SOUND-PROOFING UTILITY, ESPECIALLY A SOUND-ATTENUATING UNIT

Abstract

 The invention relates to a sound-proofing utility configured to limit, at least for a determined frequency range, the lateral emission of airborne sound caused by motorized road traffic, the sound-proofing utility comprising a plate with an acoustically hard outer surface, wherein the plate comprises at least one sound-absorbing side, wherein the sound-absorbing side has a plurality of elongate cavity structures arranged in the plate and debouching at the hard outer surface, and with resonance frequencies in the determined frequency range, for at least partially absorbing the sound incident on the sound-absorbing side.
The invention also relates to a travel surface, such as a railway or motorway, provided with such a sound-proofing utility.

Description

[0001] The invention relates to a sound-proofing utility configured to limit, at least for a determined frequency range, the lateral emission of airborne sound caused by motorized road traffic. The invention also relates to an assembly of a sound-proofing utility and a diffractor arranged or to be arranged along a travel surface at a position between the travel surface and the sound-proofing utility, and to a travel surface provided with a sound-proofing utility, optionally together with a diffractor.

[0002] Motorized road traffic can for instance be understood to mean car traffic on a motorway, train traffic on a railway or air traffic on a runway. The vehicles forming the road traffic cause a number of different sources of (airborne) sound during travel. In the case of car traffic the most significant sources are formed by the tyres (rolling noise) and the engine (engine noise). Particularly dominant at low speeds is the engine noise, and from a speed of about 50 km/h the rolling noise of the tyres. In other embodiments (not shown) the travel surface is a railway and the noise is caused by a train travelling on this railway. Railway noise is mainly caused by the rolling noise of the train or, at very high speeds, by the aerodynamic sound, for instance the sound coming from the pantograph. The different sound sources are thus situated at different heights relative to the travel surface.

[0003] It is known to arrange one or more noise-reducing screens along the travel surface in order to reduce the sound transmission from a travel surface to the surrounding area. There is a "shadow" behind such a noise-reducing screen, whereby the traffic noise is attenuated. Noise-reducing screens are reasonably effective in at least limiting the worst noise nuisance, particularly in the case of sound-sensitive objects such as houses and office buildings in the vicinity of such a travel surface. The acoustic shadow behind the screen is however not absolute. The effect of the screen is influenced by the diffraction of the sound around the upper side of the screen. The longer the path (also referred to here as the path length) which has to be covered by sound in order to arrive at the sound-sensitive object, the more effective the screen is. The height of the sound-proofing utility plays a part here. The sound-screening effect of a high noise-reducing screen is generally greater than that of a low noise-reducing screen. A high noise-reducing screen is however relatively expensive, requires special foundation and/or anchoring provisions, and sometimes cannot be applied from an aesthetic viewpoint.

[0004] It is more generally the case that noise-reducing screens are expensive utilities. They further have an adverse effect on the landscape and often deprive residents of an unobstructed view. They moreover have the drawback that their effectiveness is limited in the case of specific wind directions. Noise-reducing screens are further less readily applied in situations in which sound-sensitive objects are situated on both sides of the travel surface. This is because reflection of sound against the sound-proofing utility on a first side of the travel surface causes this sound to be transmitted to the opposite side of the travel surface, and this sound can reach the sound-sensitive objects situated there.

[0005] Different types of noise-reducing screen have been developed over the years. Reflecting noise-reducing screens particularly involve reflection of the sound against the screen, while in the case of absorbing noise-reducing screens the sound is partially (also) absorbed. Some types of noise-reducing screen provide for a combination of reflection and absorption.

[0006] In a known type of absorbing noise-reducing screen a separate layer of absorption material is arranged against the noise-impacted side of the screen. The screen itself can for instance be formed by a (non-absorbing, acoustically hard) concrete plate against which is arranged a plate or layer of another, acoustically soft material. In a known embodiment the absorbing material comprises a mixture of wood fibre and cement. Such noise-reducing screens are however relatively complex and relatively expensive to manufacture and service. The known noise-reducing screens are further often susceptible to external influences and the effect of the screens eventually decreases, for instance because the absorbing layer becomes fouled.

[0007] An example of such a sound-absorbing screen wherein use is made of acoustically absorbing material is described in the document SE 518055 C2. The known screen is constructed from successively an acoustically hard first and second layer and an acoustically absorbing third layer. Arranged in the first and second layer is a number of tubular recesses debouching into the acoustically absorbing third layer. All tubular openings have the same length. This known sound-absorbing screen also has the above stated drawbacks.

[0008] Described in the patent specification US 5 457 291 is a sound-absorbing panel in which the sound absorption is not provided by acoustically absorbing material but by a number of Helmholtz resonators provided in the noise-impacted side of the panel and distributed evenly over this side. These resonators all have the same dimensions (length). The panel is of fairly complex construction, is relatively expensive to manufacture and is fouled quickly, whereby it can lose part of its effect.

[0009] It is an object of the invention to provide a noise-reducing screen in which at least one of the above stated drawbacks is obviated.

[0010] It is a further object of the invention to provide a simple yet effective sound-proofing utility which is robust and requires little maintenance.

[0011] It is another object of the invention to provide a sound-proofing utility with relatively small dimensions yet with an effective sound screening.

[0012] It is also an object of the invention to provide a sound-proofing utility which is aesthetically attractive.

