Patent Publication Number: US-11043199-B2

Title: Sparse acoustic absorber

Description:
TECHNICAL FIELD 
     The present disclosure generally relates to acoustic metamaterials and, more particularly, to acoustic absorption metamaterials that are porous to ambient fluid. 
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it may be described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present technology. 
     Acoustic metamaterials having elastic acoustic properties that differ from those of their constituent materials are known. Such metamaterials have arrays of periodic structures, typically on a scale smaller than the target wavelength. Such metamaterials are typically solid surfaces that are impermeable to ambient fluid (e.g. air) and modulate sound in only one direction. 
     Accordingly, it would be desirable to provide an improved acoustic material having sparse (spaced apart) unit cells that allow air to flow freely between the unit cells, and that can modulate incident sound in two opposite directions. 
     SUMMARY 
     This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
     In various aspects, the present teachings provide an acoustic absorber. The acoustic absorber includes a periodic array of laterally spaced-apart, two-sided Helmholtz resonators. The periodic array further includes a plurality of unit cells spaced apart by a lateral midpoint-to-midpoint distance P, each unit cell having a maximum lateral dimension W, wherein P is greater than W. Each unit cell includes first and second Helmholtz resonators. The first Helmholtz resonator includes a first chamber portion bounded by at least one first boundary wall defining a first chamber volume. The second Helmholtz resonator includes a second chamber portion bounded by at least one second boundary wall defining a second chamber volume and a second neck forming an opening on a second side of the at least one second boundary wall and placing the second chamber portion in fluid communication with the ambient environment. The first side of the at least one first boundary wall and the second side of the at least one second boundary wall are on opposite sides of the unit cell, and the second chamber volume is greater than the first chamber volume. 
     In other aspects, the present teachings provide a dual-function sound suppression system. The system includes a substrate that is porous to a surrounding medium, the substrate having a continuous solid material having periodic apertures interspersed therein. The system also includes a periodic array of unit cells incorporated in the substrate. The periodic array includes a plurality of unit cells spaced apart by a lateral midpoint-to-midpoint distance P, each unit cell having a maximum lateral dimension W, wherein P is greater than W. Each unit cell includes first and second Helmholtz resonators. The first Helmholtz resonator includes a first chamber portion bounded by at least one first boundary wall defining a first chamber volume. The second Helmholtz resonator includes a second chamber portion bounded by at least one second boundary wall defining a second chamber volume and a second neck forming an opening on a second side of the at least one second boundary wall and placing the second chamber portion in fluid communication with the ambient environment. The first side of the at least one first boundary wall and the second side of the at least one second boundary wall are on opposite sides of the unit cell, and the second chamber volume is greater than the first chamber volume. 
     In still other aspects, the present teachings provide a fan coated with a sound suppression system. The fan includes a fan configured to move air in response to an electric current, and a sound suppression system coating or shielding the fan. The sound suppression system is as described above. 
     Further areas of applicability and various methods of enhancing the disclosed technology will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present teachings will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1A  is a schematic top plan view of a portion of a sparse acoustic absorber; 
         FIG. 1B  is a magnified view of a unit cell of the absorber of  FIG. 1A ; 
         FIG. 1C  is a schematic side cross-sectional view of three unit cells of the absorber of  FIG. 1A , viewed along the line  1 C- 1 C; 
         FIG. 1D  is a top plan view of a variant of the sparse acoustic absorber of the type shown in  FIG. 1A , having a one-dimensional array of unit cells; 
         FIG. 1E  is a perspective view of several unit cells of the one-dimensional array of  FIG. 1D ; 
         FIG. 2A  is a graph of acoustic transmission, reflection, and absorption as a function of frequency for the sparse acoustic absorber of  FIGS. 1A and 1B ; 
         FIG. 2B  is a plot of acoustic pressure distribution at the resonance frequency for the absorber of  FIGS. 1A and 1B ; and 
         FIG. 3  is a schematic top plan view of a portion of a dual-function sound suppression system incorporating a sparse acoustic absorber of the type shown in  FIG. 1A . 
