Air-transparent selective sound silencer using ultra-open meta-material

Abstract

To provide a silencer which makes air flow through the silencer while mitigating sound transmission through the silencer.SOLUTION: A meta-material sound silencer 200 includes a first transmission region 210. The region 210 specifies an opening part opened to allow flow of gas through the meta-material sound silencer 200. The first transmission region 210 specifies a liquid flow axis 211 and a fluid (gas and / or liquid, for example) can flow through the first transmission region 210 along the liquid flow axis 211. A second transmission region 220 includes a set of one or a plurality of conduits. Each conduit in the set is constructed to resonate in a target frequency. The second transmission region 220 includes an entrance and an exit, and a wave can be propagated from the entrance to the exit through the second transmission region 220.SELECTED DRAWING: Figure 2A

Description

[Technical field]

[0001] Related Applications This application claims priority to U.S. Provisional Patent Application No. 62 / 714,246, filed Aug. 3, 2018, entitled "Air-Transparent Selective Sound Silencer Using Ultra-Open Metamaterial," inventors Xin Zhang, Reza Ghaffarivardavagh and Stephan Anderson, and to U.S. Provisional Patent Application No. 62 / 863,046, filed Jun. 18, 2019, entitled "Air-Transparent Selective Sound Silencer Using Ultra-Open Metamaterial," inventors Xin Zhang, Reza Ghaffarivardavagh and Stephan Anderson, the disclosures of each of which are incorporated herein by reference in their entirety.

[0002] Technical Field FIELD OF THE DISCLOSURE This disclosure relates to sound suppression devices and, more particularly, to devices that suppress sound transmission through the device while also allowing air to flow through the device. [Background technology]

[0003] Background technology It is known to suppress sound propagation by various means, such as sound-absorbing shields and sound-deflecting surfaces. Some devices, for example noise-canceling headphones, attenuate the propagation of unwanted sounds by mixing them with a replica of the unwanted sound that is the inverse of the unwanted sound.

[0004] If the unwanted sound has a known frequency, some devices attenuate the unwanted sound at a particular frequency by mixing it with an inverted copy of the sound (e.g., a copy that is 180 degrees out of phase with the unwanted sound).

[0005] One such prior art type of device is known as a "Herschel-Quincke tube" (or "HQ tube"). An HQ tube has a first duct through which sound can propagate and a second duct through which sound can propagate. A propagating acoustic signal enters both the first and second ducts and propagates through both ducts until they meet, where the signal propagating through the second duct merges with the signal propagating through the first duct.

[0006] The ability of the HQ tube to reduce an acoustic signal propagating in a medium at a given frequency with a corresponding wavelength (λ) does not result from the length of the first duct (L1) or the length of the second duct (L2), but instead from the difference between the lengths of the first and second ducts (i.e., L2-L1). In the case of the HQ tube, the difference in length between the first and second ducts (i.e., L2-L1) is one-half the wavelength (0.5λ) of the frequency of the acoustic signal (or Nλ+0.5λ, where N is an integer), and therefore, at the point where the ducts meet and their individual signals merge, the signal propagating in the second duct is 180° out of phase with the signal in the first duct. For example, the first duct may have a length of 1.25λ and the second duct may have a length of 1.75λ, so the difference between their lengths is 1.75λ-1.25λ=0.5λ.

[0007] In particular, this means that the manufacture of the HQ tubes requires high precision in manufacturing both ducts to guarantee the required difference between their respective lengths. Moreover, such devices require a trade-off between the amount of open space through which fluid can flow and their ability to attenuate acoustic propagation (i.e., their transmission losses). In other words, the amount of open area is sacrificed to obtain the desired acoustic performance.

[0008] Some examples of prior art HQ tubes are described below.

[0009] FIG. 1A shows a schematic diagram of a prior art exhaust muffler according to FIG. 1 of U.S. Pat. No. 4,683,978 to Venter.

[0010] In the Venter device (FIG. 1A), an exhaust silencer for an internal combustion engine is generally designated by the reference numeral 10. The exhaust silencer 10 has an inlet opening 12 and an outlet opening 14 spaced axially from the inlet opening 12. The silencer includes a cylindrical shell (or casing) 16 and a core 18 within the shell 16. The core includes a central axial tube 19, which defines at least one axial flow passage 20. The core has at least one helical baffle 21, which defines a helical path 22 within the shell 16 about the axial flow passage 20. The axial flow passage 20 has an upstream axial inlet 20.1 and a transverse outlet 24 that is directed transversely outwardly into the helical path 22 at a downstream half of the helical path. The transverse outlet 24 is provided by a plurality of closely spaced openings at the downstream end of the axial passage 20 between the last two vanes 21.1 and 21.2 of the spiral baffle 21.

[0011] The Venter silencer 10 has an inlet chamber 26 including a frustoconical portion 26.1 defined by a funnel-shaped inlet connection 28, which has an axial length of about half the diameter of the cylindrical shell 16. The inlet chamber also has a cylindrical portion 26.2, which has an axial length of about half the diameter of the cylindrical shell 16. Similarly, the silencer has an outlet chamber 30, also frustoconical, extending downstream from the helical path, which is defined by a funnel-shaped outlet connection 32, which also has an axial length of about half the diameter of the cylindrical shell 16. The baffle 21 is wound in a worm screw manner around the central axial tube 19 to define the helical path 20. The upstream open end 20.1 of the axial flow passage is located at the downstream end of the cylindrical portion 26.2 of the inlet chamber 26. The central axial tube 19 defining the axial flow passage 20 is closed by a transverse partition 20.2 which is aligned with the upstream axial inlet 20.1 of the tube 19 and downstream of the transverse outlet 24 of the tube 19.

[0012] As shown, the Venter axial passage 20 is covered by its transverse bulkhead 20.2, and waves propagating through the Venter axial passage 20 can only exit the axial passage 20 radially through its transverse outlet 24 hole, which is within the boundary of its cylindrical shell (or casing) 16. As a result, the merging of the waves propagating through the axial passage 20 and the waves propagating through its helical path 22 can only occur within the silencer 10. The merging of the Venter axial passage 20 and its helical path 22 can therefore be described as "in a duct."

[0013] FIG. 1B shows a schematic diagram of a prior art noise suppression device for a gas duct 4 according to FIG. 2 of US Pat. No. 7,117,973 to Graefenstein.

[0014] The Graefenstein duct 4 includes a central pipe 44 having three helical passages 51, 53, 55 in contact with the outer surface of the pipe 44.

[0015] 1B, the helical passages 51, 53, 55 merge with the central pipe 44 in the axial direction (at the exit opening 16). As a result, the merging of the waves propagating through the Graefenstein central pipe 44 with the waves propagating through its three helical passages 51, 53, 55 can only occur within the central pipe 44. The merging of the Graefenstein central pipe 44 with its helical passages 51, 53, 55 can therefore be described as "in a duct."

[0016] FIG. 1C shows a schematic of a prior art split path silencer 10 according to FIG. 1 of U.S. Patent No. 9,500,108 to Brown. Brown's silencer 10 includes an outer shell 12 having an inlet opening 64 (with angled section 20) and an outlet opening 66. Within the outer shell 12, Brown's silencer 10 includes a baffle 63 wrapped around an inner tube 62. Sound can propagate through the inner tube 62 in the direction 28, and sound can travel in the direction 68 through a passage defined by the baffle 63. The inner tube 62 has an exhaust opening 67 that is located near, but spaced from, the outlet opening 66 of the outer shell 12.

[0017] 1C, the passageway formed by Brown's baffle 63 exits into the space inside the shell (or casing) 12. As a result, the junction of the waves propagating through Brown's inner tube 62 and the waves propagating through the passageway formed by its baffle 63 can only occur within the shell (or casing) 12. Thus, the junction of Brown's inner tube 62 and the passageway formed by its baffle 63 can be described as "in a duct." [Prior art documents] [Patent documents]

[0018] [Patent Document 1] U.S. Pat. No. 4,683,978 [Patent Document 2] U.S. Patent No. 7,117,973 [Patent Document 3] U.S. Patent No. 9,500,108 Summary of the Invention [Means for solving the problem]

[0019] Overview of Various Embodiments According to an exemplary embodiment, the sound absorbing device has a first transmission area and a second transmission area, each open to receive an incident wave (e.g., an acoustic signal having a spectrum including a frequency of interest and propagating in a fluid medium such as a gas or liquid).

[0020] The first transmission area has an inlet (first inlet) and an outlet (first outlet) and is open for waves to propagate through the first transmission area from the first inlet to the first outlet and for fluids to flow through the first transmission area from the first inlet to the first outlet. For these purposes, the first transmission area has a cross-sectional area (A1). The first transmission area is configured such that the waves propagating through the first area remain continuous. According to some embodiments, the first transmission area is configured such that it does not resonate at the frequency of interest.

