TECHNICAL FIELD

The present invention relates to an acoustic screening device for damping disturbing noise, especially such noise which arises from road traffic, rail traffic and the like. The invention especially relates to an acoustic screening device configured as a module consisting of a framework, preferably of concrete, and a noise-absorbing cassette arranged thereon. A plurality of modules can be placed in a row one after another with a view to creating an integrated acoustic screening device or barrier.

BACKGROUND ART

Traffic noise from roads and railways has become an ever increasing problem in present-day society, and the environmental effect, especially for local residents, is often obvious and very disturbing. Demands for effective measures are therefore made by members of the public and the authorities. Disturbing, high noise levels need to be effectively damped and various techniques are already currently being used. For example, excavated materials are used to build noise-damping embankments, and wooden or concrete constructions are used to a great extent to screen disturbing noise along roads and railways. However, these measures provide a limited noise protection. Noise measurements and calculations have shown that an acoustic-screening device is more effective if this is provided with noise-absorbing material against that part of the screen which faces towards the noise source. Effective noise-absorbing materials of this kind are constituted by mineral wool or the like, alternatively by foamed plastic with communicating pores, but these materials are less suitable for use outdoors and in dirty environments such as along roads and railways. The constructions, apart from damping disturbing noise, must also be able, of course, to withstand weathering, be cleaned and also function in different climates.

Technology for damping disturbing noise from roads and railways is thus previously known and, in some cases, also patent-protected.

SE513102 describes, for example, a method for producing noise-absorbing and/or vibration-damping units made of rubber waste from tyres, for example, which have been cut up and mixed with bonding agent. The units can be used as noise screens alongside roads or railways, holder elements/lifting elements being embedded in the material before it solidifies. These units do not solve all the problems associated with arranging stable and effective acoustic screening devices along roads and railways at reasonable cost.

WO2005033412 describes an acoustic screening device consisting of a concrete framework on one side of which a mesh is cast into the concrete. The mesh is integrated with a noise-damping material. The drawback with this construction is, inter alia, that the noise absorber cannot be exchanged when damaged. Moreover, the damping capacity of this acoustic screening device is limited by the structural configuration of the absorber. The width of the concrete profile also makes it difficult for the acoustic screening device to be able to be placed sufficiently close to the noise source.

FR2683368 describes an acoustic screening device for damping disturbing noise, for example noise arising from traffic on roads, railways and the like, comprising a framework, preferably of concrete, and a noise absorber arranged thereon. The noise absorber is stated to be arranged exchangeably on the framework. Spacer elements are arranged behind the noise absorber in such a way that a certain air gap is formed between the noise absorber and the framework. The air gap can be between 10 and 50 mm, but is typically 20 mm. The noise absorber is fitted against the concrete element on a substantially flat surface. The front/outer surface of the noise absorber is arranged substantially flat against the front limit surface of the concrete element. The noise absorber is stated essentially to be made of wood particles agglomerated with a bonding agent such as cement or a mixture of mineral bonding agent including cement, silicate or similar known substances and consists of a front corrugated part and a rear flat part. The noise absorber, by virtue of its configuration with corrugated front side, is intended to absorb noise, but is not expressly intended to let noise or air through to the air gap behind the plate. As a result of the properties of the incorporated material, it is not possible to control or regulate the flow resistance through the plate. Any flow of air through the plate is also, moreover, barely possible, which hence will not act as a noise absorber. The fact that the plate contains wood fibres means that it is scarcely weather- resistant and hence less suitable for outdoor use. Furthermore, the wood fibre material is relatively heavy to handle, since it is so solid and, inter alia, contains cement. The noise absorber is also stated to be mechanically fragile and is fitted with the aid of screws through the noise-absorbing material. The device does not integrate any air gap former in a cassette, nor does it form in the air gap a number of air cells which are substantially closed.

The prior art within this field does not therefore solve the problem of damping disturbing traffic noise in a flexible and effective manner and at reasonable COSt.

DISCLOSURE OF INVENTION

One object of the invention is to solve the abovementioned problem and to provide a device, an acoustic screening device, of the type stated in the introduction, having a small number of component parts and which can be configured as a simple, compact and relatively cheap unit which effectively damps disturbing traffic noise from roads and railways.