     
    
    
     It should be noted that the figures set forth herein are intended to exemplify the general characteristics of the methods, algorithms, and devices among those of the present technology, for the purpose of the description of certain aspects. These figures may not precisely reflect the characteristics of any given aspect, and are not necessarily intended to define or limit specific embodiments within the scope of this technology. Further, certain aspects may incorporate features from a combination of figures. 
     DETAILED DESCRIPTION 
     The present teachings provide a sparse acoustic absorber. The disclosed acoustic absorber provides a structure that reflects or absorbs sound (depending on direction), while allowing fluid to pass through. 
     The present technology provides an asymmetric, bidirectional noise reduction device/structure. In one direction, the structure is an acoustic reflector, reducing noise by reflecting sound waves. In the opposite direction, the structure is an acoustic absorber, reducing and dampening noise. Because of its sparse structure, fluids such as ambient air can freely pass through the structure. 
     The sparse absorber has unique applicability in any application that benefits from sound dampening, while allowing air or other fluid to pass freely through. In an example, the sparse absorber could be wrapped around or placed in front of a fan, rendering the fan silent while allowing air to blow through. 
       FIG. 1A  shows a top plan view of a portion of a disclosed sparse acoustic absorber  100 , having an array of periodic unit cells  110 , while  FIG. 1B  shows a magnified view a single unit cell  110 , viewed from the same direction as in the view of  FIG. 1A .  FIG. 1C  shows a side cross-sectional view, taken along the line  1 C- 1 C, of a portion of sparse acoustic absorber  100  of  FIG. 1A , and including only three unit cells  110 . With particular reference to  FIG. 1A , the unit cells  110  can be periodic in 2-dimensions (e.g. x,y), as in the example of  FIG. 1A . Each unit cell  110  includes at least one enclosure wall, although the unit cell  110  of  FIGS. 1A-1C  includes multiple enclosure walls, such as side walls  112 ,  114 ,  116 , and  118 , and end wall  120 , as indicated in  FIG. 1B . Each unit cell  110  further includes a neck  122 , defining an aperture passing through the end wall  120 . 
     In the example of  FIG. 1A , the periodic array of unit cells  110  has periodicity in both x and y dimensions. This can be termed a two-dimensional array. While the unit cells  110  of  FIG. 1A  are shown as having a substantially square surface profile, they can alternately have a surface profile that is non-square rectangular, circular, triangular, ovoid, or any other regular shape. In some implementations in which the periodic array of unit cells  110  is a two-dimensional array, the two-dimensional array can have 90° rotational symmetry about an axis perpendicular to the surface of the absorber  100 . 
     The period, P, of the array of periodic array of unit cells  110  will generally be substantially smaller than the wavelength of the acoustic waves that the sparse acoustic absorber  100  is designed to absorb. As shown in  FIG. 1C , the period can be equated to a center-to-center distance between adjacent unit cells. In different implementations, the period of the periodic array of unit cells  110  will be less than 0.1 or less than 0.01 of the wavelength of the acoustic waves that the sparse acoustic absorber  100  is designed to absorb, i.e. the resonance frequency/wavelength of the absorber  100 . For example, in some implementations, the sparse acoustic absorber  100  can be designed to absorb acoustic waves of a human-audible frequency, having a wavelength within a range of a few millimeters (mm) to a few tens of meters. In such implementations, the periodic array of unit cells  110  can have a period within a range of from about ten or several tens of μm to about one mm. In some implementations, the sparse acoustic absorber  100  will be designed to absorb acoustic waves in the MHz frequency range, such as those having a wavelength within a range of from about one hundred μm to about two mm. In such implementations, the sparse acoustic absorber  100  can have a period within a range of about one μm to about one hundred μm. In certain implementations, the sparse acoustic absorber  100  can have a period within a range of from about one-quarter to one-half of its resonance wavelength. 
