GB2269842A - Sound absorbing apparatus - Google Patents

Abstract

An elongate sound absorbing element for incorporation in, or addition to, traffic noise barriers provides a resonant cavity with an impermeable upper surface wall 16 extending downwardly and outwardly with respect to the rear of the cavity and with a perforated lower surface wall 18 extending outwardly with respect to the rear of the cavity and underline the upper surface wall. A layer of sound absorbent material 26 is disposed within the cavity overlying the lower surface wall. Preferably a second layer 28 of sound absorbent material is located part-way up the cavity. A traffic noise barrier includes a number of such elongate elements arranged one above the other. A sound absorbing element of this type provides flexibility of employment, protection of the absorbent material for the ingress of rain, snow, dust, etc and provides excellent sound absorbing performance with a low volumetric content of absorbent material. <IMAGE>

Description

SOUND ABSORBING APPARATUS The invention relates to sound absorbing apparatus. In particular, the invention relates to sound absorbing apparatus suitable for absorbing traffic noise.

Sound is a serious environmental problem which can cause much discomfort and annoyance to those subjected to it. Claims for compensation for noise pollution resulting from new building projects such as motorways, airports, railways and the like can significantly increase the cost of those projects. At critical points along motorways it has become commonplace to build barriers to screen adjacent areas from the worst effects of the noise pollution.

However, although conventional barriers have a screening effect, they are typically relatively inefficient when it comes to absorbing the noise generated. Indeed, conventional barriers typically reflect much of the sound energy incident on them which causes noise pollution in itself. Although people behind the barrier are troubled less by noise, people in front of the barrier will find an increase in noise levels. Sound barriers which can absorb incident sound are known.

However, conventional sound absorbing barriers are expensive and tend to be designed specifically for a particular point of application.

Accordingly, an object of the invention is to provide sound absorbing apparatus suitable for use for forming a traffic noise barrier which is simple and inexpensive in construction and installation, and versatile in application.

In accordance with the invention, there is provided sound absorbing apparatus comprising an elongate element for defining, in use, an elongate resonant cavity with an upper surface wall extending downwardly and outwardly with respect to a rear of the cavity, with a perforated lower surface wall extending outwardly with respect to the rear of the cavity and underlying the upper surface wall, and with a layer of sound absorbent material disposed within the cavity overlying the lower surface wall.

The combination of the resonant cavity with the sound absorbent material enables the absorbtion of low frequency sound (e.g.

below about 800 Hz) through acoustic diffraction combined with tuned resonance. Where a plurality of the elongate elements are mounted one above the other, high frequency sound (e.g. above about 200 Hz) can be absorbed as a result of acoustic reflection from the upper surface wall of a first element and absorbtion in the sound absorbent material of a second elongate element.

Preferably, the elongate element is configured such that1 in use, the upper surface wall slopes at an acute angle to the vertical to provide effective reflection of high frequency sound into the sound absorbing material of a further elongate element mounted above it.

Preferably, the elongate element is configured such that, in use, the lower surface wall extends substantially horizontally away from the rear of the cavity. In this way, sound reflected vertically from the upper surface wall of a lower elongate element can pass into and be absorbed by the layer of sound absorbing material.

To improve the sound absorbing properties of the apparatus, a further layer of sound absorbent material can be mounted part-way (e.g. half-way) between the lower and upper extremities of the resonant cavity.

Preferably, the elongate element defines, in use, an elongate resonant cavity of substantially triangular cross-section with the upper surface wall, the lower surface wall and a rear wall defining the three sides of the triangle. The triangular shape provides a particularly economical and effective construction.

In a preferred embodiment of the invention, the rear wall is an integral part of the elongate element to provide for ease of shipment and assembly.

However, the apparatus can include a support for the elongate member, the support including a surface for forming the rear wall of the resonant cavity.