[0021] The second transmission region has an inlet (second inlet) and an outlet (second outlet) and is open for waves to propagate through the second transmission region from the second inlet to the second outlet. According to an exemplary embodiment, the second transmission region is configured to resonate at a frequency of interest. The second transmission region has a cross-sectional area (A2).

[0022] The second transmission region is positioned relative to the first transmission region such that waves exiting the second outlet may destructively interfere with waves exiting the first transmission region at the frequency of interest. According to an exemplary embodiment, waves exiting the second outlet may destructively interfere with waves exiting the first transmission region at the frequency of interest to attenuate the incident wave by 94% (or 24 dB).

[0023] According to exemplary embodiments, the first cross-sectional area (A1) is greater than the second cross-sectional area (A2) such that the device has an open ratio of at least 0.6 [i.e., A1 / (A1+A2) is 0.6 or greater]. Some embodiments are configured to have an open ratio of 0.8 or greater, including up to 0.99, while maintaining the above-mentioned ability to attenuate an incident signal.

[0024] According to some embodiments, each second outlet is positioned such that the signal exits the second outlet in an axial direction, in such embodiments, energy from the exiting signal does not enter the first transmission region in a radial direction.

[0025] Additionally, according to some embodiments, each of the second outlets is positioned such that the signal exits the second outlet into an extra-ducted space. Some embodiments are extra-ducted in that the device does not have an integrated duct downstream thereof, and thus the signal exits the silencer into an extra-ducted space.

[0026] A first exemplary embodiment of the device includes a first passageway and one or more second passageways, the first passageway having a first inlet and a first outlet, open for a first wave at a frequency of interest to propagate therethrough and having a first cross-sectional area, and the one or more second passageways each open for a second wave at a frequency of interest to propagate therethrough and each having a second inlet and a second outlet, the one or more second passageways defining a second cross-sectional area, where each of the one or more second passageways is positioned relative to the first passageway such that a second wave at the frequency of interest exiting the one or more second outlets may destructively interfere with a first wave at the frequency of interest exiting the first passageway, and further where the first cross-sectional area is greater than the second cross-sectional area such that the device has an openness ratio of at least 0.6.

[0027] According to some implementations, the first passageway is open to fluid flow through the first passageway.

[0028] According to some embodiments, the first cross-sectional area is greater than the second cross-sectional area such that the device has an open ratio of at least 0.8. According to some such embodiments, the device has an open ratio of 0.99.

[0029] According to some embodiments, the first passage defines an axis of fluid flow through the first passage, and each of the second outlets is an outlet outside the duct.

[0030] According to some embodiments, the first passage defines an axis of fluid flow through the first passage, and each of the second outlets is an axially oriented outlet, and according to some such embodiments, each of the second outlets is an outlet outside the duct.

[0031] According to some embodiments, each of the first and second waves are acoustic waves, and destructive interference attenuates the first wave at the frequency of interest by at least 94%. According to some embodiments, acoustic energy at the frequency of interest exiting each of the second outlets destructively interferes with acoustic energy exiting the first passageway to attenuate the sound at the frequency of interest by at least 24 dB.

[0032] Another embodiment of the apparatus includes a first passageway and one or more second passageways, the first passageway being open for a first wave at a frequency of interest to propagate through the first passageway and having a first inlet and a first outlet, and the one or more second passageways each having a second inlet and a second outlet, extending along an axis defining an axial direction and open for a second wave at a frequency of interest to propagate through the second passageway, where the one or more second outlets are open in the axial direction, and further where the one or more second passageways are positioned relative to the first passageway such that the second wave at the frequency of interest exiting the one or more second outlets may destructively interfere with the first wave at the frequency of interest exiting the first passageway.

[0033] According to some of these embodiments, the one or more second passages are each configured to resonate at a frequency of interest and the first passage is configured to maintain a continuity during propagation of the first wave through the first passage. In some such embodiments, the one or more second passages are each configured to resonate at a frequency of interest and the first passage is configured to not resonate at the frequency of interest.

[0034] According to some embodiments, each of the one or more second passages is positioned relative to the first passage such that propagation of the second wave exiting the second outlet can destructively interfere with the first wave exiting the first passage at a frequency of interest to reduce transmission of the first wave by at least 94%.

[0035] According to some embodiments, each of the second passages is positioned relative to the first passage such that propagation of the second wave exiting the second outlet can destructively interfere with the first wave exiting the first passage at the frequency of interest to attenuate the first wave by at least 24 dB.

[0036] According to some embodiments, the first passage has a first cross-sectional area (A1) and the one or more second passages define a second cross-sectional area (A2), and a ratio of the first area (A1) to the sum of the first area (A1) and the second area (A2) [A1 / (A1+A2)] is greater than 0.6.

[0037] Another embodiment of the device includes a first passageway and one or more second passageways, the first passageway open for a first wave at the frequency of interest to propagate through the first passageway and having a first inlet and a first outlet opening into the volume outside the duct, and one or more second passageways each extending along an axis and open for a second wave at the frequency of interest to propagate through the second passageway and each having a second inlet and a second outlet opening into the volume outside the duct, where the one or more second passageways are positioned relative to the first passageway such that the second wave at the frequency of interest exiting the one or more second outlets may destructively interfere with the first wave at the frequency of interest exiting the first passageway.

[0038] According to some such embodiments, each second passage is configured to resonate at a frequency of interest, and the first passage is configured to maintain a continuous state during propagation of the wave through the first passage.

[0039] In some embodiments, each second passage is configured to resonate at a frequency of interest and the first passage is configured to not resonate at the frequency of interest.

[0040] According to some implementations, the first passageway is open to fluid flow through the first passageway.

[0041] According to some embodiments, the first wave is an acoustic wave, and the acoustic wave at the frequency of interest is attenuated by destructive interference.

[0042] According to some embodiments, the first passageway has a first cross-sectional area and the one or more second passageways define a second cross-sectional area, the first cross-sectional area being greater than the second cross-sectional area such that the device has an opening ratio of at least 0.8.

[0043] According to some embodiments, the first passageway has a first cross-sectional area and the one or more second passageways define a second cross-sectional area, the first cross-sectional area being greater than the second cross-sectional area such that the device has an aperture ratio of at least 0.99.

[0044] Yet another embodiment of the apparatus includes a first passageway and one or more second passageways, the first passageway open for a first wave at a frequency of interest to propagate therethrough and having a first inlet and a first outlet, where the first passageway is configured to maintain a continuous state in the presence of waves at the frequency of interest, and the one or more second passageways each open for a second wave at a frequency of interest to propagate therethrough and configured to resonate at the frequency of interest and each having a second inlet and a second outlet, where each of the one or more second passageways is positioned relative to the first passageway such that the second wave at the frequency of interest exiting the one or more second outlets may destructively interfere with the first wave at the frequency of interest exiting the first passageway.

[0045] In some such devices, the first passageway is open to fluid flow through the first passageway.

[0046] According to some embodiments, the first passage is configured to be non-resonant at the frequency of interest.

[0047] According to some embodiments, the first wave is an acoustic wave, and the acoustic wave at the frequency of interest is attenuated by destructive interference such that transmission of the acoustic wave out of the first passage is reduced by at least 94%.

[0048] According to some embodiments, the first wave is an acoustic wave, and the acoustic wave at the frequency of interest is attenuated by destructive interference such that the acoustic wave exiting the first passage is attenuated by at least 24 dB.

[0049] According to some embodiments, the first passage has a first cross-sectional area (A1) and the second passage defines a second cross-sectional area (A2), and a ratio of the first area (A1) to the sum of the first area (A1) and the second area (A2) [A1 / (A1+A2)] is greater than 0.6.

[0050] According to some embodiments, the first passage has a first cross-sectional area (A1) and the second passage defines a second cross-sectional area (A2), and a ratio of the first area (A1) to the sum of the first area (A1) and the second area (A2) [A1 / (A1+A2)] is greater than 0.8.

