Acoustic absorbing structures
In some examples, an acoustic absorbing structure comprises a surface comprising an array of apertures, each aperture defining an opening for a corresponding channel, each channel forming a non-resonant acoustic attenuation structure with an axially decreasing cross-section over at least a portion of the length of the channel.
1 . An acoustic absorbing structure, comprising:
a surface comprising an array of apertures, each aperture defining an opening for a corresponding channel, each channel forming a non-resonant acoustic attenuation structure with an axially decreasing cross-section over at least a portion of a length of the channel,
wherein a channel further comprises a series of baffles arranged along its length, each baffle being parallel to the surface.
2 . The acoustic absorbing structure as claimed in claim 1 , wherein regions of the surface between apertures are minimised whereby to reduce acoustic impedance of the surface.
3 . The acoustic absorbing structure as claimed in claim 1 , wherein the apertures are polygonal.
4 . The acoustic absorbing structure as claimed in claim 1 , wherein the apertures form a periodic or aperiodic tessellation over the surface.
5 . The acoustic absorbing structure as claimed in claim 1 , wherein a channel tapers, in an axial direction, from its opening at the surface according to a predefined tapering profile.
6 . The acoustic absorbing structure as claimed in claim 5 , wherein a rate of tapering of the channel along its length is logarithmic.
7 . The acoustic absorbing structure as claimed in claim 5 , wherein the channel tapers to a point.
8 . The acoustic absorbing structure as claimed in claim 5 , wherein the channel tapers to an opening in a side of the acoustic absorbing structure opposite the surface.
9 . The acoustic absorbing structure as claimed in claim 1 , wherein a channel defines an acoustic pathway, the length of which is longer than a dimension of the acoustic absorbing structure.
10 . The acoustic absorbing structure as claimed in claim 1 , wherein a channel is folded.
11 . The acoustic absorbing structure as claimed in claim 1 , wherein the baffles have a profile matching a profile of an interior wall defining the channel.
12 . The acoustic absorbing structure as claimed in claim 1 , wherein each baffle extends inwardly from an inner surface of the channel and wherein the length and density of baffles increases along the length of the channel.
13 . The acoustic absorbing structure as claimed in claim 1 , wherein an opening defines a contraction region between the surface of the structure and a channel.
14 . The acoustic absorbing structure as claimed in claim 1 , further comprising a first material disposed or otherwise provided within a channel, the first material having a first density presenting a first acoustic impedance.
15 . The acoustic absorbing structure as claimed in claim 14 , further comprising a second material disposed or otherwise provided within the channel, the second material having a second density presenting a second acoustic impedance.
16 . The acoustic absorbing structure as claimed in claim 1 , wherein the structure is composed of a single material with multiple regions of respective different densities.
17 . A platform comprising a source of low frequency acoustic noise, the platform comprising an acoustic absorbing structure so configured as to absorb or attenuate low frequency acoustic noise, the acoustic absorbing structure comprising:
a surface comprising an array of apertures, each aperture defining an opening for a corresponding channel, each channel forming a non-resonant acoustic attenuation structure with an axially decreasing cross-section over at least a portion of a length of the channel,
wherein a channel further comprises a series of baffles arranged along its length, each baffle being parallel to the surface.
18 . The platform as claimed in claim 17 , wherein the acoustic absorbing structure is arranged to receive the low frequency acoustic noise from the source at a normal to the surface.
19 . An acoustic absorbing structure, comprising:
a surface comprising an array of apertures, each aperture defining an opening for a corresponding channel, each channel forming a non-resonant acoustic attenuation structure with an axially decreasing cross-section over at least a portion of a length of the channel,
where the structure is composed of a single material with multiple regions of respective different densities.
20 . The platform as claimed in claim 17 , wherein the low frequency includes 200 Hz and the acoustic absorbing structure so configured as to absorb or attenuate low frequency acoustic noise with a height of less than 300 mm.