[0013] At least one of the above stated and/or other objectives is at least partially achieved in a sound-proofing utility configured to limit, at least for a determined frequency range, the lateral emission of airborne sound caused by motorized road traffic, the sound-proofing utility comprising a plate with an acoustically hard outer surface, wherein the plate comprises at least one sound-absorbing side, wherein the sound-absorbing side has a plurality of elongate cavity structures arranged in the plate and debouching at the hard outer surface, and with resonance frequencies in the determined frequency range, for at least partially absorbing the sound incident on the sound-absorbing side, wherein the plate takes a monolithic form, the inner surface of each of the cavity structures is manufactured from acoustically hard material and the cavity structures are free of acoustically absorbing material and are grouped into different groups distributed over the side of the plate, wherein the cavity structures have mutually varying lengths within each group.

[0014] This construction is easy to manufacture, offers good absorbing properties and requires little maintenance. The construction is further lighter than the current concrete screens (reducing sound by dispensing with material results in a lighter screen), requires relatively little material and is thereby relatively inexpensive and durable, requires a less heavy foundation, is cheaper to transport (from the factory to the work site, in the case of prefabricated plates) and can be placed using less heavy equipment.

[0015] The elongate cavity structures preferably extend substantially transversely of the sound-absorbing surface and/or parallel relative to each other. The cavity structures further have a number of different resonance frequencies (distributed in the above stated frequency range) in order to be able to absorb the sound over a relatively wide frequency spectrum.

[0016] The plate takes a monolithic form and/or is manufactured from a single, acoustically hard material, such as concrete or similar material. Such monolithic plates are robust and are easy to make. The plate can for instance be made by casting or pouring the plate material in a mould and, after partial curing thereof, either removing the material from the mould (for instance in the case of prefab concrete) or wholly or partially removing the mould (for instance in the case of concrete poured in situ). Said cavity structures can in both cases be co-moulded in one operation, for instance by making use of forming parts (such as plastic pipes and the like) to be optionally removed after curing.

[0017] In an embodiment of the invention the plate is a self-supporting plate configured for stable arrangement on a ground surface. A widened portion or base can for instance be formed on the underside of the plate, with which the plate can be disposed directly on a (flat) ground surface. This enables a simple and quick placing of the sound-proofing utility. In other embodiments the plates of the sound-proofing utility are configured to be mounted on a support structure, for instance an existing sound-proofing utility, anchored in the ground. It is for instance possible to provide separate plates (for instance blocks) which can be mounted on an existing sound-proofing utility in order to give the existing screen a higher absorption value. The separate plates can have dimensions of the same order of magnitude as the dimensions of the existing sound-proofing utility. In other embodiments the separate plates are however much smaller, and can be mounted on the existing sound-proofing utility at random positions so as to wholly or partially cover for instance the side of the existing sound-proofing utility directed toward the sound source with the acoustically absorbing plates.

[0018] The elongate cavity structures can be realized in a number of different ways. The sound-proofing utility can be realized by applying forming parts in a casting or pouring process in order to manufacture the cavity structures. The cavity structure can for instance be formed by a forming part such as plastic pipe, which is removed again after curing of the material of the plate. In order to make removal easier, such forming parts are often embodied with a releasing form. In other embodiments the forming parts however remain behind in the plate. The cavity structures can for instance be formed by acoustically hard pipes, for instance plastic pipes such as PVC pipes, anchored in the material of the plate. These pipes form a lost formwork and are therefore also referred to as formwork pipes. In other embodiments the cavity structures are not formed by means of (formwork) pipes, but the cavities are arranged in the cured material of the plate afterward by drilling holes in the surface thereof.

[0019] In determined embodiments the cavity structures are distributed substantially evenly over the sound-absorbing side of the plate. This means that the sound-proofing utility offers roughly the same degree of absorption over substantially the whole noise-impacted side. In further embodiments the cavity structures are grouped into different groups distributed over the side of the plate, wherein the cavity structures have mutually varying lengths within each group. Each group can essentially be built up here of the same cavity structures or even the same pattern of cavity structures (each with a different resonance frequency). A group comprises for instance a predetermined pattern of mutually adjacent cavity structures. Each cavity structure within this pattern has a different length and is thus suitable for absorbing sound of different frequency ranges. In determined embodiments there is only one pattern of cavity structures, and this pattern is repeated over the side of the sound-proofing utility. In other embodiments there are two or more different patterns of cavity structures, and the different patterns are provided at different positions of the sound-proofing utility.

[0020] The distribution of the cavity structures can vary at least partially over the height of an upright sound-absorbing side. In determined embodiments the average cross-section of the cavity structures at high positions relative to the ground is substantially smaller than the average cross-section of the cavity structures at low positions. The absorption can hereby be made dependent on the frequency content of the incident sound field. This frequency content generally varies as a function of the height relative to the ground. The absorption can in this way be improved further still.

[0021] The dimensions of the cavity structures (lengths, cross-section) are preferably chosen such that the absorption is particularly high within a predetermined frequency spectrum (for instance the shared spectrum associated with the dominant traffic noise sources). When the porosity (P_L) is defined as the overall cross-section of cavity structures of a determined length (L) (i.e. the summation of all surface areas of cavity structures (for instance pipes) of the same length, wherein the surface areas are defined in cross-section at the position of the respective mouth of the cavity structures) divided by the overall surface area of the relevant part of the sound-proofing utility (for instance the noise-impacted side of the sound-proofing utility) and expressed as a percentage, it has been found that good results are achieved if this porosity (P_L) amounts to between 0.5% and 5%, preferably between 0.5% and 2% and still more preferably about 1.4%.