As indicated above, in order to provide a sound absorbing barrier, the apparatus preferably comprises a support structure and a plurality of the elongate elements mounted one above the other on the support structure. In this way, sound can be reflected from the upper surface of one element into the sound absorbing material of another element mounted above the first element on the support structure.

Particular embodiments of the present invention will be described hereinafter with reference to the accompanying drawings: Figure 1 is a schematic diagram of a sound absorbing barrier comprising a plurality of sound absorbing elements on a support structure; Figure 2 is a cross-section through one example of a sound absorbing element for use in the barrier of Figure 1; and Figure 3 is a cross-section through a second example of a sound absorbing element for use in the barrier of Figure 1.

Figure 1 illustrates a sound absorbing barrier 10, for example a traffic noise barrier for the use at the side of a motorway, which includes a plurality of elongate sound absorbing elements 12 mounted on a support structure 14. The support structure should be relatively massive for physical support and acoustic reasons. It could, for example be an existing noise barrier, or a support structure specifically provided for supporting the elements 12. Each of the sound absorbing elements comprises an impervious upper sloping surface wall 16 and a lower, perforated, surface wall 18 which, together with walls 20 at each end and a rear wall (not shown in Figure 1), define a resonant cavity.

Figure 2 is a cross-section through one of the elongate elements 12 of Figure 1. The elongate element comprises an impervious upper sloping surface wall 16 which extends outwardly and downwardly from the top of a rear surface wall 22. A lower surface wall extends substantially horizontally from the bottom of the rear surface wall 22 to the bottom edge of the upper sloping surface wall 16.

The lower surface wall 18 is perforated with a plurality of holes. In order to be acoustically transparent, at least 25% of the lower surface wall needs to be open. This can be achieved by making individual holes in the lower surface wall. However, it is preferably achieved by making one or more large holes in the lower surface wall and then covering the large holes with a wire or other mesh 24 which can support a layer of sound absorbent material 26. The sound absorbent material 26 should preferably have an acoustic flow resistance of about 500 Rayls (Pascals/m/s). Preferably it is formed as a bonded mineral- or rockwool slab and has a thickness between 20 to 40mm, depending on the degree of sound absorbtion required (with 40mm better results can generally be achieved than with 20mm thickness).In a preferred embodiment of the invention a second layer of similar sound absorbent material 28 is supported on a mesh 30 part way (e.g. approximately half-way) between the lower and upper extremities of the cavity 32. The second layer of sound absorbent material can have a thickness of between lOmm and 20mm depending on the degree of sound absorbtion required, 20mm generally giving better performance than 10mum. The provision of the second layer of sound absorbent material enables particularly effective sound absorbtion over a wide range of frequencies. In order to complete the resonant cavity, an end wall 20 is located at each end of the elongate element 12.

The angle to the vertical of the sloping wall 16 can be chosen to suit the particular site at which the element is to be used.

However, it has been found that an acute angle e of between 450 and 150, preferably 300, to the vertical provides a good compromise between requirements of compactness and efficiency. An angle of 30 means that the elongate element is taller than it is deep. A typical element will be about 300mm high. With the upper surface wall at an angle of 30 to the vertical, this means that the depth of the element 12 (i.e. the length of the lower surface wall) is about 175mm. This compact form reduces the vulnerability of the barrier to the effects of the wind. Also, an angle of 300 still means that a significant amount of high frequency sound incident on the upper surface wall 16 is reflected into the sound absorbent material of the next higher element 12 on the barrier 10.High frequency sound which is not reflected into the sound absorbent material will be reflected harmlessly upwards. Low frequency sound can be effectively absorbed by a combination of the resonant cavity 24 and the absorbent material in the cavity. Even though the openings in the elements 12 are to be found in the lower surface, low frequency sound incident in a substantially horizontal direction will "sucked" into the cavity by diffraction. It does not matter that the opening in the cavity is not facing in the direction of the incident low frequency sound.

