ACOUSTIC ABSORBER

Abstract: An acoustic absorber panel has absorbent material (4) behind a protective corrugated steel skin (5). The steel skin (5) is formed with a series of slots (9) in it to allow the sound to penetrate the skin. The slots (9) are concealed behind the corrugations.

Description: ACOUSTIC ABSORBER This invention relates to an acoustic absorber, and more particularly to an acoustic absorber panel for use in conjunction with a telephone kiosk and in other situations where resonance and noise transmission is a problem. In a modern telephone kiosk, the transmission of sound from the exterior of the kiosk to the interior is a problem, that is to say sound is transmitted very well and resonates in the kiosk. The older style telephone kiosk was heavier, and its base did not have ventilation holes, and so it was less prone to this kind of noise problem. In the standard modern telephone kiosk, the only acoustic absorber is the person making the telephone call. This means that, in noisy environments such as railway stations and sites adjacent to busy roads, sound is trapped in the telephone kiosk. In an attempt to reduce this kind of noise problem, telephone kiosks have been fitted with acoustic absorber panels. A known acoustic absorber panel consists of an acoustically-absorbent material sandwiched between a mesh or apertured sheet and a rigid backing sheet. This type of panel is very difficult to keep clean, and nearly impossible to clean or repair after soiling or damage occurs. The acoustically-absorbent material used is often mineral wool, which because of its fibrous nature retains water very well. The presence of water in mineral wool can reduce its sound-absorbing properties and cause the wool to lose its integrity. Thus, a panel of this type requires moisture-proofing and this can reduce the sound absorbing qualities of the panel. A sound wave consists of two components, namely a velocity component and a pressure component. These must co-exist, so if the one component is destroyed the other component then ceases to exist also. The pressure component is a maximum at the rigid backing sheet of an acousric absorber panel, and the velocity is a maximum at the opposite surface of the panel. The amplitude of vibration of air molecules is, therefore, also a maximum at the surface of the absorber. The purpose of the mineral wool is to destroy the velocity component by reducing the amplitude of the vibration of the air molecules due to frictional losses in the convoluted path defined by the mineral wool. Theoretically, these frictional losses are a maximum at a distance of 1/4 of the wavelength of the sound from the rigid backing sheet (this distance is approximately 2.5m for sound at a frequency 34Hz and approximately 25cm for sound at 340Hz). In practice, it is not possible to use acoustically-absorbent material which is sufficiently thick to achieve the theoretical maximum, but clearly acoustic absorption will increase with the thickness of the material. Moreover, it is not possible to achieve the theoretical maximum for all wavelengths. The object of the invention is to provide an effective and durable vandal-resistant acoustic absorber panel. The present invention provides an acoustic absorber panel comprising an acoustically-absorbent material clad on one side with a generally corrugated protective sheet, the corrugations of the sheet defining grooves in that face of the sheet remote from the acoustically absorbant material , each of the grooves having a mouth whose width is less than the maximum width of that groove, wherein sound-transmitting apertures are formed in portions of the sheet defining groove walls, the corrugations being so sized and shaped and the apertures being so positioned that access to the apertures from said said face is restricted. Preferably, the corrugations are symmetrical. Advantageously, the corrugations each have inwardly- projecting edges at the sides defining the mouth of the groove, and the slots are formed in the side walls of the grooves behind the projecting edges. Preferably, the acoustic absorber panel has a rigid backing sheet on the opposite side of the acoustically- absorbent material to the corrugated protective sheet. The panel may further comprise a layer of dense material between the acoustically-absorbent material and the rigid backing sheet. Preferably, the corrugated protective sheet is made of stainless steel. Alternatively, the corrugated protective sheet is made of a stiff polymer. Conveniently, the corrugated protective sheet and the rigid backing sheet are made of the same material. The invention will now be described in more detail, by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a cross-sectional view of an acoustic absorber panel constructed in accordance with the invention; Figure 2 is a different cross-sectional view of the panel of Figure 1; and Figure 3 is a plan view of a cover forming part of the panel of Figures 1 and 2 before it is corrugated. Referring to the drawings, Figures 1 and 2 show an acoustic absorber which includes a layer 4 of acoustically-absorbent material (mineral wool), clad on one side with a corrugated steel protective sheet 5. The other side of the layer 4 is backed by a layer of sand 7 which is held in place by a support plate 8. As mineral wool retains water, the layer 4 is covered on each side by a respective moisture-proof protective film 6. The films 6 thus prevent moisture reaching the mineral wool layer 4 and destroying its integrity. The films 6 are made of polythene terephthalate, and are 23μm thick. The corrugations of the sheet 5 are folded about fold lines A, B, C, D and E to define channels 12 separated by raised sections 13. The raised sections 13 have ledges 14 which overhang into the channels 12. Each of the raised sections 13 is a similar shape to each of the channels 12. Each of the raised sections 13 is symmetrical about the line I-I (see Fig. 2), and each of the channels 12 is symmetrical about the line II-II. The base of each channel 12 is in the form of a shallow basin defined by the folds E and the folds D. The sides of each channel 12 then rise steeply substantially perpendicular to the base of the channel, the sides being defined by the folds D and C. At each fold C, the sheet 5 is folded through 120° back down into the adjacent channel 12. At each fold B, the sheet 5 is folded through approximately 180°. The corrugated protective sheet 5 is folded from a single sheet of metal. Slots 9 are formed in the sides of the channels 12, the slots being positioned behind the ledges 14 and between the folds D and C (see Fig. 3). The slots 9 allow sound to penetrate into the layer 4, whilst making it harder for dirt to penetrate into the mineral wool. The concealment of the slots 9 also protects the sheet 5 from vandals attempting to dismantle or destroy it. As the mineral wool layer 4 extends into the raised sections 13 defined by the corrugations of the sheet 5, the mineral wool maximises the frictional losses to the vibration of the air molecules, because mineral wool is spaced from the rigid suport plate 8 by increased amounts in these regions. Acoustic panels constructed according to the invention could be produced in large sheets which could be cut to size and fitted to existing telephone kiosks. The sand layer 7 adds extra mass to the kiosk and lowers its resonant frequency. Panels of this type could also be used to sound proof session rooms or music rooms. They could be mass-produced, and used anywhere that a sound absorbing screen or barrier is needed. It will be apparent that modifications could be made to the panel described above. In particular, an open cell foam material, wool or fibre glass wool could be used instead of mineral wool as the acoustically- absorbent material. It would also be possible to make the sheets 5 and 8 from a stiff polymer.

