Acoustic attenuator providing relatively high insertion loss at low frequencies

Claim 2

Apparatus for attenuating the acoustic energy content of acoustic pressure waves propagating through a flowing fluid comprising: a conduit having inlet and outlet openings adapted for the entrance and exit, respectively, of the flowing fluid; and, at least one series of substantially rigid, acoustically nonabsorptive plates mounted within said conduit and extending from said inlet opening toward said outlet opening, each plate in said series having a leading edge which is initially contactable by the flowing fluid and a trailing edge from which the flowing fluid can leave the plate, each plate in said series having at least one relatively flat surface extending from the leading edge to the trailing edge of the plate in a direction substantially parallel to the flow of said fluid, said plates in said series being spaced apart so that a space exists between the trailing edge of one plate and the leading edge of the next succeeding plate in the series.

Claim 3

The apparatus according to claim 1 wherein some of said plates have thicker cross sections than others.

Claim 4

Apparatus for attenuating the acoustic energy content of low frequency acoustic pressure waves propagating through a flowing fluid comprising: a conduit having inlet and outlet openings for the entrance and exit, respectively, of the flowing fluid, and, at least one series of substantially rigid, acoustically nonabsorptive plates mounted within said conduit and extending from said inlet opening toward said outlet opening, each plate in the series having a leading edge which is initially contacted by the flowing fluid and a trailing edge from which the flowing fluid leaves the plate, each plate in said series having at lEast one surface extending from the leading edge to the trailing edge of the plate in a direction substantially parallel to the flow of said fluid, said plates in said series being spaced apart so that a space exists between the trailing edge of one plate and the leading edge of the next succeeding plate in the series so that the flowing fluid forms a boundary layer flow region on the surface of the first plate encountered in the series and re-forms new boundary layer flow regions on the surfaces of each of the successive plates encountered in the series, the low frequency acoustic pressure waves causing said boundary layer flow regions to become oscillatory whereby each oscillatory boundary layer flow region in succession substantially impedes propagation of low frequency acoustic pressure waves.

Claim 5

Apparatus for attenuating the acoustic energy content of low frequency acoustic pressure waves propagating through a flowing fluid comprising: a conduit having inlet and outlet openings for the entrance and exit, respectively, of the flowing fluid, and, at least one series of substantially rigid, acoustically nonabsorptive plates mounted within said conduit and extending from said inlet opening toward said outlet opening, each plate in the series having a leading edge which is initially contacted by the flowing fluid and a trailing edge from which the flowing fluid leaves the plate, each plate in said series having at lEast one surface extending from the leading edge to the trailing edge of the plate in a direction substantially parallel to the flow of said fluid, said plates in said series being spaced apart so that a space exists between the trailing edge of one plate and the leading edge of the next succeeding plate in the series so that the flowing fluid forms a boundary layer flow region on the surface of the first plate encountered in the series and re-forms new boundary layer flow regions on the surfaces of each of the successive plates encountered in the series, the low frequency acoustic pressure waves causing said boundary layer flow regions to become oscillatory whereby each oscillatory boundary layer flow region in succession substantially impedes propagation of low frequency acoustic pressure waves.

Claim 6

Apparatus for attenuating the acoustic energy content of acoustic pressure waves, includng at least those of low frequency, propagating through a flowing fluid comprising: an elongated conduit having inlet and outlet openings spaced apart from each other and located at opposite ends of a longitudinally extending central axis of said conduit, said openings being in planes which are parallel with each other, said central axis being orthogonally disposed with respect to said planes, said inlet and outlet openings providing an entrance and exit, respectively, for the flowing fluid and the low frequency acoustic pressure waves propagating through the fluid; and, one row-and-column array of relatively thin acoustically nonabsorptive plates mounted within said conduit, each thin plate in said array having a leading edge which is initially contacted by the flowing fluid and a trailing edge from which the flowing fluid subsequently leaves the plate, each thin plate having a pair of relatively flat, parallel surfaces which extend longitudinally from its leading edge to its trailing edge, said pair of parallel surfaces being disposed in back-to-back relationship, each column of plates in said array extending along a line parallel with said central axis, the plates in each said column being arranged in a series along said line and extending from said inlet opening toward said outlet opening of said conduit with the back-to-back parallel surfaces of each plate in the series along said line being in parallel with said longitudinal central axis, the trailing and leading edges of successive serially-arranged plates in each column being separated by longitudinally extending spaces, each row of plates in said array having included therein one plate of each column in said array and extending along a line crosswise of said central axis, the flowing fluid, including the acoustic pressure waves propagating therethrough, passing through said conduit in a direction generally parallel with said central axis from said inlet opening toward said outlet opening, the flowing fluid forming, during its passage through said conduit, separate back-to-back boundary layer flow regions on the back-to-back surfaces, respectively, of each of the serially-arranged plates in each column of said array, said low frequency acoustic pressure waves exciting the boundary layer flow regions and causing said boundary layer flow regions to become oscillatory thereby forming in each oscillatory boundary layer flow region a layer of oscillating vorticity wherein flow vortices form, each said layer of vorticity being shed from the trailing edge of each plate in the form of an oscillatory wake into the longitudinally extending space between the trailing edge of one plate and the leading edge of the next succceeding plate in the column whereby propagation of the low frequency acoustic pressure waves through said oscillatory boundary layer flow regions, said layers of vorticity and said oscillatory wakes is substantially impeded so that the acoustic energy content of said low frequency acoustic pressure waves is attenuated.

