DEVICE FOR REDUCING DUCT-CARRIED AIR-BORNE SOUND

Abstract: A device for reducing duct-carried air-borne sound comprises a casing (1) enclosing a duct and a certain volume of sound absorbing material (2) attached in said casing in such a manner, that at least partially a free longitudinal passageway (3) is formed, which is enclosed by a surface constituting the internal defining surface, or corresponding surface, for the sound absorbing material. Sound attenuation is required preferably in the frequency ranges 50-500 c.p.s. For this purpose, either a greater thickness of the absorbent material or a greater total length of the attenuator is required. A solution now proposed implies that the defining surface is assembled of at least three surfaces, which are convex toward the passageway and so abut each other, that the cross-section of the passageway (3) has star shape with at least three points.

Description: Device for reducing duct-carried ' air-borne sound This invention relates to a device for damping air-borne sound carried by a duct, comprising a casing enclosing a duct and a certain amount of sound absorbing material attached in said casing in such a manner, that at least partially a free lomgitudinal passageway is formed, which is enclosed by a surface constituting the internal defin¬ ing surface, or corresponding surface, for the sound absorbing material. A great number of sound attenuators of different designs are commercially available. One usual type of sound atten¬ uators consists of a cylindric duct, the walls of which are covered with porous absorbent. At both ends of the duct area transitions and stubs to connecting ducts are mounted, and possibly also a perforated inner pipe is mounted to protect the sound absorbing porous material against erosion. In every day speech the aforesaid sound reducing devices are called pipe or duct sound attenuat- orsτ Examples of applications, for which this type of sound attenuator normally is used are ventilation installations, exhaust circuits for internal combustion engines and compressed air installations. At the aforesaid applications attenuation ' is. required ' ref¬ erably in the frequency ranges of .50-500 c.p.s. The sound attenuator types at present available yield a low attenuat¬ ion within this frequency range and.. due to sound attenuat¬ or geometry and absorbent properties., reach their attenuat¬ ion maximum at substantially higher frequencies. For obtaining the necessary attenuation, either a greater thickness of the absorbent material or a greater total length of the sound attenuator is required. This conflicts with the space requirements in buildings and other struct¬ ures. The present invention has the object to propose a duct sound attenuator without the aforesaid deficiencies. The invention is characterized in that the defining surface is assembled of at least three surfaces of convex shape tow¬ ard the passageway, which so abut each other that the pass¬ ageway has star-shaped cross-section with at least three points. In the following several imaginable embodiments are described, the characterizing features of which in the individual cases yield their special properties and advant¬ ages over the aforementioned ones. Several embodiments of the invention are described in the following with reference to the accompanying drawings, in which Fig. 1 shows a portion of a duct provided with the sound attenuating device according to the invention, Fig. 2 is a section along the line II-II in Fig. 1, Fig. 3 shows an embodiment, at which the star shape is hexagonal, and the casing of the duct has six sides. Dashed lines des¬ ignate a variant thereof with a cylindric casing. Fig. 4 shows another embodiment with a square casing, and Fig. 5 shows a further embodiment thereof. Figs. 6-8 show variants where the duct casing has cylindric shape, and the longitud¬ inal passageway has different star shapes. Fig. 1, thus, is a perspective view of a length unit of a duct with a surrounding casing 1 of sheet metal. Four bodies 2 of sound absorbing material are inserted in the duct. Each of the bodies 2 has the cross-sectional shape of a quadrant where the radius corresponds to half the side of the square cross-section of the casing. Due to the fact that the curved surfaces face inward with the convex surface, a star-shaped passageway 3 is formed within the duct. The passageway 3 extends integral through the duct, and the end pieces of the sound absorbing material 2 can be rounded so that stiea line shape faces to the afflux direction for the air through the passageway 3. This configuration of sound damping body within the sheet metal casing 1 is especially suitable from a manufacturing point of view, because each body of sound absorbing material is manufactured in a simple manner of a homogenous cylinder of sound absorbing material which is cut open longitudinally so that the quadrants in cross-section are obtained. The embodiment described is particularly advantageous, because the sound absorbing material has great thickness and the distance between covered surfaces is small. The quare casing, relative to a cylindric one, provides a greater effective sound attenuator volume without requir¬ ing more space for the attenuator in the building. The quarter-segmental tube-shaped bodies of sound absorbing material increase the effective absorbent thickness, decrease the distance between