Acoustic abatement method and apparatus

• US 3,936,606 A
• Filed: 12/11/1972
• Issued: 02/03/1976
• Est. Priority Date: 12/07/1971
• Status: Expired due to Term

• Current Assignee: Ronald L Wanke
• Original Assignee: Ronald L Wanke
• Inventors: Wanke, Ronald L.
• Primary Examiner(s): Olms, Douglas W.
• Application Number:
  US05/313,958
• Time in Patent Office: 1,149 Days
• Field of Search: 179/1 P, 181/33 L, 181/33 H, 181/33 HA, 181/33 HB
• US Class Current: 381/71.5
• CPC Class Codes: F05D 2220/323 for aircraft propulsion, e.g. jet engines
• CPC Class Codes: F05D 2260/962 by means of "anti-noise"
• CPC Class Codes: F16L 55/0333 by means of an active system
• CPC Class Codes: G10K 11/17821 characterised by the analysis of the input signals only
• CPC Class Codes: G10K 11/17857 Geometric disposition, e.g. placement of microphones
• CPC Class Codes: G10K 11/17861 using additional means for damping sound, e.g. using sound absorbing panels
• CPC Class Codes: G10K 11/17873 using a reference signal without an error signal, e.g. pure feedforward
• CPC Class Codes: G10K 2210/109 Compressors, e.g. fans
• CPC Class Codes: G10K 2210/112 Ducts vehicle exhausts G10K2210/12822
• CPC Class Codes: G10K 2210/121 Rotating machines, e.g. engines, turbines, motors; Periodic or quasi-periodic signals in general
• CPC Class Codes: G10K 2210/1281 Aircraft, e.g. spacecraft, airplane or helicopter
• CPC Class Codes: G10K 2210/3011 Single acoustic input
• CPC Class Codes: G10K 2210/32121 Fluid amplifiers, e.g. modulated gas flow speaker using electrovalves
• CPC Class Codes: G10K 2210/3216 Cancellation means disposed in the vicinity of the source

Abstract

An acoustic receiver, amplifier and transmitter generate an anti wave which is in-phase and of mirror symmetry with respect to a propagating acoustic wave. Coherent propagation means such as an acoustic duct of particular dimensions redirects and/or converts the acoustic wave and the anti wave, which otherwise propagate with incremental wavefronts at different vectors, into plane waves which are superimposed to create wave interference which itself propagates. For noise of high sound pressure level such as created by a gas turbine engine, the acoustic transmitter comprises a modulated gas flow speaker coupled to one or more bleed ports which supply gas at one or more pressures offset from ambient pressure.

Claims

1. A method for attenuating a predetermined range of frequencies within an acoustic wave propagating along different vectors through a medium, comprising the steps of:

generating an anti wave limited to only said predetermined range of frequencies and of mirror symmetry with respect to said acoustic wave,

propagating said anti wave in said medium along a given vector which is at a skew with respect to one of said different vectors,

altering a vector of at least one of said waves to produce substantially coinciding vectors of propagation only for the predetermined range of frequencies, and

aligning the wavefront of at least one of said waves to produce substantially coinciding wavefronts.

2. The method of claim 1 wherein the step of altering comprises confining said at least one wave to cause its wavefront to assume a predetermined shape which matches the shape of the wavefront of the other wave only for the predetermined range of frequencies.

3. The method of claim 2 wherein the predetermined shape comprises a plane wave having a (O,O) acoustic mode.

4. The method of claim 3 wherein the step of confining includes propagating said at least one wave in a restricted space of predetermined cross-sectional area and of a minimum length of predetermined extend greater than the longest dimension of the cross-sectional area, said cross-sectional area being selected to produce a predetermined cut-off frequency below which propagation is confined to the (O,O) acoustic mode and above which propagation can occur in a (m,n) acoustic mode, and selecting said predetermined cut-off frequency to be greater than the highest frequency acoustic wave which is to be attenuated, whereby the predetermined range of frequencies corresponds to all frequencies below the predetermined cut-off frequency.