[0022] The overall porosity can be defined as the overall cross-section of cavity structures of all different lengths (i.e. the summation of all surface areas of all cavity structures (for instance pipes) in the relevant part (for instance the noise-impacted side) of the sound-proofing utility, wherein the surface areas are defined in cross-section at the position of the respective mouths of the cavity structures) divided by the overall surface area of the relevant part of the sound-proofing utility and expressed as a percentage. This overall porosity must generally be as great as possible, depending on the number of cavity structures of different lengths which is arranged in the relevant part of the sound-proofing utility. Theoretically, the number of different lengths of the cavity structures can be no more than 1/ P_L (for instance 1/0.014 = 71). In this case the part of the sound-proofing utility would be provided with cavity structures over the whole surface area, which is of course not possible in practice. Structural standards, such as the minimum mutual distance between cavity structures which is necessary in order to maintain a strong construction, must be taken into consideration.

[0023] Besides sound-proofing utilitys with a single sound-absorbing side, sound-proofing utilitys with two or more sound-absorbing sides are also possible. In determined embodiments the sound-proofing utility comprises in the position of use for instance a first upright sound-absorbing side directed toward the travel surface, and a second upright sound-absorbing side remote from the travel surface. In further embodiments the upward directed side of the plate is additionally or alternatively provided with a number of cavities. These cavities can be formed by the mould cavities stated herein, so that additional sound absorption takes place. In other embodiments the cavities however form a diffractor. This diffractor is configured to diffract the sound caused by the traffic upward. The diffractor can comprise a number of parallel slots of different depths arranged in the plate material, as for instance described in WO 2015005774 A1, the content of which must be deemed as incorporated herein as a whole. Each of the slots has acoustically substantially non-absorbing walls and is free of acoustically absorbing material. In a situation where they are arranged along the travel surface, the recesses are arranged as seen from the travel surface in a number of successive parallel rows of resonators, wherein the depth of the recesses decreases per row in a direction away from the travel surface. Because adjoining parallel grooves have a depth decreasing in each case from the noise-impacted side of the screen in the direction of the opposite side of the screen, it is found possible to realize a particularly good diffraction of the sound.

[0024] The upper side of the sound-proofing utility can further have an oblique orientation relative to the sound-absorbing side(s) such that it is directed toward the travel surface in a situation where it is arranged along the travel surface. The sound coming from a sound source on the travel surface can in these embodiments be directly incident on the upper side of the screen and thus on the diffractor, so that a good diffraction results.

[0025] As is usual, the sound-proofing utility can be arranged parallel to the travel surface. It is however also possible to divide the sound-proofing utility into a number of different screen parts (each comprising one or more of said plates) and to dispose each of these screen parts obliquely relative to the travel surface. The screen parts are freestanding and thus not coupled to each other (although a screen part can per se consist of a number of mutually coupled plates). In embodiments of the invention the sound-proofing utility therefore comprises a number of plates disposed in a row along the travel surface, wherein each plate extends obliquely relative to the longitudinal axis of the travel surface. It is possible to dispose the screen parts (plates) such that it is possible to see through the intermediate spaces between the screen parts. The screen parts are then as it were oriented with the direction of travel of the vehicle. The angle (α) between the plates and the longitudinal axis or axis of the travel surface preferably lies in an angular range of 5 to 60 degrees, preferably an angle between 30 and 50 degrees, such as 45 degrees. The screen parts are preferably disposed such that a sound field incident on a front or rear side of a screen part is partially reflected via this screen part to respectively the rear and front side of an adjoining screen part. Every time a sound field is incident on a side of the screen which takes an absorbing form, part of the sound will moreover be absorbed. In determined embodiments both the front side and the rear side of the screen parts take an acoustically absorbing form, so that the reciprocally sound reflecting back and forth disappears as far as possible by absorption. This reflecting of sound between two adjoining screen parts can for instance be realized if said angle (α) lies in a determined angular range and said distance (b) lies in a determined distance range relative to the side of the travel surface.

[0026] It is further possible to supplement the sound-proofing utility according to one or more of the embodiments stated herein with an elongate diffractor (for instance constructed from a number of diffraction plates arranged mutually in line) arranged along the travel surface. The diffractor comprises at least one diffraction element to be disposed laterally beside the travel surface, wherein the diffraction element is provided with a pattern of cavities or recesses in the upper surface thereof for diffracting the traffic noise in a direction which differs from the lateral direction, wherein the cavities or recesses have acoustically substantially non-absorbing walls and are free of acoustically absorbing material, wherein the depth of the recesses decreases, preferably monotonically, per row as the distance relative to the travel surface increases. The porosity of a diffractor plate, being defined as the overall mouth surface area of the recesses divided by the overall upper surface area of the diffraction plate, amounts here to at least 10%, preferably more than 50% or even more than 70% to 80%. It has been found that a particularly effective diffraction of the sound field incident from the vehicle occurs at these porosity values and/or in the above stated structural embodiment of the diffractor. As a result of this diffraction the sound is diffracted upward in the relevant frequency range. This makes it possible to give the underside of the sound-proofing utilitys a lighter and/or less expensive form, to not provide it with cavity structures, or even to dispense with it completely. In the latter case it is possible to see under the noise-reducing screen, and the persons in the vehicle have a better view of the surrounding area. According to a determined embodiment, an assembly is provided of a support structure to be anchored in the ground and one or more of the above stated plates. The support structure is embodied such that it can dispose the plates at at least a predetermined minimum height above the ground. The support structure can be formed by a number of uprights which can be anchored in the ground on one side and can support the plates on the other.

[0027] In determined embodiments the sound-proofing utility is manufactured from concrete. This can be non-reinforced concrete, for instance in the case of relatively small plates, but in other embodiments use is made of reinforced concrete. The concrete plate is provided in these embodiments with an internal reinforcement, for instance of steel. The reinforcement can for instance comprise a number of parallel reinforcing bars or a reinforcing mesh. In an embodiment of the invention at least some of the cavity structures, which extend in the plate over different lengths (l_l-l_n) from the mouth in the acoustically hard outer surface of the plate, continue beyond the position of the reinforcement. The length (l) of these cavity structures is therefore greater than the distance (a) between said outer surface and the reinforcement. This has the advantage that the reinforced plate can still remain relatively thin, for instance only slightly thicker than the length of the longest cavity structure.

