SPATIALLY GLOBAL NOISE CANCELLATION

Abstract

Systems and methods for the reduction of noise arising from the fluid mechanics of the aerodynamic interactions of the relative motion of solid elements with a fluid, generally air, by generating pressure fields at or close to the source of the aerodynamic noise, these pressure fields having amplitudes and frequencies equivalent to those of the noise fields to be reduced, but phases opposite thereto. These generated pressure fields globally cancel the effect of the noise fields and this affect is propagated into the far field. Use is made of planar electro-thermal transducers, transforming a periodically fluctuating heat flux generated by Joule AC heating into an acoustic wave. The frequency, amplitude and phase of the noise field may be detected by microphones positioned close to points of generation of the noise field, such that the cancellation is effective over the same region as the noise field propagates.

Claims

1. A system for the reduction over a spatial volume, of a noise field arising from an aerodynamic interaction of an element having relative motion with its surrounding fluid, said system comprising: at least one microphone disposed on said element or in close proximity thereto, said at least one microphone adapted to produce an output signal corresponding to the noise field arising from interaction of said element with its surrounding fluid; at least one planar thermo-acoustic generator having an electrically powered heating layer, said thermo-acoustic generator being disposed on said element, or in close proximity thereto; and a control unit adapted to utilize the output signal of said at least one microphone, and to generate current correlated to the output signal for application to the electrically powered heating layer, such that said at least one thermo-acoustic generator emits a compensating noise field having the frequencies and amplitudes of that measured by said at least one microphone, but having an opposite phase, such that said noise field is globally reduced over said spatial volume.

2. A system according to claim 1 wherein said relative motion arises either from motion of said element through said fluid, or from motion of said fluid past said element, or from a combination thereof.

3. A system according to claim 1, wherein said relative motion of said fluid with respect to said element is temporally or spatially non-uniform motion.

4. (canceled)

5. A system according to claim 1, wherein said element is at least one blade of a fan or a compressor or a turbine.

6-7. (canceled)

8. A system according to claim 1, wherein said at least one thermo-acoustic generator is disposed on at least one of the stator or the rotors of a fan or a compressor, or a turbine.

9-10. (canceled)

11. A system according to claim 1, wherein said at least one thermo-acoustic generator is disposed on at least one of the surfaces of a ground vehicle or an aerial vehicle.

12. (canceled)

13. A system according to claim 1, wherein said control unit is configured to spectrally analyze said noise field, and to generate from spectral components of said noise field, waveforms of current for applying to said at least one thermo-acoustic generator, having frequency, amplitude and phase such that the spectral components of said compensating noise field emitted by said thermo-acoustic generator neutralize said spectral components of said noise field.

14. A system for the reduction over a spatial volume, of a noise field arising from an aerodynamic interaction of an element having relative motion with its surrounding fluid, said system comprising: at least one planar thermo-acoustic generator having an electrically powered heating layer, disposed on said element or in close proximity thereto; and a control unit adapted to generate a current correlated to the noise field for application to the electrically powered heating layer, such that said at least one thermo-acoustic generator emits a compensating noise field having the frequencies and amplitudes of said noise field, but having opposite phase, such that said noise field is globally reduced over said spatial volume, wherein said frequencies, amplitudes and phases of said noise field are predicted either by a simulation of said noise field as a function of the position and speed of motion of a moving element associated with the flow of said fluid, or by a set of prior measurements of said noise field as a function of the position and speed of motion of said moving element.

15. A system according to claim 14, wherein the motion of said moving element is either a rotation or displacement motion.

16-17. (canceled)

18. A system according to claim 14, wherein said moving element is at least one blade of a fan or a compressor or a turbine.

19-20. (canceled)

21. A system according to claim 14, wherein said at least one thermo-acoustic generator is disposed on at least one of the stator or rotor of a fan or a compressor or a turbine.

22. (canceled)

23. A system according to claim 14, wherein said at least one thermo-acoustic generator is disposed on at least one of the surfaces of a ground vehicle or an aerial vehicle.