Another object of the invention is to provide an acoustic screening device which can be placed in an outdoor environment, which withstands different climates and which can be easily cleaned of dirt.

A further object of the invention is to provide an acoustic screening device which can be placed close to the source of the disturbing noise, but can nevertheless allow normal maintenance of the road or railway.

Another object of the invention is that the noise absorber shall be easily exchangeable if it is damaged or if it is wished to alter the noise-damping characteristic on a certain section, or if it needs to be replaced for other reasons.

Yet another object of the invention is that the construction shall be cheap to produce, install and maintain.

These and further objects and advantages are achieved according to the invention by a device according to the distinguishing features stated in the characterizing part of the first patent claim.

The invention thus relates to an acoustic screening device for reducing disturbing noise, especially noise arising from road traffic, rail traffic and the like, comprising a framework, preferably of concrete, and a noise absorber arranged thereon. The invention is characterized in that an air gap former is arranged integrated in the cassette behind the noise-absorbing mat in such a way that an air gap is formed between the noise-absorbing mat and the framework, in that the air gap former is arranged to form a number of cells or compartments between the noise-absorbing mat and the framework, the cells or compartments being arranged to form substantially closed air volumes, and in that the noise-absorbing mat is arranged to be, at least to some degree, noise and air permeable.

Further distinguishing features and advantages of the invention emerge from the following, more detailed description of the invention, as well as from the appended drawings and other patent claims.

BRIEF DESCRIPTION OF DRAWINGS

The invention is described in greater detail below in some preferred illustrative embodiments on the basis of the appended drawings.

Figure 1 shows a section through an inventive module and a noise absorber fitted on the front side of the module.

Figure 2a shows in greater detail an inventive noise-absorbing cassette, viewed from the front. Figure 2b shows in greater detail the gap- forming mesh of the cassette.

Figure 3 shows a section through the cassette according to Figure 2a.

Figure 4 shows in greater detail a part of the cassette shown in Figures 2 and 3, from the rear.

Figure 5 shows a detail of the air gap former and its wall construction with curved, compartment- forming surfaces, which makes the noise absorber an absorber of the "locally reacting type".

Figure 6 shows the inventive module according to Figure 1 placed on a bed of gravel/macadam alongside a railway track.

Figure 7 shows a chart of the noise absorption factor as a function of frequency, the so-called noise absorption characteristic.

Figure 8 shows how a number of inventive modules have been fitted together to form noise barriers.

Figure 9 shows the noise barriers in greater detail set out by the side of a railway track.

DESCRIPTION OF PREFERRED EMBODIMENTS

Figure 1 shows in profile an inventive acoustic screening device 1 , in the form of a module, for screening and absorbing disturbing noise, for example from road traffic, rail traffic or the like. The invention especially relates to an acoustic screening device 1 consisting of a framework 2 made, preferably of fibre-reinforced concrete, and a noise absorber 3, mounted thereon, in the form of a cassette. The module is designed such that a plurality of units can be placed in a row one after another to create an integrated acoustic screening device. The fibre reinforcement preferably consists of synthetic plastic fibres in order to avoid the presence of electrically conductive material in the framework 2, whereby measures for electric earthing of the modules are rendered redundant so that they can be omitted. A future maintenance factor is thus eliminated, since earth connections would otherwise need to be checked and maintained at regular intervals.

The cassette 3, too, consists of purely non-electrically conductive material. The cassette 3 is fitted, with the aid of, for example, insulating gun nails, to the front side of the framework 2, i.e. that side of the module which is intended to be directed towards the disturbing noise source.

