Aspects relate, in general, to acoustic absorbing structures and more specifically, although not exclusively, to structures for absorption of low frequency broadband acoustic noise.
In some situations, the presence of low frequency noise can be problematic. For example, high amplitude low-frequency noise can cause fatigue to be induced on structures such as vehicular platforms (e.g., fast jet platforms) where acoustic levels high enough to damage equipment and the platform structure are typically generated in e.g., internal payload bays. Such high acoustic levels can be managed by designing the geometry of the surrounding platform structures to divert noise and to withstand the high noise levels. However, such design considerations add undesirable mass and complexity to the platform in question.
Other methods for managing the presence of undesirable low-frequency acoustic noise include the use of materials or structures to exploit resonance effects. However, these are only suitable for narrowband frequency ranges and are therefore generally unsuitable for use on or in platforms where broadband frequency control is required. Absorbers capable of broadband frequency control can be used, but they typically require spatial regions for installation that are around the same size as the wavelength corresponding to the lowest frequency of interest. That is, in order to provide a useful broadband attenuation of unwanted acoustic noise, such absorbers are typically large and bulky. Again, this adds undesirable mass, weight and cost.
According to an example, there is provided 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 the length of the channel. Regions of the surface between apertures are minimised whereby to reduce acoustic impedance of the surface. In an example, the apertures are polygonal (i.e. in two-dimensional cross-section). In other examples, the apertures are circular. The apertures can form a periodic or aperiodic tessellation over the surface. Broadly speaking, regions between apertures are minimised in order to present a surface to an incident sound wave which does not introduce a large mismatch in acoustic impedance, thereby leading to high levels of acoustic reflection from the surface.
In an example, a channel tapers, in an axial direction, from its opening at the surface according to a predefined tapering profile. The rate of tapering of the channel along its length can be logarithmic. The rate of tapering may be linear, or indeed follow any other suitable profile. A channel can taper to a point. A channel can taper to an opening in a side of the acoustic absorbing structure opposite the surface.
In an example, a channel defines an acoustic pathway, the length of which is longer than a dimension of the acoustic absorbing structure. A channel can be folded. That is, a channel can comprise a circuitous, sinuous or tortured profile. A channel can comprise a series of baffles arranged along its length. Each baffle can extend inwardly from an inner surface of the channel and the length and density of baffles can increase along the length of the channel.
An opening can define a contraction region between the surface of the structure and a channel. For example, a region of a channel can comprise a different tapering profile compared to the rest of the channel.
In an example, a first material can be disposed or otherwise provided within a channel, the first material having a first density presenting a first acoustic impedance. A second material can be disposed or otherwise provided within the channel, the second material having a second density presenting a second acoustic impedance. The structure can be composed of a single material with multiple regions of respective different densities.
According to an example, there is provided a platform comprising a source of low frequency acoustic noise, the platform comprising an acoustic absorbing structure as claimed in any preceding claim so configured as to absorb or attenuate low frequency acoustic noise. The acoustic absorbing structure can receive the low frequency acoustic noise from the source at a normal to the surface. This enables optimal noise attenuation. In an example, an acoustic absorbing structure may be arranged at other angles of incidence to the noise with reduced levels of attenuation. The acoustic absorbing structure may be placed in a reverberant field or open or closed cavities to reduce noise levels in those spaces.
For a more complete understanding of the present disclosure, reference is now made, by way of example only, to the following descriptions taken in conjunction with the accompanying drawings, in which:
Example embodiments are described below in sufficient detail to enable those of ordinary skill in the art to embody and implement the systems and processes herein described. It is important to understand that embodiments can be provided in many alternate forms and should not be construed as limited to the examples set forth herein. Accordingly, while embodiments can be modified in various ways and take on various alternative forms, specific embodiments thereof are shown in the drawings and described in detail below as examples. There is no intent to limit to the particular forms disclosed. On the contrary, all modifications, equivalents, and alternatives falling within the scope of the appended claims should be included. Elements of the example embodiments are consistently denoted by the same reference numerals throughout the drawings and detailed description where appropriate.
The terminology used herein to describe embodiments is not intended to limit the scope. The articles “a,” “an,” and “the” are singular in that they have a single referent, however the use of the singular form in the present document should not preclude the presence of more than one referent. In other words, elements referred to in the singular can number one or more, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including,” when used herein, specify the presence of stated features, items, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, items, steps, operations, elements, components, and/or groups thereof.
