GB2271387A - Noise reduction by sound wave interference. - Google Patents

Description

2271387 NOISE REDUCTION BY MEANS OF ACOUSTIC FEEDBACK This invention

relates to a method of compensating for the pressure of sound waves within a sound propagation medium, in particular air, using the principle of acoustic feedback.

The reduction and substantial prevention of noise is a particularly significant factor with respect to tircraft jet engines. Hence, a range of noise suppression systems have already been proposed for jet engines, including the ducted-fan turbine engine, also known as the turbofan, which has proved to be one of the most effective methods of noise reduction. Air flow in the peripheral area of the engine clearly reduces large velocity gradients in the jet efflux and hence effectively reduces noise.

In the case of civil aircraft jet engines, what are known as ejectors, multiple-tube nozzles and corrugated nozzles are also used. The latter are not true noise suppressors, but shift the noise into the higher frequency ranges where the absorption of air is greater. Because of the shorter sound paths, this method Is less effective for-low flying aircraft. However, It is not only low flying noise but in particular the noise generated during the take-off and landing of aircraft which causes considerable noise nuisance, especially when close to populated areas. During take-off jet noise generated as a result of the jet airstream mixing with the ambient air is dominant, whereas during landing there is also noise from the forward radiated compressor.

The object of the present invention is to provide means whereby the sound pressure of sound waves within a sound propagation medium, in particular air, may be compensated for using the principle of acoustic feedback, so that in the case of specified noise sources, for example, the noise level can be substantially suppressed by simply retrofitting the engines. Weight- saving measures are of particular importance In aircraft construction and thus fittings mounted in or on an engine must be as lightweight as possible. It should also be possible to apply the measures proposed according to the Invention to as many - different noise sources as possible to ensure the broadest possible noise reduction.

This object is achieved according to the Invention in that at least one wavegulde enclosing a volume is connected to the propagation medium via one or more openings, the volume of said waveguide being filled with a medium which facilitates a higher sound velocity than that of the propagation medium.

The method of reducing emission from a noise source by acoustic mismatch is known. one means of achieving this is to provide as small an additional impedance as possible in the area of the noise source. Acoustically, this means a parallel arrangement of radiation resistanGe and additional impedance associated with reduction and dissipation of the noise radiated to the surrounding area. An enclosed volume is used for this purpose which is connected to the propagation medium via what are termed coupled ports. A medium is provided Inside the volume, In which the sound is propagated at a clearly higher velocity than In the propagation medium, which is usually air, surrounding the additional Impedance.

A sound wave is composed of zones of higher pressure and zones of lower pressure; what are known as the compression and depression zones of a sound wave. The pressure gradients should therefore be minimised to a large extent by causing the opposed zones of high and low pressure to become superimposed in the form of - destructive Interference.

Sound propagates In the medium of air at a propagation velocity of approx. 340 m/s, whereas the propagation velocity in hydrogen gas is about 1800 m/s.

If such an atmosphere In which the velocity of sound Is higher than that in air is introduced into a sound field, then part of the sound wave spreads into that atmosphere, for example, of hydrogen, and propagates much more quickly there. Hence, a phase shift results between the two separate wave trains, such that the out-of-phase pressure fields may be superimposed in such a way that the areas of high pressure and the areas of low pressure compensate each other to a large extent. In acoustics such destructive interference-of pressure waves is also known as acoustic feedback.

According to the invention an enclosed volume in the form of a waveguide filled with a gas with a high sound velocity, e.g. hydrogen or helium gas, is connected to the. propagatIon medium air via coupling ports. It Is advantageous, if the waveguide Is generally tubular, for it to be placed so as to be parallel'of slightly Inclined In relation to the direction of propagation of the sound waves. In view of the high sound velocities In hydrogen of approx. 1800 m/sec., pressure compensation occurs in the outer propagation medium due to the different sound velocities in ambient air and waveguide gas. This pressure equalisation as a result of the feedback compensation of the H20 (or He) gas acts as an acoustic mismatch and reduces noise emission.

The sound velocity in the waveguide, and with It the effect of the feedback, may be Increased further by raising the gas temperature inside the waveguide. For example, the waveguide can be provided with an electric resistance heating unit which ensures that the waveguide medium is heated in a controlled manner.

The invention is described in more detail below, without restricting the general concept of the invention, by means of example, with reference to the drawings. The drawings are expressly referred to for disclosure of any details of the invention which are not explained further in the text. In the drawings:

Figures la, b are diagrammatic side and sectional views of a waveguide, with a slot opening according to the Invention; - Figure 2 is a partial side view of a waveguide with holes according to the invention; Figure 3 is a partial side view of a waveguide with cover units according to the invention; Figures 4a, b are schematic cross-sections through two embodiments of waveguide according to the Invention; Figures 5a, b show how waveguides according to the Invention may be used with a jet turbine; Figure 6 shows a further embodiment of waveguides according to the invention used with a jet - turbine; Figure 7 shows a waveguide according to the invention inside an inlet shaft of a jet turbine; Figure 8 shows a waveguide according to the invention used with a propeller; Figure 9 shows a waveguide according to the invention in the inlet duct of a jet turbine; Figure 10 shows a waveguide according to the invention in the inlet shaft of a jet turbine.

Figure 1 shows a waveguide 1 according to the invention with an inlet 2 at one end and an opening in the form of a slot 3 running parallel to the tube axis.

Figure lb shows the corresponding cross-section. The -6waveguide constitutes a cavity resonator and it is filled or replenished with an appropriate medium, for example hydrogen, via the inlet 2.