5. The method of claim 1 wherein the step of generating includes detecting the acoustic wave and producing a signal corresponding thereto, filtering the signal to pass frequencies below said predetermined frequency and to attenuate frequencies above said predetermined frequency, said filtered signal forming said anti wave limited to only said predetermined range of frequencies, and the step of altering includes selectively altering the vector of at least one of said waves when having a frequency less than said preselected frequency and passing without alteration said at least one wave when having a frequency higher than said predetermined frequency, whereby the predetermined range of frequencies corresponds to all frequencies below the predetermined frequency.

6. A method for attenuating an acoustic wave propagating along different vectors through a medium, comprising the steps of:

detecting the acoustic wave to produce an anti wave signal corresponding thereto,

propagating in said medium along a given vector which is at a skew with respect to one of said different vectors an anti wave of mirror symmetry with respect to said acoustic wave by producing a flow of the medium at a pressure continuously offset from the ambient pressure of the medium, and modulating the flow in accordance with variations in said signal,

altering a vector of at least one of said waves to produce substantially coinciding vectors of propagation, and

aligning the wavefront of at least one of said waves to produce substantially coinciding wavefronts.

7. The method of claim 6 wherein the step of propagating further includes producing a second flow of the medium at a second pressure continuously offset from the ambient pressure of the medium and offset from the pressure of the first named flow, one of said flows being above ambient pressure and the other of said flows being below ambient pressure, modulating the second flow in accordance with variations in said signal, and combining the first named and second flows to produce ambient pressure in the absence of a variation in said signal and a pressure deviation above or below ambient pressure for variations above and below a reference level of the signal.

8. A system for attenuating an acoustic wave propagating through a medium which is sufficiently unconfined to allow different vectors of propagation of the acoustic wave, comprising:

duct means having a pair of spaced apertures each open to said medium and a hollow passageway therebetween, the duct means being positionable to cause one of said apertures to intersect and direct the propagating acoustic wave through the passageway toward the other of said apertures, said passageway having a cross section of reduced size with respect to the cross section of the medium outside of the duct means and producing a duct cut-off frequency corresponding to the lowest order acoustic mode of propagation to confine the acoustic wave to a plane wavefront for frequencies below the duct cut-off frequency and of a predetermined vector of propagation;

receiver means for generating a signal corresponding to the propagating acoustic wave;

transmitter means responsive to said signal for generating in said passageway an interference wave of opposite pressure variation with respect to the acoustic wave and having a plane wavefront for frequencies below the duct cut-off frequency and of a predetermined vector of propagation to produce a propagating cancellation effect; and

low pass filter means connecting said receiver means to said transmitter means and passing signals less than the duct cut-off frequency.

9. The system of claim 8 wherein the cross section of said hollow passageway is generally rectangular, and the longest width of the generally rectangular cross section is less than c/2fc ) where c is the local velocity of sound and fc is the duct cut-off frequency.

10. The system of claim 8 wherein said transmitter means generates said interference wave with a pressure variation of mirror symmetry with respect to the pressure variation of the acoustic wave, said receiver means, low pass filter means and transmitter means having a transmission time equal to the propagation time in the medium for the distance between the receiver means and the transmitter means to cause the wavefront of the interference wave to be in-phase with the wavefront of the acoustic wave.

11. The system of claim 10 wherein said receiver means comprises microphone means having a sensitive surface responsive to an acoustic pressure variation to generate said signal, and delay tube means separate from said passageway for coupling said sensitive surface to the medium, said delay tube means forming an acoustic time delay which forms a part of said transmission time.

12. A system for attenuating an acoustic wave propagating through a medium, comprising:

duct means having a pair of spaced apertures each open to said medium and a hollow passageway therebetween, one of said apertures being positionable to intersect the propagating acoustic wave to cause the acoustic wave to propagate through the passageway toward the other of said apertures, said passageway having a cross section of predetermined size to confine the acoustic wave to a wavefront of predetermined shape, said duct means allowing a backward wave to propagate through the passageway in a direction opposite to the propagating acoustic wave;

means for generating an interference wave having a wavefront of said predetermined shape including

receiver means contiguous with the passageway and responsive to said acoustic wave for generating a signal corresponding to the propagating acoustic wave,

diaphragm means movable in response to said signal to produce at a transmitter output an opposite pressure variation with respect to the acoustic wave,

mounting means locating said transmitter output generally facing said other aperture to direct the interference wave through said passageway with a vector direction toward said other aperture and away from said one aperture, and

means for preventing the backward wave from producing at the transmitter output any pressure variation.