covered surfaces and yield ' a positive effect, due to the fact that the sound wave penetrates the absorbent material from the outside inward and not, as at conventional sound attenuators, from the inside outward. (The sound wave meets a decreasing area when it penetrates the absorbent material according to the present invention, while at conventional sound atten¬ uator designs the area is increasing). Fig. 3 shows a variant of the invention. The casing 4 in this case has hexagonal cross-section, and the bodies 5 of sound absorbing material are six in number and have subst¬ antially parabola shape with the point directed inward. The passageway 7 has star shape with six points. At a var¬ iant of this embodiment, the casing is of cylindric shape as indicated by the dashed lines 8. Fig. 4 shows a further embodiment where the easing 1 has square cross-section. The passageway 9 has star shape with four points. The defining surface 10 facing to the passage¬ way 9 consists of a quadrant of a cylindric plastic foil with the convex side facing inward the passageway 9. The plastic foil can be perforated for air transmission, and alternative materials to the plastic foils can be used. The object of the plastic foil is to protect the body 11 of the sound absorbing material and to maintain its shape. Fig. 5 shows a further embodiment, where the casing for the duct has square cross-section. The sound absorbing bodies 12 here have the shape of a cylinder quadrant with great wall thickness whereby an air gap 13 is obtained in each corner of the duct. The passageway 14 has star shape with four points in the same way as at the embodim¬ ent shown in Fig. 4. Fig. 6 shows a duct defined by a circular casing 15. The passageway lβ has star shape with four points, and the surface defining the sound absorbing bodies 17 toward the passageway is assembled of two surface planes 18 and 19 meeting angularly to the interior of the passageway. The defining surface for the absorbent body 17 hereby also is given convex-shaped configuration. Fig. 7 shows a variant of the embodiment according to Fig. 6 with a circular casing 15. The passageway 20 here has star shape with three points, and the defining surfaces for the absorbent bodies 21 have the cross-sectional shape of arcs. Fig. 8 shows another variant of the embodiment according to Figs. 6 and 7 with a circular casing 15. The passageway 22 has star shape with four points, and the absorbent bodies 23 have defining surfaces which face inward and have arc-shaped cross-section. The bodies of sound absorbing porous material shall be hom¬ ogenous and be manufactured so that a flow resistance is obtained which is homogenous or increases toward the corner points. The mounting can be carried out, for example, by glueing the bodies on the casing inside, and the casing can be manufactured, for example, of sheet metal. The bodies can be manufactured, for example, by compression moulding mineral or glass wool, which has been given suit¬ ably adapted density and flow resistance by choQsigg--a suitable amount of starting material. If desired, several sound attenuators according to the invention can be stacked one upon the other or be mounted laterally so that the desired total cross-section dimension is obtained. In order to reduce the total pressure drop over the sound attenuator at air flow, the mineral wool bodies in the attenuators can be given in the end areas a suitable aero¬ dynamic configuration, as already mentioned above. At low frequencies the sound reduction can be calculated according to the Sabine duct attenuation formula D = 1,05 x a 1 ' 4 x (U/A) where U = absorbent-covered perimeter A = free cross-sectional area. It can easily be shown that the absorbent geometry accord¬ ing to the present invention yields a more favourable U/A ratio than the absorbent geometry at the conventional attenuator with absorbent-covered pipe. Example A: Given: A conventional sound attenuator (Spiro attenuator). Length 1 m. Connection area diameter 100, and external pipe diameter 200. At 125 c.p.s. a = 0,2. U = 3,l4. A = 0,0078. U/A = 40,2. The attenuation is 3.78 dB. Example B: Given: A sound attenuator according to the present invent¬ ion with the length 1 m. Connection diameter 100. The outer duct is square, with the side 200 mm. Internal fill¬ ing with mineral wool body of diameter 200 mm. U = 0,628. A = 0,0086. U/A = 73. The attenuation is 6,87 dB. When the diameter of the mineral wool body can be slightly greater than 200 mm, for example 203 mm, A = 0,0078 and U/A = 80,5, which yields the attenuation 7 3 θ dB. At increased thickness of the absorbent material primarily the low frequency attenuation is improved. Also the value of a, thus, is influenced by the absorbent geometry according to the pres¬ ent invention. A certain additional increase at 125 c.p.s., thus, can be expected. It is evident that by the invention the attenuation at low frequencies is more than twice as high, primarily due to the higher value for U/A. For obtaining this increase in damping, however, the flow resistance for this absorbent geometry must be optimized. Conclusively can be said that a sound attenuator according to the present invention substantially improves the sound reduction, especially in the low frequency range, provided that the free flow area is maintained constant and the flow resistance for the new absorbent geometry is optimized.

