defining bellmout'hed orifices each converging in the direction of gas flow toward a throat section of reduced flow area, said second bafiling units being staggered relative to said first baflling units and each overlapping adjacently disposed baffling units in said first row to extend across the path of the gases flowing through the bellm-outhed orifices in said first row, each of said baffiing units comprising a panel support frame structure and each of said panels being formed of resilient, flexible material, said support frame structure being for-med with a pair of fl-ange sections rigidly joined to gether by an intermediate section and defining therewith opposed, spaced apart, open ended grooves slidably receiving opposite marginal edges of its associated panel, at least one of said flange sections clampingly engaging and compressing the panel against said intermediate section to outwardly bow the portion of the panel extending between said flange sections.

2. A sound attenuator for absorbing sound energy accompanying flow of gases and comprising a frame delimiting a passage through which said gases are adapted to flow, a plurality of first baffling units mounted on said frame in uniformly spaced apart relation extending across said passage and arranged in a straight row extending transversely of gas flow through the space delimited by said frame, and a plurality of second baffling units mounted on said frame and extending across said passage in parallel relation to said first baflling units, said second baffling units being arranged in a second row extending rearwardly of and in parallel, spaced apart relation to said first row, said first and second bafliing units having panels of sound absorbing material extending across said passage and being uniformly cambered in parallel directions facing flow of gas through said passage, the cambers on the panels of adjacently disposed bafiling units in each row defining bellmouthed orifices each converging in the direction of .gas flow toward a throat section of reduced flow area, said second baflling units being staggered relative to said first baffling units and each overlapping adjacently disposed baffling units in said first row to extend across the path of the gases flowing through the bellmouthed orifices in said first row, at least one of said baffling units at least in said second row comprising a panel support frame, and at least one of the panels of said one unit being made of resilient, flexible material, said support frame being formed with a body section having opposite marginal ends bent forwardly and toward each other to define opposed, spaced apart, open ended grooves sl-idably receiving opposite marginal edges of said one panel, each marginal edge of said one panel being compressed between opposed internal surfaces defining each groove to how the portion of the panel between sa-id grooves outwardly from said support frame.

3. A sound attenuator for absorbing sound energy accompanying flow of gases and comprising a frame delimiting a passage through which the gases are adapted to flow, a plurality of first bafliin-g unit's mounted on said frame in uniformly spaced apart relationship extending across said passage between opposed side portions of said frame, said first bafliing units being arranged in a straight row extending transversely of gas fiow through said passage, and a plurality of second bafliing units mounted on said frame and extending across said passage in parallel relation to said first baffling units, said second balfiing units being arranged in a straight row extending in parallel spaced apart, side-byaside relationship to the row containing said first bafiiing units, each of said first and second bafliing units being symmetrical about a plane extending parallel to said rows and having a pair of sound absorbing panels extending across said passage and being cambered in opposite directions respectively facing gas flow in opposite directions through said passage, the cambers of the panels on adjacently disposed baffling units in each row defining a venturi passage terminating at opposite ends in oppositely flared bellmouths, the baflling units in one row being staggered relative to the baffling units in the other of said rows and each overlapping adjacently disposed units in the other row in spaced relation to the venturi passages therein to thereby extend across the path of gases flowing through the venturi passages in the other row, at least one baflling unit in each row comprising a panel support frame structure extending across said passage, and said panels being formed from resilient flexible material, said support frame structure being formed by a pair of plates mounting respective ones of the two panels in the unit and being fixed together in back-to-back relation, each of said plates having opposite marginal end portions bent forwardly and toward each other to define opposed, spaced apart open ended grooves slidably, removably receiving opposite marginal edges of its associated panel, the width of each groove being so dimensioned as to clampingly compress the associated marginal edge of the panel between opposed groove surfaces to how the portion of the panel extending between the opposed pair of grooves outwardly from the portion of the plate extending between said bent ends.

