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An exciting sound wave generator is positioned within a nacelle of a jet engine at its inlet to generate an exciting sound wave having a primary frequency generally different from a frequency of the primary tone of the noise. The exciting sound wave is used to modulate the primary tone of the noise

An exciting sound wave generator is positioned within a nacelle of a jet engine at its inlet to generate an exciting sound wave having a primary frequency generally different from a frequency of the primary tone of the noise. The exciting sound wave is used to modulate the primary tone of the noise and, therefore, distribute sound energy of the noise from the primary tone to a broad range of side bands so that the amplitude of the primary tone of the noise is reduced.

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1. A method for suppressing noise dominated by a primary tone from a noise source within a duct housing comprising:generating an exciting sound wave dominated by a primary frequency generally within an audible range and different from a frequency of the primary tone of the noise, andmodulating the p

1. A method for suppressing noise dominated by a primary tone from a noise source within a duct housing comprising:generating an exciting sound wave dominated by a primary frequency generally within an audible range and different from a frequency of the primary tone of the noise, andmodulating the primary tone of the noise using the generated exciting sound wave to excite within the duct housing a fluid medium in which a sound wave of the noise propagates so that sound energy of the noise is re-distributed from the frequency of the primary tone to a broad range of side bands and the amplitude of the primary tone of the noise is reduced.2. A method as claimed in claim 1 wherein the fluid medium is air.3. A method as claimed in claim 1 wherein the exciting sound wave is generated by a force of a fluid flow acting on a mechanical device.4. A noise attenuation system for suppressing noise dominated by a primary tone from a noise source comprising:an elongated housing surrounding the noise source, the housing having a first and second openings on opposite ends, wherein a sound wave from the noise source propagates in air outwardly towards the first and second openings,an exciting sound wave generator associated with the generator generating an exciting sound wave dominated by primary frequency generally within an audible range and different from a frequency of the primary tone of the noise to excite the air and modulated the primary tone of the noise so that sound energy of the noise is re-distributed from the frequency of the primary tone to a broad range of side bands and the amplitude of the primary tone of the noise is reduced.5. A noise attenuation system as claimed in claim 4 wherein the exciting sound wave generator is positioned on an inner wall of the housing.6. A noise attenuation system as claimed in claim 5 wherein the exciting sound wave generator comprises a mechanical device excited by a force of air flow to generate the exciting sound wave.7. A noise attenuation system for suppressing noise dominated by a primary tone from a jet engine comprising:a nacelle surrounding the jet engine, the nacelle having an inlet and an outlet for receiving and exhausting air flow respectively, wherein a sound wave of the noise produced from the jet engine propagates outwardly towards the inlet and outlet; andan exciting sound wave generator associated with the nacelle, generating an exciting sound wave dominated by a primary frequency generally within an audible range and different from a frequency of the primary tone of the noise to excite the air flow in the nacelle and modulate the primary tone of the noise so that sound energy of the noise is re-distributed from the frequency of the primary tone to a broad range of side bands and the amplitude of the primary tone of the noise is reduced.8. A noise attenuation system as claimed in claim 7 wherein the exciting sound wave generator is positioned on an inner wall of the nacelle at the inlet.9. A noise attenuation system as claimed in claim 8 wherein the exciting sound wave generator comprises a mechanical device excited by a force of air flow to generate the exciting sound wave.10. A noise attenuation system as claimed in claim 9 wherein the mechanical device comprises a fence member exposed to the air flow entering the inlet of the nacelle.11. A noise attenuation system as claimed in claim 9 wherein the mechanical device comprises an aperture defined in the inner wall, an air flow jetting from the aperture into the nacelle.