According to an example, there is provided an acoustic absorbing or attenuation structure. The acoustic absorbing structure utilises an array of channels, 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. A surface of the acoustic absorbing structure comprises an array of apertures, with each aperture defining an opening for a corresponding channel provided in a body of the structure. In an example, each opening can be of the order of several millimetres wide, tapering to a point (or to another opening) over the length of the channel. That is, the surface comprises an array of holes that define the openings of multiple channels within the body of the acoustic absorbing structure. In an example, the channels reduce in cross-sectional area, and/or change in profile, along their length according to a tapering profile, until they terminate in a point (such as a dead end in the body) or another opening, such as an opening on the opposite side of the structure. As an acoustic wave travels down the tapered channel, which may be in the form of holes or slots, it is attenuated along the length of the channel.
In an example, in order to prevent ingress of debris into the channels, which may be in the form of narrow holes or slots for example, the surface 101 can comprise a thin membrane, mesh or other acoustically transparent material, which may be adhered, fixed or mounted to the body. Alternatively, surface 101 can comprise a layer with protected apertures. That is, surface 101 can be a layer in which the openings for channels 105 are defined by mesh (or membrane or acoustic transparent material) portions of the surface 101, which portions can be suitably profiled to match the size and shape of the mouth of channels 105. The surface 101 layer can then be fixed to body 107.
In the example of
A channel 105 attenuates sound as it passes through the channel. More specifically, as an acoustic wave passes through a channel 105 it experiences viscous damping—drag is experienced at the boundaries of the channel 105, which results in conversion of the energy from the acoustic wave to heat, which is dissipated in the body 107 and within the channel itself. The decreasing channel size also reduces the speed of sound within the channel. This reduces the acoustic wavelength corresponding to a given frequency, in turn allowing a channel of a given length to accommodate and thus attenuate sound of lower frequencies than would be possible in other types of absorber (such as porous material) with the same axial length.
As the size of the channel decreases over its length, the visco-thermal damping effect also becomes more pronounced, whilst the provision of a gradual taper in the channel means that there is a gradual change in acoustic impedance tends to minimise acoustic reflections. Minimising or eliminating acoustic reflections is advantageous in order to enable an incident acoustic wave to be attenuated or absorbed fully.
In this connection, it will be apparent that surface 101 presents a discontinuity in acoustic impedance to an incident acoustic wave. However, according to an example, this is minimised by reducing the size and/or number of regions of the surface between apertures 103. That is, with reference to
In this context, apertures may vary in size and/or shape. For example, apertures may be polygonal. Apertures can be arranged with a minimum region 113 between them, such that the region 113 represents, e.g., a thin boundary between apertures rather than a larger region of surface 101. In the case that apertures are polygonal for example, apertures may be tessellated to i) maximise the number of apertures for a given region of surface, and ii) minimise the space between apertures. Apertures may be so sized and shaped as to provide a periodic or aperiodic tessellation over the surface 101. That is, apertures may be the same or shape, at least some may comprise different shapes, and the size of apertures may vary as desired.
According to an example, the shape of an aperture can dictate the cross-sectional shape of a channel. Conversely, a given cross-sectional shape of a channel can dictate the shape and size of an aperture. For example, a circular aperture can serve as the opening of a cone shaped channel. Obviously, any number of different shapes may be selected. For example, an aperture may be hexagonal, with a correspondingly shaped channel comprising six inner faces that taper to a point 111.
The cross-sectional shape of a channel can therefore be selected and can vary as desired and, e.g., circular, 2D slots, triangular, square, pentagonal and so on cross-sections are all suitable examples. In an example, the width of the channels is small, being of the order of, e.g., a millimetre at the opening. The tapered channel can be either closed at the opposite end or terminated in a small hole. The diameter of the exit hole will influence the acoustic performance of the system. Exit holes or dead ends of the order of, e.g., 50 microns result in good attenuation across a wide frequency range. However, larger holes could be utilised in applications where a more limited attenuation characteristic would suffice.
In an example, the shape of the channel may vary along its length. For example, the channel may start, at its opening, being hexagonal in cross-section, and this may gradually change over the length of the channel to, e.g., circular, whilst tapering. There are many different permutations for the shapes and sizes of the apertures and channels, including whether they are fixed in shape (albeit tapering), or whether the shape varies long the length of the channel. The absorption characteristics of the structure may be tuned by altering at least one of: the taper gradient (which need not be constant along the length of the slit), entrance aperture and channel length.