13. The system of claim 12 wherein the diaphragm means comprises an annular surface surrounding an open central portion, said mounting means locating said annular surface in a wall defining said passageway to cause the open central portion to be contiguous with the passageway.

14. The system of claim 12 wherein said receiver means is located within said duct means and comprises a unidirectional microphone having a sensitive axis and said preventing means directs the sensitive axis toward said one aperture to orient said unidirectional microphone to discriminate against backward acoustic waves entering said passageway through said other aperture.

15. The system of claim 12 including amplifier means having an input coupled to said receiver means and an output coupled to said diaphragm means, said diaphragm means generating the backward wave which undesirably propagates through said passageway to said receiver means to produce a false output, and the preventing means corresponds to feedback means for coupling said output to said input and having an impedance to cause a feedback signal to cancel the false output from said receiver means.

16. A system for attenuating an acoustic wave propagating through a medium which is sufficiently unconfined to allow different vectors of propagation of the acoustic wave, comprising:

duct means having a pair of spaced apertures each open to said medium and a hollow passageway therebetween with central core means located within a center area of the passgeway, the duct means and the central core means defining therebetween a surrounding passageway, the duct means being positionable to cause one of said apertures to intersect and direct the propagating acoustic wave through the surrounding passageway toward the other of said apertures, said surrounding passageway having a cross section of predetermined size to confine the acoustic wave to a wavefront of predetermined shape and of a predetermined vector of propagation;

receiver means for generating a signal corresponding to the propagating acoustic wave; and

transmitter means including diaphragm means movable in response to said signal to produce at a transmitter output a pressure variation related thereto, and transmitter mounting means for locating said transmitter output within said central core means and oriented to disperse the pressure variations into the surrounding passageway to generate an interference wave of opposite pressure variation with respect to the acoustic wave and having a wavefront of the same predetermined shape and predetermined vector of propagation to produce a propagating cancellation effect.

17. The system of claim 16 wherein said duct means includes longitudinally extending wall means dividing aid passageway into a plurality of separate ducts which surround said central core means, and said transmitter mounting means includes a plurality of tube means each opening into a different one of said plurality of separate ducts for coupling the common transmitter output to each of the separate ducts.

18. The system of claim 16 wherein said central core means locates said transmitter output in the center of the cross section of said passageway and oriented to disperse the interference wave throughout the cross section of the passageway and toward said other aperture.

19. The system of claim 18 including cone-shaped disperser means spaced coaxially from said transmitter output and located within said passageway between said transmitter output and said other aperture to disperse pressure variations from the transmitter output into a surrounding interference space defined by the disperser means and the walls defining the passageway of the duct means.

20. In a gas breathing engine having rotating machinery creating a pressure gradient in a passageway which opens external to the engine and undesirably creating in said passageway acoustic noise which unless attenuated will propagate external to the engine, apparatus for attenuating the acoustic noise, comprising:

receiver means for detecting said acoustic noise and generating an audio signal corresponding thereto,

modulated gas flow speaker means having a speaker inlet for receiving gas at a pressure offset from ambient pressure at a speaker outlet, and a modulator for coupling said speaker inlet to said speaker outlet and responsive to an audio signal for modulating a flow of the gas,

bleed port means located in said passageway and open to gas at said offset pressure,

tubing means connecting said bleed port means to said speaker inlet for supplying the gas to said modulator,

anti noise means coupling said receiver means to said modulator to produce at said speaker outlet an interference wave of mirror symmetry with respect to said acoustic noise, and

coherent propagation means associated with said speaker outlet and said passageway for superimposing the acoustic noise and the interference wave to produce interference cancellation therebetween.