[0028] The sound generated by the traffic by the different sound sources (wheels, tyres, engine and so on) has different characteristic frequency ranges. For car or goods traffic the absorption will have to have a high value mainly in frequencies between 125 Hz and 2000 Hz, while for train traffic the absorption has to be maximal mainly between 125 Hz and 4000 Hz. The porosity, diameter and depth of the cavity structures are chosen here so that they absorb sound particularly in the relevant frequency range, for instance between about 400 Hz - 2000 Hz. In a preferred embodiment of the invention the porosity, diameter and depth of the cavity structures are chosen such that the absorption coefficient of the plate is optimized in a smaller frequency range, for instance between about 550 Hz - 1715 Hz. Optimizing the absorption coefficient between about 550 Hz - 1715 Hz has the advantage that, since the cavity structures resonate not only at ¼ λ frequency (wherein λ, is the wavelength) but also at the ¾ λ frequency, the ¾ λ frequency of the largest cavity structure roughly coincides with the ¼ λ frequency of the smallest cavity structure. High values for the absorption coefficient can thus also be obtained above the highest optimization frequency.

[0029] As described above, the outer side of the sound-proofing utility and the inner side of the cavity structures are manufactured from acoustically hard material. This is understood to mean material with an absorption coefficient of less than 0.15, preferably less than 0.10 and still more preferably less than 0.05 (at least in the related frequency range).

[0030] Further advantages, features and details of the present invention will be elucidated on the basis of the following description of several embodiments thereof. Reference is made in the description to the accompanying figures, in which:

Figure 1 shows a top view of a travel surface provided with a noise-reducing screen according to a first embodiment of the invention;

Figure 2 shows a top view of an alternative noise-reducing screen according to a second embodiment of the invention, wherein screen parts extend obliquely relative to the axis of the travel surface;

Figure 3 shows a side view of the travel surface with the sound-proofing utility according to the second embodiment;

Figure 4A shows a front view (left) and side view (right) of a (part of a) noise-reducing screen according to an embodiment of the invention;

Figure 4B shows a detail of the front view of figure 4A;

Figure 4C shows a detail of a cross-section through the sound-proofing utility of figures 4A and 4B;

Figure 4D shows a detail of a cross-section through a noise-reducing screen with double-sided absorption;

Figure 5 shows a number of possible forms of a cavity structure according to the invention;

Figure 6 shows a top view of the embodiment of figure 2 with a number of upright screen parts in combination with a lying diffractor placed along the travel surface;

Figure 7 shows a side view of a further embodiment, wherein a diffractor along the travel surface is combined with raised disposition of screen parts extending obliquely relative to the axis of the travel surface;

Figure 8 shows a cross-section through a further embodiment of a plate of a noise-reducing screen provided on the upper side with a diffractor;

Figure 9 shows a partially cut-away perspective view of a cavity structure which is manufactured with a pipe as lost formwork element;

Figures 10A and 10B show a schematic front view of two further embodiments of the invention;

Figure 11 shows a partially cut-away perspective view of a concrete noise-reducing screen provided with a reinforcement and a number of cavity structures according to an embodiment of the invention; and

Figure 12A shows a graph which represents the absorption coefficient as a function of the frequency of a determined embodiment of the sound-proofing utility and figure 12B shows a similar graph of another embodiment of the sound-proofing utility.

[0031] Figure 1 shows a top view of an example of a travel surface (particularly a traffic road 1) over which motorized vehicles (for instance passenger cars 2) travel. During travel, the vehicle produces several sources of (airborne) sound. The main sound sources are formed by the tyres (rolling noise) and the engine (engine noise). The engine noise dominates at low speeds, and at higher speeds the rolling noise of the tyres becomes much more significant. In other embodiments (not shown) the travel surface is a railway and the sound is caused by a train traveling on this railway. Rail noise is mainly caused by the rolling noise of the wheels of the train or, at very high speeds, by the aerodynamic sound, for instance the sound coming from the pantograph. The different sound sources are thus situated at different heights relative to the travel surface.

[0032] An elongate, upright sound-proofing utility, in particular a sound-screening unit such as a noise-reducing screen 6, is arranged along travel surface 1, for instance parallel to the imaginary longitudinal axis 20 of the travel surface (also referred to here as the axis of the travel surface), and at some distance (b) relative to the side thereof. The sound-proofing utility extends over a great length and is essentially continuous. The height of the upright noise-reducing screen can vary: a higher noise-reducing screen is generally applied at high noise loads than at low noise loads.

[0033] In the shown embodiment sound-proofing utility 6 comprises a number of mutually connecting concrete plates 7, 7', 7" arranged mutually in line. These plates are either fixed directly in the ground (o) or fixed therein indirectly via a foundation and/or support structure. The concrete plates take an absorbing form on the noise-impacted side, i.e. on the side directed toward the travel surface. The sound incident on the sound-proofing utility is therefore both partially reflected and partially absorbed.

[0034] Despite the fact that the screen is manufactured from acoustically hard material (in this case concrete), the noise-impacted side 3 of screen 6 has absorbing properties as a result of the presence of a large number of cavity structures. These cavity structures are themselves in principle not configured to absorb the sound, but together with the remaining reflecting surface of the screen form the sound-absorbing surface. The surface of the cavity structures and the remaining surface therefore co-act in absorbing the incident sound field. The cavity structures have walls of acoustically hard material (since they were formed in an acoustically hard material) and are further free of acoustically absorbing material. The remaining surface, i.e. the surface of the sound-proofing utility between the cavities, also takes an acoustically hard (and thus non-absorbing) form. In short, the cavity structures form resonators whereby, in combination with the remaining non-absorbing surface not situated in the cavities, sound around the associated resonance frequencies can be partially absorbed.