24. (canceled)

25. A system according to claim 14, wherein said control unit is configured to spectrally analyze said predicted noise field, and to generate from spectral components of said predicted noise field, current waveforms for applying to said at least one thermo-acoustic generator, said current waveforms having frequency, amplitude and phase such that the spectral components of said compensating noise field emitted by said thermo-acoustic generator neutralize said spectral components of said predicted noise field.

26-28. (canceled)

29. A method for the reduction over a spatial volume, of a noise field arising from an aerodynamic interaction of an element having relative motion with its surrounding fluid, said method comprising: providing at least one planar thermo-acoustic generator having an electrically powered heating layer, disposed on said element or in close proximity thereto; and generating a current correlated to the noise field for application to the electrically powered heating layer, such that said at least one thermo-acoustic generator emits a compensating noise field having the frequencies and amplitudes of said noise field, but having opposite phase, such that said noise field is globally reduced over said spatial volume, wherein said frequencies, amplitudes and phases of said noise field are either measured or are predicted by a simulation of said fluid flow as a function of the position and speed of motion of a moving element associated with the flow of said fluid.

30-31. (canceled)

32. A method according to claim 29, wherein said noise field is measured in real time by at least one microphone, said at least one microphone adapted to produce an output signal corresponding to the noise field arising from interaction of said element with its surrounding fluid;

33. A method according to claim 29, wherein said noise field is previously measured by at least one microphone, said at least one microphone adapted to produce an output signal corresponding to the noise field as a function of the position and speed of motion of said moving element.

34. A method according to claim 29, wherein said generated current is produced by a control unit configured to spectrally analyze said noise field, and to generate from spectral components of said noise field, waveforms of current for applying to said at least one thermo-acoustic generator, having frequency, amplitude and phase such that the spectral components of said compensating noise field emitted by said thermo-acoustic generator neutralize said spectral components of said noise field.

35. A system according to claim 1, wherein the motion of said moving element is either of rotation or displacement motion.

36. A system according to claim 14, wherein said relative motion arises either from motion of said element through said fluid, or from motion of said fluid past said element, or from a combination thereof.

37. A system according to claim 14, wherein said relative motion of said fluid with respect to said element is temporally or spatially non-uniform motion.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:

[0048] FIG. 1 shows the comparative levels of noise from the various sources in an aircraft;

[0049] FIG. 2 is a graph illustrating the spectral breakdown of the various noise components of a complete aircraft;

[0050] FIG. 3 shows schematically illustrates a representation of the generation of pressure fluctuations resulting from highly turbulent vortices generated by the interaction of two rotors and their respective stators in a multistage gas turbine engine;

[0051] FIG. 4 shows schematically an external loudspeaker prior art noise cancellation scheme;

[0052] FIG. 5 illustrates a simplified drawing of a thermo-acoustic interaction device, of the type used in implementing the systems of the present disclosure;

[0053] FIG. 6 shows a similar application to that solved by the prior art apparatus of FIG. 4, but illustrating the significant advantage of the novel systems using the thermo-acoustic interaction devices described in the present disclosure;

[0054] FIG. 7A shows a cutaway schematic drawing of a high bypass, turbo fan engine, showing where thermo-acoustic generator devices can be positioned to reduce fan noise levels, and FIG. 7B shows one exemplary method of construction and mounting of such a thermo-acoustic generator device on a blade of the engine of FIG. 7A;

[0055] FIG. 8 illustrates schematically how active noise cancellation is achieved in the example of the turbofan engine of FIG. 7A;

[0056] FIG. 9 shows schematically thermo-acoustic devices applied to various selected areas of the external panels of a motor vehicle, in order to cancel the wind noise;

[0057] FIG. 10 illustrates thermo-acoustic devices applied to external surfaces of a military aircraft, to reduce the flow noise generated by the air passing across these surfaces; and

[0058] FIG. 11 illustrates a configuration for predictive calculation of rotor-stator interaction noise in a turbojet engine.

