NOISE SUPPRESSION SYSTEMS

Abstract: A device for suppressing noise emission from a vibrating line, the device comprising means for at least partially cladding the line in a jacket of sound-absorbing material (2, 3), the jacket having an interior surface conforming to the contour of the line and spaced a predetermined distance from the line whereby the jacket is isolated (8) from the vibrating line and absorbs sound waves emitted by the line. The device is particularly suitable for use in suppressing noise emissions from railway lines. The device allows unimpeded passage of the wheel along the line.

Description: NOISE SUPPRESSION SYSTEMS This invention relates to a noise suppression system for reducing noise emitted by a vibrating rail or line, particularly a railway line, and for reducing noise emanating from a wheel/rail interface. It is known to reduce the impact noise emissions from railway lines by erecting fencing alongside such lines. This is costly and generally unsightly. It is therefore preferable to reduce the emission of noise at the track itself. DE-A-3840959 discloses a method of sound insulating the tracks by the use of reinforced rubber sound absorbing elements running the continuous length of the rail. The elements are retained by bolts welded to the web of the rail. This increases the cost of new track considerably and is also expensive to fit to existing track. DE-A-3824577 discloses the use of a soft foam strip releasably secured to the rail web. Although being low-cost and simple to apply, such strips would appear generally ineffective and to have only a short life. DE-A-3939215 discloses a two-part longitud nal soundproofing element of polyurethane and lead which is fixed to opposing sides of the rail, the upper portion being fixed to the lower portion by steel rods and pins. DE-A-4000992 discloses synthetic rubber dampers extending along both sides of a rail and held against the ra l by spring clamps. The above publications are generally concerned with suppressing noise by damping the vibrations of the rail. In most cases, the damping element is secured in contact with the rail and will therefore vibrate with it. While this may produce some reduction in the amplitude of the vibrations, it can never eliminate the noise completely. In one alternative Known arrangement, slabs of sound-absorbing material are placed along the track between the two rails and along each side of the track such that each rail effectively lies within a sound-absorbing duct. This not only uses large quantities of sound-absorbing material but the slabs must be lifted and relai each time the track is maintained by tamping the underlying substrate. According to one aspect of the present invention, there is provided a device for reducing noise emission from a vibrating line, the device comprising means for at least partially cladding the line in a jacket of sound absorbing material, the jacket having an interior surface spaced a predetermined distance from the line whereby the jacket is isolated from the vibrating line and absorbs sound waves emitted by the 1ine. Preferably, the interior surface of the jacket substantially conforms to the contour of the line. When the vibrations are caused by the passage of a wheel along the line, as in a railway, a portion of the jacketed line remains accessible to allow unimpeded passage of the wheel along the line. In a preferred embodiment of the invention, the cladding is releasably secured to the line by a resilient clamping mechanism, and vibration isolating spacers are preferably inserted between the line and the interior surface of the cladding. The cladding may consist of an assembly of discrete components angularly spaced around the line and held together by the resilient clamping mechanism. In the case of a railway line, one of these components positioned on the outside of the railway track may include a limb which extends upwardly above the head of the rail to provide a fence or barrier for reducing impact noise emanating from the rail/wheel interface of a train. preferred sound absorbing material for the cladding is a wood-fibre concrete. One alternative is a lightweight, porous concrete sometimes known as no fines concrete and made from cement, water and coarse aggregate only. However, any suitable sound absorbing material can be used, including aerated rubber which has the added advantage of flexibility. By way of example only, an embodiment of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 is a diagrammatic cross-section showing a railway line fitted with a noise suppression device embodying the invention, and Figure 2 is a perspective sketch of the device shown in Fig. 1. Referring to these figures, the noise suppression device consists of cladding formed by two separate sound-absorbing components 2 and 3 which combine to form a jacket partially encasing a railway line shown generally at 4. The components are held together and against the railway line by a series of longitudinally spaced spring clips 9 (only one of which is illustrated in the drawings) engaging longitudinal grooves 10 in the respective side components 2 and 3. The clips 9 are spaced at intervals along the length of the line. Where the line is secured to wooden or concrete sleepers, the clips are positioned between the sleepers. The profile of the railway line 5 has a head 4 joined to a foot 7 by a web 6. The sound-absorbing components 2, 3 are positioned on opposing sides of the web 6. As illustrated, for optimum performance, each component 2, 3 is provided with an interior surface which conforms to the contour of the line. Components 2 and 3 are formed from a sound absorbing material which might be no fines concrete or an aerated rubber, such as neoprene. However, we have found that a wood-fibre concrete (a concrete and wood fibre composition), such as that sold under the trade name Dumex, is particularly effective. Each component is separated from the rail by longitudinally spaced vibration isolation spacers 8. In the embodiment shown, vibration isolation spacers 8 are in the form of strips of resilient material fixed to the rail by, for example, double-sided adhesive tape. In alternative embodiments (not shown) the spacers 8 may be in the form of discrete resilient buttons or pads each with a projecting nipple that locates in a hole provided in the sound absorbing material. In any event, it is important to ensure that the spacers remain in place and cannot become displaced due to vibrations from the track. The spacers 8 are formed from a comparatively soft material, such as neoprene rubber or a mastic material, and isolate the sound absorbing material from the vibrations of the rail caused by the passage of a train. Where the sound absorbing material is itself formed of rubber, the spacers 8 are preferably moulded as part of the respective components 2 and 3. The side components 2 and 3 are formed in lengths of, for example, 2 to 3 metres. It is preferable for them to be as long as possible to maintain the continuity of noise suppression, but they must be sufficiently short to avoid the possibility of fracture, and to enable the device to be fitted to curving railway lines. The rail 4 is conventionally secured to wooden or concrete sleepers or slab track by spring clips or bolted clips. Accordingly, sound- absorbing components 2 and 3 may be provided with cut-away portions to allow for the components to locate over the securing means, so as not to impede the coupling of components 2 and 3 to the rail 4. Such cut¬ away portions may be provided by cutting components 2 and 3 in-situ. The side component 2 additionally has a generally upwardly extending limb 11 providing an acoustic barrier against noise emanating from the wheel/rail interface of a passing train. The noise suppression device reduces the noise emanating from the track by isolating track vibrations and absorbing the sound waves to suppress the noise. Reductions of several decibels have been observed. Typically, a reduction of up to 8 decibels is obtainable. The device has the further advantages of low-cost and low-maintenance, and is easily fitted to existing track without requiring any modifications of the track and without interfering with subsequent track maintenance.