Claims: Claims 1. A device for reducing duct-carried air-borne sound, comprising a casing enclosing a duct and a certain volume (2;5;11;12;17;21;23) of sound absorbing material attached therein in such a manner, that at least partially a free longitudinal passageway is formed, which is enclosed by a surface constituting the internal defining surface for the sound absorbing material, c h a r a c t e r i z e d i n that the defining surface is assembled of at least three surfaces of convex shape toward the passageway, which surfaces so abut each other that the passageway (3;7;9;l4;l6;20;22) has star-shaped cross-section with at least three points. 2. A device as defined in claim 1, c h a r a c t e r ¬ i z e d i n that the convex surfaces have arc-shaped cross-section. 3. A device as defined in claim 2, c h a r a c t e r ¬ i z e d i n that the arc shape consists of a circular arc. 4. A device as defined in claim 2, c h a r a c t e r ¬ i z e d i n that the arc shape consists of an ellipse arc. 5. A device as defined in claim 1, c h a r a c t e r ¬ i z e d i n that the cross-section of the convex surf¬ aces consists of an open polygon (l8,l9)(at least a _r * ._ * _e broken line) . 6. A device as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the points of the cross-section are located on the defining lines for the casing cross-section. 7. A device as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the casing (14) has the cross-section of a polygon. 8. A device as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the casing has the cross-section of a circle (8;15). 9. A device as defined in claim 2,. c h a r a c t e r ¬ i z e d i n that the absorbent material is arranged as a homogenous cylindric body with circular or elliptic cross-section, which is symmetrically cut open longitudin¬ ally into four identical parts (2;11;12) and attached with the cylindrically curved surface toward the centre of the duct, so that a star-shaped passageway (3;9;l4) is formed between the four absorbent bodies (2;11;12). 10. A device as defined in any one of the preceding claims, c h a r a c te r i z e d i n that the flow resistance of the body of sound absorbing material increases at inter¬ vals or continuously inward to the corners of the cut-open quadrant-shaped cylindric partial body (2;11). 11. A device as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the star-shaped pass¬ ageway between the four absorbent bodies is given aerodynam__- ic shape, so that low total pressure drop and low natural sound generation over the sound attenuator at air flow are obtained. 12. A device as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the absorbent mater¬ ial in the casing consists of a plurality of partial bodies. 13. A device as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the convex surfaces consist of the defining surface of the sound absorbing mater¬ ial. 14. A device as defined in the claims 1-8 and 10-12, c h a r a c t e r i z e d i n that the convex surfaces (10) consist of a material other than the. sound absorbing material, and that said material is sound pervious or formed sound pervious. 15. A device as defined in any one of the claims 1-8 and 10-12, c h a r a c t e r i z e d i n that the convex bodies (12) of sound absorbing porous material are formed as cylindric shells, so that an air space (13) or air gap is formed between the attenuator shell (1) and mineral wool bodies (12) .

Inventors: NILSSON NILS-AOKE, NILSSON, NILS-AOKE
Applicants: VIAK AB
Application year: 1985
Main classification: F16L55
IPC codes: F16L55