GB2028049A

GB2028049A - Sound absorbing resonators

Info

Publication number: GB2028049A
Application number: GB7926179A
Authority: GB
Original assignee: Messerschmitt Bolkow Blohm AG
Current assignee: Airbus Defence and Space GmbH
Priority date: 1978-08-09
Filing date: 1979-07-27
Publication date: 1980-02-27
Also published as: DE2834823B1; IT1162762B; FR2433217B2; GB2028049B; DE2834823C2; FR2433217A2; IT7924674A0

Abstract

An individual volumetrically variable resonator comprises a chamber defined by wall elements (1) at least one of the wall elements being resilient and at least one of the wall elements being curved and externally convex, the height h of curvature of the or each said curved wall element being 0.5-5 times its thickness s and the diameter d of said curved wall element being 30-3000 times its thickness s. Preferably at least one of the wall elements is provided with an external shoulder (2). A composite resonator comprises a plurality of said individual resonators arranged together. The wall elements may be of metal, or of reinforced plastics to give high damping. Damping may also be provided by means within the chamber e.g. a low pressure gas or a friction mesh, or by eddy currents. Damping material may be sandwiched between the joined edges of the wall elements, and the chambers of a composite resonator may be in communication. <IMAGE>

Description

SPECIFICATION Improvements in or relating to individual and composite volumetrically variable resonators The present invention relates to an individual volumetrically variable resonator comprising a reservoir defined by wall elements and also to a composite volumetrically variable resonator comprising a plurality of said individual resonators arranged together.

The object of the present invention is to secure one or more of the following advantages: to save material and production costs; to reduce the mass of the resonant material and to increase the resonant surface(s); to influence the damping favourably; to compensate adequately for fiuctuations of external air pressure; to integrate anti-noise louvres; to construct sound-absorbing walls.

According to one aspect of the invention there is provided an individual volumetrically variable resonator comprising a reservoir defined by wall elements, at least one of the wall elements being resilient and at least one of the wall elements being curved and having an external convexity, the height of curvature of the or each said curved wall element being 0.5-5 times the thickness of said curved wall element and the diameter of said curved wall element being 30-300 times the thickness of said curved wall element. Preferably said wall element has an external convexity of conical or spherical shape.

Expediently at least one of the wall elements is resilient and curved and desirably at least one other wall element is curved or planar.

Preferably at least one of the wall elements is provided with an external shoulder; for instance the reservoir may be one that is defined by wall elements joined together at their edges and where at least one of the wall elements is provided with an external shoulder adjacent one of said edges. The reservoir may be under vacuum or reduced pressure. In general the reservoir may be defined by two wall elements joint together at their edges and preferably where both of said wall elements are curved.

In one desirable embodiment of the invention the reservoir is defined by wall elements joined at their edges and for at least one of the wall elements there is an intermediate region of the edges of reduced thickness as compared with the thickness of the edge region(s).

A central internal region of at least one of the wall elements defining the reservoir may be reinforced by a flexurally resistant strut. At least one of the wall elements may be substantially disc-shaped and may be reinforced by a spoke-shaped strut.

The internal space of the reservoir may be provided with obstructing means to impede flow of fluid medium within the reservoir. For instance, such obstructing means may comprise a residual gas of high viscosity, wire mesh or netting, a drop of liquid, one or more permanent magnets fixed into or onto the internal surface of at least one of said wall elements.

For a resonator embodying the invention and where the reservoir is defined by wall elements joined at their edges such obstructing means may comprise a damping means or material sandwiched by the joined edges.

In one desirable form of the invention where the reservoir is defined by wall elements joined at their edges annular springs in a flexible fluid tube are interpositioned between the joined edges so as to compensate for external changes in the atmospheric pressure. In another desirable form of the invention, where the reservoir is defined by wall elements joined at their edges, in order to compensate for changes in the atmospheric pressure the reservoir contains a residual gas at least one wall element is set to a decreasing negative elasticity constant.

