Sound absorbing device

Apparatus for damping a sound field in a fluid propagation medium within a main channel having a closed cross-sectional periphery, comprising at least two lattice works spaced from each other by a longitudinal section of the free main channel cross-section, each of said lattice works being comprised of first and second partition wall means crossing each other and subdividing the free main channel cross-section into a plurality of juxtaposed and superposed partial channels running in the direction of the main channel and being open at both longitudinal ends and having a length which is larger than the cross-sectional width dimension of said partial channels, wherein at least predetermined ones of adjacent partial channels have different characteristic dimensions for producing different sound oscillation states therein, whereby in said predetermined ones of adjacent partial channels local sound pressure differences are produced and distributed over their length, said partition wall means between said predetermined ones of adjacent partial channels of different characteristic dimensions being permeable for the propagation medium, at least over a partial length to cause at least partly local sound pressure difference compensation of said local sound pressure differences by a flow of the propagation medium from one of the predetermined ones of said adjacent partial channels into the adjacent one thereof, said wall means comprising a damping material, such as felt or sintered material, for frictionally damping said flow of the propagation medium, said longitudinal section having a longitudinal dimension corresponding approximately to a quarter or half of the wave length of a sound oscillation in the lower range of the frequency spectrum of said field.

Apparatus for damping a sound field propagating in a fluid propagation medium within a main channel having a closed cross-sectional periphery, comprising at least two lattice works spaced from each other by a longitudinal section of the free main channel cross-section, each of said lattice works being comprised of first and second partition wall means crossing each other and subdividing the free main channel cross-section into a plurality of juxtaposed and superposed partial channels running in the direction of the main channel and being open at both longitudinal ends and having a length which is larger than the cross-sectional width dimensions of said partial channels, wherein at least predetermined ones of adjacent partial channels have different lengths for producing different sound oscillation states therein, whereby in said predetermined ones of adjacent partial channels of different lengths local sound pressure differences are produced and distributed over their length, said partition wall means between said predetermined ones of adjacent partial channels of different lengths being permeable for the propagation medium, at least over a partial length to cause at least partly local sound pressure difference compensating of said local sound pressure differences by a flow of the propagation medium from one of the predetermined ones of said adjacent partial channels into the adjacent one thereof, said wall means comprising a damping material, such as felt or sintered material, for frictionally damping said flow of the propagation medium said longitudinal section having a longitudinal dimension corresponding approximately to a quarter or half of the wave length of a sound oscillation in the lower range of the frequency spectrum of said sound field.

Apparatus for damping a sound field propagating in a fluid propagation medium within a main channel having a closed circular cross-section periphery, comprising at least one first lattice work arranged in said main channel for extending between a first longitudinal section and a second longitudinal section of the free main channel cross-section, said lattice work being comprised of a plurality of partition walls extending radially with respect to the main channel cross-section, and a plurality of wallsheets running concentrically or in a spiral with respect to the main channel axis and crossing said radial partition walls, thereby subdividing the free main channel cross-section in the manner of a guide blade into a plurality of partial channels for branching between said first and second longitudinal sections, said partial channels running in the direction of the main channel and being open at both longitudinal ends and having a length which is larger than the cross-sectional width dimensions of said partial channels and which is at least a quarter of the wave length of the sound oscillation having the lowest fundamental frequency of said sound field, wherein at least predetermined ones of adjacent partial channels have different characteristic dimensions for producing different sound oscillation states therein to be out of phase in said predetermined ones of adjacent partial channels, thereby producing in said predetermined ones of adjacent partial channels local sound pressure differences distributed over their lengths, said partition walls between said predetermined ones of adjacent partial channels of different characteristic dimensions being permeable for the propagation medium, at least over a partial length, to cause at least partly local sound pressure difference compensation of said local sound pressure differences by a flow of the propagation medium from one of the predetermined ones of adjacent partial channels into the adjacent one thereof, said partition walls comprising a damping material, such as felt or sintered material, for frictionally damping said flow of the propagation medium.