US2882989A - Sound absorber - Google Patents

Description

April 1959 PER VILHELM BRUEL EI'AL 2,882,989

SOUND ABSORBER I 2 Sheets-Sheet 1 Filed Nov. 16, 1956 INVENTORS P, K Dru e L C141: Tejlzz B '1 O y I I I ATTORNEY-3 April 21, 1959 PER VILHELM BRUEL ETAL SOUND ABSORBER 2 Sheets-Sheet 2 Filed Nov. 16, 1956 INVENTORS P 1 .131 L Le C 4.15. Tejuar ATTORNEY3 United States Patent 2,882,989 SOUND ABSORBER Per Vilhelm Bruel, N aerum, Denmark, and Carl-Axel Elof Tegner, Stockholm, Sweden Application November 16, 1956, Serial No. 622,581 Claims priority, application Sweden November 16, 1955 6 Claims. (Cl. 181-33) It is known to use sound absorbers based upon the principle of resonance in order to reduce the reverberation time in places with a high noise level, for instance, offices and industrial premises, and in places where it is desirable, due to special circumstances, to have a short reverberation time, for instance, in conference rooms, auditoriums, and so on. The most common procedure is to apply so called acoustic panels in the ceiling of the room. Such panels are comparatively expensive and besides their mounting call for comprehensive, timeconsuming, and expensive preparatory work, and also great painting costs. Furthermore, the method is limited by the fact that the acoustic panels, being flat and rigid, may only be applied on a plane base structure.

An acoustic absorber designed for highest efficiency comprises a perforated front panel mounted some distance away from the solid back wall and with a layer of porous absorbing material applied to the interior side of the front panel. The main principle of its function is that the vibrations of the volume of air caught between the front panel and the back wall are obstructed by the acoustic impedance which the absorbing material olfers to the movements of the air particles, part of the sound energy being converted into heat energy. The ratio between the size of perforations and in some degree also the thickness of the perforated panel on one hand, and the volume of air enclosed behind the same unit area on the other hand will decide within which frequency range the greatest absorbing effect will appear. If the air volume caught behind a certain unit area is great, maximum absorption will be obtained at the low end of the audio frequency spectrum. If said volume is small, maximum absorption will be obtained at the right end of the spectrum.

A generally applicable absorber must not have a pronounced maximum of absorption but its absorption curve should extend as even as possible. This requires a varying ratio between the area of the front panel and the air volume present at different spots behind the surface of theabsorber. Moreover, the absorbing effect should be satisfactory over as great a range of the frequency spectrum as possible. In this respect the greatest difficulty is to obtain a sufficient absorption at low frequencies since these require a comparatively large volume of enclosed air.

The most favourable absorber of this kind from a theoretical point of view is a hollow body with a large cubic capacity in relation to the covered surface and having walls which, firstly, are rigid-in order not to irresistently follow the vibrations of the air and, secondly, are designed in such a manner in respect to the absorption that their acoustic impedance will attain approximately 41 acoustical ohms per square centimeter. An acoustical ohm is the unit of acoustic impedance and is expressed, in the c.g.s. system as g. cm.- sek" (Per V. Briiel,.Lydisolation och Rumakustik (Sound Insulation and Acoustics of Rooms), Trans. No. 55, 1946, M.l15. of Chalmers University of Technology).

By itself there is nothing new involved in designing an absorber in the shape of a hollow body. It has been proposed in prior art to design absorbers as hollow bodies with walls comprising perforated material, such 2,882,989 Patented Apr. 21, 1959 as a shell with a rear porous absorbing material of various kinds. Hence, the external wall of these known absorbers shaped as hollow bodies only functions as a carrying and covering member for the porous absorbing material proper.

The main object of the present invention is to provide a sound absorber in which the acoustic impedance necessary for the sound absorption is established already by the front panel itself which is made of thin material such as cardboard, metal foil, or the like, having very small and very narrowly spaced holes, apertures or perforations. Such a sound absorber constructed in accordance with the absorption principle is the object of a co-pending application filed by the same applicants. In applying this principle of absorption, there is no need for any special, porous absorbing material, which will involve a considerable simplifying and saving of costs. As a matter of course there is on the other hand nothing to prevent applying a porous absorbing material in an absorber designed according to the present principle if a special effect is desired for some particular reason.

In order to render the thin material rigid enough not to follow the vibrations of the sound, the absorber according to the invention is constructed as a cone, part of a sphere, part of an ellipsoid or another hollow body of revolution with a straight or curved generatrix, said body being pressed against a solid surface, for instance a ceiling or a Wall, by means of a resilient fastening member; the absorber may also possibly comprise a combination of several such bodies. According to a further feature of the invention one end of said resilient attachment is connected to a support or attachment in the ceiling or the wall while the other end is connected to a wire or the like, forming a hook or the like which is adapted to engage the rim portion of an opening of said hollow body of revolution.

