Acoustic panel

Aug. 1, 1961

R. B. BOURNE ET AL

ACOUSTIC PANEL

2,994,401

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INVENTORS

ROLAND B. BOURNE JOHN P.TYSKEWICZ

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ATTORNEYS

Aug. 1, 1961

2,994,401

R. B. BOURNE ET AL

ACOUSTIC PANEL

Filed May 26, 1958

Sheets-Sheet 2

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ATTORNEYS

United States Patent Office

2,994,401

Patented Aug. 1, 1961

2,994,401 ACOUSTIC PANEL

Roland B. Bourne, West Hartford, and John Paul Tyskewicz, Hartford, Conn., assignors, by mesne assignments, to American Machine &amp; Foundry Company, Inc., New York, N.Y., a corporation of New Jersey

Filed May 26, 1958, Ser. No. 737,677

Claims. (CI. 181—42)

This invention relates to an improved acoustic panel of the type employed in the construction of acoustic conduits and engine test cells or the like.

It is the general object of the invention to provide an acoustic panel of the aforementioned type which is unusually strong and rigid but which can be fabricated easily and economically of lightweight materials and which can be employed to advantage in many different ways in the construction of various acoustic conduits and cells.

The drawings show preferred embodiments of the invention and such embodiments will be described, but it will be understood that various changes may be made from the construction disclosed, and the drawings and description are not to be construed as defining or limiting the scope of the invention, the claims forming a part of this specification being relied upon for that purpose.

Of the drawings:

FIG. 1 is a perspective view of one form of acoustic panel constructed in accordance with the present invention;

FIG. 2 is a transverse, cross-sectional view of an acoustic conduit wherein a plurality of panels of the type shown in FIG. 1 are employed;

FIG. 3 is a perspective view of another form of acoustic panel provided in accordance with the present invention;

FIG. 4 is a transverse sectional view of an acoustic conduit employing the panel shown in FIG. 3;

FIG. 5 is another transverse sectional view of an acoustic conduit employing a panel of the general type shown in FIG. 3 in a manner different from that shown in FIG. 4; and

FIG. 6 is an additional transverse sectional view of an acoustic conduit wherein panels of the general type shown in FIG. 3 are employed in still a different manner

A common form of acoustic panel is one of generally rectangular shape formed of six sheet metal sides and having two side walls of major area. In this common form panel, all of the sides or walls are flat and generally at least one of the major area walls is perforated to pass sound energy to the interior of the panel which may be filled with sound absorbing material. A fiber glass pack or body is a preferred sound absorbing material for use in panels of the type mentioned, but there are many other fibrous materials used, such as metallic wool, and there are particulate sound absorbing materials which are sometimes but less frequently used.

When the common form acoustic panel is used in the construction of acoustic conduits and test cells for jet engines or the like or in other installations wherein the panels are subjected to severe buffeting and vibration, the common form of fibrous sound absorbing material may be worked out of the panels through the perforate walls thereof. Moreover, the vibration and buffeting may well cause fatigue failure of the metallic wall structure of the panel, and the vibration causes the sound absorbing pack in the panel to be redistributed so that the attenuation characteristics of the panel changes.

To overcome some of these disadvantages, the common flat panels have been made more rigid by utilizing internally disposed braces or the like to· secure the major area walls against vibration, and relatively heavy and inherently stiff sheet metal has been used in the fabrication of the major area walls. However, these steps have failed to completely overcome the vibration and, of course, the modified panels are more expensive to manufacture.

In addition, materials have been used to retain the sound absorbing material within the panel during vibration thereof. That is, fine mesh screening has been employed in sheets laid adjacent the major area perforated wall or walls on the inner side thereof and additional fibrous sheet material, such as fiber glass cloth, has been disposed between the screening and the sound absorbing material to retain the sound absorbing material within the panel while still permitting sound waves to enter the pack through the perforations. But difficulty has been encountered in disposing the screening and cloth material in the flat side acoustic panels and in retaining the screening and cloth material in desired position after construction of the panels and after use in which they have been subjected to buffeting and vibration. That is, it is difficult to make the glass cloth and screening lie in close contact with each other and with the perforate sides of the panel, and the cloth and screening tend to separate as a result of vibration of the perforate sheet metal sides. Recourse has been made to the use of cloth bags of sound absorbing material as a solution, but when the filled bags are used in the panels, the sound absorbing material is easily displaced and frequently there are open spaces within the panel wherein there is no sound absorbing material disposed.

In accordance with the present invention, there is provided a generally rectangular acoustic panel having at least six sides and wherein the walls of major area are perforated for the passage of sound wave energy but wherein the said major area perforate walls are made in arcuate form to· resist vibration. In the panel construction provided here, the concave surfaces of the major area walls face outwardly and the convex surfaces thereof face toward each other which provides and advantage in the construction of the panel and an additional advantage in maintaining the structural integrity of the finished panel.