The triangular section of the elongate element can be formed as an extrusion. The holes in the lower surface wall can be formed by drilling, stamping or any other suitable technique. The layer of sound absorbing material can then be introduced into the triangular section before fixing of the end walls 20 to the end of the triangular section.

Alternatively, the triangular section can be formed from sheet material. The hole(s) in the lower surface wall 18 can be formed by drilling stamping, etc. The sheet material can be folded or stamped into the required triangular shape. The elongate edges of the sheet material can be joined in an appropriate manner as required.

For example, in Figure 2, the elongate edges 33 and 34 are folded to form a lower mounting bracket for the elongate member 12. The longitudinal edges 33 and 34 can, if required, be joined together by welding, rivetting, or other appropriate techniques as required depending on the materials used. In the example shown in Figure 2, the edges are preferably held together by means of the fixing means (not shown) used to attach the elongate member to the support 14. The fixing means can take the form of screws, bolts, or any other appropriate fixing means depending on the materials used. Figure 2 also illustrates a further fixing bracket 36 adjacent to the upper edge of the triangular member 12. This fixing bracket can be used, with the bracket formed by the edges of the elongate material 33 and 34 for attaching the elongate member to the support member 14.The support bracket 36 can be attached to the elongate member 12 by any suitable means such as welding, rivetting, etc. However, the brackets are not essential as other fixing arrangements (e.g. hangers) could be used.

The elongate member 12 can be formed of any suitable impermeable material. For example, the elongate element could be made of steel, the steel being protected from corrosion by conventional methods such as dipping and/or painting with protective materials.

However, any other suitable material could be used. Examples of suitable materials having a higher resistance to environmental effects than steel are aluminium and glass reinforced plastics. The configuration of the impermeable sloping upper surface member of each element allows rain to drip from the outer edge without wetting the absorbent material. It also reduces the deposition of falling dust.

The deposition of moisture and dust into the absorbent material can rapidly degrade the performance of absorbent materials. The present construction provides significant protection from the elements.

However, absorbent materials which are resistant to environmental effects should preferably be used.

Figure 3 illustrates an alternative embodiment of the present invention. This embodiment comprises an elongate element 40 which includes an upper, sloping, surface wall 42 and a lower surface wall 44. The upper surface wall is formed with a flange 46 and the lower surface wall 44 is formed with a flange 48 for attaching the elongate element 40 to a support structure 38. In this example of the invention the support structure 38 provides a rear wall which, in combination with the upper wall 42 and the lower wall 44 form a resonant cavity 50. The elongate member 40 is attached to the support member 38 by means of fixing elements 52, 54 which can be in the form of screws, bolts, etc. as appropriate. As can be seen in Figure 3, the lower surface wall 44 and the flange 48 are configured to overlie the upper flange of a further elongate element 40'.This construction prevents the ingress of moisture into a lower elongate element such as that labelled 40'. In a similar manner, a cover member 56 can be provided for the topmost elongate element 40 of a noise barrier to prevent the ingress of moisture into the resonant cavity 50. The cover member can be attached in any suitable manner, for example by means of fixing elements 58.

As in the case of the embodiment of Figure 2, the upper surface wall 42 is impermeable and extends downwardly and outwardly from the rear of the cavity 50, preferably at an acute angle of about 30O to the vertical. In this embodiment of the invention, the lower surface wall 44 also extends downwardly and outwardly with respect to the rear of the cavity 50. Depending on the direction of incident noise which the elongate element is intended to absorb, providing the lower surface wall at an angle in this manner can improve the sound absorbing qualities of the sound barrier. Preferably, the lower surface wall is, however, at a relatively shallow angle to the horizontal in order to enable effective location of a first layer of sound absorbent material 60 over the lower surface 44 of the elongate element 40.In this example the lower surface wall is formed with a plurality of holes 62 rather than a mesh. As with the first embodiment, however, a second layer of sound absorbent material 64 is supported on a mesh 66 part way up the cavity 50.

