Porous acoustic element and a method of controlling aerodynamic noise in a flowing gas

A method of reducing aerodynamic noise in a gas flowing from a region of relatively high pressure to a region of relatively lower pressure by limiting the gas flow velocity comprising:

• disposing a first porous layer in juxtaposition to said high pressure region to cause said gas to flow through said first porous layer, said first porous layer being comprised of a plurality of layers of windings of wire, each of said windings being spaced apart at the same pitch with successive layers being wound to cross each other to create a multiplicity of first spaces, said first spaces having a total predetermined cross-sectional area;
• disposing a transition layer adjacent said first porous layer remote from said high pressure region, said transition layer being formed with a multiplicity of plenum openings of total cross-sectional area at least as great as the total predetermined cross-sectional area of said multiplicity of first spaces in said first porous layer;
• disposing a second porous layer, said second porous layer being comprised of a plurality of layers of windings of wire in juxtaposition to said transition layer, each of said windings being spaced apart at the same pitch, said pitch being greater than the pitch of said first porous layer windings, with successive layers being wound to cross each other to create a multiplicity of second spaces, said second spaces having a total predetermined cross-sectional area greater than said total predetermined cross-sectional area of said first spaces, such that the difference in total cross-sectional area corresponds to the increase in the volume of said gas as said pressure decreases in flowing through said first porous layer and said transition layer;
• said transition layer further being oriented to said first porous layer so as to preclude an interference pattern producing blockages or openings of smaller area than either of said first or second porous layers;
• bonding together said first and second porous layers and said transition layer;
• and
• then passing said gas through a sufficient number of alternating porous and transition layers from said high pressure region to said lower pressure region to decrease said gas pressure to that existing in said low pressure region without exceeding a predetermined gas flow velocity.

A method of manufacturing a porous acoustic element comprising:

• winding a plurality of windings of wire on a mandrel at a constant spaced apart pitch, successive windings crossing each other to create a first porous layer;
• placing a transition plenum layer over said porous layer;
• winding a plurality of windings of wire on said mandrel over said transition layer at a constant spaced apart pitch differing from the pitch of said first porous layer windings, successive windings crossing each other to create a second porous layer;
• bonding said windings in said first and second porous layers and said transition layer together to create a unitary structure.