Sound absorption barriers

Abstract: The present invention aims at providing a sound barrier member that will enable a sound barrier to be constructed that provides good prevention of sound transmission as well as prevention of sound reflection while remaining relatively inexpensive to produce, the sound barrier member comprising an enclosure in a generally panel like configuration having a rectangular outer perimeter adapted to engage with other similar members to produce a sound barrier, the enclosure being formed by a front panel (10) and a rear panel unit (16) defining an enclosed space therebetween, the front panel (10) having a plurality of openings (12) in a desired shape and array allowing sound transmission into the enclosed space, a sound absorbing material (25) being positioned in the enclosed space overlying at least the openings in the front panel (10) and being spaced forwardly of the rear panel unit (16), the sound absorbing material (25) having at least one sound absorbing batten (28,29) of fibreglass mat construction and a layer of solid plastics material (31) on a rearward face of at least one batt (batten) whereby a space is left between the layer of solid plastics material and the rear panel unit (16).

Description: The present invention relates to sound absorption barriers and more particularly to an improved panel construction adapted, in association with other similar panels, to form a sound absorption barrier. The present invention has been developed particularly for use in controlling noise generated by road or other ground transport machines but it will be apparent from the following that the invention could well be used in other applications. Situations other than adjacent roadways where the invention could be employed include industrial applications where noisy machines are used, enclosures around generators, sound barriers at airports and adjacent railway lines. Again, this list should not be regarded as exhaustive. Unwanted sound is commonly referred to as noise. This can take the form of aircraft noise, train noise, motor vehicle noise and even loud music can be noise to some people. The level where sound becomes noise can be very subjective. At a certain level some individuals can be severely affected while other people may not react until the noise level becomes much greater. Road traffic noise generated by cars, trucks and motorcycles can take two main forms, continuous background noise and individual vehicle noise. Noise associated with heavily trafficked freeways and arterial roads is a mix of many sources, and is mainly continuous bulk traffic noise with an overlay of individual noisy vehicles. The object of the present invention is to create a panel adapted for use in a barrier which will absorb noise from any desired location and will not reflect such noise to any significant extent while reducing such transmitted noise. Plain, hard, dense noise barriers inserted between the noise source and the receiver, tend to reduce the transmission of noise between the source and the receiver. The insertion loss is not only dependent upon the barrier density but also the barrier height and length. The barrier integrity is important, as porous or badly joined barriers will leak noise, increasing the level at the receiver. A well constructed hard barrier will reflect traffic noise back towards and beyond the motor vehicle source. In some circumstances this may increase the noise levels at a second receiver opposite the noise barrier across the arterial road or freeway. If this occurs it may be necessary to erect a second noise barrier to protect the second receiver, resulting in parallel barriers on the sides of the roadway. The presence of high parallel reflective barriers adjacent to the roadway, can cause the multiple reflection of traffic noise between the barriers. In some circumstances the noise levels between the barriers could be higher than noise levels at the source without the barriers. If the noise level at the source is effectively increased then the noise level at the receiver will be proportionately increased. What this means, is that the erection of a second barrier will be detrimental to the receiver behind the first barrier which shields it from direct traffic noise. The reduction of the reflection capabilities of a noise barrier will lower noise levels opposite the barrier in the single barrier situation and the noise levels on both sides of the road in the parallel barrier situation. Reducing the reflection potential of a barrier involves increasing the absorption qualities of that barrier. An absorptive barrier must also be dense enough to achieve an effective sound transmission loss through it. Noise absorption and to a much lesser degree noise reflection is frequency dependent. Absorption coefficients are expressed in a range of 0.0 to 1.0 at a specific frequency, normally octave or third octave points. Traffic noise is louder in the low frequency range of 100 Hz to 1000 Hz, so for an absorption barrier to be effective it must perform very well in this range. The principle function of a sound barrier, particularly for use adjacent roadways or the like, is to attenuate noise between the source and a receiver while minimising or preventing reflection of the noise. In achieving this basic requirement it is of course also desirable to achieve a sound barrier which is relatively inexpensive to produce, durable, maintenance free, aesthetically acceptable and has high absorption coefficients at low frequencies. Prior art sound absorbing panel members are disclosed in U.S. Patent Nos. 4042061 and 3804196. U.S. Patent No. 4042061 discloses a rigid box like structure containing a compartment which may be divided into two sub compartments by a metal partition. The partition is either integrally extruded with the metal forming the rigid outer casing or may be a separate perforated panel. A free spacing plate is provided spaced from the rear wall of the box structure and is isolated there from by elastomeric mounting members. The front face of the box like structure is perforated by a series of holes to allow sound to be received therein. U.S. Patent No. 3804196 shows a moulded plastics material enclosure with at least one perforated face and one or two integrally moulded internal partition walls to divide the interior of the enclosure into two or three adjacent compartments. The compartment (or compartments) adjacent an outer perforated wall are filled