[0035] Figures 4A-4C show an example of such a plate of a noise-reducing screen according to the embodiment of the invention. The figures show that a large number of cavity structures 10 is arranged in the surface of the sound-proofing utility. The cavity structures have a substantially elongate form (figure 4C) with a substantially circular cross-section which is constant over its length (figures 4B and 4C). Such cavity structures together form a number of resonators for providing a desired absorption spectrum, wherein the absorption can be accounted for using a mass balance just in front of the sound-absorbing surface, the resonances of the medium situated in the cavity structures and the viscous and thermal properties of the medium. The absorption caused by a determined cavity structure depends among other things on the length (l) of the pipe forming the cavity structure. In order to be able to absorb the incident sound field over a relatively wide absorption spectrum, pipes of different lengths are applied, wherein each pipe of a determined length is suitable for absorbing a relatively narrow frequency range.

[0036] In a determined embodiment the absorbing side of the sound-proofing utility is divided into a large number of characteristic areas 5 (shown with a broken line in figure 4B). Areas 5 can each have the same surface area, although varying surface areas are also possible. The porosity for instance has to decrease for obliquely incident sound waves. Because in higher noise-reducing screens the angle of the incident sound waves is greater at higher positions (and is thus more obliquely incident), a lower porosity can be opted for at higher positions. A collection of cavity structures, each having a different length, is arranged in each area. In the embodiment shown in figure 4B 16 cavity structures are arranged in each area, although this number can be greater or smaller in other embodiments. Each of the cavity structures is thus suitable for absorption in its own associated frequency range. The cavity structures in a determined area 5 thus together provide for a relatively wide-band absorption. The pattern of cavity structures in area 5 can be repeated in the other areas of which the sound-absorbing surface of the sound-proofing utility is built up, and thus realize a wide-band absorption distributed evenly over the noise-impacted side of the sound-proofing utility.

[0037] If the number of pipes is for instance equal to 16, the radius of the (cylindrical) pipes is equal to 5.5 mm and the lengths (l_i with i=1-16) of the respective pipes are equal to 47, 50, 53, 56, 60, 64, 68, 73, 78, 85, 91, 99, 108, 119, 131 and 145 mm, the characteristic area for instance becomes a square area of about 85x85 mm². This square characteristic area can be repeated over the whole surface of the sound-proofing utility, or a part thereof. With this choice of lengths and radii of the pipes, the distance between the underlying pipes is about 1 cm. This means that when the sound-proofing utility is manufactured from for instance concrete, the walls between the different pipes are sufficiently thick to enable a structurally strong construction. Figure 12A shows a graph with the absorption coefficient of this embodiment as a function of the frequency. The graph clearly shows the short quarter-wavelength resonance peaks and the three-quarter-wavelength resonance peaks caused by each of the cavity structures of this embodiment. As follows from the graph, a relatively high absorption coefficient is realized over a relatively wide spectrum.

[0038] If the number of pipes is for instance equal to 25, the radius of the (cylindrical) pipes is equal to 7 mm and the lengths (l_i with i=1-25) of the respective pipes are equal to 45, 47, 49, 51, 53, 55, 58, 60, 63, 66, 69, 72, 76, 79, 83, 88, 92, 97, 103, 109, 115, 122, 129, 137 and 144 mm, the characteristic area for instance becomes a square area of about 120x120 mm² (porosity about 27%). This square characteristic area can be repeated over the whole surface of the sound-proofing utility, or a part thereof. With this choice of lengths and radii of the pipes, the distance between the underlying pipes is once again about 1 cm, so that a strong noise-reducing screen is obtained. Figure 12B shows a graph with the absorption coefficient of this embodiment as a function of the frequency. The graph clearly shows the short quarter-wavelength resonance peaks and the three-quarter-wavelength resonance peaks caused by each of the cavity structures of this embodiment. As follows from the graph, a relatively high absorption coefficient over a relatively wide frequency spectrum is in this embodiment also realized.

[0039] In a determined embodiment the absorption coefficient of the sound-proofing utility is optimized as a function of the frequency, i.e. the addition of individual absorptions of the cavity structures of the sound-proofing utility, in a frequency range between about 550 Hz and 1715 Hz. The optimization of the absorption within this frequency range has the advantage that, since the pipes resonate not only at a ¼ λ but also at ¾ λ, the ¾ λ frequency of the largest pipe roughly coincides with the ¼ wavelength frequency of the smallest pipe. The pipes thus act twice in the absorption of the incoming sound. This means that relatively high absorption values can be obtained, such as beyond the highest optimization frequency (i.e. above 1715 Hz).

[0040] The sound-proofing utility can be provided on one upright side with said cavity structures, as for instance shown in figure 4C. In other embodiments, one of which is shown in figure 4D, the sound-proofing utility can also be provided with cavity structures on two or more sides. This embodiment can otherwise be the same as that of figures 4A and 4B, for instance in that it is provided with a widened base. In determined embodiments the screen takes a double-sided form, i.e. is provided with the cavity structures on the two upright sides situated opposite each other, so that there is sound absorption on both sides. In a preferred embodiment the lengths of the cavity structures are adjusted to each other on both sides of the sound-proofing utility. Relatively long cavity structures in a first side of the screen can be positioned opposite relatively short cavity structures in a second, opposite side of the screen and vice versa. This is possible because the distribution of lengths is in principle the same on both sides. A particularly light construction which absorbs on both upright sides can in this way be realized. This construction further requires only a simple foundation because the wind load decreases.