Viewed in profile, the acoustic screening device 1 is configured with a base 4, a middle section 5 and a top part 6. The base 4, or the ground support, constitutes a compact and heavy part and is intended to hold its acoustic screening device 1 in position despite, for example, powerful wind blasts from passing trains or vehicles. The forces in the wind blasts can be approximately as large out from the track, which is caused by the pressure created in front of the train, as in towards the track, by suction forces created once the train has passed. The module 1 is therefore heavy and very stable so as not to be dislodged from its position when a train passes. Should the acoustic screening device 1 move, this can create serious risk of accident. Trains must be able to pass at speeds of up to 250 km/h and the acoustic screening device 1 must also be able to endure various weather phenomena and be stable in all conceivable weather situations, for example withstand hurricane winds. In order to achieve maximum stability, the centre of gravity of the framework 2 should lie as close to its geometric centre as possible.

The middle section 5 is located on or close to the front edge 7 of the base 4 in such a way that the cassette 3 can be arranged as close to the noise source as possible. The cassette 3 is preferably fitted with the aid of insulating bolts, but can also, of course, be fitted differently. By virtue of the fact that the front side of the acoustic screening device 1 is substantially flat and does not have any projecting parts, the module is more easily put in place than previously known acoustic screening devices. The front side of the noise-absorbing cassette 3 is thus arranged substantially in line with the front edge 7 of the base 4 and with the front edge 8 of the top part 6. This means that the module requires less space on the ground, which is advantageous when placed on a railway embankment.

The configuration of the framework 2 with a base 4 which projects forward (to the left in the figure) to a lesser degree means that the foundation for the acoustic screening device 1 in, for example, a railway embankment can be placed slightly farther out from the railway track than was previously possible. This enables, inter alia, the maintenance work which often needs to be carried out on the railway embankment, inter alia with the aid of a so-called macadam plough, to be done without affecting the acoustic screening device 1 or its foundation. It is thus possible to use the module more effectively for damping of disturbing noise from rail traffic. The front surface of the acoustic screening device 1 is substantially flat and vertical by virtue of the fact that the front edge 7 of the framework 2 lies substantially level with the front surface of the cassette 3 and the front edge 8 of the top part 6.

In the upper surface of the base 4, behind the middle section 5, an attachment fitting 9 for a lifting yoke or the like can be placed in a recess provided for this, for example a sleeve rod anchor.

Acoustic screening devices 1 can be set out, for example, with the aid of two slings hooked into eyelets screwed onto the modules, which eyelets are removed from the module following assembly. No special lifting device is therefore required for handling of the modules, which means that the full capacity of the lifting gear is not reduced by the weight, for example, of a lifting yoke.

Figure 2a shows from the front, and in greater detail, the noise-absorbing cassette 3. The cassette 3 consists of an outer frame 11 of, for example, plastic.

On the front side of the cassette 3, i.e. that side which, following fitting against the framework 2 of the acoustic screening device 1, is facing towards the noise source, a grille or mesh 12 (see Figure 2b), also preferably made of plastics material, is arranged. In or against the mesh 12 there is suitably arranged a noise-absorbing material, for example a rubber material. The noise-absorbing material can, for example, be integrated with the mesh 12, or cast into the mesh 12, or alternatively glued or otherwise applied to the mesh 12 with a view to stiffening the mesh 12, which allows even a thin mat 13 to be used. If the mat 13 is sufficiently rigid, it can be placed loose behind the mesh 12. The cassette 3 is fitted on the front side of the middle section 5 of the framework and is protected at the top against rain, for example, by a protruding portion on the top part 6 of the framework 2 (see Figure 1). Figure 3 shows a section through an inventive cassette 3 along the line A-A shown in Figure 2. In the frame 11, the mesh 12, the noise-absorbing material in the form of a mat 13 and an air gap former 14 are arranged.

The noise-absorbing material in the mat 13 consists at least partially of a fine granulate of, for example, reused rubber, which is held together by a bonding agent intended for this purpose. Such a bonding agent can be a dual- component polyurethane glue, which makes the mat 13 flexible and resilient even after hardening. The mat 13 is at least partially air permeable by virtue of the fact that it contains small communicating pores or channels, so that a flow resistance arises in the mat 13. The material in the mat 13 is also durable and withstands, for example, cleaning with water without the noise-absorbing effect being affected or impaired. The granulate size and degree of compaction affects the flow resistance and can be adapted for different applications. In the case of damping of traffic noise from railways, granulate of about 3 mm size is used and the mat 13 has a density of about 650 kg/m ³ . With a mat thickness of about 10 mm, its weight is about 6.5 kg/m .