[0051] According to some embodiments, the first passage has a first cross-sectional area (A1) and the second passage defines a second cross-sectional area (A2), and a ratio of the first area (A1) to the sum of the first area (A1) and the second area (A2) [A1 / (A1+A2)] is greater than 0.9. [Brief description of the drawings]

[0052] The features of the embodiments hereinbefore described will be more readily understood by reference to the following detailed description when taken in conjunction with the accompanying drawings, in which: [Figure 1A] 1 is a schematic diagram of a prior art exhaust muffler; FIG. [Figure 1B] 1 is a schematic diagram of a prior art noise suppression device for a gas duct; [Figure 1C] FIG. 1 is a schematic diagram of a prior art split path silencer. [Figure 2A] FIG. 1A is a schematic diagram illustrating a cross-sectional view of one embodiment of a metamaterial acoustic silencer. [Figure 2B] 1 is a graph illustrating the transmission of acoustic energy through a metamaterial silencer 100 for various impedance ratios. [Figure 2C] 1 is a graph illustrating the transmission of acoustic energy through a metamaterial silencer 100 for various refractive index ratios. [Figure 3A] FIG. 1 is a schematic diagram illustrating one view of an embodiment of a metamaterial acoustic silencer. [Figure 3B] 1A and 1B are schematic diagrams illustrating alternative views of an embodiment of a metamaterial acoustic silencer. [Figure 3C] 1A and 1B are schematic diagrams illustrating alternative views of an embodiment of a metamaterial acoustic silencer. [Figure 3D] FIG. 3B is a schematic diagram showing the embodiment of FIG. 3A in cross-section; [Figure 4] FIG. 4A is a graph showing the transmission of acoustic energy through the metamaterial silencer 100 at frequencies of interest, FIG. 4B is a graph showing the transmission of acoustic energy through the metamaterial silencer 100 at frequencies of interest, FIG. 4C is a graph showing the transmission and reflection of acoustic energy through the metamaterial silencer 100, and FIG. 4D is a graph showing sound transmission through a dual layer metamaterial silencer 100 having various degrees of structural openness. [Figure 5A] 1A-1D are schematic diagrams illustrating alternative embodiments of metamaterial acoustic silencers. [Figure 5B] 1A-1D are schematic diagrams illustrating alternative embodiments of metamaterial acoustic silencers. [Figure 6A] 1A-1D are schematic diagrams illustrating alternative embodiments of metamaterial acoustic silencers. [Figure 6B] 1A-1D are schematic diagrams illustrating alternative embodiments of metamaterial acoustic silencers. [Figure 7] FIG. 1 is a schematic diagram illustrating one embodiment of a sound silencer system having multiple metamaterial acoustic silencers arranged in series. [Figure 8A] 1A-1D are schematic diagrams illustrating alternative embodiments of metamaterial acoustic silencers. [Figure 8B]1A-1D are schematic diagrams illustrating alternative embodiments of metamaterial acoustic silencers. [Figure 9A] FIG. 1 is a schematic diagram of one embodiment of a metamaterial sound silencer disposed within a tube. [Figure 9B] 13 is a graph showing the results of operation of a metamaterial sound silencer disposed within a tube. [Figure 10A] FIG. 1 illustrates a schematic diagram of an apparatus having a metamaterial acoustic silencer. [Figure 10B] FIG. 1 is a schematic diagram of a bulkhead having multiple metamaterial acoustic silencers. [Figure 11A] 1A-1D are schematic diagrams illustrating alternative embodiments of metamaterial acoustic silencers. [Figure 11B] 1A-1D are schematic diagrams illustrating alternative embodiments of metamaterial acoustic silencers. [Figure 11C] 1 is a graph showing noise pressure inside an enclosed automobile wheel; [Figure 11D] 1 is a graph showing one embodiment of a metamaterial sound deadening device disposed inside an enclosed pneumatic wheel. [Figure 11E] 11B is a graph showing the pressure inside the wheel normalized to the pressure when the wheel does not have the metamaterial sound dampening device 1100 of FIG. 11A. [Figure 11F] FIG. 1 is a schematic diagram illustrating one embodiment of a metamaterial sound suppressor disposed on the hub of a pneumatic wheel. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0053] Detailed Description of Specific Embodiments The devices included in various embodiments allow sufficient fluid flow (e.g., air flow) through the device while reducing noise transmission through the device and while providing a significantly more compact form factor than known devices.

[0054] Additionally, embodiments allow a designer to specify and adjust one or more frequencies at which the device reduces noise transmission and / or a bandwidth around one or more frequencies at which the device reduces noise transmission.

[0055] Definition: The term "external to the duct" means that the space downstream from the apparatus is not surrounded by a duct, for example a duct that is an integral part of the apparatus.

[0056] The term "acoustic waves" refers to waves that propagate through a fluid by adiabatic compression and decompression.

[0057] The term "acoustic energy" refers to energy carried or propagated by acoustic waves.

[0058] The term "axial" means parallel to an axis.

[0059] The term "axially oriented" means oriented with respect to an axis in a direction parallel to that axis.

[0060] The term "axis of fluid flow" means the direction in which a fluid is allowed to flow.

[0061] The term "continuous state" refers to a signal having a frequency spectrum, in which the signal maintains energy at frequencies throughout the spectrum.

[0062] The terms "destructive interference" or "destructively interfere" refer to the phenomenon in which two individual waves incident on a common point overlap to form a composite wave having an amplitude equal to the difference in the individual amplitudes of each of the individual waves.

[0063] The term "fluid" refers to any medium capable of flowing and through which waves can propagate, including but not limited to gases, liquids, or combinations thereof.

[0064] The term "free space" (or "unenclosed" space), in relation to a metamaterial silencer, means the space outside the metamaterial silencer, the space outside a duct from which acoustic energy is received at the metamaterial silencer, or the space outside a duct downstream of the metamaterial silencer.

[0065] The term "open ratio" refers to a device having a first transmission area having a first area (A1) and a second transmission area having a second area (A2), and refers to the ratio of the first area (A1) to the sum of the first area and the second area (A1+A2) [i.e., open ratio=A1 / (A1+A2)].

[0066] For purposes of this disclosure and any claims appended hereto, "open ratio" means, with respect to a device having a first region having a first cross-sectional area (A1) and a second region having a second cross-sectional area (A2), the ratio of the first cross-sectional area (A1) to the sum of the first and second cross-sectional areas (A1+A2) [i.e., open ratio=A1 / (A1+A2)].

[0067] The term "radial" means perpendicular to the axis.

[0068] "Maintaining a continuous state" means that, with respect to a path through which a signal propagates, the path is configured to pass the signal while maintaining a signal-continuous state, as opposed to a path that is resonant at a frequency in the signal's spectrum, which would not maintain the signal in a signal-continuous state.

[0069] A "set" contains at least one element. For example, a set of passages contains at least one passage.

[0070] A "frequency of interest" is a frequency of acoustic energy of interest when tuning or configuring a bidirectional metamaterial silencer to produce destructive interference.

[0071] The term "transmittance" refers to the ratio of the energy of a signal incident on a device that passes through the device to the energy that is incident on the device.

[0072] Some embodiments below are described using gas as the fluid medium through which the signal propagates and as the fluid medium that flows through the metamaterial silencer, however, the embodiments are not limited to gas as the fluid medium as the fluid medium may also be a liquid, and thus, the exemplary embodiments described with respect to this type of gas are not intended to limit such embodiments.

[0073] Figures 2A, 2B, and 2C: Transverse double-layer metamaterial sound silencer. FIG. 2A shows a schematic cross-sectional view of one embodiment of a metamaterial acoustic silencer 200.

[0074] The metamaterial acoustic silencer 200 has a first transmission region 210 that defines an opening that is open to allow gas flow through the metamaterial acoustic silencer 200 .

[0075] For this purpose, the first transmission area 210 is open so that even a solid object, such as a straight rigid rod, can pass through the first transmission area 210 without bending and without colliding with the metamaterial silencer 200. For example, the first transmission area 210 can have a hollow cylindrical shape, defined by an inner ring 302 having an inner radial surface 325 and a thickness 227 ("t") (in this embodiment, the thickness can be considered as the height of the cylinder). In the case of an exemplary embodiment, the thickness 227 is also the height of the cylinder and therefore the length of the first passage 210. In the case of an exemplary embodiment, the thickness 227 of the device 200 is less than one-quarter of the wavelength of the frequency of interest, according to some embodiments, the thickness 227 is less than one-eighth of the wavelength of the frequency of interest, and further according to some embodiments, the thickness 227 is less than one-sixteenth of the wavelength of the frequency of interest. In a preferred embodiment, the passages 210, 220 are less than one-half of the wavelength of the frequency of interest.

[0076] In the embodiment of FIG. 2A , the first transmission region 210 defines a fluid flow axis 211 along which a fluid (e.g., gas and / or liquid) can flow through the first transmission region 210, and thus through the metamaterial silencer 200.

[0077] The first transmission region 210 has a first acoustic impedance (Z 1 ) and the first acoustic refractive index (n 1 In contrast to the second transmission region 220, the first transmission region 210 is configured (e.g., based on its dimensions) to not resonate at the frequency of interest.