[0041] Figure 4A shows that the sound-proofing utility is provided on the underside with a widened base 20. If the sound-proofing utility is for instance manufactured from concrete, this base 20 can be co-moulded at the same time as the manufacturing process. The base and the rest of the sound-proofing utility in both cases form a monolithic whole. The plate takes a self-supporting form in these embodiments, so that it can remain in place on the ground (o) in stable manner and without further technical support means. This enables a rapid and simple placing of the sound-proofing utility, which has a positive effect on the total cost of realizing the sound-proofing utility. It is further for instance possible to make sound-absorbing partition walls (preferably provided with double-sided absorption by means of cavity structures on both noise-impacted sides, as shown in figure 4D) between two traffic lanes, which walls are lighter and require less material than the existing, solid concrete partition walls and moreover absorb an additional portion of the sound.

[0042] In other embodiments (not shown in the figures) the plates are fastened to a separate support structure. The support structure can for instance consist of a number of support posts arranged at regular mutual distances in the ground. The rear side of the plates is arranged against these support posts and coupled thereto so that a stable whole is created. In further embodiments the sound-proofing utility consists of plates with limited dimensions which can be arranged against an already existing noise-reducing screen. An existing noise-reducing screen, for instance of the solely reflecting type, is in this way converted into a noise-reducing screen of the absorbing type.

[0043] Figure 9 shows a further embodiment of the invention, wherein the cavity structure is formed by a plastic pipe 21 which has remained behind in the material of plate 7 as lost formwork. In the case of a noise-reducing screen consisting of concrete plates, these concrete plates being manufactured by pouring liquid concrete into a mould, it is possible to provide the mould with a large number of such plastic pipes 21. The plastic pipes are of suitable length (corresponding to the length of the desired cavity structure) and are arranged at suitable positions, so that a plate with the desired absorbing properties can be realized in one operation after filling of the mould with liquid concrete and curing thereof. The plastic pipes can be removed from the plate after the manufacturing process, but they preferably remain behind in the material. If the plastic pipes are sufficiently acoustically hard, the formed cavities can also function as sound-absorbing cavity structures when the pipes have remained behind. In embodiments in which it is desirable to remove the pipes, they preferably have a releasing form, for instance the form of a truncated cone 23 (figure 5), such that the pipes can still be pulled from the material of the screen afterward.

[0044] The cavity structures can have a cross-section constant over the length thereof, but in other embodiments the cross-section increases as the distance from the mouth to the outer end of the cavity structure increases. These forms are releasing and are thus often used if the pipes have to be pulled from the plate material again at the end of manufacturing. The cavity structures can further have diverse forms in cross-section, including a substantially circular 24, oval 25, rectangular 26, 28 or triangular 27 cross-section, as shown in figure 5.

[0045] Figure 10A shows a view of a noise-reducing screen wherein the average cross-section of cavity structures 28 at relatively high positions relative to the ground (o) is smaller than the average cross-section of cavity structures 29 at the low positions. The angle of incidence is generally greater for higher positions. This means that the porosity has to be lower. A lower porosity can for instance be realized by a smaller diameter of the cavities while the distance between the cavities remains the same, the same diameter while the distance between the cavities becomes greater and/or diverse tubes of a greater number of lengths. What is often recommended in respect of simplicity is that the distance between cavities remains the same and cavities have a smaller diameter, as shown in figure 10. A still further improved general sound absorption can be obtained in this manner. Figure 10B shows a similar noise-reducing screen as figure 10A. In this embodiment the rows of cavity structures are alternately offset relative to each other (over about half the intermediate distance between adjoining cavity structures). This makes it possible to achieve more cavity structures and thus a higher porosity while the structural standards remain the same.

[0046] Figure 2 shows an embodiment wherein the plates of the sound-proofing utility are not placed substantially parallel along the travel surface (such as the situations in figure 1), but obliquely relative to the longitudinal axis 20 of the travel surface. In the shown embodiment a number of plates 7, 7', 7" are arranged at some mutual distance (M) relative to each other. In other embodiments groups of two or more sound-absorbing plates placed one behind the other are disposed obliquely relative to the longitudinal axis 20. Plates 7-7" are placed one behind the other such that a row of sound-absorbing plates results. The angle (α) between longitudinal axis 20 of the travel surface and the respective plates can vary, for instance between 30 and 50 degrees. In the shown embodiment the angle is equal to about 45 degrees.

[0047] Further shown in figure 2 is how sound coming from car 2, for instance engine and/or tyre noise, is transmitted in the direction P₁ to the rear side of a plate 7' of the sound-proofing utility. The sound reflects on the sound-absorbing rear side 9 of plate 7' and is sent in the direction P₂ of a further absorbing plate 7". The incident sound field is at least partially absorbed by the absorbing side 8 of this plate 7". The rest of the sound is reflected and disappears in further direction P₃. The absorption quality of the second absorbing plate 7" and the loss as a result of the reflection against first plate 7' ultimately determines how much sound disappears in direction P₃. An advantage of this embodiment is that the driver of the passing sound source can look through the sound-proofing utility and maintains a view of his/her surroundings. By providing the screen on both upright sides with the cavity structures defined herein, the overall absorption of the sound-proofing utility can be increased relative to embodiments in which only one upright side of the sound-proofing utility is provided with said cavity structures. A further advantage of the oblique placing of the plates of the sound-proofing utility is therefore that use can be made of both the front side and the rear side of the plates, which can increase the sound absorption and thereby the sound-screening effect of the whole noise-reducing screen.

[0048] As already stated above, in a further embodiment (not shown) the opposite (rear) side 9 of each of the plates 7-7" is not provided with an absorbing side (due to the presence of cavity structures). In this embodiment the sound can also be absorbed by the plates, although this happens only on one single side of the plate in question.