If the width of the frame 11 is dimensioned to about 40 mm, an air gap 15 of about 30 mm width is formed behind the mat 13, i.e. the distance between the mat 13 and the concrete framework 2 of the module is about 30 mm. The width of the air gap 15 is here achieved by an air gap former 14, for example made of a plastics material, which forms cells or compartments 16 and which makes the absorber acquire the character of a "locally reacting absorber". The width of the air gap 15 and the size of the cells 16 is determinant for the range of frequency at which the noise will most effectively be absorbed. The width of the air gap 15 can be altered and adapted by the width of the frame 11 and of the air gap former 17 being changed, or by the mat 13 being otherwise arranged at a certain distance from the middle section 5 of concrete framework 2. Also determinant for the total noise absorption effect is the air flow resistance of the noise-absorbing mat 13, in combination with the width of the air gap 15. The flow resistance in the noise-absorbing material in combination with the air gap forms an acoustically equivalent electric, so-called RC circuit.

The absorber or the cassette 3 thus has a better adjustable and adaptable noise absorption capacity than previously known systems, and, above all, it damps a frequency range which, more easily than previous systems, can be adapted to the character of the disturbing noise, so that maximum reduction, in terms of dB(A) units, is achieved. Should the noise absorber/the cassette 3 be damaged in any way, it is also easy to replace it with a new cassette 3. Previously known alternatives require, for example, whole concrete blocks to be exchanged.

Figure 4 shows the rear side of the mat 13 and the air gap former 14 placed thereon, which is configured with a number of substantially circular cells 16 or compartments. In the assembled state, the cells 16 thereby form a number of closed volumes between the mat 13 and the concrete framework 2. The cells 16 can also, of course, have a shape other than that shown. Each cell 16 receives some of the noise and spreading of the noise inside the air gap 15 is prevented with the aid of the wall construction of the air gap former 14, having curved surfaces which, by virtue of their thereby acquired rigidity, minimize the noise transmission between the cells. The noise is thus absorbed primarily in each cell 16 individually. This type of noise absorber is termed "absorber of the locally reacting type". This absorber type is characterized in that the frequency for the absorption maximum can be more easily adapted to the desired frequency range, at the same time as the absorption factor is higher at absorption maximum. Figure 5 shows in greater detail a part of the air gap former 14, here made of plastic, and its walls 17, which have curved surfaces with high rigidity and form closed cells 16, which air gap former, at the same time as it forms the gap, also makes the noise absorber 3 a so-called "locally reacting absorber". By this is meant that the noise which enters through the mat 13 at one place is prevented by the air gap former 14 from spreading sideways in the noise absorber 3. Reflected noise must now exit along the same path as it came into the noise absorber 3. In this way, the noise absorber 3 is "locally reacting". For a locally reacting absorber, it is easier to control the absorption maximum to the frequency range which leads to a maximum reduction in a number of dB(A) units. Even obliquely incoming noise will find its absorption peak at the frequency which is determined by the thickness of the mat 13 and the width of the air gap to the solid wall, here, for example, the concrete framework.

Figure 6 shows the inventive acoustic screening device 1 according to Figure 1 placed on a bed 18 of gravel/macadam alongside a railway track 19, placed on sleepers 20. The railway embankment 21 usually consists of coarse macadam. The coarseness in the material makes noise able to pass through and, moreover, cannot be smoothed. The acoustic screening device 1 is therefore placed preferably on a bed 18 of finer material, for example shingle, which can also be compacted and smoothed so that it does not let through noise to an undesirable extent. This finer material is kept separate from the macadam, for example, with the aid of a geotextile 22.