Claims: CLAIMS 1. A device for suppressing noise emission from a vibrating line, the device comprising means for at least partially cladding the line in a jacket of sound-absorbing material, the jacket having an interior surface spaced a predetermined distance from the line whereby the jacket is isolated from the vibrating line and absorbs sound waves emitted by the 1 ine. 2. A device according to claim 1 wherein the interior surface of the jacket substantially conforms to the contour of the line. 3. A device according to claim 1 or claim 2 wherein the line comprises a railway line and a portion of the jacketed line remains accessible to allow unimpeded passage of a wheel along the line. 4. A device according to claim 1, 2 or 3 further comprising means for releasably securing the cladding to the line. 5. A device according to claim 4 in which the releasable securing means includes a resilient clamping mechanism. 6. A device according to claim 5 in which the resilient clamping mechanism includes a plurality of spring clips longitudinally spaced along the line. 7. A device according to any one of the preceding claims in which vibration isolating spacers are inserted between the line and the interior surface of the cladding. 8. A device according to claim 7 in which the spacers are in the form of longitudinally spaced elongate resilient strips adhered to the 1 ine. 9. A device according to claim 7 or claim 8 in which the spacers are formed of rubber or a mastic material. 10. A device according to any one of the preceding claims in which the sound absorbing material is a wood fibre concrete. 11. A device according to claim 7 in which the sound absorbing material is aerated rubber and the spacers are integrally moulded as part of the interior surface of the cladding. 12. A device according to claim 3 in which the railway line profile includes a head portion connected to a foot portion by a web portion, and the cladding consists of discrete components respectively assembled on opposing sides of the rail web below the head portion. 13. A device according to claim 12 in which one of the side components positioned on the outside of the railway track includes a limb which extends upwardly above the head portion of the rail to provide a fence or barrier for reducing noise emanating from the wheel of a train. 14. A method of reducing the emission of noise from a vibrating line, particularly a railway line, the method comprising at least partially cladding the line in a jacket of sound absorbing material and isolating the jacket from the vibrations of the line whereby the jacket absorbs sound waves emitted by the vibrating line. 15. A railway line fitted with a noise suppression device according to any one of the claims 1-13. 16. A device according to any one of the claims 1-13 and substantially as herein described with reference to the accompanying drawings.

Inventors: HOOD ROBERT, HOOD, ROBERT
Applicants: ACAPELLA ENTPR LTD, ACAPPELLA ENTERPRISES LTD.
Application year: 1993
Main classification: E01B19
IPC codes: E01B19