with a sound absorbing material such as glass fibre, rock wool or a foamed plastics material. The central or rear compartments depending on the configuration is left open or filled by a particulate material such as sand. Both these specifications provide arrangements which will permit noise absorption over only a relatively narrow frequency spectrum. The present invention aims at providing an acoustic barrier member which, with other similar members, is adapted to form a sound barrier capable of absorbing noise over low, mid and high frequency ranges and which is relatively simple and inexpensive to produce bearing in mind in any barrier numerous such members are required. The acoustic barrier member according to this invention is defined in claim 1 below, and preferred additional features are defined in the subsidiary claims. Accordingly the present invention provides an acoustic barrier member comprising an enclosure formed by a front panel member having at least one open area, and a continuous rear panel member spaced rearwardly from said front panel member by top, bottom and side edge walls, said acoustic barrier member further comprising a sound absorbing unit located within said rigid enclosure, characterised in that said sound absorbing unit comprises at least one sound absorbing batt located so that a forwardly directed face of the or at least one said sound absorbing batt faces toward the front panel member of said rigid enclosure, a solid thin layer of plastics material being arranged adjacent a rearwardly directed face of the or at least one said sound absorbing batt, and in that said front panel member is secured to said rear panel member so as to retain said sound absorbing unit between said front and rear panel members overlying the or each open area within the front panel and in that an air gap is provided between an inner rear face of the rear panel member and a rearwardly directed face of said sound absorbing unit. Conveniently, the enclosure is formed from a moulded fibre reinforced cement which provides a relatively inexpensive but durable and rigid outer casing. Advantageously, the solid thin plastics material layer is formed from polyvinyl chloride which provides appropriate performance characteristics and is relatively inexpensive in production. The layer may be about 400 µm thick. Preferably the plastics material layer is bonded or otherwise secured to a said sound absorbing butt. The invention in providing an open front wall, a closed rear wall and sound absorbing material in spaced relation therebetween achieves a high degree of prevention of sound transmission and sound reflection which together with the selection of and disposition of the sound absorbing materials provides good performance over low, mid and high frequency noise ranges. In a preferred arrangement the sound absorbing unit may be at least as large as the internal dimensions of the enclosure forming the panel. In a further preferred arrangement, a forward face of the sound absorbing unit is spaced rearwardly of the front face of the enclosure. In this manner, air gaps are formed between the sound absorbing unit and the rear face of the enclosure or between the sound absorbing unit and both the front and rear faces of the enclosure. In accordance with a further preferred arrangement, the sound absorbing unit may comprise at least two fibre glass sound absorbing batts with a layer of polyvinyl chloride located therebetween. Conveniently the sound absorbing batts, each have a layer of polyvinyl chloride adhered to one face of the batt, the two batts being so arranged that the layers of polyvinyl chloride are adjacent one another. Preferably the enclosure is formed by moulded a glass fibre reinforced cement (GRC) is moulded in two parts with the front face being formed separately from the rear face. The rear face may be moulded integrally with forwardly extending side, top and bottom edge walls with the front face being moulded separately and securable to the side, top and bottom edge walls to complete the enclosure. The open area of the front face is preferably at least 10% of the total area of the front face and preferably is about 40% of the aforesaid total area. The maximum open area is dependent upon mechanical design constraints for the panel itself but might be up to 60%. Conveniently the open area may be comprised of a plurality of discrete spaced openings in the front face. The aforesaid openings may have a diamond shape. Preferably a fine mesh material may cover the openings inwardly of the enclosure to prevent or minimise the entry of contaminants such as dust, water, insects or the like. The invention will now be described with reference to the accompanying drawings which illustrate one particularly preferred embodiment adapted to form a sound barrier adjacent roadways or the like. In the drawings: Figure 1a is a schematic front elevation of a front section of a panel produced according to a preferred embodiment of the present invention; Figure 1b is a detail view of the area marked B in Figure 1a; Figure 1c is a further detail view of part of Figure 1b; Figures 2a and 2b are outer elevation views of rear sections of panels adapted to cooperate with the front section shown in Figure 1a; Figure 2c is a top plan view of the rear section shown in Figure 2a; Figure 2d is an end elevation view of the rear section shown in Figure 2a; Figure 2e is a detailed sectional view of the area marked D in Figure 2c; Figure 3a is an inside elevation view of the rear section shown in Figure 2a; Figure 3b is a cross-sectional detail view of a reinforcing element included in the rear section construction; and Figure 4 is a cross-sectional view showing schematically the transverse location of the various elements making up the sound absorption panel. Referring to the drawings, the preferred noise barrier panel is constructed of three main components namely, a front panel 10, a rear unit 16 and located therebetween a sound absorbing unit 25. Each component is described below in some detail. The first component is the front panel 10 which is an open lattice constructed from GRC. The basic overall dimensions may be 1990 mm x 980 mm while the depth of the panel will vary according to the overall dimensions which then is dependent upon the application. The front panel 10 has been arranged with a surrounding solid frame 11 and a plurality of openings 12 along the diagonals in the panel (Figure 1a). The solid diagonal