     With reference to  FIGS. 1D and 1E , the periodic array of unit cells  110  can alternatively be periodic in one dimension only.  FIG. 1D  shows a top plan view of such a one-dimensional periodic array of unit cells  110 , periodic in the x-dimension, and  FIG. 1E  shows a perspective view of the array of  FIG. 1D . As shown in the example of  FIGS. 1D and 1E , when an array is periodic in one-dimension (e.g. the x-dimension), each unit cell  110  will typically be elongated in the y-dimension. 
     Each unit cell  110  of the periodic array of unit cells  110  will generally have a maximum lateral dimension, or width W. It will be understood that in the case of a one-dimensional array, such as that of  FIGS. 1D and 1E , the maximum lateral dimension is only in the direction of periodicity (e.g. the x-dimension), and not in the elongated direction (e.g. the y-dimension). The periodic array of unit cells  110  is further characterized by a fill factor equal to P/W. In general, the fill factor will be 0.5 or less. In some implementations, the fill factor will be 0.25 (i.e. 25%) or less. It will be appreciated that the resonant frequency of the periodic phase—i.e. the periodic array of unit cells  110 —is substantially determined by the fill factor of the periodic array of unit cells  110 ; the ratio of period to width of unit cells  110 . As noted above, the period of the periodic array of unit cells  110  is smaller than the wavelength corresponding to the desired resonance frequency (period&lt;wavelength). At the same time, in many implementations the period and width of unit cells  110  will be chosen so that the periodic array of unit cells  110  has a fill factor of at least 0.2 (i.e. 20%). 
     In some implementations, the unit cells  110  of the sparse acoustic absorber  100  can be positioned periodically on a porous substrate, through which ambient fluid  170  can pass with little constraint. Such a porous substrate could be a mesh or screen, such as an air screen of the type used in a window, a sheet of material having periodic apertures or perforations, or any other suitable substrate. 
     Referring now more particularly to  FIG. 1C , each unit cell  110  of the sparse acoustic absorber  100  includes first and second Helmholtz resonators  130 A and  130 B. Each of the first and second Helmholtz resonators  130 A,  130 B includes a chamber  132 A,  132 B, respectively, bounded by the at least one enclosure wall  111  and by at least one partition wall  134 . In the example illustrated in  FIG. 1B , the first Helmholtz resonator  130 A is bounded by side walls  112 A and  116 A; by the end wall  120 A; and by the partition wall  134 ; as well as by side walls  114 A and  118 A which are not visible in the view of  FIG. 1C . Similarly, the second Helmholtz resonator  130 B is bounded by side walls  112 B and  116 B; by the end wall  120 B; and by the partition wall  134 ; as well as by side walls  114 B and  118 B which are not visible in the view of  FIG. 1C . Each of the first and second Helmholtz resonators  130 A,  130 B includes a neck  122 A,  122 B passing through the end wall  120 A,  120 B, and thereby placing the chamber  132 A,  132 B in fluid communication with the ambient environment. Thereby, an ambient fluid  170 , such as air, can pass in and out of the chambers  132 A,  132 B through the necks  122 A,  122 B. However, because the partition wall  134  is impermeable to ambient fluid  170 , ambient fluid  170 , such as air, cannot pass directly between the first and second Helmholtz resonators  130 A,  130 B. 
     While the unit cell  110  of  FIGS. 1A and 1B  defines a substantially rectangular prismatic shape, it is to be understood that a unit cell  110  of the present teachings can include any suitable shape, such as cylindrical, conical, spherical, ovoid, or any other shape that is suitable to enclose first and second Helmholtz resonators  130 A,  130 B separated by at least one partition wall  134 . It will therefore be understood that a unit cell  110  need not necessarily have first and second end walls  120 A,  120 B and that therefore first and second necks  122 A,  122 B need not necessarily pass through an “end wall”. In general, the first and second necks  122 A,  122 B will be positioned on opposite sides of the unit cell  110 , and will be substantially parallel to an axis, z, that is perpendicular to the x-axis or x,y-axes defining periodicity of the array of unit cells  110 . In general, the maximum width of a chamber  132 A,  132 B will be substantially greater than the maximum width of its associated neck  122 A,  122 B. 