Figure 7 shows a chart of the noise absorption factor as a function of frequency, the so-called noise absorption characteristic for an inventive noise absorber/cassette 3 with a noise-absorbing mat 13 having a thickness of about 10 mm and having an air gap 15 between the mat 13 and the concrete framework 2 of about 30 mm. A noise absorption factor > 0.9 is here achieved for noise within a frequency range of 700-1050 Hz, which is especially well suited for optimal damping of noise from rail traffic. If the width of the air gap 15 is increased or reduced, a more effective noise damping is obtained within other frequency ranges which can be relevant to other types of disturbing noise, such as, for example, road traffic noise, which requires a somewhat lower frequency of noise absorption maximum compared with, for example, railway noise. The dashed curve in the chart shows what absorption factor is obtained if the absorber is of the so-called "non-locally reacting type" without air gap former, i.e. with only air in the gap and without local cells 16. From the continuous black curve, it can be seen that with an air gap former and absorption of a "locally reacting character ^" ", a more well formed noise absorption maximum is obtained, which, through the altered width of the air gap, can be easily adapted to the particular noise source type, for example road traffic or rail traffic.

Figure 8 shows how a number of inventive modules have been fitted together to form noise barriers placed along either side of a railway track. The top part 6 of the modules is provided with a substantially flat upper surface 10, which allows it to be trodden, for example by persons who need to be evacuated/cleared from a train which has come to a halt out on the line, for example, due to a minor accident, mechanical faults, fire or the like. The upper surface 10 of the top part 6 can also be provided with an anti-slip pattern, such as, for example, a chequered plate pattern, to facilitate treading and reduce the risk of slipping. An appropriate anti-slip protection of a suitable non- electrically conductive material can be fitted onto the upper surface 10 of the top part 6. The patterning can also have a certain effect on the propagation of the noise which escapes between the train and the acoustic screening device 1. The module can also be provided with steps 23 on the rear side of the middle section 5, which makes it easier for evacuating persons to come down to the railway embankment.

The modules can also be supplied complete with evacuation doors 24. The module is in this case provided with recesses/openings 25 through the middle section 5 and top part 6 of the framework 2. The openings 25 are closed off by doors 24, which are self-closing by, for example, spring action (not shown), and which are locked in the closed state to prevent them from being accidentally opened by, for example, the air pressure from a train and from thus reducing the noise-damping effect of the acoustic screening device 1. The locking can be released and the door 24 opened, for example, by a vertical pressure against the top edge of the door 24.

Figure 9 shows the noise barriers in greater detail and how doors 24 are arranged in the openings 25 of the modules to permit evacuation of staff from the track region.

Modules which are placed on the end sections of the acoustic screening device 1 endure significantly greater forces than other modules in the barrier and are preferably configured with a sloping upper surface (not shown). These modules, too, can be provided with cassettes/noise absorbers 3.

With the present invention, it is thus possible, with a small number of component parts, to configure an acoustic screening device 1 as a simple, compact and cheap unit which effectively damps disturbing noise from roads and railways. Moreover, an acoustic screening device 1 according to the invention, by virtue of its construction, withstands different climates and can easily be cleaned of dirt with, for example, water. The acoustic screening device 1 according to the invention, by virtue of the fact that the base 4, the ground support, is substantially located on the rear side of the framework 2, can also be placed close to the source of the disturbing noise, and can thus to a greater degree prevent noise from escaping between the bogie space formed by the bottom side of a rail coach and the side of the coach and the acoustic screening device, yet nevertheless allow normal maintenance of the railway tracks.

The fact that the acoustic screening device 1 is placed so close to the track means that, at "platform-close" distance, the screen forms a seal for the noise which escapes from the bottom edge of the body edge of the railway coach. By virtue of the fact that the bottom side of the coach, together with the body sides drawn down into the bottom edge, forms the so-called bogie space, a new type of damping is obtained, which is termed chamber damping by virtue of the fact that the ballast, too, provides certain noise absorption. Moreover, the acoustic screening device 1 provides, together with the body side, a so-called channel damping.

By virtue of the construction with a noise absorber in the form of a cassette 3, this can be exchanged, if damaged or worn out, in order to adapt the noise damping characteristic. The construction is, in its entirety, relatively cheap to produce, install and maintain.

The description above is primarily intended to facilitate understanding of the invention. The invention is not, of course, limited to the specified embodiments, but rather other variants of the invention are also possible and conceivable within the scope of the inventive concept and the scope of protection of the following patent claims.