GRC struts 13,14 alternate from about 20 mm to about 25 mm, and increase in width with the depth of the panel (Figure lb). These dimensions are variable and future lattice designs may have the struts being a uniform size or the alternate strut 13 being much greater than the narrow strut 14. Holes 11a provide a recess for the nuts 33 that are attached to bolts 23 moulded into a rear panel unit 16. The nuts 33 threadably engaged on the bolts 23 secure the front panel 10 to the rear panel unit 16. The open area of the panel 10 should be at least 10% and preferably about 40%, while the individual openings 12 are diamond or square/rectangular in shape measuring 45 mm x 45 mm having bevel edges on the front of the panel, while the opening is slightly bevelled at a much lesser angle through the depth of the panel. The thickness of the front panel is dependent on the designed open area and strength. A suitable range may be between 10 mm and 20 mm. The percentage open area and the openings dimension may vary according to barrier application. A mesh 15, for example a plastic fly mesh may be glued to the rear of the front panel 10. The mesh 15 serves two purposes. It reduces the amount of rain and water splash entering the barrier and also reduces the opportunity for nesting insects and birds to enter the panel. The rear and side sections of the noise barrier panel are shown in Figures 2a to 3b of the drawings. The rear and side sections of the barrier are moulded in a single unit 16 from glass fibre reinforced cement (GRC). Basically the overall dimensions may be 1990 mm x 1000 mm with a maximum cross-sectional depth of 230 mm. The GRC material may be a nominal 10 mm in thickness. Figure 2a gives an overall impression of the architectural treatment of the rear panel and the size of the individual components in the panel treatment. The rear outer surface of the unit 16 has preferably three upraised panels 17, 18 and 19 separated by grooves 20 and 21. Figure 2b gives an overall impression of an alternative architectural treatment of the rear panel and size of the individual components in the panel treatment. The rear outer surface of the unit 16a has three upraised prismoidal sections 17a and 18a separated by grooves 20a and 20 21a. Other alternative rear panel treatments of the rear panel unit will be used from time to time depending on the panels application. Figure 2c shows the top elevation and the relative size of an internal reinforcing component 22 shown in more detail in Figures 3a and 3b. The end elevation, Figure 2d demonstrates where the front panel 10 attaches on to the rear panel unit 16 using the bolts 23. In this manner the front panel 10 is secured to the rear unit 16. An end post slot 24 is shown in Figures 2c and 2e. The size of the moulded slot 24 will vary according to the size of the support post. The higher the barrier the larger the post cross-section, so the larger the slot. The configuration shown in Figure 2e is designed for a 2m high barrier supported by a 76 mm x 76 mm galvanized square post. In use, a plurality of noise barrier panels are supported one on the other between two support posts. The length of the barrier is increased by arranging more support posts with panels located therebetween. The reinforcing channel 22 attached to the centre of the rear unit 16 is shown in Figures 3a and 3b. This channel is moulded independently of the rear unit 16 and is attached when the rear unit 16 is being moulded. The channel 22 reinforces the unit 16 and assists in locating the sound absorbing unit 25 away from the rear internal surface 26 so that a cavity 27 is created. The cavity 27 enhances the acoustic performance at mid to high frequencies. The size of the cavity is believed not to be too critical although a spacing of at least 50 mm is currently regarded as optimal. The uniformity of the cavity 27 is not essential and spacings greater than 50 mm will not change the performance characteristics greatly. The sound absorbing component or unit 25 is best seen in Figure 4. Preferably the unit 25 includes two batts (battens) 28,29 of fibreglass mat construction sandwiching between them a pair of layers 31 of solid, that is non-perforated, plastics material, preferably sheets of polyvinyl chloride, and located inside the enclosure formed by the front panel 10 and the rear unit 16. The sound absorbing fibre batts 28 may be 50 mm thick and have a nominal density of between about 32 to 35 kg/m³. This dimension and density range are standardly produced and commonly available having acoustic performance characteristics that are adequate for the present purposes. The acoustic performance will drop if the density or thickness is less than the foregoing figures. The front batt may have a scrim 30 glued to the surface facing the front lattice panel 10. This scrim 30 is used to reduce the amount of rain water penetrating the front batt 28. A substantially rigid 400 µm thick PVC sheet 31 is glued to each of the surfaces of the front and rear batts 28 and 29 which face one another. A plastics sheet thicker than 400 µm may be more expensive but not add greatly to the acoustic performance of the panel. Each sheet 31 acts as a low frequency absorber as well as providing a small amount of rigidity to the overall absorbing unit 25. The absorbing components 25 are located firmly in the rear panel unit 16 by being slightly larger than the internal dimensions. The overall dimensions of absorbing components 25 may vary depending upon the barrier application. The arrangement is also such as to preferably provide an air space or cavity 32 between the front batt 28 and the front panel 10. The spacing assists in improving rain resistance. Road traffic noise tends to predominate in the frequency range of 100 Hz to 1000 Hz, then decreases in loudness upwards from 1000 Hz. Traffic noise loudness varies depending upon the road surface and the vehicle mix, as well as the source and receiver exposure. Most previous designs of absorbing barriers have been constructed of materials other than GRC, for example fibreglass composites or metal sheet. These barriers require on-going maintenance, and may eventually deteriorate to a stage where the barrier needs to be replaced. Many of these barriers have been designed on a fundamental performance basis rather than a performance and aesthetic concept. The barrier described in this document is designed to be long-lasting, maintenace free and aesthetically appealing.