In FIG. 1 there is shown a first embodiment of a generally rectangular acoustic panel provided in accordance with the present invention. The panel is provided with six walls, two of which comprise the major area perforate sheet metal walls 10, 10. As shown in the drawings, the said perforate major area walls of the panel are arcuate and are formed as identical sections or arcuate segments of equal cylinders so that in transverse section, each perforate major area wall 10 constitutes a segment of a large diameter circle, the arch of the wall being relatively shallow. The longitudinal dimension of each wall 1® corresponds to the longitudinal dimension of the completed panel. In the panel construction, the arcuate walls 10, 10 are rigidly supported in spaced side-by-side relationship with their convex surfaces facing toward each other. The means supporting the arcuate major area perforate walls m the aforesaid relationship comprise the other four walls of the panel, these including a pair of opposed rectangular walls 12, 12 which connect the associated longitudinally extending straight edges of the arcuate walls 10, 10 and a pair of opposed walls 14, 14 which connect the associated curved edges of the arcuate walls 10, io at the ends of the panel. All of the aforedescribed walls of the panel are preferably formed of sheet metal and may be connected together in any suitable way as by welding or the like.

As shown by the cut-away of FIG. 1, the interior of the panel defined by the aforesaid walls is filled with a suitable sound absorbing material 16 such as fiber

3,994,401 glass. Preferably, a layer of fiber glass cloth 18 and a layer of fine mesh screening are disposed between the sound absorbing material and the major area perforate walls 1®, 1®, the screening 2© being disposed adjacent the said walls. As previously mentioned, the glass cloth and screening materials are provided to prevent loss of the sound absorbing material through the perforations of the major area walls due to any movement thereof and due to the passage of gas and sound along and through the panel. While the cloth and screening materials may not be essential in the construction of a panel in accordance with the present invention wherein the arcuate wall configuration prevents vibration, if such materials are used, they need only be used adjacent the perforate walls.

The arcuate form of the major area walls of the panel facilitate construction thereof. That is, when the panel is partially constructed by the connection of one perforate wall 10 with the two straight edge walls 12, 12 and the curved edge end walls 14, 14, the panel can be rested upon the arcuate wall 10 to receive the filler of sound absorbing material and the protecting cloth and screening material. The screening material 20 can easily be placed in position against the convex surface of the connected arcuate wall 10 and the cloth material 18 can as easily be laid thereover. Then, a uniform layer of the sound absorbing material 16 is placed within the open panel and the detached perforate wall 10 with its layer of screening 20 and cloth 18 is placed in position thereover. Then, by pressing the last mentioned perforate wall against the sound absorbing material and into position for welding to the walls 12,12 and 14, 14, the sound absorbing material is properly distributed within the panel. It will be apparent that by compressing the sound absorbing material in the panel, the sound absorbing pack is more dense at the central portion of the panel wherein the space between the convex surfaces of the walls 10, 10 is the least and the pack is less dense adjacent the straight edges of the arcuate walls 10, 10. The panel thus formed with variable density pack provides for attenuation over a greater frequency range than is the case with the common form flat side panel mentioned above. Preferably, a panel formed in accordance with the present invention should be at least twice as thick along the straight edges of the arcuate side walls as it is along the center line of said arcuate side walls.

Obviously, a panel of the form shown in FIG. 1 can be provided in any size desired. A typical panel may be eight feet long, three feet wide, sixteen inches at its thickest portions and four inches thick along the longitudinal center line. The relatively shallow arch provided in the arcuate side walls 1®, 10 is sufficient to prevent vibration of even very thin sheet metal when the panel is used in installations where experience has shown the most severe conditions to exist. In constructing relatively large panels, it may be desirable to employ tie rods to assist in pulling the arcuate major area walls toward each other in compressing the pack. There may be a plurality of such tie rods spaced longitudinally along the panel and extending transversely thereof at the thinnest portion of the panel. Such a tie rod is shown in FIG. 2 and may comprise an elongated bolt 22 extending from one arcuate wall 10 through the panel and the opposed arcuate wall 10 wherein the threaded end of the bolt 22 is engaged by a nut 24. Preferably, a spacer sleeve. 26 surrounds the bolt within the panel to prevent drawing the arcuate side walls 1®, 10 too closely together. That is, the length of the spacer sleeve determines the distance separating the arcuate walls at their closest point. When such tie rods are used, they further increase the rigidity and structural integrity of the finished panel.

The sound absorbing panel shown in FIG. 1 can be used to advantage in the construction of a variety of sound absorbing conduits and the like. For example, in FIG. 2 there is shown a conduit 28 of rectangular cross section, the walls of which may be made of concrete. In this conduit, the sound-carrying gas stream 5 flows normal to the plane of the drawing and the panels are arranged in longitudinally extending partitions within the conduit, each partition comprising two vertical courses of panels and there being five partitions shown. The panels may be connected together and to the top and 10 bottom walls of the conduit 28 by any suitable means (not shown) which form no part of the present invention. It should be observed that a panel in one course is connected to the panel above or below it by engagement along their associated straight edge walls 12, 12. 15 It should also be noted that the major area walls

I®, 10 of the panels reside in vertical planes parallel to the gas and sound path.