Claim 7

Apparatus for attenuating the acoustic energy content of acoustic pressure waves, includng at least those of low frequency, propagating through a flowing fluid comprising: an elongated conduit having inlet and outlet openings spaced apart from each other and located at opposite ends of a longitudinally extending central axis of said conduit, said openings being in planes which are parallel with each other, said central axis being orthogonally disposed with respect to said planes, said inlet and outlet openings providing an entrance and exit, respectively, for the flowing fluid and the low frequency acoustic pressure waves propagating through the fluid; and, one row-and-column array of relatively thin acoustically nonabsorptive plates mounted within said conduit, each thin plate in said array having a leading edge which is initially contacted by the flowing fluid and a trailing edge from which the flowing fluid subsequently leaves the plate, each thin plate having a pair of relatively flat, parallel surfaces which extend longitudinally from its leading edge to its trailing edge, said pair of parallel surfaces being disposed in back-to-back relationship, each column of plates in said array extending along a line parallel with said central axis, the plates in each said column being arranged in a series along said line and extending from said inlet opening toward said outlet opening of said conduit with the back-to-back parallel surfaces of each plate in the series along said line being in parallel with said longitudinal central axis, the trailing and leading edges of successive serially-arranged plates in each column being separated by longitudinally extending spaces, each row of plates in said array having included therein one plate of each column in said array and extending along a line crosswise of said central axis, the flowing fluid, including the acoustic pressure waves propagating therethrough, passing through said conduit in a direction generally parallel with said central axis from said inlet opening toward said outlet opening, the flowing fluid forming, during its passage through said conduit, separate back-to-back boundary layer flow regions on the back-to-back surfaces, respectively, of each of the serially-arranged plates in each column of said array, said low frequency acoustic pressure waves exciting the boundary layer flow regions and causing said boundary layer flow regions to become oscillatory thereby forming in each oscillatory boundary layer flow region a layer of oscillating vorticity wherein flow vortices form, each said layer of vorticity being shed from the trailing edge of each plate in the form of an oscillatory wake into the longitudinally extending space between the trailing edge of one plate and the leading edge of the next succceeding plate in the column whereby propagation of the low frequency acoustic pressure waves through said oscillatory boundary layer flow regions, said layers of vorticity and said oscillatory wakes is substantially impeded so that the acoustic energy content of said low frequency acoustic pressure waves is attenuated.

Claim 8

The apparatus according to claim 4 wherein some of said plates have relatively thicker cross sections Than others.

Claim 9

The apparatus according to claim 5 wherein the plates having the thicker cross sections are included in each row and column of said array and are arranged in symmetrical relationship with respect to said central axis.

Claim 10

The apparatus according to claim 4 further comprisng a second row-and-column array of like plates, the rows and columns of said second array of plates being staggered with respect to the rows and columns of said one array.

Claim 11

Apparatus for attenuating the acoustic energy content of acoustic pressure waves, including at least low frequency waves, propagating through a flowing fluid comprising: a conduit having inlet and outlet openings for the admission and discharge of the flowing fluid; and, a plurality of plates mounted within said conduit, each plate having a leading edge and a trailing edge and a pair of parallel, relatively flat, acoustically nonabsorptive back-to-back surfaces essentially parallel to the fluid flow extending from the leading edge to the trailing edge, said plates of said plurality being spaced apart from one and other so that the flowing fluid can establish individual pairs of boundary layer flow regions on said back-to-back surfaces of each plate successively encountered as the fluid flows from said inlet opening toward said outlet opening, the low frequency acoustic pressure waves causing each boundary layer flow region to become an oscillatory boundary layer flow region wherein fluid flow vorticities are formed and shed from the trailing edges of each plate.

Claim 12

Apparatus for attenuating the acoustic energy content of acoustic pressure waves, including at least low frequency waves, propagating through a flowing fluid comprising: a conduit having inlet and outlet openings for the admission and discharge of the flowing fluid; and, a plurality of plates mounted within said conduit, each plate having a leading edge and a trailing edge and a pair of parallel, relatively flat, acoustically nonabsorptive back-to-back surfaces essentially parallel to the fluid flow extending from the leading edge to the trailing edge, said plates of said plurality being spaced apart from one and other so that the flowing fluid can establish individual pairs of boundary layer flow regions on said back-to-back surfaces of each plate successively encountered as the fluid flows from said inlet opening toward said outlet opening, the low frequency acoustic pressure waves causing each boundary layer flow region to become an oscillatory boundary layer flow region wherein fluid flow vorticities are formed and shed from the trailing edges of each plate.

Claim 13

The apparatus according to claim 8 wherein some plates in said plurality have thicker cross section dimension between the parallel surfaces thereof than other plates in said plurality.