DETAILED DESCRIPTION

[0059] Reference is now made to FIGS. 3 and 4, which illustrate details of a typical exemplary source of the noise pollution which arise from aerodynamic interactions, and prior art active methods of combating such noise pollution. Reference is first made to FIG. 3, taken from an article by Marco Ernst, et al., of Aachen University, entitled “Analysis of Rotor-Stator-Interaction and Blade-to-Blade Measurements in a Two Stage Axial Flow Compressor” published by ASME in J. Turbomach 133(1), 011027 (Sep. 27, 2010) (12 pages). This drawing illustrates schematically a representation of the generation of pressure fluctuations resulting from highly turbulent vortices generated in the compressor stages of a multistage turbine engine. In FIG. 3, there is shown the air flow 35 entering the engine through the Inlet Guide Vanes (IGV) 30, and flowing through the first rotor 31, the first stator 32, the second rotor 33 and the second stator 34. Highly turbulent vortices 37, 38 are generated along the air stream through the compressor, with noise 39 resulting from the pressure fluctuations generated in several locations within the engine, mostly at the interaction points of each rotor blade with a stator blade which it passes, this resulting in either impingement 36 of the flow vortices with the leading edge of the next blade set, or lack of impingement 37, depending on the mutual and constantly changing positions of the next encountered leading edge. The resulting noise is produced from different phenomena such as blade-pass interaction between the stages, separation points, vortex wake interactions, large-scale turbulence, boundary layer radiation and shockwaves. Such a complex sound field configuration would be highly difficult to compensate for, using a local cancellation field externally applied by loudspeakers located remotely from the source positions, outside of the engine. The same considerations apply to airframe noise, generated at or close to moving surfaces, especially at the leading and trailing edges of airfoils. A particularly predominant source of noise is the flap-airfoil interaction, where highly turbulent motion is generated, this being substantially noticeable at landing approach.

[0060] Such a prior art external loudspeaker noise cancellation scheme is illustrated schematically in FIG. 4. The noise field 40 generated by a fan 41 is propagated in both directions upstream and downstream of the fan. Loudspeakers 42 are positioned outside of the fan, since such prior art sources cannot be positioned in the close proximity to the points of origin of the noise. The sound waves 44 which the loudspeakers generate interact with the noise field 40 generated by the fan, and the resultant output noise 43 should be at a reduced level compared with the original noise field 40. However, it should be clear that the ability to significantly cancel a complex noise field, such as that shown in FIG. 3, using discretely positioned sound sources, is very limited. Furthermore, since the location of the loudspeakers 42 is remote from the source of the noise field 41, the cancellation can only be effective over a limited set of predetermined positions where destructive interference takes place, leading to output noise 43 at a reduced level compared with the original noise filed 40. Outside of this small range of acoustic “fields of view”, the noise will not be cancelled, and moreover at specific other locations 45, may even be amplified due to constructive interference. Therefore, although this active prior art scheme may be effective for cancellation of noise in very specific locations resulting from a localized source, the cancellation effect will be very position dependent, and the prior art systems are thus ineffective in dealing with global environmental noise pollution such as that generated by a moving source such as an aircraft or a vehicle.

[0061] In contrast to the use of the prior art vibro-acoustic transducers remotely positioned relative to the position of the noise source, the devices described in this application, namely surface-deposited thermo-acoustic transducers, comprising a periodically heated electrically conductive thin layer, deposited directly on the noise source itself, addresses both of the shortcomings of the prior art cancellation systems shown hereinabove, namely (i) the ability to compensate for highly complex acoustic noise fields, and (ii) the ability to do so over a wide “field of view” space, such that the cancellation is generally effective no matter where the listening person is situated.

