Anechoic broadband compact (ABC) muffler with moving blade and vehicle applications

Abstract

A muffler has a first plate with multiple holes; a second plate with multiple holes; the first plate mounted at a distance across an entire area having holes of first and second plate that is less than or equal to two millimeters above the second plate; and an additional feature selected from a back cavity and an acoustic membrane over holes of the first plate. In embodiments, none of the holes of the first plate are directly aligned with holes of the second plate. In embodiments, the first plate is mounted at a distance above the second plate that is no more than twice the Stokes boundary layer thickness estimated at a first predetermined frequency. The muffler may be disposed on a surface of a blade of a fan or wind turbine. The muffler may also be used to quiet a centrifugal blower or a vehicle.

Description

Anechoic Broadband Compact (ABC) Muffler with Moving Blade and Vehicle ApplicationsCross-Reference to Related Applications

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 537,342, filed on 08 September 2023, which is incorporated herein by reference in its entirety.Background

[0002] Sound mitigation techniques are important for addressing the noise challenges posed specially by many devices including airplanes and wind turbines. Most noise generated by airplane wings are ultra-low frequencies (below 100 Hz). Wind turbines and HVAC systems generate noise typically between 20 and 400 Hz. These low and mid-frequency noises are a significant concern due to their heightened annoyance factor compared to higher-frequency noise. The invention aims to solve the problem of ultra-low frequency (below 100 Hz) and low-frequency (100-1000 Hz) noise emitted by wind turbines and airplanes during their operation, as well as noises from many other devices. Published measurements of infrasound (noise at frequencies less than 20 Hz) indicate that levels at typical residential set-back distances from wind farms may be too low to be directly audible but may be perceived through window rattling. Low-frequency noise levels in the frequency range of 20-200 Hz may exceed audibility thresholds and be annoying or interfere with conversations. This annoyance can lead to complaints, public opposition, and legal challenges, hindering further expansion of wind power projects. Moreover, low- and mid-frequency noises pose health risks to nearby residents, including sleep disturbances, stress, and potential cardiovascular and cognitive effects.

[0003] Sound propagating through media other than air can also cause issues and require damping. For example, much submarine detection and antisubmarine warfare relies on detection of sound emitted by submarines and propagated through water; such noises can be emitted by both unducted and ducted propellers or thrusters of the submarine as well as internal machinery. Similarly, engines and propellers of boats and ships may emit sounds for which damping can be desirable.

[0004] In addition to air and water, sound can propagate through other media such as oil; many of these media require pumping that can produce sound. Similarly,heavy-duty power transformers often vibrate at powerline frequencies of 50 Hz or 60 Hz, or harmonics thereof; these vibrations can also produce objectional sounds.

[0005] Typically, passive mufflers contain Helmholtz resonators; quarter-wave resonators; duct area expansions (this relates to horns with a gradual or stepwise increase of the wave front area); aerodynamic modifications, acoustic absorbing materials (fibrous, porous, reticulated, micro-perforated, etc.), and “acoustic black hole” constructions (typically consisting of a range of discs with holes of decreasing diameters separated by cavities).

[0006] Sabine (US2966954) in 1963 described an acoustical correction element that serves as a basic sound-absorbing element. It has two perforated sheets of material mounted together in spaced relation to form a tortuous path through the element, with a spacing on the order of 0.005 inches (0.13 mm) or, in another embodiment, 0.026 inches between sheets. The base sheet material is any stiff material that, when composited, forms an element with sufficient stiffness to serve as a wall or ceiling surface. A preferred construction is using 0.015-inch-thick vinyl resin sheets. The perforations are staggered between sheets, can vary in size, spacing, and pattern, with the most efficient reported being approximately 3 / 16 inch in diameter, spaced one-half inch on centers, providing around 11% open area. The unit can be composited of various materials such as paper, cardboard, hardboard, wood, plywood, aluminium, asbestos-cement, and so on. Suitable spacing sheet can also be a porous sheet material.