Claim 14

Apparatus for attenuating the acoustic energy content of acoustic pressure wave propagating through a flowing fluid comprising: one series of stationary plates mounted in a flow path adapted to be traversed by the flowing fluid and by the acoustic pressure waves propagating through the flowing fluid, said series of plates extending longitudinally in the general direction of the flow of the fluid, each plate in said series having a leading edge and a trailing edge longitudinally spaced therefrom, each plate having a pair of relatively flat, acoustically nonabsorptive parallel surfaces disposed in back-to-back relationship and extending longitudinally from said leading edge thereof to said trailing edge thereof, in a direction substantially parallel to the flow of said fluid, the distance between said pair of surfaces being relatively smaller than the distance from said leading edge to said trailing edge, the plates in said series being longitudinally separated so that a longitudinally extending space exists between the trailing and leading edges of successive plates in said series.

Claim 15

Apparatus for attenuating the acoustic energy content of acoustic pressure wave propagating through a flowing fluid comprising: one series of stationary plates mounted in a flow path adapted to be traversed by the flowing fluid and by the acoustic pressure waves propagating through the flowing fluid, said series of plates extending longitudinally in the general direction of the flow of the fluid, each plate in said series having a leading edge and a trailing edge longitudinally spaced therefrom, each plate having a pair of relatively flat, acoustically nonabsorptive parallel surfaces disposed in back-to-back relationship and extending longitudinally from said leading edge thereof to said trailing edge thereof, in a direction substantially parallel to the flow of said fluid, the distance between said pair of surfaces being relatively smaller than the distance from said leading edge to said trailing edge, the plates in said series being longitudinally separated so that a longitudinally extending space exists between the trailing and leading edges of successive plates in said series.

Claim 16

Apparatus according to claim 10 wherein the acoustic pressure waves include at least low frequency acoustic pressure waves and wherein said distance from the leading edge to the trailing edge of each plate as well as said longitudinally extending space between the leading and trailing edges of successive plates are of sufficient length to insure the formation of new pairs of boundary layer flow regions on the surfaces of each plate successively encountered by the flowing fluid.

Claim 17

Apparatus according to claim 10 wherein the distance between the pair of surfaces is greater for some plates in the series than for others.

Claim 18

Apparatus according to claim 10 further comprising additional series of like plates said additional series together with said one series forming one row-and-column array of plates.

Claim 19

Apparatus according to claim 13 wherein some plates in said one and additional series have greater distance between the pairs of surfaces thereof than other plates in said one and additional series.

Claim 20

Apparatus according to claim 13 wherein said plates having the greater distance between the pairs of surfaces thereof are symmetrically spaced throughout the row-anD-column array.

Claim 21

Apparatus according to claim 13 further comprising a second row-and-column array of like plates arranged in staggered relationship relative to said one row-and-column array.

Claim 22

Apparatus according to claim 16 wherein in each array said longitudinally extending space between the trailing and leading edges of successive plates in the same column is substantially equal to the distance L between the leading and trailing edges of each plate and wherein in each array the distance 2b between the surfaces of successive plates in the same row is substantially twice the distance b between the trailing edge of a plate in one row of one array to the leading edge of a corresponding plate in a corresponding row of the second array whereby the plates in the columns of said second array divide the fluid flowing between adjacent columns in the one array.

Claim 23

Apparatus according to claim 17 wherein in each column of both arrays there are N plates in series.

Claim 24

Apparatus according to claim 18 wherein N ranges from 100 to 300 inclusive and the ratio L/2b ranges from 3 to 1.03, inclusive.

Claim 25

Apparatus according to claim 19 wherein some plates in both arrays have more thickness than the remaining plates.

Claim 26

Apparatus according to claim 20 wherein said plates are formed from metal corrugations.

Claim 27

Apparatus according to claim 21 wherein said plates are formed from strips of corrugated metal adjacently arranged.

Claim 28

Apparatus for attenuating the acoustic energy contact of acoustic pressure waves propagating through a flowing fluid comprising: a series of longitudinally spaced acoustically nonabsorptive surfaces mounted in a flow path substantially parallel thereto adapted to be traversed by the flowing fluid, said surfaces in said series having sufficient length and sufficient spacing between successive surfaces to enable the flowing fluid to establish a new boundary layer flow region on each surface successively encountered.

Claim 29

Apparatus for attenuating the acoustic energy contact of acoustic pressure waves propagating through a flowing fluid comprising: a series of longitudinally spaced acoustically nonabsorptive surfaces mounted in a flow path substantially parallel thereto adapted to be traversed by the flowing fluid, said surfaces in said series having sufficient length and sufficient spacing between successive surfaces to enable the flowing fluid to establish a new boundary layer flow region on each surface successively encountered.

Claim 30

The apparatus accoading to claim 23 wherein said surfaces include a leading edge first contacted by the flowing fluid and forming separation bubbles thereat.

Claim 31

The apparatus according to claim 23 wherein said acoustic pressure waves are of low frequency and cause said boundary layer flow regions to become oscillatory.