[0078] The metamaterial silencer 200 includes a second transmission region 220. Generally, the second transmission region 220 includes a set of one or more conduits, each conduit in the set configured to resonate at a frequency of interest. The second transmission region 220 has an inlet and an outlet such that waves can propagate through the second transmission region 220 from the inlet to the outlet. In an exemplary embodiment, a fluid can flow through the second transmission region 220 from the inlet to the outlet.

[0079] Some notable properties of the metamaterial sound absorber 200 are described below.

[0080] degree of openness The first transmission area 210 has a first area dimension ("A1") facing the incident acoustic signal, and the second transmission area 220 has a second area dimension ("A2") facing the incident acoustic signal.

[0081] The ratio (A1 / A1+A2) of the area of ​​the first transmission area 210 (A1) to the sum of the area of ​​the first transmission area 210 (A1) and the area of ​​the second transmission area 220 (A2) can be considered as a measure of the openness of the metamaterial silencer 200 to the fluid flow. This ratio can be referred to as the "open" ratio and can be expressed, for example, as a decimal or a percentage of the device that is open to the liquid flow. According to the exemplary embodiments described herein, the metamaterial silencer 200 can have an openness ratio of at least 0.6 (or 60%) or more. For example, some embodiments have an openness ratio of 0.7 (70%), 0.8 (80%), 0.9 (90%) or more, such as up to 0.99 (99%), while maintaining its ability to attenuate signals for all. Such metamaterial sound deadening devices can be referred to as “ultra-open metamaterials” (“UOMs”) and are in stark contrast to prior art devices that could have, for example, no more than 40% openness.

[0082] Impedance and Refractive Index Similarly, as will be described in more detail below, when metamaterial silencer 200 is placed in a fluid (e.g., gas) environment, first transmission region 210 has a first acoustic impedance (hereinafter referred to as “Z 1 ") and the first acoustic index of refraction (which can be referred to as "n 1 "), and the second transmission region 220 has a second acoustic impedance (which may be referred to as "Z 2 ") and a second acoustic index of refraction (which can be referred to as "n 2 The first acoustic impedance (Z 1 ), the first acoustic refractive index (n 1 ), the second acoustic impedance (Z 2 ) and the second acoustic refractive index (n 2 ) is determined at least in part by the physical dimensions of the metamaterial silencer 200.

[0083] Transmittance The transmission is a quantitative measure of the transmission of wave energy (e.g., acoustic energy) of an incident signal through the metamaterial silencer 200 from the upstream side 221 to the downstream side 222. For example, the transmission can be defined as the ratio of the energy transmitted from the metamaterial silencer 200 (e.g., output from the downstream side 222 of the metamaterial silencer 200) to the energy received by the metamaterial silencer 200 (e.g., input into the first transmission region 210). In other words, the acoustic transmission is the ratio of the transmitted energy to the incident energy. For example, if a signal is incident on the metamaterial silencer 200 with a given amount of energy, and the energy transmitted from the metamaterial silencer 200 is only six percent (6%) of the energy accepted in the first transmission region 210, then the ratio is 6 / 100 or 0.06. In other words, the metamaterial silencer 200 attenuates the signal by 94% or 24.4 dB. where dB is calculated as 20log(input energy / output energy). In this example, the ratio of input energy to output energy is 100 / 6=16.66, or 20log(16.66)=24.4 dB.

[0084] The embodiments of Figures 2B and 2C are based on an acoustic plane wave incident on the upstream side 221 of a metamaterial silencer 200 that has distinct acoustic properties.

[0085] For these examples, the following assumptions are made: the metamaterial silencer 200 has an axially symmetric configuration with respect to the X axis with a thickness t, in which the first transmission region 210 (r<223) has a length of Z 1 and the acoustic impedance of n 1 and the second transmission region 220 (223 <r<224)は、Z 2 and the acoustic impedance of n 2It should be noted that the axially symmetric configuration is chosen only for simplicity and that other configurations, such as a honeycomb shaped rectangular prism, can be considered without loss of generality. As mentioned above, the interface (r=223) between the first and second transmission regions 210 and 220 is considered to be a hard boundary and the entire structure is taken as C for the purpose of deriving the acoustic transmission coefficient. o and p 0 is confined within a rigid cylindrical (i.e., circular in cross section) waveguide filled with a medium having a density of

[0086] As a first step to derive the permeability, the following definitions are adopted for the acoustic pressure and velocity fields at the interface (x=0 and x=t) in order to remove the transverse field variations:

number

number

[0087] where p and u are the acoustic pressure and velocity fields, respectively. P 1,2 and U 1,2 are the averaged pressure and volume velocity at the interface between the first transfer region 210 and the second transfer region 220. Then, if we consider these regions to be separated by a hard boundary, we can write in a decoupled manner the transfer matrices that connect the output pressure and velocity to the input conditions for the first region 210 and the second region 220.

number

[0088] where k o is the wave number associated with the medium inside the duct, and ω / C o is defined as n 1 and n 2 are the refractive indices of the transmission regions 210 and 220, respectively, t is the thickness, and Z1 and Z 2 are the characteristic impedance values ​​of the transfer regions 210 and 220, respectively. Applying the Green's function technique, the following relationships can be derived:

number

[0089] Here, the Green's function is

number

[0090] Here, the eigenmode is φ n (r)=J 0 (k n r) / J 0 (k n r 2 ) where k n J(k n r 2 )=0 as the solution.

[0091] By solving the above equations, the averaged pressure and volume velocity defined above can be readily calculated, from which the acoustic transmission coefficient can be readily derived as follows:

number

[0092] 2B and 2C show graphs of transmission from a two-layer metamaterial silencer 200 for various values ​​of refractive index and acoustic impedance. In FIG. 2B, the effect of the characteristic impedance ratio is depicted, with respect to which the Q factor (or "quality factor") of the filtering can be adjusted. In FIG. 2C, the effect of the refractive index ratio is depicted, with respect to which the frequency regime of the filtering can be adjusted.

[0093] In the case of Figure 2B, n 2 / n 1= 10 is considered, and the permeability is the dimensionless quantity n 2 In the case of FIG. 2C, the impedance ratio remains constant (Z 2 / Z 1 = 10), the transmission is plotted for three different values ​​of the refractive index ratio. In particular, for these examples, the background medium in the waveguide is considered to be air, and the medium in the first transmission region 210 is considered to be identical to this background medium. Thus, the characteristic acoustic impedance of the first transmission region 210 is Z i =ρ o c o / πr 1 2 The reflectance (n 1 ) is equal to the identity element.

[0094] From FIG. 2B and FIG. 2C, the following can be observed: 2 and n 2 For values ​​of , if the acoustic properties of the transmission region 210 are different from those of the transmission region 220, an asymmetric transmission profile is obtained, in which case destructive interference can result in a transmission zero based on Fano-like interference. 2 It appears when t ≒ λ / 2. The refractive index of the two regions (n 1 and n 2 ), the first transmission region 210 will sustain a continuous state, resulting in Fano-like interference. During this state, the portion of the acoustic wave traveling through the second transmission region 220 will interact with the local modes induced by resonance in this region, resulting in an out-of-phase state after traveling through this region. The portion of the incident acoustic wave traveling through region 210 passes through metamaterial 200 with negligible phase shift, thus resulting in destructive interference at the transmission side of the metamaterial. Notably, the destructive interference initially occurs in the first resonant mode of region 220, n 2occurs at t ≒ λ / 2, but n 2 If t≈Nλ / 2, where N is an integer, then higher resonant modes will also occur.

[0095] From Figure 2B, by comparing the transmission for different values ​​of impedance ratio, it can be seen that increasing the difference between the characteristic acoustic impedances of the two regions increases the quality factor (Q factor) of the attenuation performance. This attribute provides a degree of freedom to adjust the impedance difference to achieve the desired filtering bandwidth. Interestingly, the characteristic impedance ratio can be increased to a significantly larger value (Z 2 / Z 1 =∞), filtering performance is suppressed and orifice-like behavior is achieved, given that the filtering is of a significantly narrowband nature. However, an orifice structure with a similar open area geometry results in relatively poor acoustic filtering performance, which results in only a small reduction in the attenuation of the transmitted acoustic wave.

[0096] FIG. 2C shows the effect of the difference in refractive index between two media on the transmittance, n 2 It has been shown that a high degree of filtering is obtained when t ≈ λ / 2.Thus, the inventors have found that by adjusting the refractive index in the proposed structure, high performance acoustic attenuation can be achieved at any desired frequency.