[0062] The structure of such heat flux transducers is simple and can be formed by conventional deposition techniques. Since the device is thin and its surface does not need to move mechanically in order to generate its output with respect to the application media, it can be deposited or attached to any surface in the vicinity of which the noise field is being generated. Such emitters do not create any macroscopic or microscopic mechanical motion during acoustic generation and create distortion-free sounds without any resonances that are characteristic to mechanical structures of vibrating elements. The sound is generated in the solid boundary at the solid-fluid interface, unlike conventional thermo-acoustic sources where the sound originates from the temperature-driven pressure gradients in the fluid, as in in Rijke tubes. Furthermore, the elements of the present application, neither take up significant space within the system in which they are installed nor increase the weight. They can also be constructed to withstand high temperatures. The above characteristics all point to their suitability for a wide range of applications, including turbomachinery, UAVs, drones, rotating systems, cars, wind turbines and printing mills.

[0063] Pressure field stimulation and sound production by means of Joule heating has been studied since the late 19th century. The term “thermophone” was coined two decades later, to define an acoustic transmitter capable of producing sound through high frequency thermal oscillations. Such thermo-acoustic interaction devices behave as electrical resistors, in which an alternating electrical current is converted to produce surface heat flux fluctuations and, consequently, pressure waves in the surrounding fluid, without requiring any mechanical motion. However, while there is no clear consensus in the literature as to the correct approach to modelling thermo-acoustic interaction device sound production, for the purposes of this disclosure, it is sufficient to describe such sound production as being electrically driven and capable of generating a spectral range of sounds, having both high and low frequencies. At the beginning of the present century, thermo-acoustic interaction devices regained the interest of the scientific community and advanced designs, such as suspended arrays of aluminum wires, carbon nanotubes, and graphene, were developed to explore the efficiency and performance envelopes of such heat flux sound sources. Moreover, significant efforts have been invested to characterize the impact of the deposition substrate, and thermo-acoustic device behavior in different gaseous and liquid media.

[0064] Reference is now made to FIG. 5, which illustrates a simplified drawing of such a thermo-acoustic device, of the type used in implementing the systems of the present disclosure, and showing schematically its component parts. The device is produced on a substrate 50 having a thin metallic heating element 51 deposited on its upper surface through which is passed the alternating electric current 52 to generate the desired oscillating heat fluxes 53 at the surface. These oscillating surface heat fluxes then generate acoustic waves 54, which are emitted from the surface. If a conducting substrate is used, a thin insulating layer 55 is formed on the surface of the substrate. The structure is simple and can be formed by conventional microelectronic techniques, or by any other suitable fabrication method. The device substrate should have as high a heat conduction as possible, since this feature enables the heating element to cool down rapidly as the AC drive current falls through its zero point, such that the amplitude of its temperature fluctuations should be as large as possible compared to its mean temperature. The thermal conduction characteristic of the substrate which should be maximized is the thermal effusivity, es, which is also known as the thermal product, and is given by the expression

es=κρCp,

where
κ is the thermal conductivity of the substrate,
ρ is the density, and
Cp is the specific heat capacity.

[0065] In addition, the thermo-acoustic interaction effect is generated only at the boundary between the substrate and the air, and the thermal condition of the deeper layers is thus only of secondary consequence to the effect of the thermo-acoustic interaction. Therefore, it is predominantly the boundary of the substrate which determines the thermal efficiency of the thermo-acoustic interaction. One particular property which has an effect on the thermo-acoustic effect is the atomic order of the surface layer, which has an important effect on the phonon transmission through the boundary. Along these lines, the thinner the substrate, the larger is the relative contribution of the ordered layer associated with ballistic phonon transfer with respect to the total phonon transfer across the heating element. Thus, for example, a liquid metal surface layer may be more efficient for use in this application than a solid metal substrate itself, since the liquid metal has a pool of electrons and phonons unbound to any structural imperfections which could be present in a solid metallic substrate. Such liquid metal surface layers are known, for instance, in a eutectic mixture of the metals gallium, indium, and tin, which can remain liquid down to −19° C. Such a mixture can be obtained as the commercial material known as “Galistan”, as supplied by Geraberger Thermometerwerk Gmbh of Geschwenda, Germany, and as described in U.S. Pat. No. 6,019,509, for “Low Melting Gallium, Indium, and Tin Eutectic Alloys, and Thermometers Employing Same”. Other similar compositions are also known. Such a substrate could be produced by depositing a thin layer of such a liquid alloy on the surface of a high thermal conductivity substrate, on which the liquid metal forms a tenacious layer because of the very high surface tension of the liquid alloy. The layer is so tenacious that it withstands removal by the fingers of a user or someone involved in the handling during assembly of the device. The positive and negative electrodes could be submerged into the liquid metal coating, which then serves as the heating element. In the scope of this disclosure, all references to solid thermophones can also be attributed to stationary liquid transducers held in place by viscous, capillary or electromagnetic forces.