[0007] Sabine mainly focuses on creating an acoustic panel, and their optimal opening area is around 11%. Sabine does not mention applications like moving appliances or rotating fan blades, which have inherent acoustic pressure differences on both sides, and Sabine fails to discuss the influence of a back cavity, an acoustic membrane for dust prevention and maintaining flow and power efficiencies, or applications like acoustic tapes. Further, Sabine fails to mention muffling sound in media other than air.Summary

[0008] A muffler has a first plate having a plurality of holes; a second plate having a plurality of holes; the first plate mounted at a distance across an entire area having holes of first and second plate that is less than or equal to two millimeters above the second plate; and an additional feature selected from an acoustic membrane over holes of the first plate and a back cavity. In embodiments, none of the holes of the first plateare directly aligned with holes of the second plate. In embodiments, the first plate is mounted at a distance above the second plate that is no more than twice the Stokes boundary layer thickness estimated at a first predetermined frequency.

[0009] An alternative muffler has a first plate having a plurality of holes; a second plate having a plurality of holes; the first plate mounted at a distance across an entire area having holes of first and second plate that is less than or equal to two millimeters above the second plate by providing a spacer layer comprising a plurality of openings therethrough disposed between the first and second plates; wherein at least one opening of the spacer layer connects at least one hole of the first plate with at least one non-overlapping hole of the second plate; and wherein the first and second plates and the spacer layer are flexible. In embodiments, the muffler designated B further including a flexible back cavity, where the flexible back cavity comprises a bottom layer with one or more projections extending in the direction of the plates, and in embodiments the muffler further includes a flexible acoustic membrane.

[0010] An article of manufacture includes a core and a muffler of the type described in the above paragraphs curved or wound about the core.

[0011] In another embodiment, a blade includes a muffler disposed on a surface of the blade where the muffler has a first plate having a plurality of holes; a second plate having a plurality of holes; the first plate mounted at a distance across an entire area having holes of first and second plate that is less than or equal to two millimeters above the second plate; and an additional feature selected from an acoustic membrane over holes of the first plate and a back cavity. In embodiments, none of the holes of the first plate are directly aligned with holes of the second plate. In embodiments, the first plate is mounted at a distance above the second plate that is no more than twice the Stokes boundary layer thickness estimated at a first predetermined frequency. In embodiments, a wind turbine includes blades of this type.

[0012] In another embodiment, a method of fabricating a blade includes forming a first plate with a plurality of holes; forming a second plate with a plurality of holes; mounting the first plate with a gap from the second plate to form a perforated structure; assembling the perforated structure as at least part of a trailing surface of the blade.Brief Description of the Figures

[0013] Fig. 1 is a schematic cross section of a flat muffler.

[0014] Fig. 2 is a cross section of a dome-shaped muffler.

[0015] Fig. 3 is a graph showing reflection coefficient versus frequency for a fixed hole pattern (Diameter 2 mm with pitch 7 mm) with variable plate spacing at 20C and 1 atm.

[0016] Fig. 4 is a graph showing reflection coefficient versus frequency for a fixed hole pattern (Diameter 2 mm with pitch 7mm) and a fixed 0.25 mm plate spacing with several dynamic viscosities of air.

[0017] Fig. 5 illustrates a flat muffler having round holes in a rectilinear pattern simulated to determine reflection, absorption and transmission coefficients and transmission losses.

[0018] Fig.6 illustrates performance as simulated of the muffler of Fig. 5 with a first design (hole diameter 2 mm with pitch 7 mm and plate spacing 0.25 mm with and without the back cavity of a depth of 4 [cm]).

[0019] Fig. 7 illustrates performance as simulated of the muffler of Fig. 5 with a second design (hole diameter 1 mm with pitch 4 mm and plate spacing 0.25 mm with and without the back cavity of a depth of 2 [cm]).

[0020] Fig. 8 illustrates muffler parameters versus frequency of the muffler of Fig. 5 with a third design (hole diameter 2 mm with pitch 3 mm and plate spacing 0.07 mm with and without the back cavity of a depth of 0.5 [cm]).

[0021] Fig. 9A and 9B illustrate a flat muffler having a pattern of slits simulated to determine reflection, absorption, and transmission coefficients together with transmission losses.

[0022] Fig. 9C is a plot of reflection coefficient versus frequency for several plate-to-plate spacings determined by spacer thicknesses mentioned in Fig. 9A.