According to another aspect of the invention there is provided a composite volumetrically variably resonator comprising a plurality of individual resonators arranged together, the individual resonators being of the kind defined by the first aspect of the invention. The individual resonators may be so constructed that when they are arranged areally the wall elements are hexagonal, quadrangular or strip-shaped in horizontal projection. In one expedient form of this second aspect of the invention the reservoir of an individual resonator is arranged to communicate with the reservoir of a neighbouring individual resonator via a conduit or a channel, the internal pressure being set to an appropriate level in order to compensate for external changes in the atmospheric pressure.The individual resonators may be fixedly connected to one another and the individual resonators may be so constructed that the wall elements are adapted to be bent, curved or twisted so as to alter the degree of prestressing to thereby compensate for the elasticity constant and the natural frequency.

By way of example only, various embodiments of the invention will now be described with reference to the accompanying diagrammatic drawing, wherein: Figure lisa sectional elevation of a first form of individual volumetrically variable resonator embodying the invention, Figure 2 is a sectional elevation of a second form of individual volumetrically variable resonator embodying the invention, Figure 3 which is similar in shape to Figure 1, is a sectional elevation of a third form of individual volumetrically variable resonator embodying the invention, Figure 4 is a sectional plan view of a fourth form of individual volumetrically variable resonator embodying the invention, Figure 5 is a horizontal projection of a first form of composite volumetrically variable resonator embodying the invention, Figure 6 which is similar in shape to Figures 1 and 3, is a sectional elevation of a fifth form of individual volumetrically variable resonator embodying the invention, Figure 7 which is similar in shape to Figures 1,3 and 6, is a sectional elevation of a sixth form of individual volumetrically variable resonator embodying the invention, Figure 8 which is similar in shape to Figure 2, is a sectional elevation of a seventh form of individual volumetrically variable resonator embodying the invention, Figure 9 is a sectional elevation of an eighth form of individual volumentrically variable resonator embodying the invention, Figures 10 and 11 are sectional elevations of ninth and tenth forms of individual volumetrically variable resonators embodying the invention, Figure 12 is a sectional elevation of a second form of composite volumetrically variable resonator embodying the invention, and Figures 13, 14 and 15 are sectional elevations of third, fourth and fifth forms of composite volumetri cal ly variable resonators embodying the invention.

It will be understood that with reference to all of the illustrated embodiments, even where no dimensions are stated, and in accordance with the invention, the height of curvature h of the or each curved wall element is 0.5-5 times the thickness s of said curved wall element and the diameter d of said curved wall element is 30-300 times the thickness s of said curved wall element.

The final digits of the reference numerals used in the drawings, including the basic numbers 1-9, denote the following: 1 = curved wall element.

2 = external shoulder.

3 = supporting point.

4 = planar wall element.

5 = reinforcements.

6 = damping means.

7 = pressure-compensating springs.

8 = flexible annular substance.

9 = passages.

The individual resonator of Figure 1 relates to one which can be prepared by a simple method of production and more complete utilization of the material. It consists of two externally convexly curved wall elements 1 interconnected in a vacuum-proof manner at supporting points 3. At points 3 the wall elements 1 are joined together at their edges. The internal vessel thus formed constitutes a reservoir and may be evacuated. Adjacent the said edges are external shoulders 2. Owing to the under-pressure to which they are subjected the wall elements 1 are practically flat in the relevant load zone, with a low elasticity constant. Generally without the shoulders 2 the curved wall elements 1 would be unable to participate unimpeded in the vibration. In such an event spacers would be generally required at the supporting points 3.The curved wall elements 1 themselves are advantageously constructed on rotationally symmetrical principles. The course taken by the external convexity is optional within very wide limits. Instead of the approximate frusto-conical shape of the resilient wall elements, part-spherical shapes can be adopted, as well as surfaces of a higher order. By "surface of ann th order", in this context, is meant that the external convexity takes its course in accordance with a powerfunction ofthen th order. With surfaces of a higher order the edge zones participate more intensively in the vibration, resulting in a better degree of utilization. When the curved wall elements 1 are made of steel the height h of the externally curved convexity amounts to 0.5-5 times (preferably 1.5 times) and the diameter d to 30 to 300 times the wall thickness s.Materials with a low elasticity modulus result, owing to the route ratio, in smaller diameters for the curved wall elements 1. It is of advantage to use gas-tight materials of low density, a high elasticity modulus with a high yield point and a high vibratory load, e.g. light metals such as beryllium, aluminium or magnesium, as well as glass and fibrous materials.