Absorbers designed as hollow bodies of revolution are known by themselves but in none of these cases the capacity of the absorber as a hollow body of revolution (or perhaps as having a curved shape) is dependent on the necessity of the wall of the absorber to obtain adequate rigidity, as this wall already by itself is sufficiently thick as to render the required rigidity, irrespective of the shape.

According to the present invention the simplest shape of an absorber is the conical one, said shape being accom plished by joining together one or more flat sheets of the perforated basic material, said sheets substantially having the form of sectors of a circle with a total peripheral angle of less than 360, and the joining together being effected along the radial edges of the sheets. By providing the corners of the sector elements facing the tip of the contemplated cone with a notch, an opening is formed at the assembling for introducing of a resilient fastening member which holds the absorber on to the ceiling after said member has been caused to engage the edge portion about this opening. The absorber may be assembled of one or more sector elements having the same or differing arc lengths, and the number of sector elements of the absorber may be varied according to the desired top angle of the cone. As an important advantage of the conical shape, it may be pointed out that the basic material, comprising the flat sector elements, is easy to pack up and transport and that assembling and mounting on plane as well as on curved surfaces readily may be carried out by unskilled personnel without any preparatory work.

As the absorber is constructed of a fiat and thin basic material which will obtain its curved shape when assembled into a cone or another body of revolution and will obtain its rigidity when pressed against the carrying surface, the fastening device itself constitutes an integrating part of the invention.

In order to further explain the invention some embodiments thereof will now be described with reference to the accompanying drawings. In connection therewith further advantages associated with the invention will be evident.

Figure l is a central sectional elevation of a sound absorber designed according to the invention;

Figure 2 is a top plan view of the same absorber in a flattened state consisting of sector shaped, substantially fiat sheets or disks, the total peripheral angle of which being less than 360;

Figure 3 is a fragmentary cross sectional elevation along the line III-III of Figure 2;

Figure 4 is a diagrammatic cross sectional elevation of another embodiment of the absorber;

Figure 5 is a perspective view of part of a further embodiment; and

' Figure 6 is a diagrammatic cross sectional elevation of the last mentioned embodiment in an installed position.

The sound absorber 1 may, for instance, comprise three annular sector shaped elements 3, 4 and 5, each provided with perforations, apertures or slits 2, which for instance may be grouped into long and narrow triangular or otherwise shaped fields so as to cause at the same time a decorative effect. However, the invention is not confined to this embodiment. The three sector elements 3, 4, 5, cover each a centre angle less than 120 and their radial edges 3a and 4a are folded. Abutting edges 3a, 4a of adjacent sector elements are joined together in pairs by means of U-shaped channels 6 or spring clips slipped onto the edges and serving as clenching or fasting means. A conical screen will be formed when the edges of sector elements 3 and 5, shown on the left of Figure 2, are brought together and joined or assembled in the manner described. The apex portions or corners 7 at the top of the sector elements are punched away so that the conical screen formed after joining together the sector elements will obtain a corresponding opening at the top. As a matter of course the invention is by no means confined to the number of sector elements shown in the drawing or to the shape of these elements.

In the case illustrated in Figure l the absorber is attached underneath a ceiling 10 by means of a fastening device comprising a helical spring 14, the top end of which merges into a loop engaging a wire 12 which forms a loop at its central part. The free parts of the wire also form loops 13 through which fastening pins 11 pass, driven obliquely into the ceiling 10. One end of helical spring 14 also forms a loop through which a wire 15 is threaded, the fastening end of which at the helical spring is bent backwards and twined around wire part 15 and at the outer part bent into a hitch or fastening hook 16. In installing the screen, wire part 15 is caused to protrude through opening 7 in the apex or top of the screen, and a small shield or hood 8 in the shape of a truncated conical envelope surface in put on to this top; wire part 15 being pulled out through opening 9 of said envelope surface. On pulling wire part 15, fastening hook 16 is caused to hook on to the edge portion about opening 9 of hood 8 while simultaneously stretching spring 14, whereupon the freely protruding part of wire 15 may be cut off. This simple fastening device will retain and press the screen effectively against the ceiling and the same fastening device may be usedfor various screen sizes.