[0097] As shown in Figures 2B and 2C, the transmission of the acoustic signal is zero or near zero at the frequencies of interest, so what can be said is that the acoustic waves are attenuated at the frequencies of interest by destructive interference, reducing transmission of the acoustic wave silencer 200 by at least 94%.

[0098] It should be noted that the metamaterial silencer 200 is a passive device, which means that the device does not require an energy supply, but instead operates using only the energy in the incident signal.

[0099] From the disclosure above, and in light of the examples presented below, it is possible to characterize the metamaterial silencer 200 in terms of its physical dimensions (radius, thickness, helix angle) and other properties (Z 1 , Z 2 , n 1 , n 2 It can be seen that the design of the metamaterial sound absorber 200 can be specified by selection of parameters of the device, such as the frequency at which the device's damping effect is most pronounced, the bandwidth at the frequency, and the openness of the metamaterial sound absorber 200, by making an informed selection of such parameters. For example, by making an informed selection of such parameters, a designer can specify the target frequency of the metamaterial sound absorber 200 (the frequency at which the device's damping effect is most pronounced), its bandwidth at the frequency, and its openness. Moreover, by specifying the physical dimensions, the first transmission region 210 of the metamaterial sound absorber 200 can be configured such that waves propagating through the first transmission region 210 remain continuous (e.g., the first transmission region does not resonate at the frequency of interest) (such a first transmission region can be said to remain or remain continuous), and the second transmission region 220 can be configured such that it resonates at the frequency of interest.

[0100] FIG. 3A-3D: Cylindrical embodiment of a metamaterial sound silencer. Figure 3A shows a schematic front view of one embodiment (300) of the cylindrical two-layer metamaterial sound silencer 200. Figure 3B shows a schematic cutaway side view of the cylindrical two-layer metamaterial sound silencer 300, and Figure 3C shows a schematic rear view of the cylindrical two-layer metamaterial sound silencer 300.

[0101] 3A has a cylindrical shape and includes an outer ring 301 with an outer surface 326. The outer ring 301 defines an interior space that includes two transmission regions (or "layers") 210 and 220.

[0102] The first transmission region 210 in this embodiment includes an inner ring 302 and is defined by an inner radius 223 .

[0103] According to a preferred embodiment, the inner ring 302 acoustically isolates the first transmission region 210 from the second transmission region 220 by substantially preventing gas and acoustic energy therefrom in the first transmission region 210 from being transmitted to the second transmission region 220 and by substantially preventing gas and acoustic energy therefrom in the second transmission region 220 from being transmitted to the first transmission region 210. The inner ring 302 may be referred to as an "acoustically rigid spacer." According to an exemplary embodiment, the inner ring 302 is made of acrylonitrile butadiene styrene.

[0104] The second transition area 220 in this embodiment is defined by an outer radius 224 and an inner radius 223. As shown in Figures 3A and 3C, the second transition area 220 has an upstream face 221 on a first side and a downstream face 222 on a side opposite the first side.

[0105] The second transition region 220 includes a set of helical passages 341, 342, 343, 344, 346. Each of the helical passages 341-346 of the helical passage set has a corresponding passage inlet opening (331-336, respectively) that opens towards the upstream face 221, and a corresponding passage outlet opening (351-356, respectively) that opens towards the downstream face 222.

[0106] The upstream surface 221 of the first transition region 210 has an area (A1) defined as the square of the inner radius 223 times π. As shown, the second transition region 220 includes a set of spiral passages 341-346, each of which has a radial height defined as the distance between the inner ring 302 and the outer ring 301 (or the inner radius 223 and the outer radius 224). Thus, when viewed in cross section (FIG. 3D, shown along the X-axis of FIG. 3A), the set of passages represents a cross section having an area (A2) of 2π times the square of the difference between the inner radius 223 and the outer radius 224. In other words, the second transition region 220 of the metamaterial silencer 300 of FIG. 3A is annular in shape and has an area of ​​2π times the square of the outer radius (224) minus 2π times the square of the inner radius (223) [i.e., 2π(R 2 2 -R 1 2 ), where R 1 is the inner radius 223, R 2 has an outer radius 224]. In fact, the second transfer region 220 would have the same area (A2) even if the metamaterial silencer 300 of FIG. 3A had only a single helical duct (e.g., 341), because this single helical passage, when viewed in cross section, would represent a cross section with an area (A2) of 2π × [the square of the difference between the inner radius 223 and the outer radius 224].

[0107] The spiral passages 341-346 may be referred to as "resonant passages" because, during operation, one or more frequency components (each a "frequency of interest") of an acoustic wave incident on the upstream face 221 will resonate in one or more of the spiral passages 341-346.

[0108] Each of the helical passages 341-346 of the helical passage set has a helical axis, and in the exemplary embodiment, the helical passages 341-346 have the same helical axis.

[0109] Each of the helical passages 341-346 of the helical passage set has a helical angle 347. For the embodiment of Figure 3A, each of the helical angles 347 for each of the helical passages 341-346 is the same, however, in some embodiments, any one or more of the helical passages 341-346 can have a different helical angle 347 than the helical angle 347 of one or more of the other helical passages in the set.

[0110] Each of the helical passages 341-346 of the helical passage set also has a passage length, the length of a given helical passage being the distance along the helical axis between a corresponding passage inlet opening and a corresponding passage outlet opening of that helical passage. According to exemplary embodiments, each of the helical passages 341-346 of the helical passage set is a sub-wavelength structure, where the passage length is less than the wavelength of the frequency at which the passage operates as a silencer. Moreover, according to some exemplary embodiments, the passage length of each of the passages 341-346 is one-half (½) of the wavelength of the frequency at which the passage operates as a silencer, and according to preferred embodiments, is less than one-half (½) of such wavelength (but greater than ¼).

[0111] The operation and some characteristics of a bidirectional metamaterial silencer 300 configured to have a frequency of interest of 460 Hz are described below, although it should be understood that the operation and characteristics of metamaterial silencer 200 in general are not limited to this particular embodiment. The embodiment of metamaterial silencer 300 used to provide these characteristics had a thickness (t) 327 of 5.2 cm, an inner radius 223 of 5.1 cm, an outer radius 224 of 7 cm, and a helix angle 347 of 8.2°. The impedance ratio Z 2 / Z 1 is 7.5, the refractive index ratio n 2 / n 1 was 7.

[0112] Figures 4A-4D: Performance of the metamaterial sound suppressor According to an exemplary embodiment of operation, the metamaterial silencer 300 is positioned in the path of an acoustic signal propagating through a gas. Specifically, the metamaterial silencer 300 is positioned such that the acoustic signal is incident and enters the first and second transmission regions 210, 220 (in this example, the passage entrance openings 331-336 of the spiral passages 341-346). The portion of the wave propagating in the first transmission region 210 may be referred to as the first wave, and the portion of the signal propagating in the second transmission region 220 may be referred to as the second wave. It should be noted that acoustic energy from the acoustic signal may enter the passage entrance openings 331-336 without first entering the cylinder of the first transmission region 210.

[0113] The gas itself may travel in a direction along the gas flow axis 211. Such a direction may be referred to as the "downstream" direction. The acoustic signal may have a spectrum that includes multiple frequency components. According to an exemplary embodiment, the metamaterial silencer 300 is configured to allow the gas to pass through the first transmission region 210 while attenuating or silencing at least one frequency (a "frequency of interest") in the acoustic signal spectrum.

[0114] As previously mentioned, the helical passages 341-346 may be referred to as "resonant passages" because, during operation, one or more frequency components of an acoustic wave incident on the upstream face 221 resonate in one or more of the helical passages 341-346. At the same time, the acoustic signal propagates through the first transmission region 210 without resonating (i.e., in "continuous" fashion). Moreover, when gas is moving, the gas can pass through the first transmission region 210 substantially unimpeded.

[0115] Acoustic energy from the helical passages 341-346 exits the metamaterial silencer 300 at passage exit openings 351-356. Specifically, the acoustic energy exits the downstream face 222 of the metamaterial silencer 300 into an unenclosed volume 205 downstream of the metamaterial silencer 300. Moreover, according to an exemplary embodiment, the acoustic energy exits the second passage 220 of the metamaterial silencer 300 in a tangential direction. A tangential direction is defined as a direction tangent to a radius (223, 224) extending from the center of the metamaterial silencer 300 and substantially parallel to the downstream face 222. However, the direction of energy exiting the second passage 220 of the metamaterial silencer 300 may still be described as axial (or axially oriented), at least in that it is not radial.

[0116] The acoustic energy from each of the spiral passages 341-346 has a frequency equal to the resonant frequency of the passage from which it exits, and cancels out the acoustic energy at this frequency in the gas from the first transfer region 210 due to Fano interference.

