EP0013513B1

EP0013513B1 - Sound absorbing structure

Info

Publication number: EP0013513B1
Application number: EP79303073A
Authority: EP
Inventors: Kurt Hjelholt; Niels Christian Dyrving
Original assignee: Daempa AS
Current assignee: Daempa AS
Priority date: 1979-01-04
Filing date: 1979-12-28
Publication date: 1984-08-15
Also published as: JPS6310258B2; ES256657Y; FI800007A; AU533557B2; AU5426880A; MX149377A; DE2967185D1; ES256657U; EP0013513A1; ATE9017T1; EG14896A; DK4379A; NO794220L; JPS5592446A

Abstract

A sound absorbing structure, for instance in the form of a ceiling lining element, consisting of perforated sheet material (3) covered on one side by the foil (4), suitably of plastics, which by means of a netting (5) or in another manner, is spaced a short distance from the sheet material.

Description

During the last two or three decades many proposals have been presented for the purpose of reducing or controlling the noise level in buildings by means of sound absorbing panels that are usable in the construction of ceilings, partitions or other structures. An example of such sound absorbing or acoustic panels is disclosed in the specification of U.S. Patent No. 3,948,347 according to which the panel comprises a fibrous matrix, a diaphragmatic plastic skin overlying this matrix, a perforated metal face sheet spaced from the plastic skin, and means such as a galvanised wire grid disposed between the metal sheet and the plastic skin to create an air space of approximately 1/4 inch (0.6 cm) therebetween to permit sound- activated air to be distributed over the plastic skin.
Another solution is known from the specification of Danish Patent No. 128,947 which discloses a sound absorbing ceiling lining consisting of elements of thin perforated sheet material and an absorption material, such as mineral wool, batts, disposed in these elements, and an air and particle intercepting foil inserted between the sheet material and the absorption material, said foil being adhered to the sheet material in a cross pattern consisting of two intersecting sets of continuous glue stripes. The specification states that the foil is preferably a plastic foil having a thickness of about 10,um or less, and that it forms a kind of checkered coating on the upper side of the sheet material, the glue stripes dividing the foil into a large number of generally rectangular, possibly square, sections having normally a side length of the order of one or a few centimetres. The stated purpose is to prevent an air flow along the upper side of the sheet material and thereby also to counteract an air circulation through this perforated sheet material, without substantially reducing the sound absorbing effect, especially in the high frequency range, as would be the case when a tight foil were glued flat to the perforated sheet material.
The known ceiling lining, when applied in practice, has proved not to come up, in all cases, to expectations with regard to the sound absorbing effect. Thus, the plastic foil has turned out to have a reducing effect on the sound absorbing capacity of the mineral wool, to such extent, in some cases, that the mineral wool becomes almost ineffective, irrespective of its thickness. Furthermore, this reducing effect varies substantially dependent on the physical characteristics of the plastic foil, especially its composition, its melting point, and its thickness and variations in thickness, and it has been found that these factors can vary in one and the same roll of plastic foil to such an extent as to strongly influence the said reducing effect. The glue stripes constitute a further important factor, as it can be difficult to control their thickness and width with sufficient accuracy, and because they must be expected, by providing a direct contact between the foil and the sheet material, to reduce the effective absorption area proportionally to the area of the glue stripes.
Starting from the above known technique, the present invention has for its object to provide a sound absorbing structure consisting also of perforated sheet and foil but having a sound absorbing capacity which even under practical conditions can be controlled adequately.
A sound absorbing structure is known from US-A-3 827 999 which comprises a perforated face plate of sheet metal, a backing sheet covering the perforated face plate and spacing means provided between the opposed surfaces of the face plate and the backing sheet and forming a firm bond therebetween. This liner is of circular configuration so that it may be positioned about the periphery of a bladed rotor generating an airflow and acoustic field from the tips of the blades, and the spacing means of this known liner are formed by a honeycomb or another layer of partitioned cellular material, sandwiched in between the face plate and the backing sheet. A particular feature of the liner is that the axes of the cavities, bounded by the perforated face plate, the backing sheet or plate, and the partitions of the intermediate layer, are slanted relative to the local tangential direction of the liner.
The sound absorbing structure of the present invention comprises only a thin perforated sheet material, such as aluminium or steel sheet, a plastics foil secured to one side of the sheet having a thickness of 10-50 µm, and spacing means secured between the sheet material and the foil and dividing the space between the sheet material and the foil into a great plurality of cellular cavities, the spacing between the sheet material and the foil being chosen so as to give the optimum sound absorption of the sound to be absorbed.
This can in practice be conveniently achieved by the insertion, between the sheet material and the foil, of a netting which by glueing or welding is attached to the sheet material as well as to the foil and which has a mesh size of the order of 1-5 mm.
By comprehensive experiments and theoretical analyses, it has been found that in this structure there is such an interaction between the perforated sheet material, the air-filled cavities, and the foil that a satisfactory sound absorption can be achieved without using any mineral wool or other porous material. This is, needless to say, of great interest not only from an economic point of view but also for the quality of environment, as such materials are liable to cause dust nuisances.
It can be regarded as established that the sound absorbing effect, at any rate to a great extent, is due to the fact that the foil is divided into a great number of small areas or small diaphragms, which vibrate or oscillate independently and out of time with the vibrations of the perforated sheet material. When using a netting as stated above, the said diaphragms or cells are given a regular shape, for example a quadratic or hexagonal shape, but this is of minor importance, since a gauze-like material, such as glass fiber gauze, has also proved suitable instead of a netting provided with regular meshes.
An important advantage of the sound absorbing structure according to the invention is that unintended minor variations originating from slightly varying conditions during the production of the foil, especially unintended variations in its thickness and composition, have practically no importance for the sound absorbing capacity of the finished structure. Said capacity may, on the other hand, be varied within a rather wide range, if so desired, in particular according to the prevailing frequency range, by changing the thickness and perforation ratio of the sheet material, the thickness of the foil and the netting, and the size and shape of the net meshes. This enables an economical manufacture of structures having specific sound absorbing characteristics, in order, for instance, to meet requirements as to tailored sound absorption.
The desired distance between the sheet material and the foil can also be secured in another way than by sandwiching of netting or the like between the parts in question, e.g. the netting may form part of the foil, or said foil and/or the sheet material may be provided with low ribs or protuberances to secure the required spacing.
A preferred embodiment of the sound absorbing structure is illustrated on the accompanying drawing.
In the embodiment of the drawing the structure comprises a rectangular sheet 1 of steel or aluminium which is perforated between the parallel lines 2, 2 so that a pair of opposite marginal portions 3, 3 are left without perforations.
A piece of plastic foil or film 4 having a thickness of e.g. 25 ,um and provided with a netting 5 on its lower surface, covers the perforated area of the metal sheet 1 and extends slightly therebeyond as indicated by the lines 6, 6. The netting 5 is adhesively attached to both the plastic foil 4 and the metal sheet 1 and forms a layer of air-filled cells therebetween which are closed by the plastic foil or film but communicate with the surrounding air through the perforations of the metal sheet 1.

Claims

A sound absorbing structure comprising only a thin perforated sheet material (1), such as aluminium or steel sheet, a plastics foil (4) secured to one side of the sheet having a thickness of 10-50 µm, and spacing means (5) secured between the sheet material and the foil and dividing the space between the sheet material and the foil into a great plurality of cellular cavities, the spacing between the sheet material and the foil being chosen so as to give the optimum sound absorption of the sound to be absorped.