The versions shown in Figures 2-5 relate to a reduction of the resonant mass and an increase in the resonant surface. Smaller resonant masses increase the admittance. Larger resonant surfaces increase the volumetric stroke. In Figure 2 the thickness of the curved resilient wall element 11 tapers towards the middle in the zone of the maximum vibration amplitude. The opposite wall element 14 is planar and preferably is also resilient. In Figure 3, on the other hand, central reinforcements 25 are provided. These consist of struts, with a high polar moment of inertia, situated perpendicularly to each curved resilientwall element 21 and connected thereto. They cause the internal portion of the curved wall element 21 to move like a piston, which increases the volumetric stroke.Figure 4 is a sectional plan view of a resilient wall element 31 with an externally shouldered zone 32 and a supporting zone 33; spoke-shaped struts 35 are provided. The intermediate material is of reduced wall thickness, in order to reduce the resonant mass. In Figure 5 the base surfaces of the wall elements 41 of several individual resonators arranged to form a composite resonator are hexagonal. By reducing the bearing surface 43 of the resonant surface is increased. In addition to hexagonal disposition they may instead be quadrangular, triangular, strip-shaped or asymmetrical.

The individual resonators illustrated in Figures 6 to 9 show possible ways of producing an internal damping effect. This enables the propogation of sound to be absorbed. If the system is adapted to the wave resistance of the surrounding medium, wall elements could be constructed which are free of reflection. In Figure 6 the internal space formed by the curved wall elements 51 may not be completely evacuated. Furthermore, obstacles to the flow, such as a wire netting 56, are provided. As the central parts of the curved resilient wall elements 51 have greater vibration amplitudes than the edges, the residual gas is caused to move transversally. This movement is damped by friction and additionally by the flow impeding devices 56. As is known, the viscosity of a gas is largely independent of the pressure, so that even with low internal pressures an adequate damping effect is guaranteed. With gases having a very small molecule diameter the viscosity coefficient is higher. In Figure 7 a drop of fluid 66 is provided in the interior of the curved resilient wall elements 61. Owing to the vibratory movement this drop undergoes continuous deformation, thus again producing a damping effect. In Figure 8 a magnetic vorticial damping system is adopted. For this purpose small-scale magnets 76 are embossed in the preferably resilient, planar wall element 74. If the vibrating, curved resilient wall element 71 consist of an electrically conductive material, currents are induced therein. In Figure 9 damping materials 86, such as adhesive agents or plastics, are provided at the bearing points 83 between the edges of curved resilient wall element 81 and the, preferably resilient, planar wall element 84.Vibrations of the wall element 81 are accompanied by a relative movement between the walls 84 and 81, which is damped. A further possible means of damping is the selection of a suitable material for the said wall elements; for example, plastics reinforced with glass or with carbon fibre provide a high degree of material damping.

Figures 10 and 11 show constructional characteristics adopted for the purpose of compensating pressure differences due to weather conditions or to heights. By altering the external pressure the operating point of the resonator participating in the vibration is likewise altered. Owing to the non-linear elasticity characteristic of the wall elements a different elasticity constant and thus a different resonance frequency are obtained. In the case of wide-band noise with a use of a number of resonant frequencies graduated to different natural frequencies a change in pressure is generally negligible, since all the frequencies are displaced collectively, thus maintaining the wide-band effect. In the case of narrow-band noise, considerable pressure differences and wall elements of low eight per unit of the area, however, pressure compensators have to be employed.