[0066] The heating element itself should have an impedance equal to the source impedance of the AC power supply, if that is what is being used to drive the device, such that the energy transfer is optimal. The thinner the element, the higher its resistance, and therefore the higher the voltage required from the power supply to input a predetermined level of power.

[0067] Since the device is thin, it can be formed in large planar sheets, even flexible, and thus applied to any surface at which, or in the vicinity of which, the noise field is being generated. A particularly convenient construction could be in the form of layers of a thermo-acoustic device printed or assembled onto adhesive tapes which can be applied to the surfaces close to the source of the acoustic noise. There would then be need for a power supply connected to the applied tape, such as by means of a flexible flat cable connection.

[0068] Additionally, such devices, unlike prior art vibro-acoustic sources, not being dependent for the sound generation on any macroscopic mechanical motion, generate distortion-free sounds, without any resonances characteristic of the mechanical structure of a vibrating element. Furthermore, such elements do not require an opening in the structure on which they are mounted to enable their free vibrational motion, and therefore do not compromise the mechanical strength of the part on which they are mounted. Furthermore, they do not take up any significant space within the system in which they are installed nor increase the weight significantly. They can also be constructed to withstand high temperatures for use in those locations where such conditions exist. The above characteristics all point to their eminent suitability for application within rotating machinery such as fans and jet engines, or on the surfaces of vehicles or aircraft generating acoustic noise.

[0069] Reference is now made to FIG. 6, which shows a similar application to that solved by the prior art apparatus of FIG. 4, but illustrating the significant advantage of the novel systems described in the present disclosure. Similar to the situation in FIG. 4, in FIG. 6, there is shown a noise field 60 generated by a fan 61, being propagated in both directions upstream and downstream of the fan. However, unlike the situation of FIG. 4, in FIG. 6, thermo-acoustic generators 62 applied to the sound emitting elements of the fan 61, generate a compensation field 64 having the same amplitude and frequency as the noise emitted by the fan and emanated from the same source, but being in anti-phase to that noise, such that the noise field 60 of the fan is almost completely cancelled out, as shown by the small resultant field 63. Unlike the situation of FIG. 4 which uses remotely positioned loudspeakers, in FIG. 6, the compensation sound field is generated almost superposed on the noise sound field, such that the compensation is not only essentially complete, but also propagates with the propagating noise field, thereby providing its compensation effect globally over a large volumetric space.

[0070] Reference is now made to FIG. 7A, which is a cutaway schematic drawing taken from the above mentioned U.S. Pat. No. 5,478,199 of a high bypass, turbo fan engine, showing the fan rotor 71, the fan stator 73, and the various compressor stages. According to the noise compensation configuration of the present disclosure, planar thermo-acoustic transducers 72 can be applied to any of the surfaces shown in the engine, such as on the rotor blades 71 of the fan, in which case the power may be supplied to the transducers through a slip ring, or the stator blades 73, in which case the power can be supplied to the transducers through hard wiring. In the prior art engine, the compensation sound sources 36a, 36b, are located in the inner wall of the fan duct, remote from the location of the sources of the noise at the fan rotor and stator blades, such that the compensation is expected to be approximate and spatially dependent.