     It will further be understood that each chamber  132 A,  132 B defines a volume, corresponding to the volume of ambient fluid  170  that can be held in the chamber  132 A,  132 B, exclusive of the neck  122 A,  122 B. The volume of the second chamber  132 B will generally be greater than the volume of the first chamber  132 A. It will further be understood that each of the first and second necks  122 A,  122 B has a length. In general, the length of the first neck  122 A will be greater than the length of the second neck  122 B. Thus, the first Helmholtz resonator  130 A generally has a longer neck  122 A and a smaller (lower volume) chamber  132 A than does the second Helmholtz resonator  130 B. 
     The at least one enclosure wall and the end wall  120  will typically be formed of a solid, sound reflecting material. In general, the material or materials of which the at least one enclosure wall and the end wall  120  are formed will have acoustic impedance higher than that of ambient fluid  170 . Such materials can include a thermoplastic resin, such as polyurethane, a ceramic, or any other suitable material. 
     Referring to  FIG. 1C , when an acoustic wave approaches the device from the direction indicated by the arrow, A, the device operates in what can be termed “absorption mode”. When an acoustic wave approaches the device from the opposite direction, the device operates in what can be termed “Reflection mode.” In absorption mode, sound is blocked by the absorption of the structure, while the ambient fluid  170  can flow. The incident acoustic energy is dissipated to heat in the first neck  122 A via viscous loss. It will be appreciated that the first Helmholtz resonator  130 A has higher viscous loss than does the second Helmholtz resonator  130 B. The sound propagation direction shown in  FIG. 1  is for acoustic absorption mode. 
       FIG. 2A  is a graph of acoustic transmission, reflection, and absorption as a function of frequency for a sparse acoustic absorber  100  of the present teachings. The simulated results of  FIG. 2A  are for an absorber having a fill factor of 25%, with acoustic waves approaching from the direction of the arrow, A that is shown in  FIG. 1C . It will be observed that the absorber  100  demonstrates strong acoustic absorption at the resonance frequency—in this example centered at 2.5 KHz, and allows very low transmission at the resonance frequency. It will further be observed that reflection is very low at the resonance frequency, such that nearly all of the sound is absorbed at the resonance frequency.  FIG. 2B  shows acoustic pressure distribution at the resonance frequency (2.5 KHz) for the absorber whose acoustic properties are shown in  FIG. 2A . As can be seen from the schematic image of  FIG. 2B , acoustic energy is concentrated primarily around the neck  122 A of the first Helmholtz resonators  130 A, but also significantly around the neck  122 B of the second Helmholtz resonators  130 B. This result highlights the contribution that both Helmholtz resonators  130 A,  130 B make to the absorption properties of the absorber  100  when operating in absorption mode. 
     However, if acoustic waves impinge on the absorber  100  from the opposite direction, indicated by the arrow, R, in  FIG. 1C , the absorber  100  has an altered function, operating primarily as a reflector. In this instance, the incident acoustic waves arrive at the side of the second Helmholtz resonator  130 B. When the absorber  100  is used in this manner, the absorption and reflection curves of  FIG. 2A  are substantially switched with one another, so that the incident acoustic waves are predominantly reflected, rather than absorbed, as described above in reference to absorption mode and reflection mode. Thus, depending on whether acoustic absorption or reflection is desired, the absorber  100  can be positioned relative to an acoustic source in either of two general orientations, to achieve the desired outcome. An absorber  100  of the present teachings can thus be alternatively referred to as a “reversible, dual-function acoustic absorber/reflector”. While not shown graphically here, both Helmholtz absorbers  130 A,  130 B likewise contribute to the reflective properties of the absorber  100  when operating in reflection mode. 
