[0001] The specification of Danish Patent No. 128 947 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 10v 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.
[0002] 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.
[0003] 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.
[0004] More specifically, the invention relates to a sound absorbing structure consisting,
in a conventional manner, of thin, perforated sheet material, such as aluminium sheet,
and a foil secured to one side of said sheet, preferably a plastic foil having af
thickness of about 10-50p. The structure according to the invention differs from the
known designs of same type in that between its securing points, the foil is spaced
a distance from the sheet material which is optimum as far as sound absorption is
concerned. 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.
[0005] In spite of comprehensive experiments and theoretical analyses, it has not been possible
to find an unambiguous explanation of the reason why the distance between the foil
and the sheet material is of decisive importance to the sound absorbing capacity,
but it has been found that in the structure according to the invention, there is such
an interaction between the perforated sheet material, the air-filled cavity, and the
foil that a satisfactory sound absorption can be achieved without using any mineral
wool or other porous material. This is, needles 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.
[0006] 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. A further probable explanation
may be that the provision is made between the foil and the sheet material of a layer
of air-filled cells to which an access is provided through the perforations of the
sheet material. These individual cells have a sound quenching effect. It has also
been established that the cavity absorption generally obtained by mineral wool is
also obtained by the structure according to the invention, so that additional measures
to that effect are reduced to a minimum. 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.
[0007] An important advantage of the sound absorbing structure according to the invention
is that unintended variations in the applied foil, especially variations in 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.
[0008] 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,
as 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.
[0009] A preferred embodiment of the sound absorbing structure is illustrated on the accompanying
drawing.
[0010] 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.
[0011] A piece of plastic foil or film 4 having a thickness of e.g. 25u 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.
1. A sound absorbing structure consisting of thin, perforated sheet material, such
as aluminium or steel sheet, and a foil secured to one side of said sheet, preferably
a plastic foil having a thickness of about 10-50p, characterized in that between its
securing points the foil is spaced a distance from the sheet material which is optimum
as far as sound,absorption is concerned.
2. A structure according to claim 1, characterized in that a netting is sandwiched
between the sheet material and the foil and by glueing or welding is attached to the
sheet material as well as to the foil, said netting having a mesh size of the order
of 1-5 mm.