By the described assembling and fastening method, the sector elements may be readily mounted and dismounted again without being damaged. This involves the eminent advantage that absorbers with different acoustic properties may be obtained from one single type of basic material and that the design which in the best way solves the actual problems of absorption may be worked out by practical tests without damaging absorbers or ceilings. For instance, by only utilizing sector elements 3 and 4 a more acute cone angle and a larger volume of air in relation to the surface are obtained. The maximum value of absorption is displaced towards lower frequencies in this embodiment.

Furthermore, according to Figure 4 a cylindrical ring 17 may be arranged for screen 1, said ring having approximately the same width as the base of the screen and possibly being provided with one or more depressions 18, 18a, extending around the ring. This ring 17 is applied between screen 1 and the ceiling so that the screen will rest in or adjacent to the depression 18, if any. In conjunction with this ring two screens, one Within the other, may be utilized; one (indicated by lines 1a) at the base 18a of the ring and the other 1 at the top edge of the ring or in the aforesaid depression 18. All these alternatives will involve a change of the ratio between surface and volume and thus a displacement of the maximum value of absorption and/ or an increase of the absorption effect.

The shapes of the two embodiments now described, that is a cone and a cone with a cylindrical base, respectively, are in first line intended to be made of non-ductile material such as cardboard or the like. If, instead, a material is used, such as aluminium foil or the like, which is shaped into a curved configuration by means of a pressing procedure it will be possible to follow the same principle as previously described in assembling the absorber, but in this case a peripheral edge 20 at every mainly sector shaped unit 19 is already bent or pressed up during the manufacturing step, as best seen in Figure 5. Figure 6 shows an absorber according to this design installed.

It will be easily appreciated that in assembling, for instance, three such units, a cone is obtained with a cylindrical or somewhat conical basis 20 without employing the previously described loose ring 17. In joining together such an absorber, straight U-shaped clips 21, 22 (Figure 5) may be used, being of a type somewhat differing from the fastening means previously described, wherein, for instance, a clip 22 is fastened at each joint onto the folded abutting edges 24 of the cylindrical part 20 and three or more clips 21 are fastened onto the abutting edges 23 of the conical part 1. Instead of these clips also the clips 6 described may be employed.

It will be evident from the foregoing that it is desirable not only to obtain a varying ratio between the front panel surface and the air volume behind the surface of the absorber at different spots thereof, but also that the air volume contained behind each unit area is great in relation to the unit area itself. This condition is complied with, not only by the cone and the combination of cone and cylinder but also by hollow bodies of revolution with a generatrix in the shape of a broken line or a curved line, for instance, a hemisphere, a hemiellipsoid, or the like, which in relation to the first mentioned bodies involve the advantage that every part of their surface is curved in two planes which renders a greater rigidity than one-plane curvature. Hence, it will be possible in employing this design to obtain the desired rigidity with a thinner material than would be required by curving in one plane.

If the absorber is made as a spherical, ellipsoidal, or similar segment surface, it is suitably pressed integrally from a perforated basic material, since units having this shape readily may be stacked onto each other for transportation. If the absorber is designed as a hemisphere, hemiellipsoid, or the like, which cannot advantageously be stacked onto each other, it should preferably be made in two or three sections with completely inturned edges. These sections may be readily stacked onto each other for transportation, and when they are to be assembled the bent metallic clips 21, 22 are utilized, engaging the inturned edges.

In assembling as well as installing this embodiment, the same further steps should be taken as previously described in connection with the cone-shaped absorber. For aesthetical reasons, however, it would be advisable not to design the small hood 8, positioned in the center of the absorber, as a truncated conical envelope surface but instead to give it the shape of a cup conforming to the shape of the absorber.

Any of the previously disclosed embodiments may have a layer of porous sound absorbing material applied therein, as shown in Figure 1 for example.

The detailed description herein of the various embodiments of the invention for the purpose of explaining the principles thereof is not to be construed as restricting or limiting the invention, since many modifications may be made by the exercise of skill in the art.

We claim:

1. A sound absorber for covering a surface, comprising a cup-shaped hollow revolution body of sheet material conically tapering in at least one direction with respect to its axis of rotation and terminating in a circular edge in the opposite direction, at least said conically tapering part being composed of annular sectors of perforated sheet material curved relative to the plane of said sheet material and having radially extending edge portions substantially perpendicular to said curved surface, said annular sectors being juxtaposed with said edge portions in abutment in adjacent sectors so as to form a truncated conical wall having two end openings of different diameter, resilient clenching means engaging remote sides of adjacent edge portions so as to maintain the latter in abutment, resilient suspending means having one end fixed relative to said surface and the other end provided with fastening means extending through one of said end openings having a smaller diameter, said fastening means engaging said cup-shaped body so as to resiliently maintain it in engagement with said surface along said circular edge.