[0049] Figure 6 shows yet another embodiment of the invention. This embodiment is based on the embodiment shown in figure 2, i.e. the embodiment wherein the sound-proofing utility consists of a number of parts arranged obliquely relative to the longitudinal axis of the travel surface. It is however also possible to apply the embodiment of figure 6 to the embodiment shown in figure 1, i.e. the embodiment wherein the sound-proofing utility consists of a long row of parts placed one behind the other. Figure 6 shows that an elongate strip 35 of diffractor plates 36, 36' placed one behind the other is arranged on the ground (o) in the roadside shoulder between the sound-proofing utility and the travel surface. Diffractor plates 36 are arranged in the ground such that the upper side of diffractor plates 36, 36' lie at roughly the same height as the upper side of the ground. Diffractor plates 36 consist of a number of parallel slots of different depth arranged adjacently of each other. The slots form resonators with resonance frequencies in the range of the frequencies of the sound to be diffracted, particularly frequencies around about 1 kHz. The slots are embodied as cavities with walls which are substantially non-absorbing and are further free of any acoustically absorbing material whatsoever. The plates ensure that the sound coming from the sound source (for instance car 2) is diffracted in the direction which differs from the lateral direction. In other words, the sound propagating along the upper side of the diffractor plate is diffracted upward. It is otherwise not the case that it is only possible to dispose a row of diffractor plates adjacently of the sound-proofing utility. In further embodiments (optional) additional diffractor plates (shown with broken lines in figure 6) are arranged adjacently of the row of diffractor plates 36, 36' in order to diffract the sound propagating through the openings between the noise-reducing screen parts upward.

[0050] Shown in the embodiment of figure 7 is how the sound can be diffracted upward. This embodiment largely corresponds to that of figure 6, with the difference that plates 37 are placed at a distance (h) above the ground (o), for instance by arranging them on a separate support structure (legs). The sound coming from the car is transmitted to the resonators in the diffractor (direction P₄). Depending on the wavelength of the sound, this sound is diffracted upward (direction P₅) by an associated slot-like resonator 37. The sound thus reaches the lower section of plate 37 and is there absorbed by the mould cavities. No or only very little sound will thus be incident in an area from the ground up to the minimum height H. The sound-proofing utility thus need not be provided on the underside with a sound-absorbing layer or, as in the embodiment shown in figure 6, the sound-proofing utility can be wholly dispensed with on the underside. The overall construction of the sound-proofing utility hereby becomes lighter, and traffic on the travel surface has a view of its surroundings via the underside of the sound-proofing utility. For further details of the diffractor and diffractor plates stated herein reference is made to the international patent application WO 2015005774 A1 of applicant, the content of which must be deemed as incorporated herein as a whole.

[0051] Figure 8 shows a further embodiment of the invention, wherein the upper side of plate 17, which is provided at least on front side 18 with absorbing cavity structures (but is in some embodiments also provided with such cavity structures on the rear side), likewise has special provisions on upper surface 30. The upper surface of the sound-proofing utility extends obliquely relative to upright side 8 and thereby relative to the ground during use. The angle of inclination (β), as shown in figure 8, is chosen here such that the sound transported from the sound source on the travel surface to the upper side of the screen can be diffracted by a number of diffractors 31 provided in upper surface 30. The diffractors can for instance be built up of a number of elongate and parallel resonator slots 32. The angle β will generally have to be greater when the distance between the travel surface and the sound-proofing utility is small than when this distance is great. For further details of such diffractors reference is made to the above stated patent publication WO 2015005774 A1.

[0052] Diffractors 31 on the upper side of the sound-proofing utility consist of slots 32 extending in longitudinal direction of the sound-proofing utility. Just as the slots in the above stated diffractor plates 36, 36', these slots 32 are manufactured from acoustically hard material and moreover take a substantially empty form, or at least no acoustically absorbing material is arranged therein. The depth (length) of diffractor slot 32 decreases in each case from the visible side (side 8) of the sound-proofing utility in the direction of the rear side. The mouth of each of the diffractor slots 32 is in each case situated at a greater height than the mouth of the previous diffractor slot. The depths of the reactor slots preferably decrease monotonically, although some variation in depth can occur in other embodiments. The sound transported along the upper side of the sound-proofing utility is diffracted upward as a result of the presence of diffractor 31, so that the sound-screening effect of the sound-proofing utility is increased still further.

[0053] Figure 11 shows a further embodiment of the invention, wherein the sound-proofing utility is manufactured from reinforced concrete. This noise-reducing screen thus comprises a per se known reinforcement 44, this reinforcement for instance consisting of a metal latticework. The reinforcement is for instance arranged in the centre of the plate, but can also be arranged closer to the front side or the rear side in other embodiments. Cavity structures 45 are in this embodiment arranged such that at least a part of the cavity structures can extend beyond the position of reinforcement 44 (as is indicated with broken lines 46). Either no cavity structures are therefore situated at the position of the reinforcement, or only a group of cavity structures with a relatively short length. This makes it possible on the one hand to reinforce the plate, but on the other to use almost the whole thickness of the plate to provide cavity structures therein. This otherwise applies not only to embodiments wherein the cavity structures extend on only one side (visible side) of the sound-proofing utility, but also to the above stated embodiments wherein the cavity structures are provided on both sides (and the screen is thus absorbent on both sides).

[0054] The present invention is not limited to the embodiments thereof described herein. The scope of protection is defined by the appended claims, within the scope of which numerous modifications can be envisaged.