[0071] Reference is now made to FIG. 7B, which illustrates schematically the structure of a thermo-acoustic layer 72 mounted on a surface profile 79 of such a stator 73, illustrating the ease with which a conformal configuration can be obtained with minimal disturbance to the functionality of the element on which it is mounted. The thermo-acoustic element itself is made up of the substrate 76, on which is formed the transducer itself 77, which is made up of the Joule heating current electrodes, and a protective cover layer. The thermo-acoustic element 76, 77 is bonded to the stator surface 75 by means of a bonding layer 78, such that it becomes a thin additional layer on the stator, without significantly interfering with the gas flow through the fan. The construction and materials can be made such that the thermo-acoustic layer withstands the temperatures expected at its location within the engine.

[0072] Reference is now made to FIG. 8, which illustrates schematically how the active noise cancellation is achieved in the example of the turbofan engine of FIG. 7. FIG. 8 is a schematic cross section of the inlet nacelle 80 and fan of such an engine, showing thermo-acoustic transducers 83 attached to rotor blades 81, stator blades 82 and the hub 86 of the inlet fan. The sounds generated by the inlet flow and from the fluid dynamic interaction of the rotating fan blades with the stator, are detected by the array of sensor microphones 84 which should be located close to the sources of the noise. The outputs of these sensor microphones are input to the system controller 85, where they are processed, and signals are distributed from the controller to the thermophones mounted in the engine to provide acoustic outputs which cancel as best as is possible, the noise generated by the flow and the fan. The microphones detect multi-tonal sounds, which must be converted by the controller 85 into current signals for driving the thermo-acoustic devices. The complex spectrum can be Fourier analyzed into its main spectral components, and each component may then be converted by the controller into a waveform containing a train of electric current pulses which should correspond to the temporally fluctuating heat flux train to be delivered to the thermo-acoustic device, for generating that component of the cancellation sound wave. The three important features of the temporal form of the current input waveform to the thermo-acoustic devices, for generating the temporally fluctuating heat flux, are that:

(i) the frequencies of the components of the electric current waveform input to the device should be the same as the frequencies of corresponding components of the tonal sound which it is desired to counterbalance;
(ii) in order to effect the noise cancellation itself, the controller should arrange that the phase of each component of the electric current waveform for generated the heating effects be opposite to that of the corresponding components of the sound waveform detected by the microphone; and
(iii) to ensure as complete cancellation as possible, the amplitude of each spectral component of the heating current should be controlled such that the sound generated by the device will be equal to that expected at the source position, either as directly measured by the microphone or as position-extrapolated by the microphone measurement, if the microphone is not close to the exact noise source.

[0073] As previously stated, if the Joule heating current supplied by the controller has a conventional AC sinusoidal voltage waveform without a DC component, i.e. a waveform generally symmetrical around the zero level, the classically understood voltage frequency of the sine wave should be half of the sound frequency which that spectral component of the noise that waveform is intended to cancel, since both the positive and the negative currents of the waveform generate successive heating pulses. This is analogous to the effect of a full-wave voltage rectification which generates successive positive pulses of output voltage relative to the zero voltage line, at twice the AC input frequency. On the other hand, if the controller is such as to supply heating current in the form of a series of positive-going current pulses, then the effective frequency of those pulses should be equal to the frequency of the sound wave component it is intended to cancel. In the scope of this disclosure, all references to correlated current imply a relation between the frequency and phase of the noise field and the current source.

[0074] The temporal position of the antiphase cancellation waveform is then determined by temporally positioning the heating waveform with its peaks at the same point of time as the troughs of the sound component waveform, and vice versa. It is to be understood that in any temporal determination of the phase of the input current waveform to the thermo-acoustic devices, there is to be taken into account any inherent phase delay that may be generated by the thermal characteristics of the device, between the current input itself and the resulting acoustic output from the device, such that it is the acoustic output waveform components that are in opposite phase to those of the noise waveform components which it is desired to cancel.

[0075] Such planar thermo-acoustic generators can be applied to other fan configurations, or other noise generating machinery, having less stringent environmental conditions than those expected within a turbojet engine, such as household fans or fans for electronic instrument cooling. They can also be used on wind turbines, for reducing the noise level of the turbine, applied either to the rotor blades, or to the gearbox, or to the tower of the turbine.

