The invention relates to a sound damping arrangement for the damping of noise from the motor and blower of a vacuum cleaner, wherein the motor and the blower are incorporated in a housing, and wherein a flow of air, to- gether with the noise, is conveyed from the interior of the housing out to the surroundings.

Noise from vacuum cleaners in operation is a well-known problem. Many attempts have therefore been made at constructing these such that the noise is minimized to the greatest extent possible.

A structure having good noise damping properties is known e. g. from WO 01/74025. This known structure includes a noise damping arrangement for a vacuum cleaner having a motor which drives a blower incorporated in a housing. With a view to reducing the noise from the vacuum cleaner in op- eration, the housing is constructed as a double-walled structure which forms a channel for the transport of air and thereby also the noise. The air and the noise flow from the bottom of the housing upwards in a first channel section to the top of the housing and from there downwards in a second channel section, whereby the direction of the air and the noise at the top of the housing is changed 180°. The overall principle (which will be explained later) of this noise damping is based on a Helmholtz resonator.

Although the structure of the WO publication damps the noise quite con- vincingly, there are certain limitations with respect to the position of the ex- haust opening and the exhaust filter, which must either be positioned within the top of the housing or at the bottom of it, which is not always desirable.

Accordingly, an object of the invention is to provide a vacuum cleaner which has a convincing noise damping, and where it is also possible to arrange

the exhaust elsewhere than at the bottom or within the top of the housing.

The object of the invention is achieved by a sound damping arrangement of the type defined in the introductory portion of claim 1, which is character- ized in that the housing is formed by inner and outer walls, wherein inner and outer, vertically extending gaps are provided in the inner and outer walls, respectively, and wherein one edge of the gaps is connected with a common wall member.

Hereby, the flows of air and the noise flow from one gap concentrically around the housing forwards to the other gap, where the flow of air leaves the housing and reaches the surroundings. Thus, it is possible e. g. to con- nect a filter on the outer side of the housing at the outer gap, which may be an advantage if e. g. the design of the vacuum cleaner is to have a cubic appearance.

In addition, a horizontal motor shaft allows the exhaust opening to be posi- tioned either on top of, below or at the side of the vacuum cleaner.

When, as stated in claim 2, the distance between the walls is t/4 wave- length, the noise damping may be dimensioned to damp particularly incon- venient frequencies merely by adapting the distance between the walls cor- responding to precisely 1/4 wavelength of the given frequency.

When, as stated in claim 3, at least one intermediate wall having an inter- mediate gap is provided between the inner and outer walls, it is advan- tageously ensured that the noise is divided into at least two propagation parts, where one noise part runs through a distance which is greater than the distance of the other noise part. This means that a phase shift takes place between the noise parts, and as a result of this the noise is damped additionally at some frequencies and most strongly where the phase shift is

180° between the noise parts.

When, as stated in claim 4, the intermediate wall has a length which is adapted to a given noise characteristic, the dimensioning of the noise damping characteristic may be adapted individually to the characteristic of the noise sources.

Additionally, it is an advantage if, as stated in claim 5, the intermediate gap is divided into two intermediate gaps which are defined by the common wall member, which involves a structural advantage, since the intermediate wall may be inserted into and removed from the housing, if e. g. structural changes of the vacuum cleaner cause the noise characteristic to be changed, which results in a need for the intermediate wall to have other dimensions.

Further expedient embodiment are defined in claims 6 and 7.

The invention will now be explained more fully with reference to the draw- ing, in which fig. 1 shows the structure known from WO 01/74025, fig. 2 shows the basic principle of the structure of a housing for a vacuum cleaner according to the invention, fig. 3 shows the housing of fig. 1 with incorporated motor, while fig. 4 shows a filter mounted on the housing of fig. 2.

In fig. 1, the numeral 1 designates a motor which drives a blower 2 that sucks air in as shown at 3. The air is conveyed through an exhaust filter

where the air flows vertically downwards in a channel as shown by the ar- row 6, following which the direction of the air and the noise is changed 180° and extends upwards in another channel as shown by the arrow 7 and from there to the surroundings shown at 8.

The noise damping of this known structure is based on a so-called Helm- holtz resonator, the principle being that air in a volume moved into a chan- nel is equivalent to the air in the volume behaving like a spring and the air in the channel behaving like a mass.

With this principle it is hereby possible to dimension the size of the volume and the dimensions of the channel so as to cause damping of the noise which originates from a source in the volume when it is transported from the volume into and out of the channel to the surroundings.

Referring now figures 2 and 3, the structure of the noise damping arrange- ment of the invention will be explained.

9 designates an outer wall of a housing in which a motor having a blower is incorporated and shown by the reference numeral 16 in fig. 3. As will be seen, the housing has an inner wall 17 which is concentric with the wall 9.

The outer wall 9 has a vertically extending gap 13, while the inner wall has a vertically extending gap 14. As will be seen, the two gaps are connected with each other along their one edge by a partition 12.

Figures 2 and 3 additionally show an intermediate wall 10 which has a gap defined by the partition to form two gaps 18 and 19. As will be seen, these gaps have different widths. However, it is possible that the gaps may have the same width.

The transport of air and the propagation of noise from the motor and the

blower will now be explained more fully.

The air and the noise are conveyed from the interior of the housing out through the inner gap 14 concentric with the housing, and from there it ex- tends into the gap 19 and is then divided into two paths that extend con- centrically in the space between the outer wall 9 and the intermediate wall 10, shown at 20, as well as in the space between the intermediate wall 10 and the inner wall 17, shown at 21. The two flows of air meet again at the gap 18 and is conveyed out through the gap 13. By adaptation of the gap dimensions and lengths of the outer, inner and intermediate walls it is pos- sible to provide a desired noise damping characteristic.

Since the noise is divided into outer and inner propagation paths, they will, when joined again, be phase-shifted relative to each other, which means that part of the noise originating from the motor and the blower will be damped. Further, the distance between the inner and outer walls at the dis- charge from the inner volume defined by the intermediate member 12 will damp noise having wavelengths which are'/4 of the distance. It is hereby possible to dimension the walls of the housing with gaps so that a given noise characteristic from a motor and a blower may be compensated.

Since, moreover, the flow of air leaves the housing longitudinally of it, there is freedom to connect a filter, e. g. as shown in fig. 4 at the reference nu- meral 22, and thus not necessarily inside the housing or at the bottom of it, even though this is also possible.

It should also be observed that even though the invention has been ex- plained with the use of an intermediate wall, it will also have noise-damping properties if the intermediate wall is omitted.