21. The apparatus of claim 20 wherein said coherent propagation means comprises a plurality of duct means each of cross section less than the cross section of said passageway and located in said passageway in abutting relation to force the acoustic noise to flow through the plurality of duct means during travel through the passageway, each duct means having a size to redirect the acoustic noise into a wavefront of predetermined shape, and said outlet being contiguous with said plurality of duct means to cause the interference wave to have a similar predetermined shape.

22. The apparatus of claim 21 for a gas turbine engine wherein said plurality of duct means comprise a plurality of vanes for dividing said passageway into a plurality of separate ducts, the rotating machinery comprises rotating blades which produce acoustic noise having a spinning pressure variation of one or more lobes which rotate while propagating through said passageway, said plurality of vanes being spaced to convert said spinning pressure variation into separate plane waves of (O,O) mode and each of which propagate independent of the propagation of the remaining plane waves of (O,O) mode in the remaining of the separate ducts, said modulated gas flow speaker means comprises a plurality of modulated gas flow speaker devices each associated with a different duct, said receiver means comprises a plurality of transducers each associated with a different duct, and said anti noise means independently couples each transducer to the modulated gas flow speaker device associated with the same duct.

23. The apparatus of claim 22 wherein said passageway comprises an annular duct located between an outer cylindrical housing and a coaxial inner cylindrical housing, said gas flowing parallel to the coaxial housings and through the annular duct under control of the rotating blades, each of said vanes extending radially from the inner housing of the outer housing with adjacent vanes defining therebetween said ducts, shroud means forming a wall between said inner and said outer cylindrical housings to divide each duct into at least two sections in order to increase the duct cut-off frequency for all modes above the (O,O) mode, and low pass filter means located between said transducers and said modulated gas flow speaker devices for attenuating audio signals greater than said duct cut-off frequency.

24. The apparatus of claim 20 wherein said coherent propagation means is formed by adjacent vanes logated in said passageway to define tehrebetween an anti noise duct which alters the vector of the acoustic noise, said modulated gas flow speaker means being mounted external to said passageway, and tube means coupling the outlet of said modulated gas flow speaker to the anti noise duct.

25. The apparatus of claim 24 wherein said tube means comprises a speaker vane spaced generally equidistant between the adjacent vanes forming said anti noise duct and having an aperture within said speaker vane leading to a speaker opening for coupling the interference wave to a generally center position within the anti noise duct.

26. The apparatus of claim 25 wherein said receiver means comprises a microphone having an active transducer surface, said microphone being mounted external to said anti noise duct, and said speaker vane includes an elongated microphone tube for coupling said active transducer surface to a microphone opening in said speaker vane, said micropnone opening being on an opposite facing surface to the speaker opening.

27. The apparatus of claim 20 wherein said modulated gas flow speaker means includes chamber means having a plurality of slots and port means forming said inlet for coupling the chamber means to said tubing means, said modulated comprises a movable diaphragm having a plurality of slots generally offset from the slots of said chamber means and movable into overlapping positions to pass an amount of gas dependent on the position of the diaphragm, magnetic flux generator means for establishing a magnetic flux field, voice coil means located on said diaphragm and within the magnetic flux field, said voice coil means being coupled to said source of audio signal to prodcue movement of the voice coil and attached diaphragm with the extent of movement being proportional to the signal to cause the amount of gas passed through the overlapping slots of the chamber means and diaphragm to be proportional to said signal.

28. The apparatus of claim 27 wherein said chamber means is annular and has a coaxial inner wall and outer wall, said port means being located on said outer wall, said plurality of slots surrounding said inner wall to pass the gas to a hollow central core, said diaphragm being cylindrical and located in said hollow central core for passage of gas through the slots to a hollow interior of the cylindrical diaphragm, and tube means forming said outlet for coupling the hollow interior of the cylindrical diaphragm to an opening for the acoustic waves.

29. A system for attenuating an acoustic wave propagating through a medium, comprising:

source means including receiver means for detecting said acoustic wave, transmitter means responsive to said receiver means for producing an anti wave which interferes with said acoustic wave to produce a cancellation effect, and time delay means for varying the transmission time of the source means to change the position of said anti wave in said medium,

sensing means for sensing a condition which indicates misalignment between the anti wave and the acoustic wave which were initially aligned, and

correction means responsive to said sensing means for automatically varying said time delay means to again align the anti wave with the acoustic wave.

