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An acoustic structure that includes a septum having an acoustic opening which defines an open area that varies in response to changes in the velocity of noise-containing media passing through the acoustic opening. The septum includes a fixed portion and one or more movable flapper portions wherein t

An acoustic structure that includes a septum having an acoustic opening which defines an open area that varies in response to changes in the velocity of noise-containing media passing through the acoustic opening. The septum includes a fixed portion and one or more movable flapper portions wherein the fixed portion and/or the flapper portion(s) include surfaces that define an acoustic opening through the septum. The acoustic opening has an open area which varies due to movement of the movable flapper(s) in response to changes in velocity of the noise-containing media that passes through the acoustic opening. The resulting septum has a relatively low non-linearity factor (NLF).

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1. An acoustic structure comprising a septum having an acoustic opening that varies in size in response to changes in the velocity of noise-containing media passing through said acoustic opening, said acoustic structure comprising: a septum comprising a fixed portion and one or more movable flapper

1. An acoustic structure comprising a septum having an acoustic opening that varies in size in response to changes in the velocity of noise-containing media passing through said acoustic opening, said acoustic structure comprising: a septum comprising a fixed portion and one or more movable flapper portions that surround said acoustic opening wherein said one or more movable flapper portions are hinged to said fixed portion so that said one or more movable flapper portions move in order to vary the size of said acoustic opening in response to changes in the velocity of said noise-containing media passing through said acoustic opening. 2. An acoustic structure according to claim 1 wherein said movable flapper portion is hinged to said fixed portion by way of a fold line in said septum that forms a transition between said fixed portion of said septum and said flapper portion. 3. An acoustic structure according to claim 2 wherein a plurality of movable flapper portions surround said acoustic opening. 4. An acoustic structure according to claim 3 wherein said acoustic opening is surrounded by at least three flapper portions and wherein said fold lines form a regular polygon. 5. An acoustic structure according to claim 1 wherein said septum comprises a plurality of said acoustic openings. 6. An acoustic structure according to claim 1 wherein a plurality of movable flapper portions surround said acoustic opening. 7. An acoustic structure according to claim 1 wherein said structure comprises a honeycomb having a cell in which said septum is located. 8. A jet engine nacelle comprising an acoustic structure according to claim 7. 9. A method for making an acoustic structure comprising a septum having an acoustic opening that varies in size in response to changes in the velocity of noise-containing media passing through said acoustic opening, said method comprising the steps of: providing a septum;forming an acoustic opening through said septum, said septum comprising a fixed portion and one or more movable flapper portions that surround said acoustic opening, wherein said one or more movable flapper portions are hinged to said fixed portion so that said one or more movable flapper portions move in order to vary the size of said acoustic opening in response to changes in the velocity of said noise-containing media passing through said acoustic opening. 10. A method for making an acoustic structure according to claim 9 wherein said movable flapper portion is hinged to said fixed portion by way of a fold line in said septum that forms a transition between said fixed portion of said septum and said flapper portion. 11. A method for making an acoustic structure according to claim 10 wherein a plurality of said movable flapper portions surround said acoustic opening. 12. A method for making an acoustic structure according to claim 11 wherein said acoustic opening is surrounded by at least three flapper portions and wherein said fold lines form a regular polygon. 13. A method for making an acoustic structure according to claim 9 wherein a plurality of said acoustic openings is formed in said septum. 14. A method for making an acoustic structure according to claim 9 wherein a plurality of said movable flapper portions surround said acoustic opening. 15. A method for making an acoustic structure according to claim 9 which includes the additional step of securing said septum into a cell of a honeycomb to form an acoustic honeycomb structure. 16. A method for making a jet engine nacelle comprising the step of using the acoustic honeycomb structure of claim 15 to form at least a part of said jet engine nacelle. 17. A method of attenuating noise from a source wherein the velocity of noise-containing media emitted from said source is variable, said method comprising the step of locating an acoustic structure near said noise source, said acoustic structure comprising a septum having an acoustic opening that varies in size in response to changes in the velocity of said noise-containing media passing through said acoustic opening, said acoustic structure comprising: a septum comprising a fixed portion and one or more movable flapper portions that surround said acoustic opening wherein said one or more movable flapper portions are hinged to said fixed portion so that said one or more movable flapper portions move in order to vary the size of said acoustic opening in response to changes in the velocity of said noise-containing media passing through said acoustic opening. 18. A method of attenuating noise from a source according to claim 17 wherein said acoustic structure comprises a honeycomb having a cell in which said septum is located to provide an acoustic honeycomb. 19. A method of attenuating noise from a source according to claim 18 wherein said acoustic honeycomb forms at least part of a nacelle for a jet engine.