Claims

1. Sound-proofing utility, in particular a sound-screening unit, configured to limit, at least for a determined frequency range, the lateral emission of airborne sound caused by motorized road traffic, the sound-proofing utility comprising a plate with an acoustically hard outer surface, wherein the plate comprises at least one sound-absorbing side, wherein the sound-absorbing side has a plurality of elongate cavity structures arranged in the plate and debouching at the hard outer surface, and with resonance frequencies in the determined frequency range, for at least partially absorbing the sound incident on the sound-absorbing side, characterized in that the plate takes a monolithic form, that the inner surface of each of the cavity structures is manufactured from acoustically hard material and that the cavity structures are free of acoustically absorbing material and are grouped into different groups distributed over the side of the plate, wherein the cavity structures have mutually varying lengths within each group and wherein the porosity (P_L) of a part of the plate, defined as the summation of all surface areas of cavity structures of the same length divided by the overall surface area of the relevant part of the plate, amounts to between 0.5% and 5%.

2. Sound-proofing utility as claimed in claim 1, wherein the porosity (P_L) of the plate amounts to between 0.5% and 2%, still more preferably about 1.4%.

3. Sound-proofing utility as claimed in claim 1 or 2, wherein the cavity structures are formed by elongate tubular cavities with a substantially releasing form, and preferably have a conical form, wherein the tubular cavities preferably extend perpendicularly of the absorbing side.

4. Noise-reducing screen as claimed in any of the foregoing claims, wherein the cavity structures are provided on both opposite and upright sides of the plate, wherein the lengths of the cavity structures on both sides of the sound-proofing utility are preferably adjusted to each other and/or wherein relatively long cavity structures in a first side of the screen are positioned opposite relatively short cavity structures in a second, opposite side of the screen and vice versa.

5. Sound-proofing utility as claimed in any of the foregoing claims, comprising a first upright sound-absorbing side and a second, opposite upright sound-absorbing side and/or comprising a number of plates configured to be mounted on a support structure, for instance an existing noise-reducing screen, anchored in the ground.

6. Sound-proofing utility as claimed in any of the foregoing claims, wherein the distribution of the cavity structures varies at least partially over the height of an upright sound-absorbing side.

7. Sound-proofing utility as claimed in any of the foregoing claims, wherein the sound-proofing utility is a noise-reducing screen which can be anchored in the ground, wherein the noise-impacted side of the noise-reducing screen has a lower area and an upper area relative to the ground and wherein the porosity of the cavity structures in the upper area is lower than the porosity of the cavity structures in the lower area.

8. Sound-proofing utility as claimed in any of the foregoing claims, wherein the average cross-section of the cavity structures at high positions relative to the ground is substantially smaller than the average cross-section of the cavity structures at low positions.

9. Sound-proofing utility as claimed in any of the foregoing claims, wherein
the side of the plate directed upward relative to the ground is provided with a number of cavities configured to diffract and/or absorb the sound caused by the traffic, wherein the cavities are diffractors and the cavities are formed by one or more parallel elongate recesses in the upper surface of the plate, wherein each of the recesses has acoustically substantially non-absorbing walls and is free of acoustically absorbing material and/or wherein, in a situation where they are arranged along the travel surface, the recesses are arranged as seen from the travel surface in a number of successive parallel rows of resonators, wherein the depth of the recesses decreases per row in a direction away from the travel surface.

10. Sound-proofing utility as claimed in any of the foregoing claims, wherein
the upper side of the plate has an oblique orientation relative to the sound-absorbing side(s) such that it is directed toward the travel surface in a situation where it is arranged along the travel surface and/or wherein the screen parts are disposed in mutually freestanding manner.

11. Sound-proofing utility as claimed in any of the foregoing claims, comprising a number of screen parts disposed in a row along a travel surface and consisting of one or more plates placed one behind the other, wherein each screen part extends obliquely relative to the longitudinal axis of the travel surface, wherein the screen parts preferably extend at an angle (α) relative to the longitudinal axis, wherein the angle (α) lies in an angular range of 5 to 60 degrees, preferably an angle between 30 and 50 degrees.

12. Assembly of a sound-proofing utility and a diffractor arranged or to be arranged along a travel surface at a position between the travel surface and the sound-proofing utility, the diffractor comprising at least one diffraction element to be disposed laterally beside the travel surface, wherein the diffraction element is provided with a pattern of recesses in the upper surface thereof for diffracting the traffic noise in a direction which differs from the lateral direction, wherein the recesses have acoustically substantially non-absorbing walls and are free of acoustically absorbing material, wherein the depth of the recesses decreases, preferably monotonically, per row as the distance relative to the travel surface increases.

13. Assembly as claimed in claim 12, comprising a support structure to be anchored in the ground and configured to dispose the one or more plates at at least a predetermined minimum height above the ground, wherein the space between the underside of the plate and the ground is preferably substantially transparent and/or wherein the plate is manufactured from concrete, preferably reinforced concrete, wherein the cavity structures preferably extend in the plate over different lengths (l₁-l_n) from the mouth in the acoustically hard outer surface of the plate, wherein at least one of these lengths is greater than the distance (a) between said outer surface and the reinforcement.

14. Sound-proofing utility as claimed in any of the foregoing claims, wherein the porosity, diameter and length of the cavity structures are embodied to absorb sound in the frequency range of about 400 Hz - 2000 Hz and/or wherein the porosity, diameter and length of the cavity structures are embodied to optimize the absorption coefficient of the plate in a frequency range between about 550 Hz - 1715 Hz and/or wherein the acoustically hard outer surface has an absorption coefficient of less than 0.15, preferably less than 0.10 and still more preferably less than 0.05.

15. Travel surface provided with at least one sound-proofing utility as claimed in any of the foregoing claims, wherein the travel surface is for instance a railway or a motorway.

Drawing