[0117] For purposes of visualizing the sound-attenuating performance of one embodiment of metamaterial sound-attenuator 300, Figures 4A and 4B show a schematic of sound transmission through metamaterial sound-attenuator 300. In Figures 4A and 4B, a cutaway view of metamaterial sound-attenuator 300 is shown. In other words, cutaway planes are used in these figures to represent the resulting pressure and velocity fields in two dimensions (2D).

[0118] Figure 4A is a graph showing the transmission of a first frequency of a plane wave incident on a bidirectional metamaterial sound suppressor. Figure 4B is a graph showing the transmission of a second frequency (the "interesting" frequency) of a plane wave incident on a bidirectional metamaterial sound suppressor. In Figures 4A and 4B, the background color represents the absolute value of the pressure field normalized by the amplitude of the incident wave, and the white lines represent the flow and orientation of the local velocity field.

[0119] 4A depicts a plane wave with a frequency of 400 Hz incident on the metamaterial silencer 300 from the left side, as indicated by the black arrow, which results in high pressure transmission in the 400 Hz frequency regime, according to the analytically and experimentally expected behavior of the metamaterial silencer 300 structure.

[0120] In this state, the helical portion 220 of the structure of the metamaterial silencer 300 is in a state where the acoustic impedance (Z 1 ) with a significantly larger acoustic impedance (Z 2 ), the incident wave will travel primarily through the open central portion 210 of the metamaterial silencer 300. This behavior can be visually confirmed by the local velocity field flow shown in FIG. 4A, where the velocity field is minimally disturbed both in front of and behind the metamaterial silencer 300 structure, except for changes in cross-sectional area.

[0121] 4B shows a similar case for a plane wave incident from the left, but at a frequency of 460 Hz. Based on the theoretical and experimental results obtained above, it is expected that at this frequency, the wave transmitted through the helical portion 220 of the metamaterial silencer 300 will be out of phase with the wave transmitted through the central open portion 210 of the metamaterial silencer 300. The results obtained show that the wave transmission is attenuated in the unenclosed volume 205 as a result of destructive interference on the transmitting side of the metamaterial silencer 300 (the right side of these figures).

[0122] In particular, the out-of-phase transmission through the two regions 210, 220 of the metamaterial silencer 300 can be better understood by referring to the velocity profiles shown in white lines in FIG. 4B. It is immediately recognized that the local acoustic velocities of the waves transmitted from the two regions 210, 220 of the metamaterial silencer 300 are in opposite directions, resulting in a pronounced curvature in the velocity flow and reduced far-field radiation. It is noted here that due to the presence of destructive interference based on Fano-like interference, the metamaterial structure 300 reproduces a situation similar to the case of an open-ended acoustic termination, where a near-zero effective acoustic impedance results in a dominant reflection of the incident wave.

[0123] In other words, Fig. 4A shows, using a color map, the absolute pressure values ​​normalized by the incident wave magnitude resulting from a plane wave incident on the metamaterial silencer 300 from the left side at a frequency of 400 Hz. The local velocity flow is shown by the white line. At this frequency, the transmission coefficient (ratio of transmitted pressure to incident pressure) is about 0.85, and therefore about 72% of the acoustic wave energy is transmitted.

[0124] In Figure 4B, pressure and velocity profiles are depicted with an incident plane wave of similar amplitude to the incident wave depicted in Figure 4A, but with a frequency of 460 Hz. At this frequency, based on Fano-like interference, the transmitted wave has a significantly reduced amplitude and is effectively silenced. According to this embodiment, the phase difference between the waves transmitted from the two regions 210, 220 of the metamaterial silencer 300 results in a curvature of the wave velocity field and reduced far-field radiation.

[0125] 4C is a graph showing the normalized amount of acoustic energy transmitted by the two-layer metamaterial silencer 300 and the amount of acoustic energy reflected by the two-layer metamaterial silencer 300. As shown, at the frequency of interest of 460 Hz, significantly little acoustic energy is transmitted by the metamaterial silencer 300 (less than about 5%), while the majority of the acoustic energy is reflected by the metamaterial silencer 300 (about 94% or more).

[0126] 4D is a graph showing acoustic transmission through a two-layer metamaterial silencer 300 having various degrees of structural openness. The transmission was analytically derived using Green's function techniques. In particular, the two-layer metamaterial silencer structures considered herein are characterized by the same refractive index ratio in their transverse two-layer metamaterial models, but have different impedance ratios.

[0127] According to an exemplary embodiment, the openness percentage correlates with the acoustic impedance ratio, and even with a significantly higher openness percentage, sound attenuation can be achieved within the scope of the present embodiment. For example, as shown in FIG. 4D, even for a two-layer metamaterial sound absorber 300 with a significantly higher percentage of open area (approaching nearly full open area where the openness approaches 0.99 or 99%), the sound attenuation function remains intact, although the frequency band that is attenuated is narrowed as a result. The following table shows the relationship between openness (open area / total area, column labeled "openness") and sound transmission (transmission) at various frequencies as shown in FIG. 4D. [Table 1]

[0128] Although the previous figures show an embodiment of the silencer 200 having a target frequency of 460 Hz, the embodiment is not limited to silencers having this target frequency. As described above, the target frequency of the silencer 200 can be set by specifying the parameters of the silencer.

[0129] 5A-5B: An embodiment of a cylindrical metamaterial sound silencer with non-uniform passages. 5A and 5B, another embodiment (500) of the metamaterial silencer 200 is shown in schematic form. In this embodiment, the helical passages 341-346 in the second transfer region 220 do not have the same physical dimensions. For example, some passages are longer than others. To accommodate the different passage lengths, the passage inlets 331-336 for the helical passages 341-346 are non-uniformly spaced around the upstream face 221. Alternatively or additionally, the passage outlets 351-356 are non-uniformly spaced around the downstream face 222. In addition, the six passages 341-346 each have a different helix angle 347. In this design, the different passage front angles have different effective lengths (and therefore reflectivity, n) and cross sections (and therefore impedance, Z). This model of the silencer can therefore be designed for multiple frequencies of interest simultaneously, each with a different silencer bandwidth.

[0130] 6A-6B: An embodiment of a cylindrical metamaterial sound silencer having multiple radially arranged conduits. 6A and 6B, another embodiment (600) of the metamaterial silencer 200 is shown in schematic form. In this embodiment, the spiral passages 341-342 in the second transition region 220 include individual passages wound around the inner ring 302. Each of the individual passages 341, 342 has a top panel 510 and two side panels 511, 512. Each of the two side panels extends radially outward from the inner ring 302, and the top panel 510 extends between the radially outer ends of the two side panels 511, 512, thereby forming a spiral passage having a rectangular cross section. The spiral passages 341, 342 can be identical or can have different helix angles and / or helix lengths and / or different cross-sectional areas. This embodiment can be desirable when the goal is to minimize pressure loss in the central passage 210. In this case, the passage inlet openings 331, 332 and passage outlet openings 351, 352 are arranged radially and the silencer features two passages 341, 342 with different lengths (passage 342: 0.75 turns) (passage 341: 1.1 turns). By adjusting the passage lengths and the passage cross sections, the desired silencer can be realized, either multi-band or single-band with appropriate bandwidth.

[0131] FIG. 7: An embodiment with metamaterial silencers arranged in series. 7 shows a schematic of a stack 700 of multiple metamaterial sound silencers 200 such as those shown in FIG 3A. Each metamaterial sound silencer 200 can be configured to attenuate different frequencies than the other two metamaterial sound silencers 200. The multiple metamaterial sound silencers 200 in the stack 700 have a synergistic effect such that the stack 700 is configured to attenuate the transmission of multiple target frequencies.

[0132] 8A-8B: An embodiment of a cylindrical metamaterial sound silencer with a centrally located second transmission region. 8A and 8B, another embodiment (800) of the metamaterial silencer 200 is shown in schematic form. This embodiment includes a second transmission region 220 and a first transmission region 210 arranged radially outward of the second transmission region 220. The first transmission region 210 is surrounded by an outer ring 301 and defines a non-resonant path around the second transmission region 220. According to this embodiment, the second transmission region 220 is a hub suspended from the outer ring 301 by one or more spars 810.

[0133] 9A-9B: An embodiment of a cylindrical metamaterial sound silencer placed inside a tube. While the above-described embodiments (200; 300; 500; 600; 800) are external to the duct and require an outer casing to provide the performance and achieve the results described, exemplary embodiments can be used within a casing as described in connection with Figures 9A and 9B.

[0134] FIG 9A shows a schematic of an embodiment of a metamaterial sound silencer 200 disposed within a tube 910. The metamaterial sound silencer 200 may be any of the cylindrical sound silencers disclosed herein. FIG 9B shows a graph illustrating the sound-absorbing effect of the metamaterial sound silencer 200 within the tube 910.