30. The system of claim 29 wherein said sensing means monitors a condition of said medium which changes with changes in the velocity of sound in said medium, said correction means controlling said time delay means to cause the transmission time of said source means to equal the transmission time in the medium between the receiver means and the transmitter means.

31. The system of claim 30 wherein said sensing means monitors a temperature T of said medium, said correction means comprises a function generator for controlling the transmission time of said source means in proportion to ##EQU2## where K₁ and K₂ are constants.

32. A system for attenuating an acoustic wave propagating through a medium, comprising:

receiver means positionable to intersect the acoustic wave for generating a receiver signal corresponding thereto,

transmitter means spaced from said receiver means and responsive to an amplified signal for generating an interference wave which propagates toward an interference zone spaced away from the receiver means and the transmitter means, generation of the interference wave undesirably creating a traveling wave which propagates to said receiver means and produces a false signal not resulting from the acoustic wave, and

amplifier means having an amplifier input coupled to said receiver means and an amplifier output coupled to said transmitter means for amplifying the receiver signal to generate the amplified signal, including feedback means generating a feedback signal having a time delay corresponding to the propagation time of the traveling wave for cancelling the false signal produced when the traveling wave reaches the receiver means.

33. A system of claim 32 wherein the feedback means includes time delay means coupled to the amplifier output and having a time delay corresponding to the propagation time and an impedance which reduces the value of the feedback signal to equal the value of the false receiver signal produced by the traveling wave, and subtraction means for subtracting the receiver signal and the feedback signal.

34. The system of claim 32 including a duct having an elongated passageway with the acoustic wave propagating therethrough, an acoustically downstream section of the passageway corresponding to the interference zone, said receiver means being positioned acoustically upstream from said interference zone to detect the acoustic wave before it propagates to the interference zone, and the transmitter means being located intermediate the receiver means and the interference zone and having a transmitter output which directs the interference wave generally toward the interference zone, the passageway extending between the transmitter output and the receiver means corresponding to an undesired acoustic path for the traveling wave.

35. A system for attenuating an acoustic wave propagating through a medium, comprising:

a plurality of separate ducts surrounding a central core and each having a separate passageway with an opening contiguous with the central core;

receiver means for generating a signal corresponding to an acoustic wave propagating through at least one of the separate passageways; and

transmitter means located in said central core and responsive to said signal for generating an interference wave which propagates through all of the contiguous opening into the plurality of separate passageways to produce cancellation between the interference wave and the acoustic wave propagating in the associated passageway.

36. The system of claim 35 wherein each of the plurality of ducts has an interference zone located in the associated pasageway and spaced acoustically downstream from the opening contiguous with the central core, each of the opening being defined by walls shaped to direct the interference wave through the associated passageway with a vector direction towards the interference zone, and the receiver means having a receiver aperture open to at least one of the passageways and spaced acoustically upstream with respect to the contiguous openings.

37. The system of claim 36 wherein the central core has a receiver chamber containing the receiver means and at least one receiver aperture contiguous with at least one of the passageways, and the central core has a transmitter chamber located generally between the receiver chamber and the interference zones and separate from the receiver chamber for containing the transmitter means.

38. The system of claim 35 wherein the plurality of separate ducts are formed by a unit having four exterior walls which join perpendicularly to form a rectangular or square exterior cross-section and a plurality of interior walls extending between the exterior walls and the central core to define the separate passageways forming each separate duct, the exterior walls and the interior walls and the central core defining at one end of the unit a plurality of apertures positionable to intersect the propagating acoustic wave to cause the acoustic wave to separate and propagate separately through the plurality of passageways towards the opposite end of the unit, the opposite end of the unit having corresponding apertures open to the medium, and the transmitter means being located in said central core between the ends of the unit.

39. The system of claim 38 wherein an exterior part of the central core which intersects the propagating acoustic wave has surfaces which taper smoothly toward a point to minimize the introduction of bending waves.