Claims: CLAIMS 1. An acoustic absorber panel comprising an acoustically-absorbent material clad on one side with a generally corrugated protective sheet, the corrugations of the sheet defining grooves in that face of the sheet remote from the acoustically absorbant material , each of the grooves having a mouth whose width is less than the maximum width of that groove, wherein sound-trans itting apertures are formed in portions of the sheet defining groove walls, the corrugations being so sized and shaped and the apertures being so positioned that access to the apertures from said said face is restricted. 2. An acoustic absorber panel as claimed in claim 1, wherein the corrugations are symmetrical. 3. An acoustic absorber panel as claimed in claim 2, wherein the corrugations each have inwardly-projecting edges at the sides defining the mouth of the groove, and apertures are formed in the side walls of the grooves behind the projections. 4. An acoustic absorber panel as claimed in any one preceding claims, further comprising a rigid backing sheet on the opposite side of the acoustically-absorbent material to the corrugated protective sheet. 5. An acoustic absorber panel as claimed in any one of the preceding claims, wherein the corrugated protective sheet and the rigid backing sheet are made of the same material. 6. An acoustic absorber panel as claimed in claim 4 or claim 5, further comprising a layer of dense material between the acoustically-abs orbent material and the rigid backing sheet. 7. An acousti c abs orber panel as claimed in claim 6, wherein the dens e material is s and. 8. An acousti c abs orber panel as claimed in any one of the preceding claims , wherein the acoustically-absorbent material comprises mineral wool . 9. An acoustic absorber panel as claimed in any one of claims 1 to 7, wherein the acoustically-absorbent material comprises an open cell foam material. 10. An acoustic absorber panel as claimed in any one of claims 1 to 7, wherein the acoustically-absorbent material comprises wool. 11. An acoustic absorber panel as claimed in any one of claims 1 to 7, wherein the acoustically-absorbent material comprises fibre glass wool. 12. An acoustic absorber panel as claimed in any one of the preceding claims , wherein the corrugated protective sheet is made of stainless steel. 13. An acoustic absorber panel as claimed in any one of claims 1 to 12, wherein the corrugated protective sheet is made of a stiff polymer. 14. An acoustic absorber panel as claimed in any one of the preceding claims , wherein the acoustically-absorbent material is protected from moisture on both sides by a moisture proof skin. 15. An acoustic absorber panel as claimed in claim 14, wherein the moisture proof skin is of the order of 23μm thick. 16. An acoustic absorber panel as claimed in claim 14 or claim 15, wherein the moisture proof skin is made of polythene terephthalate. 17. A telephone kiosk provided with an acoustic absorber panel as claimed in any one of the preceding claims. 18. A kit of parts comprising an acoustic absorber panel as claimed in any one of the preceding claims and means for attaching the acoustic absorber panel to a telephone kiosk. 19. An acoustic absorber panel constructed substantially as hereinbefore described with reference to, and as illustrated by, the accompanying drawings.

Inventors: CROSS ROBIN CHRISTOPHER, CROSS, ROBIN, CHRISTOPHER
Applicants: BRITISH TELECOMM, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1991
Main classification: G10K11
IPC codes: G10K11