In Figure 10 a resonator is formed by two curved resilient wall elements 91. Between the bearing points 93 is an intermediate spring 97. This is accommodated in a flexible annular tube 98 filled with fluid. The intermediate spring 97 at the same time subdivides the quantity of fluid into two separate rings and contains small passages 99. If the external pressure is (slowly) increased the intermediate spring 97 is compressed. The flexible annular tube 98 is thus widened, thus reducing the effective diameter of the wall elements 91. The operating point and thus the resonance frequency thus remain in the same position. The rapid changes in the sonic pressure, on the other hand, cannot alter the operating point, owing to the small time constants of the fluid equalization flow.

Figure 11 shows a further possible means of compensating the pressure. In this system the curved resilient wall element 101 is held by a comparatively heavier annular spring 107. The internal space formed by the curved resilient element 101 and the planar, preferably resilient, wall element 104 has not been completely evacuated. For compensating the spring action of the enclosed volume of gas likewise, the curved wall element 101 is set to a negative elasticity constant. If the ambient pressure is increased the internal volume is reduced, by the deflection of an annular spring 97 which is present here as well but not shown in the drawing, the elasticity constant of the enclosed gas thus being increased.A further increase in negative deflection of the curved resilient wall element 101 and increasing spring action of the gas cancel each other out, so that the resonance frequency remains constant. The same principle can be carried out without an additional annular spring 107 and solely with the static complete deflection of the curved wall element 101.

A further means of compensation is the interconnection, by gas channels, of the reservoirs of individual resonators arranged together. The reduced pressure may thus be set in accordance with the change in the external pressure. This is done, for example, by means of a vapour/fluid system: if the temperature of the fluid is increased this results in an increase of the vapour pressure and thus of the reduced pressure of the arranged resonators. Automatic pressure compensation can be obtained by means of a pressure chamber connected up to the system of channels. In this version the pressure chamber is built up on the same principle as for curved resilient wall elements. If the external air pressure changes the internal volume and thus the pressure change as a result of the static pressure movement.The said pressure is transmitted via the channel system and the individual resonators are thus readjusted. It is advisable for each separate zone of the resonators to be provided with its own pressure chamber, so that in the event of leakage the fault remains confined to the separate zone in question.

A further means of pressure compensation is to bend, curve or twist together the individual resonators intergrated into strips or surfaces. A measure of this kind changes the natural frequency of the individual resonators, via the prestressing.

The deformation of the strips or surfaces can be automatically effected by the change in the air pressure. Forthis purpose a barometer spring is provided, for example, which undergoes deformation in accordance with the external pressure. The same principle can be used for the compensation of temperature influences. In this case the adjusting element will consist of a bimetall element instead of the barometer spring.

Finally, Figures 12 to 15 show examples of individual resonators joined together to form composite volumetrically variable resonators constituting wall surfaces. Figure 12 illustrates a louvre arrangement serving to damp the sound when the air is given free passage. Applications of this kind arise in connection with windows, ventilating shafts, suction intake apertures and outlet apertures. Convex zones of the same kind as in the case of externally convexly curved resilient wall elements, such as described in greater detail in connection with Figure 1, are embossed in strips of sheet metal. Each two strips are joined together to form a gas tight pair, with vacuum or reduced pressure. The surface of the strip is positioned parallel to the direction of flow. The embodiments shown in Figures 13to 15 represent composite resonators consisting sound-absorbing walls, particularly with an additional increase in the surface area. In Figure 13 the curved resilient wall elements 121 are mounted on a planar, preferably resilient wall element 123. The resilient wall elements are damped in accordance with Figures 6-9. In Figure 14 plates of sheet metal having embossed all curved resilient wall elements 131 on both sides are joined together. In this system air apertures 139 are provided in the bearing surfaces, so that the air also acts on the curved resilient wall elements and these latter likewise take effect. Finally, the curved resilient wall elements 141 shown in Figure 15 are attached to the wall like scales.

Claims

1. An individual volumetrically variable resonator comprising a reservoir defined by wall elements, at least one of the wall elements being resilient and at least one of the wall elements being curved and having an external convexity, the height of curvature of the or each said curved wall element being 0.5-5 times the thickness of said curved wall element and the diameter of said curved wall ele ment being 30-300 times the thickness of said curved wall element.