As with the first embodiments, the second embodiment can be constructed using any suitable manufacturing technique and any suitable materials. The overall dimensions will be similar to those of the first embodiment.

There has been described sound absorbing apparatus comprising a sound absorbing element. The element is suitable for incorporation in, or addition to, traffic noise barriers. It provides flexibility of employment in that the element may simply be applied to existing, non-absorbent, noise barriers or can be applied to a new traffic noise barrier structure. It provides protection of the absorbent material from the ingress of rain, snow, and dust and the like. Also, it provides natural drainage. Excellent low frequency performance can be achieved with a low volumetric content of absorbent material and with an attendant reduction of cost. The absorbent material can be chosen to provide optimised acoustic flow resistance.

For a particular implementation, the angle of slope of the upper sloping surface wall and the dimensions of the cross section can be selected to optimise the performance in relation to the topology of the site of installation. The physical construction exploits low frequency acoustic diffraction combined with tuned residence plus high frequency acoustic reflection in vertical direction into the absorbent element.

By stacking elements one above another to form an array, elements may be permanently incorporated into a barrier construction, or attached to existing barriers by means of simple hangers or other fixing means. The relatively small height of each element allows the total number of elements in the array to be selected to accommodate variations in barrier height associated with variations of ground level along the length of a barrier. The "venetian blind" appearance of an array is aesthetically attractive and the elements can be rendered or decorated as desired.

The sloping of the surface of each element allows rain to drip off the outer edge without wetting the absorbent element and thus reduces the deposition of falling dust. The construction and installation of this type of sound absorbent element is simple and inexpensive, as is the replacement of damaged elements.

Although particular embodiments of the present invention have been described herein, it will be appreciated that many additions and or modifications are possible within the scope of the invention. For example, the embodiment of Figure 1 could be provided with a lower surface wall 18 which slopes at an angle to the horizontal. In the same manner, the lower surface wall 44 of the example of Figure 3 could be substantially horizontal.

Claims (11)

1. Sound absorbing apparatus comprising an elongate element for defining, in use, an elongate resonant cavity with an impervious upper surface wall extending downwardly and outwardly with respect to a rear of the cavity, with a perforated lower surface wall extending outwardly with respect to the rear of the cavity and underlying the upper surface wall, and with a layer of sound absorbent material disposed within the cavity overlying the lower surface wall.

2. Apparatus as claimed in claim 1 wherein the elongate element is configured such that, in use, the upper surface wall slopes at an acute angle to the horizontal.

3. Apparatus as claimed in claim 1 or claim 2 wherein the elongate element is configured such that. in use, the lower surface wall extends substantially horizontally away from the rear of the cavity.

4. Apparatus as claimed in one of the preceding claims wherein the lower surface wall comprises one or more apertures spanned by a mesh for retaining the sound absorbent material.

5. Apparatus as claimed in any one of the preceding claims comprising a second layer of sound absorbent material supported part way between the upper and lower extremities of the resonant cavity.

6. Apparatus as claimed in any one of the preceding claims wherein the elongate element defines, in use, an elongate resonant cavity of substantially triangular cross-section with the upper surface wall, the lower surface wall and a rear wall defining the three sides of the triangle, and with first and second end walls being disposed at opposite ends of the elongate element.

7. Apparatus as claimed in any one of the preceding claims wherein the elongate element includes an integral rear wall.

8. Apparatus as claimed in any one of claims 1 to 6 comprising a support structure for the elongate member, the support structure including a surface for forming a rear wall of the resonant cavity.

9. Apparatus as claimed in any one of the preceding claims comprising means for securing the elongate member to a support structure.

10. Apparatus as claimed in any one of the preceding claims, comprising a support structure and a plurality of the elongate elements mounted one above another on the support structure.

11. Sound absorbing apparatus substantially as hereinbefore described with reference to the accompanying drawings.