According to an example, the cross-sectional area of a channel varies over its length according to a tapering profile. That is, the cross-sectional area of a channel will gradually reduce along its length from the opening towards the terminal point or exit hole in line with a predetermined set of parameters. Each channel may have a different tapering profile. In an example, a tapering profile can define, e.g., a rate of taper of different regions of a channel, and a form of tapering. For example, tapering can be linear, logarithmic or based on any other suitable function. A rate of tapering may be more pronounced (i.e., higher) at a region of a channel nearer to the opening 103 than at the tail end of the channel near surface 109. Accordingly, the rate of tapering can slow or reduce along the length of the channel.
According to an example, the required lowest frequency of absorption dictates the overall length of a channel 105. For example, for absorption down to approximately 200 Hz a channel length of approximately 600 mm is required. However, as noted above, and in the context of the example shown in
The length of the acoustic pathway of a channel 105 shown in
In the example of
In the case that the channel of
In the case of other channel shapes, the baffles can be so profiled as to match the profile of the inner wall of the channel. For example, a hexagonal channel would lead to use of hexagonally shaped baffles (with holes through the centre in order to maintain the acoustic pathway, which holes in the baffles need not be the same shape as the outer edge of the baffle—i.e., the outer edge of a baffle could be hexagonal to match the profile of a portion of a channel, whereas the hole in the baffle could be circular and so on). Similar considerations with respect to the positioning of the aperture in a baffle relative to the aperture in a succeeding and/or preceding baffle apply as noted above. While the side wall 303 of the channel proximate the opening 103 is shown to be vertical, in other embodiments it is sloped. For example, the side wall 303 in the embodiment of
The example of
In the example of
A broadband absorber/attenuator can be produced using one or more of the examples described above that is able to operate down to around at least 200 Hz with an overall thickness of around 100 mm or less.
An acoustic absorbing structure according to examples can be used in space critical applications such as aircraft and other transport applications. Other applications, where space is less critical but a small absorber is beneficial, may include, e.g., internal spaces in buildings where control of the acoustic environment is required. These may include, but are not limited to, entertainment venues such as theatres, concert halls, cinemas, recording studios and other spaces such as offices, restaurants etc.
According to an example, a channel may be defined by a cavity in the body of the acoustic absorbing structure. That is, a channel may be an opening in the body of the structure. In an example, a channel may be partially or fully filled or composed of another material (that differs from the material used for the body and/or which is not free space). For example, a channel may be partially of fully filled or composed from an aerogel or other porous material. The density of the material used to fill the channel may be varied axially and/or radially in order to alter the acoustic impedance of the channel. For example, a channel can be partially filled (from the region adjacent the surface 109 upwards towards surface 101, or vice versa) with a material such as an aerogel. The material can have a constant or varying density. For example, in the latter case, the density of the material can increase towards surface 109. As such, with a relatively lower density closer to the opening 103, the acoustic impedance mismatch between the material and, e.g., air or another material in the rest of the channel can be regulated in order to minimise reflections from the surface of the material in the channel.
In this connection, in an example, body 107 and/or channels 105 can comprise a porous material, such as aluminium sponge for example. The porous material can vary in density. For example, the body of the acoustic absorbing structure can comprise porous material with a first density, whilst the channels can comprise porous material with a second density which is lower than the first density and so on. So, for example, the whole acoustic absorbing structure can be composed from one material with differing densities whereby to define the channels and body regions of the structure. The material used may be the same or may differ (e.g., aluminium sponge for the body, aerogel for the channels and so on, or aluminium sponge of a first density for the body, and aluminium sponge of a second (lower) density for the channels).
As signal 605 is incident on the surface 101 of structure 607, it travels into the channels where it experiences a gradual reduction in the cross-section of the channel. Due to visco-thermal effects, the signal is thus absorbed along the length of the channel. As noted above, due to the various possible implementations in the way in which the acoustic pathways of channels may be increased without increasing a dimension (e.g., the depth, d) of the structure, the structure 607 can be effective in absorbing the sound within the portion of the platform 601 whilst taking up much less space than would otherwise be the case.
Unless otherwise defined, all terms (including technical and scientific terms) used herein are to be interpreted as is customary in the art. It will be further understood that terms in common usage should also be interpreted as is customary in the relevant art and not in an idealized or overly formal sense unless expressly so defined herein.
Number | Date | Country | Kind |
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21275034.3 | Mar 2021 | EP | regional |
2104412.8 | Mar 2021 | GB | national |
Filing Document | Filing Date | Country | Kind |
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PCT/GB2022/050724 | 3/23/2022 | WO |