Claims: An acoustic barrier member comprising an enclosure formed by a front panel member (10) having at least one open area (12), and a continuous rear panel member (16) spaced rearwardly from said front panel member (10) by top, bottom and side edge walls, said acoustic barrier member further comprising a sound absorbing unit (25) located within said rigid enclosure, characterised in that said sound absorbing unit (25) comprises at least one sound absorbing batt (28) located so that a forwardly directed face of the or at least one said sound absorbing batt (28) faces toward the front panel member (10) of said rigid enclosure, a solid thin layer of plastics material (31) being arranged adjacent a rearwardly directed face of the or at least one said sound absorbing batt (28), and in that said front panel member (10) is secured to said rear panel member (16) so as to retain said sound absorbing unit (25) between said front and rear panel members (10, 16) overlying the or each open area (12) within the front panel (10) and in that an air gap (27) is provided between an inner rear face of the rear panel member (16) and a rearwardly directed face of said sound absorbing unit (25). An acoustic barrier member according to claim 1, characterised in that said enclosure (10,16) is formed from moulded fibre reinforced cement. An acoustic barrier member according to claim 1, characterised in that said layer of plastics material (31) is polyvinyl chloride. An acoustic barrier member according to any one of claims 1 to 3 wherein said solid thin layer of plastics material layer (31) is about 400 µm thick. An acoustic barrier member according to any one of claims 1 to 4 wherein an air gap (32) is provided between an inner face of the front panel member (10) and a forward face of the sound absorbing unit (25). An acoustic barrier member according to claim 1, characterised in that said solid thin layer of plastics material (31) is adhered to said at least one sound absorbing batt (28). An acoustic barrier member according to claim 1, characterised in that said sound absorbing unit (25) has a surface area at least as large as internal dimensions of said enclosure. An acoustic barrier member according to claim 1, characterised in that the forwardly directed face of said at least one sound absorbing batt (28) is spaced rearwardly from an inner face of said front panel member (10). An acoustic barrier member according to claim 1, characterised in that said sound absorbing unit (25) comprises two sound absorbing batts (28,29) formed from fibre glass mat, and said solid thin layer of plastics material (31) is located between said two sound absorbing batts (28,29). An acoustic barrier member according to claim 1, characterised in that said front panel member (10) includes a plurality of openings (12) forming said at least one open area (12) whereby a total open area of said front panel member (10) is between 10 % and 60% of a total area of said front panel member (10). An acoustic barrier member according to claim 10, characterised in that said plurality of openings (12) are diamond shaped. An acoustic barrier member according to claim 10, characterised in that a fine mesh material (15) is secured to or disposed adjacent an inner face of said front panel member (10). An acoustic barrier member according to claim 1, characterised in that a water repellent material (30) is applied to a front face of said sound absorbing material (25). An acoustic barrier member according to claim 1 characterised in that said sound absorbing unit (25) is comprised by two said sound absorbing batts (28,29) with a said solid thin layer of plastics material (31) being adhered between said sound absorbing batts (28,29). An acoustic barrier member according to claim 1 characterised in that said sound absorbing unit (25) is formed by two said sound absorbing batts (28,29) with two solid thin layers of said plastics material (31) located between said sound absorbing batts (28,29). An acoustic barrier member according to claim 15 characterised in that each said sound absorbing batt (28 or 29) has a respective said solid thin layer of plastics material (31) secured thereto. An acoustic barrier member according to claim 1 characterised in that an outer rear face of the rear panel member (16) includes angular protuberances (17a,18a) arranged to reflect noise directed towards said outer rear face in either an upward or a downward direction.

Inventors: BARDEN RONALD, WEST PHILIP J, BARDEN, RONALD, WEST, PHILIP J.
Applicants: ROADS CORP, ROADS CORPORATION
Application year: 1989
Main classification: G10K11
IPC codes: G10K11