When the panels are employed in a conduit such as the conduit 28 which is not lined with sound absorbing ma20 terial, one set of panels is disposed closely adjacent each side wall of the conduit so as to prevent short circuiting of the sound through the panel assembly. It will be observed that the arrangement of the panels is such that there is a substantial variation in the width separating ad25 jacent sound absorbing walls comprising the panel courses.

In FIG. 3 there is shown another form of panel provided in accordance with the present invention. In the form of FIG. 3, the panel is still generally rectangular 30 and comprises six sheet metal walls enclosing a sound absorbing pack. As in the first described embodiment, the major area walls 30, 30 are perforate so as to pass sound to the interior of the panel. Also, as in the first described embodiment, the major area walls are arcuate 35 sections of cylinders which are disposed with their convex surfaces facing toward each other and rigidly held in spaced relationship by the other walls of the panel. The said other walls of the panel include a wall 32 and a wall 34 which join or connect associated straight longitudinal 40 edges of the arcuate walls 30, 30. However, the wall 32 is substantially wider than the wall 34 whereby the panel in addition to being generally rectangular may be said to be somewhat wedge-shaped. The widest straight edge wall 32 is preferably arcuate and perforate with its con45 vex surface facing inwardly or outwardly of the panel.

The walls 36, 36 closing the ends of the panel by connection to the curved edges of the major area walls 30, 30 are preferably imperforate. As in the first described embodiment, the panel of FIG. 3 is filled with a sound ab50 sorbing material such as fiber glass and a layer of fine mesh screening is disposed adjacent the arcuate perforate walls and a layer of cloth, such as glass cloth, is disposed between the pack and the screening. In this embodiment, the screening and cloth materials can also be em55 ployed between the pack and the widest straight edge wall 32 when such wall is perforate, but is most anticipated uses this is unnecessary.

The generally rectangular wedge-shaped panel of FIG. 3 lends itself to use in many different ways in the con60 struction of sound absorbing conduits and the like. For example in FIG. 4, there is shown an acoustic conduit 38 of circular cross section having a sound absorbing wall comprising a sound absorbing filler material 40 disposed between an imperforate outer shell and a perforate cy65 lindrical inner shell 42. The panels of FIG. 3 can be secured together as shown in FIG. 4 within the conduit 38 by welding or other securing means may be employed forming no part of the present invention. It will be observed that the panels extend in radial planes in the con70 duit 38 with their widest straight edge walls 32, 32 engaging the inner wall 42 of the conduit. The radially inner walls 34, 34 of the panels are connected together and a sound absorbing material is preferably disposed in the space 44 defined by the interconnected walls 34, 34 75 of the several panels. When the panel of FIG. 3 is to

2,994,401 be used as shown in FIG. 4, the widest straight edge wall 32 is preferably perforated and formed arcuately with its convex surface facing outwardly in engagement with the inner perforate wall of the conduit. The arcuate shape provides for better connection between the conduit wall and the panel and the provision of the perforations in the wall 32 of each panel provides a sound path through the panel and into the walls of the conduit.

The panel of FIG. 3 can also be employed in the conduit 38 as shown in FIG. 5 with the widest straight edge wall 32 of each panel disposed radially inwardly and with the narrowest straight edge wall 34 disposed outwardly. When the panels are employed as shown in FIG. 5, the wide straight edge wall 32 is again preferably perforated but it is arched so that the concave surface faces radially inwardly of the conduit. The space 46 defined between the connected walls 32, 32 is preferably filled with a sound absorbing material. The narrow straight edge wall 34 of each panel when employed as shown in FIG. 5 is preferably perforated to provide a sound path through the panel into the sound absorbing material of the conduit.

The generally rectangular wedge-shaped panel of FIG. 3 can also be used in a rectangular conduit as shown in FIG. 6 like the conduit 28 of FIG. 2. In the construction illustrated, there are six vertical panel partitions provided within the rectangular conduit 28, each partition comprising three courses of the generally rectangular wedge-shaped panels. It will be observed that the straight edge walls in each panel are flat and preferably imperforate and the panels are connected so that the widest straight edge wall 32 of one course engages the widest straight edge wall 32 of the next course and the narrowest straight edge wall 34 in one course engages the narrowest straight edge wall of the adjacent course. It will further be observed that the preferred wall arrangement provides that the wide portions of one wall constituting three panel courses is disposed adjacent the narrowest portions of the adjacent wall comprising three panel courses. Thus, there is provided a vertical zigzag path between the bottom and top wall of the rectangular conduit 28. This arrangement prevents sound waves from deflecting off the inner surface of the bottom or top wall and reaching the top or bottom wall without encountering acoustically dissipative material.

The arrangement of the sound absorbing panels may vary considerably for different conduits, one arrangement being more desirable for conduits of a certain size and shape than another arrangement. However, in all arrangements shown, the panels are disposed so that only arcuate surfaces are exposed to the gas stream carrying the sound energy and in each instance, the arcuate surface exposed to the gas stream and sound energy is concave and parallel to the normal path of the stream.