[0076] Reference is now made to FIGS. 9 and 10, which illustrate different applications of the thermo-acoustic generator systems of the present disclosure, to situations in which the noise is generated by the flow of air over parts of the moving object or inside ducts.

[0077] In FIG. 9, there are shown thermo-acoustic devices 90 applied to various selected areas of the external panels of a motor vehicle, in order to cancel the wind noise generated in the region of those selected areas. The regions from which the flow noise is generated are determined by means of preliminary wind tunnel investigations of the vehicle. Thermo-acoustic devices can also be applied along parts of the length of the exhaust pipe 91 of a motor vehicle, in order to compensate locally for the noise generated by the flow of the exhaust gases down the exhaust pipe.

[0078] In FIG. 10, there are shown thermo-acoustic devices applied to the nose cone 101, canard wings 102, and rudder 103 of a military aircraft, to reduce the flow noise generated by the air passing across these surfaces.

[0079] Reference is now made to FIG. 11, which illustrates a configuration for generating predictive noise cancellation in a turbojet engine, by evaluation of the relative rotor-stator phase angle. FIG. 11 shows a fan having twelve rotor blades 110, shown in FIG. 14 with cross-hatched lines, and eleven stator blades 111. The relative phase angle between the rotor and the stator blades is different for each angular blade position, because of the unequal numbers of blading. At the 3 o'clock position of the fan, 112, the rotor blade and the stator blade are at the same angular position. At the opposite, 9 o'clock position of the fan, 113, the rotor blade and the stator blade are in antiphase. The relative location of rotor blades with respect to stators at each point of time and the relative speed of rotation, enables prediction of the fundamental frequency and phase expected from each particular blade passage event, and hence, if desired for the most comprehensive noise cancellation, the noise field at each single stator position. This can be determined either from a prediction based on a previous simulation of generated noise, or can be based on previous measurements of actually generated noise levels. Alternatively, if a less accurate noise cancellation is desired, the predicted noise characteristics from separate sectors of the fan can be used by combining the noise expected from a number of blades, and using that noise profile for cancellation of the noise in that sector of the fan.

[0080] In order to predict the noise field arising from the varying flow at each single stator position, or any other aero-acoustic interaction, either previous measurements of actually generated noise levels or computational simulation tools can be used. Numerous mathematical approaches to simulating complex aero-acoustic interactions are known in the art. These range from computationally expensive Direct Navier Stokes solvers, to reduced hybrid models that combine two separate numerical approximations, first a dedicated Computational fluid dynamics (CFD) tool and secondly an acoustic solver. The initial flow field is solved via various CFD solvers. Both steady state (Reynolds Averaged Navier-Stokes, Stochastic Noise Generation and Radiation) and transient (Direct Navier-Stokes, Large Eddy Simulation, Detached Eddy Simulation, Unsteady Reynolds Averaged Navier-Stokes) fluid field solutions can be used. These results include the sources of aero-acoustic noise and thus serve as inputs to the second acoustic solver, which calculates the sound propagation. The sound propagation can be characterized via various methods such as Lighthill's analogy, Kirchhoff integral, Linearized Euler Equations and others.

[0081] FIG. 14 illustrates for the case of a rotation generated noise field, the application of a predictive construction of the noise field, rather than its measurement by microphones. However, a noise field originating from a linearly moving element can also be considered. For instance, the linear motion of the pistons of an internal combustion engine can be used as the reference motion in order to relate to, for instance, the exhaust noise from that engine, since the nature of the exhaust noise will be related to the position of the pistons, or any other measurable quantity synchronized thereto, such as the flywheel position. Similarly, the noise generated by a linear pump can be related to the position of the piston in the compression cylinder. The predictive determination of the noise field can therefore be performed either by acoustic simulation or by predetermined measurements, relating the noise spectrum and phase to the position of the linearly moving elements of the mechanism.

[0082] It is appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and sub-combinations of various features described hereinabove as well as variations and modifications thereto which would occur to a person of skill in the art upon reading the above description and which are not in the prior art.