[0135] The tube 910 is a cylinder with two openings 911 and 912 at its ends. For purposes of describing this embodiment, an acoustic source (e.g., a speaker) 920 is disposed at a first end 911 of the tube 910 such that an acoustic signal generated by the source 920 travels into the tube 910 through the first opening and then propagates downstream down the tube 910 to a second opening 912 at the other end of the tube 910. The acoustic signal in this embodiment has a spectrum that covers a range of frequencies including the frequencies of interest of the metamaterial silencer 200. An acoustic load 910, which may be, for example, a cap, is disposed in or on the opening 912.

[0136] The metamaterial silencer 200 is disposed within the tube 910 such that its upstream surface 221 faces the sound source 920. The metamaterial silencer 200 in this embodiment has a frequency of interest of 460 Hz.

[0137] In FIG. 9A, a tube 910 is fitted with a number of microphones 931-935 positioned to measure the strength of an acoustic signal at various points within the tube 910. Microphones 931, 932 and 935 are positioned upstream of metamaterial silencer 200, and microphones 933 and 934 are positioned downstream of metamaterial silencer 200. As shown in FIG. 9B, metamaterial silencer 200 substantially attenuates an acoustic signal at a frequency of interest (460 Hz) downstream of the metamaterial silencer. Specifically, metamaterial silencer 200 transmits approximately 90% of the acoustic energy of the acoustic signal at frequencies below the frequency of interest, transmits approximately 50% of the acoustic energy of the acoustic signal at frequencies above the frequency of interest, but transmits very little (zero or near zero percent) of the acoustic energy of the acoustic signal at the frequency of interest, and transmits less than 50% of the acoustic energy of the acoustic signal in a band around the frequency of interest. 9A and 9B illustrate that metamaterial silencer 200 works well even if its downstream surface 122 is an enclosed space rather than free or unenclosed space. For example, the operation of metamaterial silencer 300 in unenclosed space 205 as described above also applies to operation in an enclosed space, such as the interior of tube 910.

[0138] FIG. 10A and FIG. 10B: Practical application embodiment of a metamaterial acoustic silencer. 10A and 10B are schematic diagrams of practical applications of various embodiments of metamaterial sound silencer 200 (e.g., 300; 500; 600; 800). FIG. 10A is a schematic diagram of metamaterial sound silencer 200 disposed at the outlet 1012 of a tube 1010. The tube 1010 can be a sound source or the tube 101 can include a sound source. For example, the tube 1010 can be an exhaust pipe of an automobile or a jet engine, just to name a few. The metamaterial sound silencer 200 operates as described above to attenuate noise exiting the tube 1010, but still allow gas flow (e.g., exhaust gases, jet streams) exiting the tube 1010.

[0139] 10B shows a schematic of a sound barrier 1020 having a set of metamaterial sound absorbers 200 (e.g., 300; 500; 600; 800), each of which operates to attenuate noise incident on the sound barrier 1020 as described above, but still allows gas to flow through the sound barrier 1020. According to some embodiments, the set of metamaterial sound absorbers 200 are positioned near the ground surface to allow animals to pass through the metamaterial sound absorbers 200.

[0140] FIG. 11A-11E: An embodiment of a metamaterial sound deadening device in a wheel 11A and 11B, another embodiment of a metamaterial silencer 1100 is shown in schematic form. This embodiment includes an outer ring 201 having an inner radial surface 225 that defines an interior region 1101. An arc-shaped resonator 1120 is disposed on the inner radial surface 225 and includes one or more serpentine resonating passages 1141. In this exemplary embodiment, a single passage 1141 is wound within the arc-shaped resonator 1120. The arc-shaped resonator 1120 forms an angle 1147 at the center of the outer ring 201, which in this embodiment is about 45°. According to other embodiments, the angle 1147 can be greater or less than 45°, for example, 30°, 60°, 90°, or 120°.

[0141] During operation, acoustic energy enters the passages 1141 and resonates within those passages. The acoustic energy then exits the arc-shaped resonators 1120, attenuating the acoustic energy within the interior region 1101.

[0142] One application of such an embodiment is in the wheels of an automobile, and to this end, FIG. 11C shows the noise pressure inside an enclosed automobile wheel 1150. According to this embodiment, a metamaterial sound suppressor having three arc-shaped resonators 1120 is placed inside the wheel 1150.

[0143] Figure 11E is a graph 1160 showing the pressure inside a wheel normalized to the pressure when the wheel does not have the metamaterial sound dampening device 1100 of Figure 11A. Trace 1161 shows the normalized pressure when the metamaterial sound dampening device 1100 of Figure 11A is not housed within the wheel 1150. In contrast, trace 1162 shows the normalized pressure inside the wheel 1150 when the metamaterial sound dampening device 1100 of Figure 11A is housed within the wheel 1150, as shown generally in Figure 11D. As shown, the presence of the metamaterial sound dampening device 1100 within the wheel 1150 reduces the acoustic pressure by approximately 90%.

[0144] FIG. 11F shows a schematic representation of an embodiment of a wheel 1150 having an arc-shaped resonator 1120 disposed on a wheel hub 1171 of the wheel 1150 within a tire 1152 attached to the hub. [Explanation of symbols]

[0145] 200 Metamaterial acoustic silencer 205 Unenclosed Space 210 First Transmission Area (or "Through Path") 211 Gas flow direction 220 Second Transmission Area 221 Upstream surface of metamaterial acoustic silencer 222 Downstream surface of metamaterial acoustic silencer 223 inner radius 224 outer radius 301 Outer Ring 302 Inner Ring 325 Inner radial surface of metamaterial acoustic silencer 326 Outer radial surface of metamaterial acoustic silencer 327 Thickness 328 Acoustically rigid members (or acoustically rigid spacers) 331~336 Passage entrance Aisles 341-346 347 Helix angle 351~356 Passage exit 810 Spar 910 Acoustic load 920 Sound Source 931~935 Microphone 1010 Tube (e.g. hollow cylinder) 1011 First end of cylinder 1012 Second end of cylinder 1020 Soundproof wall 1101 Internal area 1120 Arc-shaped resonator 1147 Arc angle 1150 wheels 1151 Wheel hub 1152 Tires

[0146] Various embodiments may be characterized by potential claims listed in the paragraphs following this paragraph (and preceding the actual claims presented at the end of this application). Those potential claims form part of the written description of this application. Accordingly, the subject matter of the following potential claims may be presented as actual claims in a later proceeding involving this application or any application claiming priority from this application. The inclusion of such potential claims should not be construed to mean that the actual claims do not cover the subject matter of the potential claims. Thus, a decision not to present those potential claims in a later proceeding should not be construed as a donation of subject matter to the public.

[0147] Without intending to be limiting, potential subject matter that may be claimed (prefaced with the letter "P" to avoid confusion with the actual claims presented below) includes:

[0148] P1 A transverse bilayer device for reducing transmission of an acoustic wave in a gaseous medium, the acoustic wave having a predetermined frequency and corresponding wavelength, the device having a first transmission area and a second transmission area; The first transition region defines a gas flow axis, defines a substantially open non-resonant path for gas to flow along the gas flow axis, and further defines a first acoustic impedance (Z 1 ) and the first acoustic refractive index (n 1 ) The second transition region has an axial upstream surface and an axial downstream surface opposite the upstream surface, and a thickness (t) of less than 50% of the wavelength, and includes a set of helical resonator passages within the second transition region, each helical resonator passage in the set of helical resonator passages having a passage inlet opening open to the axial upstream surface, a passage outlet opening open to the axial downstream surface, a helix axis parallel to the gas flow axis, and a second acoustic impedance (Z 2 ) and the second acoustic refractive index (n 2 ) The second acoustic refractive index (n2 ) times said thickness (t) is equal to one-half said wavelength; Contrast (Z 2 / Z 1 ) is at least 1 and less than 100; Transverse bilayer device.

[0149] P2 The transverse bilayer device of P1 further comprises an acoustically rigid spacer positioned to acoustically separate the first transmission region from the second transmission region.

[0150] P3 A transverse bilayer device as described in P2, wherein the acoustically rigid spacer comprises a cylinder made of acrylonitrile butadiene styrene resin.

[0151] P4 A transverse bilayer device according to any one of P1 to P3, wherein the upstream axial surface is perpendicular to the helix axis and the downstream axial surface is perpendicular to the helix axis.

[0152] P5 P4, a transverse bilayer device, wherein the second transfer region has an annular body having an inner radius defining the non-resonant path and an outer radius defining a ring, the ring having the axial upstream surface and the axial downstream surface.