       FIG. 3  shows a schematic, top plan view of a disclosed, dual-function sound suppression system  300 . The dual-function sound suppression system  300  includes a substrate  310  that is porous to a surrounding medium, such as air. Examples of such a porous substrate can include a mesh or screen, such as an air screen of the type used in a window, a sheet of material having periodic apertures or perforations, or any other suitable substrate, as described above. The substrate  310  is generally composed of a continuous solid material, that may be, but need not necessarily be, flexible. Suitable solid materials for the substrate  310  and can include metals, plastics, and the like. The system further includes periodic apertures  320  that provide the substrate  310  with its porosity. 
     The system  300  further includes unit cells  110  of a sparse acoustic absorber  100 , as described above, positioned in the apertures  320  of the substrate  310 . The unit cells  110  can be positioned so that first and second necks  122 A,  122 B are substantially perpendicular to the two-dimensional surface of the substrate  310 , and may be positioned on aperture edges, as shown in  FIG. 3 . The system can define a substrate fill factor, which is the two-dimensional surface of the system occupied by substrate, divided by the two dimensional surface of the system that is occupied by aperture (i.e. that is unoccupied). This can alternatively be referred to as inverse substrate porosity. In general, the substrate fill factor will be substantially lower than is the fill factor of the absorber  100  that is incorporated in the substrate. For example, the fill factor of the absorber  100  as incorporated in the substrate  300  can have a fill factor in a range of about 0.1 to 0.25, while the substrate fill factor may be 0.05 or less. This allows the system to remain porous with the incorporated absorber  100 . 
     The substrate  310  will generally be substantially planar—although as noted above, it can be flexible—having first and second planar surfaces. Due to the dual absorption mode/reflection mode of the array of unit cells  110 , as described above, the system will predominantly absorb acoustic waves at or near a resonant frequency when such waves are incident on one of the planar sides; and will predominantly reflect acoustic waves at or near the resonant frequency when such waves are incident on the other of the two planar sides. 
     In an example, a dual-function sound suppression system  300  can be used as a window screen that allows air flow through an open window. In such an implementation, the screen can absorb sound arriving at the window from one side, and reflect sound arriving at the window from the opposite side. It will be understood that such a sound suppression system  300  can have utility in any scenario where fluid flow is desirable, and either or both of sound absorption and sound reflection is useful. For example, a disclosed sound suppression system  300  can be useful as a coating or shield for any device that benefits from air or fluid flow and also produces sound, such as a fan or other mechanical blower, or a noise producing mechanism having an air intake. In an example, a fan that is shielded with a sound suppression system  300  could be deployed in a motor vehicle, such as a fan that circulates air in a passenger cabin, a turbocharger, or a turbine fan on a jet engine. 
     The preceding description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical “or.” It should be understood that the various steps within a method may be executed in different order without altering the principles of the present disclosure. Disclosure of ranges includes disclosure of all ranges and subdivided ranges within the entire range. 
     The headings (such as “Background” and “Summary”) and sub-headings used herein are intended only for general organization of topics within the present disclosure, and are not intended to limit the disclosure of the technology or any aspect thereof. The recitation of multiple embodiments having stated features is not intended to exclude other embodiments having additional features, or other embodiments incorporating different combinations of the stated features. 
     As used herein, the terms “comprise” and “include” and their variants are intended to be non-limiting, such that recitation of items in succession or a list is not to the exclusion of other like items that may also be useful in the devices and methods of this technology. Similarly, the terms “can” and “may” and their variants are intended to be non-limiting, such that recitation that an embodiment can or may comprise certain elements or features does not exclude other embodiments of the present technology that do not contain those elements or features. 
     The broad teachings of the present disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the specification and the following claims. Reference herein to one aspect, or various aspects means that a particular feature, structure, or characteristic described in connection with an embodiment or particular system is included in at least one embodiment or aspect. The appearances of the phrase “in one aspect” (or variations thereof) are not necessarily referring to the same aspect or embodiment. It should be also understood that the various method steps discussed herein do not have to be carried out in the same order as depicted, and not each method step is required in each aspect or embodiment. 
     The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations should not be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.