GB2121475A - Element having resonators for silencing gaseous flow - Google Patents

Abstract

A sound-deadening and sound- damping element suitable for incorporation into a ventilation or i.c. engine exhaust duct has resonators 7.11,7.12...7.21,7.22... formed by two thin flat opposed plates 7.1, 7.2 mounted on a frame structure 5, 6.1, 6.2 to enclose a gas volume 8, so that the resonators act on the enclosed gas volume. A bore 9 provides pressure equalisation between the volume 8 and the duct in which the element is mounted. The frame part 5 may be resilient and a plurality of elements (11, 13 16, Figures 3 to 5) may be mounted longitudinally in the duct (10, 12, 15). <IMAGE>

Description

SPECIFICATION A sound-deadening and sound-damping element having resonators This invention relates to a sound-deadening and sound damping element having resonators made of thin plates for incorporation into channels through which gas flows.

To lower the level of sound in channels or ducts, it is known to design the walls with soundabsorbing materials. Relative to these materials, it is a matter predominantly of an open-pore material, which, however, easily becomes fouled and more particularly needs a large constructional volume.

Furthermore, a sound absorber having plate resonators in general, as well as an appropriate absorber also made of plastics foil in particular, is described by F. Mechel and N. Kiesewetter in "ACUSTICA", vol. 47 (1981), pages 83-88. In such sound absorbers, the sound field excites a plate into bending or flexural oscillations, which are damped by internal friction and thus bring about sound absorption. In the above-mentioned experiments described in ACUSTICA, a plastics foil has been so deformed that there arise rectanguiar surface elements, or elementary areas or surface segments, of a few centimetres in length and width, which are bounded at the edge by a kink or jog or knee. The kink at the edge of these plates acts as a fastening and inhibits free movement of the foil at this point.The plate is therefore excited into natural oscillations by sound waves. The wave lengths of these natural oscillation shapes are in the frequency range up to 5000 Hz considerably smaller than the basic wave length of the impinging air sound wave. The amplitude of oscillation of the plate, and thus the sound-absorbing effect thereof, becomes particularly great at the natural frequencies.

So that such a plate is excited by air sound into natural oscillation, incoherent pressure impingement on the plate from both sides is necessary. For this, the plate is securely fixed at its edge into a frame. The frame is provided with a rigid terminal wall so that a cavity filled with air arises between the plate and this terminal wall.

The enclosed air then acts like a simple spring on the reverse side of the plate.

To use such plate resonators in channels through which gas flows, a major part of the inner walls of the channel would have to be designed with such plate resonators.

The air of the invention is, therefore, to provide a sound-deadening and sound-damping element having resonators made of thin plates for incorporation into channels through which gas flows, which element acts over a wide frequency range and only needs a slight construction volume. This aim is achieved, in accordance with the invention by providing a sound-deadening and sound-damping element having resonators in the form of thin plates for incorporation into channels through which gas flows, characterised in that it comprises at least two opposite flat plates as well as at least one bending-resistant frame connected in a force-locking manner to the plates to enclose a gas volume, and in that the plates, which are in each case acted upon from the outside with sound waves, act jointly on the enclosed gas volume.

The sound-deadening and sound-damping elements in accordance with the invention are excited into oscillation in a sound field in their respective natural frequency. The co-oscillating resonators become secondary sound sources, and this leads, through phase reversal, to a sound pressure compensation between the incident sound wave and the sound wave which is radiated back. The damping accordingly arises as a combination of the said sound pressure compensation, and energy loss in the plate.

As mentioned above, a single or simple plate resonator remains ineffectual if the same sound pressure is present on both sides. However, if the plates are joined together, in accordance with the invention, into a ciosing housing, then the sound pressure is always present unilaterally (or on one side) on the respective outsides of the plates, so that these are excited into oscillations. The entire sound-absorbing area of such an element is therefore twice as great as in the case of the known plate resonators. As a result of an appropriate choice of the thickness of the enclosed gas volume, the resonance frequency of the system consisting of plate mass and spring action of the enclosed gas volume can be adjusted at will.

The invention will be described further, by way of example, with reference to the accompanying schematic drawings, in which: Fig. 1 is a cross-section through a simple or single plate resonator element conforming to the present invention; Figs. 2a and 2b are respectively a cross-section on a top view illustrating a unit consisting of several plate resonator elements of the invention; Fig. 3 is a perspective view illustrating a channel or duct through which gas flows, having plate resonator elements in accordance with the invention; Fig. 4 is a perspective view illustrating a channel or duct through which gas flows, having plate resonator elements in accordance with the invention and which act to by-pass and which act on a common gas volume; and Fig. 5 shows a waste-gas duct or exhaust pipe for an internal-combustion engine, having plate resonator elements which are arranged as a bypass.