[0153] P6 A transverse two-layer device as described in P5, wherein the non-resonant path defines a first two-dimensional area (A1) and the axial upstream surface defines a second two-dimensional area (A2), and the ratio of the first two-dimensional area to the sum of the first two-dimensional area (A1) and the second two-dimensional area (A2) is at least 0.6 (i.e. A1 / (A1+A2)×100≧60%).

[0154] P7 A transverse two-layer device as described in any one of P1 to P6, wherein the first transmission region is arranged radially outside the second transmission region, and the non-resonant path is arranged around the second transmission region.

[0155] P8 A transverse bilayer device as described in P7, wherein the non-resonant path has an annular shape around the second transmission region.

[0156] P9 The transverse bilayer device described in P7 further comprises an outer ring and a set of spars; the outer ring is disposed coaxially with the second transmission region and radially outward of the second transmission region, the outer ring defining a radially outer boundary of the non-resonant path; The set of spars extends from the outer ring to the second transition region and suspends the second transition region from the outer ring.

[0157] P10 The transverse bilayer device of any one of P1 to P9 further comprises an outer ring having an inner surface and defining an inner region (1101), the second transmission region comprising an arc-shaped resonator subtending an angle of less than 365°.

[0158] P11 The transverse bilayer device according to P10, wherein the arc-shaped resonators form an angle of less than 45°.

[0159] The embodiments of the invention described above are intended to be merely exemplary and numerous variations and modifications will be apparent to those skilled in the art, all of which are intended to be within the scope of the invention as defined in any appended claims.

Claims

1. 1. An apparatus having a first passageway and one or more second passageways, the first passageway has a first inlet and a first outlet, is open for propagation of a first wave at a frequency of interest therethrough, and has a first cross-sectional area; each of the one or more second passages is open to propagation of a second wave at the frequency of interest therethrough, each having a second inlet and a second outlet, the one or more second passages defining a second cross-sectional area; each of the one or more second passages is positioned relative to the first passage such that the second waves at the frequency of interest exiting one or more of the second outlets may destructively interfere with the first waves at the frequency of interest exiting the first passage; the first cross-sectional area is greater than the second cross-sectional area such that the device has an open ratio of at least 0.6; Device.

2. the first passageway is open to allow fluid to flow therethrough; 2. The apparatus of claim 1.

3. the first cross-sectional area is greater than the second cross-sectional area such that the device has an open ratio of at least 0.8; 2. The apparatus of claim 1.

4. the first cross-sectional area is greater than the second cross-sectional area such that the device has an open ratio of 0.99; 2. The apparatus of claim 1.

5. the first passage defines an axis of fluid flow through the first passage, and each of the second outlets is an outlet outside a duct.

2. The apparatus of claim 1.

6. the first passage defines an axis of fluid flow through the first passage, and each of the second outlets is an axially oriented outlet.

2. The apparatus of claim 1.

7. each said second outlet is an outlet external to a duct; 7. The apparatus of claim 6.

8. each of the first wave and the second wave is an acoustic wave, and the destructive interference attenuates the first wave at the frequency of interest by at least 94%; 2. The apparatus of claim 1.

9. each of the first and second waves is an acoustic wave, and acoustic energy at the frequency of interest exiting each of the second outlets destructively interferes with acoustic energy exiting the first passageway to attenuate sound at the frequency of interest by at least 24 dB; 2. The apparatus of claim 1.

10. 1. An apparatus having a first passageway and one or more second passageways, the first passageway is open for propagation of a first wave at a frequency of interest through the first passageway and has a first inlet and a first outlet; each of the one or more second passages has a second inlet and a second outlet, extends along an axis defining an axial direction, and is open for propagation of a second wave at the frequency of interest through the second passage; The one or more second outlets are open in the axial direction, the one or more second passages are positioned relative to the first passage such that the second waves at the frequency of interest exiting one or more of the second outlets may destructively interfere with the first waves at the frequency of interest exiting the first passage. Device.

11. each of the one or more second paths is configured to resonate at the frequency of interest, and the first path is configured to maintain a continuous state during propagation of the first wave through the first path; 11. The apparatus of claim 10.

12. each of the one or more second paths is configured to resonate at the frequency of interest and the first path is configured not to resonate at the frequency of interest.

12. The apparatus of claim 11.

13. each of the one or more second passages is positioned relative to the first passage such that propagation of the second wave out of the second outlet can destructively interfere with the first wave out of the first passage at the frequency of interest to reduce transmission of the first wave by at least 94%.

12. The apparatus of claim 11.

14. each of the one or more second passages is positioned relative to the first passage such that propagation of the second wave out of the second outlet destructively interferes with the first wave out of the first passage at the frequency of interest to attenuate the first wave by at least 24 dB.

12. The apparatus of claim 11.

15. the first passage has a first cross-sectional area (A1); the one or more second passages define a second cross-sectional area (A2); A ratio [A1 / (A1+A2)] of the first area (A1) to the sum of the first area (A1) and the second area (A2) is greater than 0.6; 12. The apparatus of claim 11.

16. 1. An apparatus having a first passageway and one or more second passageways, the first passageway is open for a first wave at a frequency of interest to propagate through the first passageway and has a first inlet and a first outlet opening into a volume outside the duct; one or more of the second passages each extend along an axis and are open for second waves at the frequency of interest to propagate through the second passage, each having a second inlet and a second outlet opening into the volume outside the duct; the one or more second passages are positioned relative to the first passage such that the second waves at the frequency of interest exiting one or more of the second outlets may destructively interfere with the first waves at the frequency of interest exiting the first passage. Device.

17. each of the second paths is configured to resonate at the frequency of interest, and the first path is configured to maintain a continuous state during propagation of the wave through the first path; 17. The apparatus of claim 16.

18. each of the second paths is configured to resonate at the frequency of interest and the first paths is configured not to resonate at the frequency of interest; 17. The apparatus of claim 16.

19. the first passageway is open to allow fluid to flow therethrough; 17. The apparatus of claim 16.

20. the first wave is an acoustic wave, and the destructive interference attenuates the acoustic wave at the frequency of interest.

17. The apparatus of claim 16.

21. the first passageway has a first cross-sectional area and the one or more second passageways define a second cross-sectional area, the first cross-sectional area being greater than the second cross-sectional area such that the device has an aperture ratio of at least 0.8; 17. The apparatus of claim 16.

22. the first passageway has a first cross-sectional area and the one or more second passageways define a second cross-sectional area, the first cross-sectional area being greater than the second cross-sectional area such that the device has an aperture ratio of at least 0.99; 17. The apparatus of claim 16.

23. 1. An apparatus having a first passageway and one or more second passageways, the first passageway is open for propagation of a first wave at a frequency of interest therethrough, the first passageway having a first inlet and a first outlet, the first passageway being configured to maintain a continuous state in the presence of waves at the frequency of interest; each of the one or more second passages is open to propagation of a second wave at the frequency of interest through the second passage and configured to resonate at the frequency of interest, each having a second inlet and a second outlet; each of the one or more second passages is positioned relative to the first passage such that the second waves at the frequency of interest exiting one or more of the second outlets may destructively interfere with the first waves at the frequency of interest exiting the first passage; Device.

24. the first passageway is open to allow fluid to flow therethrough; 24. The apparatus of claim 23.

25. the first passage is configured to be non-resonant at the frequency of interest; 24. The apparatus of claim 23.

26. the first wave is an acoustic wave, and the destructive interference attenuates the acoustic wave at the frequency of interest such that transmission of the acoustic wave out of the first passage is reduced by at least 94%.

24. The apparatus of claim 23.

27. the first wave is an acoustic wave, and the destructive interference attenuates the acoustic wave at the frequency of interest such that the acoustic wave exiting the first passage is attenuated by at least 24 dB.

24. The apparatus of claim 23.

28. the first passage has a first cross-sectional area (A1); the second passage defines a second cross-sectional area (A2); A ratio [A1 / (A1+A2)] of the first area (A1) to the sum of the first area (A1) and the second area (A2) is greater than 0.6; 24. The apparatus of claim 23.

29. the first passage has a first cross-sectional area (A1); the second passage defines a second cross-sectional area (A2); A ratio [A1 / (A1+A2)] of the first area (A1) to the sum of the first area (A1) and the second area (A2) is greater than 0.8; 24. The apparatus of claim 23.

30. the first passage has a first cross-sectional area (A1); the second passage defines a second cross-sectional area (A2); A ratio [A1 / (A1+A2)] of the first area (A1) to the sum of the first area (A1) and the second area (A2) is greater than 0.9; 24. The apparatus of claim 23.

Images