2. A sound absorber for covering a surface, comprising a cup-shaped conical hollow body composed of annular sectors of perforated sheet material curved relative to the plane of said sheet material and having radially extending edge portions substantially perpendicular to said curved surfaces, said annular sectors being juxtaposed with said edge portions in abutment in adjacent sectors so as to form a truncated conical wall having two end openings of different diameter, resilient clenching means engaging remote sides of adjacent edge portions so as to maintain the latter in abutment, resilient suspending means having one end fixed relative to said surface and another end provided with fastening means extending through one of said end openings having a smaller diameter, said fastening means engaging a rim portion around said smaller diameter opening so as to resiliently maintain said cupshaped body in engagement with said surface along a circular edge surrounding said end opening having a greater diameter.

3. A sound absorber for covering a surface, comprising a cup-shaped hollow revolution body of sheet material conically tapering in one direction of its axis of rotation, and having a cylindrical part adjacent to said conically tapering part terminating in a circular edge in the opposite direction, said conically tapering part being composed of annular sectors of perforated sheet material curved relative to the plane of said sheet material and having radially extending edge portions substantially perpendicular to said curved surface, said annular sectors being juxtaposed with said edge portions in abutment in adjacent sectors so as to form a truncated conical wall having two end openings of different diameter, said cylindrical part being connected with said conically tapering part along an edge portion of the latter surrounding one of said end openings having a greater diameter, resilient clenching means engaging remote sides of adjacent edge portions so as to maintain the latter in abutment, resilient suspending means having one end fixed relative to said surface and another end provided with fastening means extending through the other of said end openings having a smaller diameter, said fastening means engaging a rim portion around said smaller diameter opening so as to resiliently maintain said cup-shaped body in engagement with said surface along the circular edge of said cylindrical part.

4. A sound absorber for covering a surface, compris ing a cup-shaped hollow revolution body of sheet material conically tapering in one direction of its axis of rotation and having a cylindrical part adjacent to said conically tapering part terminating in a circular edge in the opposite direction, said conically tapering part consisting of a hollow wall having the shape of an annular envelope confined between two concentric conical walls of different tapering angle and said cylindrical part, said conical walls being composed of annular sectors of perforated sheet material curved relative to the plane of said sheet material and having radially extending edge portions substantially perpendicular to said curved surfaces, said annular sectors being juxtaposed with said edge portions in abutment in adjacent sectors so as to form two truncated conical walls of different tapering angle each having two opposite end openings of different diameter said two truncated conical walls being in engagement near said end openings having a smaller diameter while in engagernent with said cylindrical part along their edge portions surrounding said end openings having a greater diameter, resilient clenching means engaging remote sides of adjacent edge portions so as to maintain the latter in abutment, resilient suspending means having one end fixed relative to said surface and another end provided with fastening means extending through said openings having said smaller diameter, said fastening means engaging a rim portion around said smaller diameter openings so as to resiliently maintain said cup-shaped body in engagement with said surface along the circular edge of said cylindrical part.

5. A sound absorber for covering a surface, comprising a cup-shaped hollow revolution body of sheet material conically tapering in one direction of its axis of rotation and having a cylindrical part connected to said conically tapering part terminating in a circular edge in the opposite direction, said hollow body being composed of sections of perforated sheet material each having a conical annular sector and a cylindrical segment having a central angle which is the same as said conical sector, said sections having edge portions substantially perpendicular to the surface of said conical sector and said cylindrical segment, respectively, said sections being juxtaposed with said edge portions in abutment in adjacent sections so as to form said cup-shaped hollow body and leaving an opening at the tapering end of said hollow body, resilient clenching means engaging remote sides of adjacent edge portions of adjacent sections so as to maintain the latter in abutment, and resilient suspending means having one end fixed relative to said surface and another end provided with fastening means extending through said opening at the tapering end and engaging a rim portion around said opening so as to resiliently maintain said cup-shaped body in engagement with said surface along said circular edge of said cylindrical part.

6. A sound absorber according to claim 1, wherein porous material is introduced into the absorber in order to increase its acoustic impedance.

References Cited in the file of this patent UNITED STATES PATENTS 1,363,782 Handel Dec. 28, 1920 1,467,689 Scherer Sept. 11, 1923 1,509,744 Wilson Sept. 23, 1924 2,161,708 Heerwagen June 6, 1939 2,186,511 Welch Ian. 9, 1940 2,390,262 Mazer Dec. 4, 1945 2,610,695 Grue Sept. 16, 1952 FOREIGN PATENTS 43,323 Denmark Oct. 27, 1930 111,603 Sweden Aug. 29, 1944

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