The basic element, shown in Fig. 1, of a plate resonator in accordance with the invention comprises a frame 1 having a frame height h and a frame diameter D. The frame is covered on both sides with thin plates or foils 2.1 and 2.2, so that these, together with the frame enclose a gas volume 3. So that the plates 2.1 and 2.2 are, at all times, undeformed in their normal state, the enclosed gas volume 3, without impingement by sound pressure, has to correspond at all times, with the environmental pressure. For this, provided in the frame 1 is a narrow bore 4 through which pressure differences can slowly equalise. The resilience of the enclosed gas volume is substantially determined by the height h of the frame.

Fig. 2a shows a cross-section through a unit having several plate resonators, which unit in turn comprises a frame 5 acting as a distance layer or spacer layer. Fastened on this frame 5 on both sides are respective latticed bending-resistant frame parts 6.1 and 6.2, at top view of which is shown in Fig. 2b. The latticed frame parts 6.1 and 6.2 are, in each case, covered on the outside with a respective thin plate or foil 7.1 or 7.2 and are fastened along all the frame webs. In this way plate resonators 7.11,7.12,... 7.21,7.22,...

which are oscillatory or capable of oscillation independently of one another, arise. The individual resonators act as a whole on an enclosed gas volume 8, which by way of a pressure equalisation bore 9, without pressure impingement, assumes environmental pressure.

The resonance frequencies of the individual resonator plates are determined, inter alia, by the size of the window apertures fixed by the lattice frames 7.1 and 7.2. These window apertures may be round, rectangular or square; in the case of angular dimensions, the greatest surface utilisation is achieved. The distance frame 5 can be designed to be formed by springs or by a rubber layer in such a way that subsequent adjustment of the thickness of the enclosed gas volume is possible. In this way, the spring properties of the encosed gas volume are varied.

Fig. 3 shows a channel or duct 10 through which gas flows and in the interior of which are plate resonator elements 11, arranged parallel and at intervals. As a result of such an arrangement, for example ventilation or air shafts can be insulated against transmission of fan or ventilator noises or other sound waves, without the flow resistance being significantly increased.

In the exemplified embodiment, shown in Fig. 4, of a sound-damped ventilation channel or duct 12; plate resonator elements 13 in accordance with Fig. 2 are arranged in a manner similar to the arrangement of Fig. 3.

However, the elements 13 fill only about half the height of the ventilation channel or duct 12 and therefore act partially in by-pass. The individual plate resonator elements 13 are open at their undersides and are connected to one another in such a way that they enclose, together with the bottom wall of the channel, a common air cushion 14. In this way, the spring action or resilient action of the enclosed air cushion is distinctly reduced.

Fig. 5 shows an end of a waste-gas or exhaust gas pipe 15, of an internal combustion engine, which is widened by two-sided outwards bulges 1 5.1 and 15.2. Inside these outward bulges, plate resonator elements 16, similar to those provided in Fig. 4, are arranged in accordance with the by pass system. Such an arrangement contributes, without effecting the flow conditions, to a considerable reduction in intensity of the exhaust gas noises.

Claims (7)

Claims

1. A sound-deadening and sound-damping element having resonators in the form of thin plates for incorporation into channels through which gas flows, characterised in that it comprises at least two opposite flat plates as well as at least one bending-resistant frame connected in a force-locking manner to the plates to enclose a gas volume, and in that the plates, which are in each case acted upon from the outside with sound waves, act jointly on the enclosed gas volume.

2. An element as claimed in claim 1, characterised by two opposite continuous thin plates, and by at least one bending-resistant supporting frame having a plurality of window apertures of the same size and/or different sizes and characterised in that the plates are connected on the supporting frame or frames in a force locking manner, more particularly by adhesion, in such a way that a multiplicity of independently oscillatory resonator elements arises.

3. An element as claimed in claim 1 or 2, characterised in that the frame and/or the plates have at least one comparatively small opening through which pressure differences, between the enclosed gas volume and the environment, which differences in time change more slowly than the sound pressure can be equalised.

4. An element as claimed in claim 1, 2 or 3, characterised in that it comprises a multiplicity or plurality of plates having resonates, two each of which enclode a gas volume, arranged side-by side at predetermined spacings and arranged with at least one of the non-oscillating sides thereof on the inner wall of a channel or duct through which gas flows.

5. An element as claimed in claim 4, characterised in that all the resonators act on a common enclosed gas volume.

6. An element as claimed in claim 4 or 5, characterised in that the resonators are arranged as a by-pass system on a channel through which gas flows.

7. A sound-deadening and sound-damping element substantially as hereinbefore described with reference to and as illustrated in Fig. 1, in Figs. 2a and 2b, in Fig. 3, in Fig. 4 or in Fig. 5 of the accompanying drawings.