With respect to the dimensioning of the waveguide, It must be ensured that Its length Is greater than the longest sound pressure wave still to be compensated.

Figure 2 shows a further embodiment of a waveguldt 1 according to the Invention with linearly arranged holes 4 as openings. The distance between these holes 4 should be less than half the sound wave length.

Figure 3 shows a further example of a waveguide 1 with respective scalelike cover units 5 over each opening in the waveguide. Such cover units inhibit the waveguide medium from being discharged from the wavegulde 1.

Figures 4a and 4b also show cross-sections of waveguides according to the invention with additional structures to inhibit or impede the waveguide medium from being discharged from the inside of the wavegulde to the outside. Hence, in Figure 4a, the opening 12 is provided with a collar 6 projecting inside the waveguide. Loss of waveguide gas can also be considerably reduced, if the volume of the waveguide 1 is almost divided into two by webs 7, as shown in Figure 4b.

Figure 5a represents a lateral view of a possible application of the waveguide 1 according to the invention in conjunction with a jet turbine 8. In this case, the waveguide 1 is disposed underneath the jet turbine 8 in such a manner that at least the sound pressure field (shown by dotted lines) radiated downwards Is covered by the effective range of the wavegulde 1. Figure 5b shows an example of four tubular waveguides 1 positioned radially around the lowtr section of the jet turbine 8 in such a way that the distance between adjacent waveguides Is less than half the smallest sound pressure wave still to be compensated. This measure ensures that the sound pressure field radiated downwards Is suppressed considerably.

Figure 6 shows a possible arrangement of waveguldes according to the invention in the wake of a jet turbine 8. These are annular waveguides 1 which ensure radial damping of the sound pressure field generated by the jet turbine 8.

Figure 7 shows a diagrammatic cross-section through a jet turbine 8 having a radially disposed waveguide 1 forming an oval shape, particularly to compensate for the sound pressure which occurs in the forward area of the turbine during landing with reverse thrust. The diagram shows the turbine having a compressor 9.

The device 1 according to the invention may also be attached as a radial waveguide in the wake of a -8propeller 10 (see Figure 8).

Figure 9 shows a further configuration of the wavegulde 1 used with a jet turbine 8. A waveguide attached radially to the inlet is also provided here, however in this case It Is provided with a number of holes 4.

Finally, Figure 10 shows a tubular waveguide 1 positioned In the inlet shaft of a jet turbine 8 which to a large degree reduces a forwardly directed sound wave field, as in the embodiments shown in Figures 7 and 9.

Particularly where the waveguide Is used with jet turbines according to the invention, the ambient temperature at the waveguide may be above the ignition temperature of hydrogen gas, which is the preferred medium supplied to the waveguide. This results In the hydrogen discharged from the wavegulde being ignited and its waste heat raising the temperature of the waveguide, which can cause an Increase in the sound velocity In the waveguide.

Further applications not restricted to jet engine technology are conceivable in principle.

Claims (19)

CLAIMS:

1. A method of compensating for the pressure of sound waves within a sound propagation medium using the principle of acoustic feedback, whereby at least one waveguide enclosing a volume is connected to the propagation medium via one or more openings, the volume of said waveguide being filled with a medium which facilitates a higher sound velocity than that of the propagation medium.

2. A method according to Claim 1, wherein the waveguide is positioned so as to be parallel or slightly Inclined in the direction of the sound waves.

3. A method according to Claim 1 or 2, wherein the medium is hydrogen, helium or superheated steam.

4. A method according to Claim 1, 2 or 3, wherein - the waveguide is tubular and Is longer than the longest sound pressure wave to be compensated.

5. A method according to any preceding claim, wherein the opening of the waveguide Is a narrow slot running parallel to the axis of the tube.

6. A method according to any of claims 1 to 4, wherein the openings of the waveguide are circular holes arranged parallel to the axis of the tube.

7. A method according to any preceding claim, wherein scale-like cover units are provided over the opening(s).

8. A method according to any preceding claim, wherein the or end opening is provided with a collar or layer directed Inwardly of the volume enclosed by the waveguide.

9. A method according to any preceding claim, wherein the volume enclosed by the waveguide is formed by two connected chambers.

10. A method according to any preceding claim, wherein a heating unit is connected to or provided in the waveguide.

11. A'method according to Claim 10, wherein the heating unit is an electric resistance heater.

12. A method according to any preceding claim, wherein when hydrogen is used as the waveguide medium, the hydrogen discharged from the opening(s) is burned and the resulting combustion heat is passed through the waveguide wall to raise the temperature of the waveguide medium.

13. A method according to any preceding claim, wherein the waveguide is a hollow body formed in the shape of a ring.

14. A method according to Claim 13, wherein the annular waveguide is incorporated into the air Inlet housing of a jet turbine.

15. A method according to Claim 13, wherein the annular waveguide is attached axially behind a propeller of a jet engine.

16. A method according to any preceding claim, wherein the waveguide is attached In front of a jet turbine.

-1117. A method according to Claim 1, wherein the sound velocity of the propagation medium is less than is the case In air, and air is used as the waveguide medium.

18. A method according to any preceding claim, wherein several waveguides are placed In a sound pressure field to be compensated in such a way that the distance between adjacent waveguides Is less than half the smallest sound pressure wave to be compensated.

19._ A method of compensating for the pressure of sound waves within a sound propagation medium substantially as hereinbefore described with reference to the accompanying drawings.