2. Resonator according to Claim 1, wherein said wall element has an external convexity of approximate frusto-conical shape or part-spherical shape.

3. Resonator according to Claim 1 or Claim 2, wherein at least one of the wall elements is resilient and curved.

4. Resonator according to Claim 3, wherein at least one other wall element is curved or planar.

5. Resonator according to any preceding claim, wherein at least one of the wall elements is provided with an external shoulder.

6. Resonator according to Claim 5, wherein the reservoir is defined by wall elements joined together at their edges and at least one of the wall elements is provided with an external shoulder adjacent one of the said edges.

7. Resonator according to any preceding claim, wherein the reservoir is under vacuum or reduced pressure.

8. Resonator according to any preceding claim, wherein the reservoir is defined by two wall elements joined together at their edges.

9. Resonator according to Claim 8, wherein both of said wall elements are curved.

10. Resonator according to any preceding claim, wherein the reservoir is defined by wall elements joined at their edges and for at least one of the wall elements there is an intermediate region of the edges of reduced thickness as compared with the thickness of the edge region(s).

11. Resonator according to any preceding claim, wherein the central internal region of at least one of the wall elements defining the reservoir is reinforced by a flexurally resistance strut.

12. Resonator according to any preceding claim, wherein at least one of the wall elements is substantially disc shaped and is reinforced by a spokeshaped strut.

13. Resonator according to any preceding claim, wherein the internal space of the reservoir is provided with obstructing means to impede flow of fluid medium within the reservoir.

14. Resonator according to Claim 13, wherein the obstructing means comprises a residual gas of high viscosity.

15. Resonator according to Claim 13, wherein the obstructing means comprises wire mesh or netting.

16. Resonator according to Claim 13, wherein the obstructing means comprises a drop of liquid.

17. Resonator according to Claim 13, wherein the obstructing means comprises one or more perma nent magnets fixed into or onto the internal surface of at least one of said wall elements.

18. Resonator according to Claim 13, wherein the reservoir is defined by wall elements joined at their edges with inter-positioning of a damping means or material sandwiched by the joined edges.

19. Resonator according to any preceding claim, wherein the reservoir is defined by wall elements joined at their edges with interpositioning of annular springs in a flexible fluid tube to compensate for external changes in the atmospheric pressure.

20. Resonator according to any one of Claims 1 to 18, wherein the reservoir is defined by wall elements joined at their edges and to compensate for external changes in the atmospheric pressure the reservoir contains a residual gas and at least one wall element is set to a decreasing negative elasticity constant.

21. A volumetrically variable resonator comprising a reservoir defined by wall elements, at least one of the wall elements being resilient and at least one of the wall elements being curved and having an external convexity substantially as herein described with reference to any one of Figures 1-4 or any one of Figures 6-11 of the accompanying drawings.

22. A composite volumetrically variable resonator comprising a plurality of individual resonators arranged together, the individual resonators being of the kind defined in any preceding claim.

23. Composite resonator according to Claim 22, wherein the individual resonators are so constructed that when they are arranged areally the wall elements are hexagonal, quadrangular, triangular or strip-shaped in horizontal projection.

24. Composite resonator according to Claim 22 or Claim 23, wherein the reservoir of an individual resonator communicates with the reservoir of a neighbouring individual resonator via a conduit or channel, the internal pressure being set to an appropriate level in order to compensate for external changes in the atmospheric pressure.

25. Composite resonator according to Claims 22 to 24, wherein the individual resonators are fixedly connected to one another.

26. Composite resonator according to Claim 25, wherein the individual resonators are so constructed that the wall elements are adapted to be bent, curved or twisted so as to alter the degree of prestressing to thereby compensate for the elasticity constant and the natural frequency.

27. A volumetrically variable composite resonator comprising a plurality of individual resonators arranged together substantially as herein described and with reference to any one of Figures 5, 12, 13, 14 and 15 of the accompanying drawings.