4. A sound attenuator for absorbing sound energy accompanying flow of gases through a confined space and comprising a plurality of bafliing modules staggeringly arranged in a plurality of side-by-side spaced apart rows extending transversely of the direction of gas flow, each of said modules comprising a pair of panels formed from defiormable, acoustically absorbent material and with oppositely facing side surfaces, and a relatively rigid frame mounting said panels in back-to-back relation transversely of the gas fl-ow through said space and with the side sur faces of said panels which face away from each other respectively confronting and being impinged by gas flowing from opposite directions, said frame being formed with a panel-like section sandwiched between and separating said panels, said section extending transversely of gas flow in either direction through said space, and said frame being further formed with means extending from said section and overlapping the transversely extending marginal edges of each of the panel side surfaces confronting gas flow to engage and confine said panels against said section.

5. The sound attenuator define-d in claim 4 wherein said firame is formed by a pair of separately formed members having rectangular, fl-at sided portions fixed in abutting, back-to back relation to define said section, and wherein said means extending from said section comprises a pair of lips formed integral with each of said rectangular portions.

6. The sound attenuator defined in claim 4 wherein said means extending from said section comprises a pair of lips for each of said panels, each lip cooperating with said section to define a transverse groove receiving one of the marginal edges of an associated panel.

'7. The sound attenuator defined in claim 6 wherein each of said panels are slidably received in their associated grooves and wherein the grooves associated with each panel are open at least at corresponding ends to provide for the endwise removal of each panel from said frame.

8. The sound attenuator defined in claim 4 wherein adjacent ones of said modules in each row define a bellmouthed orifice converging in the direction of gas flow from a relatively wide, outwardly flared entrance opening toward a throat region of reduced flow area.

9. The sound attenuator defined in claim 4 wherein each of said panels is bowed away from said section to form between adjacent modules in each row a bellmouthed orifice converging in the direction of gas flow from arelatively wide, outwardly flared entrance opening toward a throat region of reduced flow area.

10. A sound attenuator for absorbing sound energy accompanying flow of gases through a confined space and comprising a plurality of bafliing modules staggeringly arranged in a plurality of side by-side spaced apart rows extending transversely of the direction of gas flow through said space, each of said modules comprising a pair of separately formed panels of acoustically absorbent material and a relatively rigid frame mounting said panels in back-to-back relation transversely of the gas flow through said space and having a panel-like section sandwiched between and separating said panels, said panels having side surfaces facing away from said section and impinged by gas flowing from opposite directions, said frame including means extending from said section and over lapping only peripheral portions of said side surfaces to retain said panels in said backt-o-back relation.

11. The sound attenuator defined in claim 10 wherein said panels are symmetrically disposed on opposite sides of a plane medially intersecting the associated baflling unit.

12. The sound attenuator defined in claim 10 wherein said means extending from said section comprises a pair of lips for each of said panels and extending over opposite edges of each panel, said lips cooperating with said sect-ion to define spaces receiving and confining said opposite edges,

13. The sound attenuator defined in claim 12 wherein said section is of substantially rectangular configuration and wherein said pair of lips .are bent toward each other and terminate in spaced apart relation, the panel associated with each pair of lip-s being continuous in the region between the ends of said lips.

14. The sound attenuator defined in claim 12 comprising a structure supporting said modules and having spaced apart portions, said modules extending between said portions of said structure, and said frame being fixed at opposite ends to said portions of said structure, said lips extending transversely between said port-ions of said structure and delimiting passages providing for the flow of the gas around said modules.-

15. The sound attenuator defined in claim 10 wherein said means extending from said section delimits passagew ays providing tor the passage of the gas around said modules.

References Cited UNITED STATES PATENTS 1,865,677 7/ 1932 Cheyney 181-60 2,656,004 10/1953 Olson 181-33 2,759,554 8/1956 Baruch 181-56 2,916,101 1 2/1959 Naman 18'1-46 XR 2,942,682 6/1960 Bergh et al. 181-56 2,959,243 11/1960 Smith 181-56 XR FOREIGN PATENTS 638,407 6/1950 Great Britain. 721,859 1/ 1955 Great Britain.

ROBERT S. WARD, 111., Primary Examiner.