[0023] Fig. 10A illustrates a flat muffler having a back cavity and Fig. 10B illustrates absorption coefficient of the flat muffler of Fig. 10A having a pattern of holes versus frequency for several sizes of back cavity from 2 Hz to 500 Hz. The figures illustrate that larger volumes of the back cavity can be used to absorb lower frequencies.

[0024] Fig. 11A illustrates a muffler according to the present disclosure including a multitude of plates having the design of Fig. 5 with the spacing between theplates of 0.25mm, simulated to determine reflection, absorption and transmission coefficients and transmission losses.

[0025] Fig. 11B and 11C illustrate reflection, absorption, and transmission coefficients together with transmission losses of a muffler with multiple sheets having a pattern of holes.

[0026] Fig. 12 is a rendering of a CAD model of a commonly-used blade design for wind turbines with a section taken near the tip of the blade. The section near the tip of the blade is used for noise simulations of the muffler as applied to the blade.

[0027] Fig. 13 is a perspective view of a wireframe model of the airfoil section near the tip of the blade of Fig. 12.

[0028] Fig. 14 is a sound intensity plot (dark being more intense sound intensity) of sound generated by airflow over the airfoil section of Fig. 12 without a muffler at 100 Hz.

[0029] Fig. 15A is a sound intensity plot of sound generated by airflow over the airfoil section of Fig. 12 with the muffler section in place on the windward side, or top, of the blade at 100 Hz.

[0030] Fig. 15B is a 3D view of the velocity profile by airflow over the airfoil section of Fig. 12 with the muffler section in place on the windward side, or top, of the blade.

[0031] Fig 16A is an illustration of an additional muffler sections added on the windward side, or to the full or part of the top front, top rear, bottom front, bottom rear of the blade.

[0032] Fig. 16B is an illustration of axial fans and propeller that the muffler merge to the blades.

[0033] Fig. 17 is an illustration of a vehicle with muffler sections incorporated in the hood of a vehicle and into a shroud of a rear wheel of the vehicle.

[0034] Fig.l8A is a schematic cross section drawing illustrating a two-layer- with-spacer implementation of the muffler with a dimpled back plate forming a back cavity.

[0035] Figs. 18B, 18C, and 18D are schematic cross section drawings illustrating individual layers that may be used to make the assembly of Fig. 18A.

[0036] Fig. 18E is a schematic cross section drawing illustrating a two-layer- with-spacer implementation of the muffler that, if made from flexible materials, be formed as an acoustic tape that can be adhered to objects needing sound dampening.

[0037] Fig. 18F is a perspective view showing the muffler assembled from stacked layers.

[0038] Figs. 18G, 18H, 18J, and 18K show layers that may be used to form the assembly of Fig. 18E.

[0039] Fig. 18M is a schematic illustration of an assortment of possible shapes of holes in spacer layers and a top and bottom plate.

[0040] Fig. 18N is a schematic illustration of alpha versus frequency for several embodiments of the ABC muffler.

[0041] Fig. 19A and 19B illustrate placement of the ABC muffler of Fig. 19C on a compressor of heat pump unit, while Fig. 19D illustrates performance of the muffler at reducing emitted noise from the heat pump.

[0042] Fig. 20A illustrates absorption coefficients of 6 ABC mufflers all having a 5-centimeter backspace and differing hole size and spacing.

[0043] Fig. 20B illustrates absorption coefficients of several ABC mufflers having different backspace dimensions and different material.

[0044] Figs. 21A, 21B, 21C and 21D illustrates application of the ABC muffler to quieting centrifugal blowers, with Figs. 21A and 21C illustrating performance and Figs. 2 IB and 2 ID illustrating disassembled blowers with ABC mufflers as used to obtain the data shown in Figs. 21 A and 21C.

[0045] Fig. 22 illustrates application of the ABC muffler to quieting fan blades.

[0046] Fig. 23A and Fig. 23B illustrate performance of the ABC muffler to quieting exhausts.

[0047] Fig. 24 is a photograph of an ABC muffler embodiment formed of a first and second perforated layer of fabric separated by a spacer layer of fabric, stretched and adhered to a semiflexible grid.Detailed Description of the Embodiments

[0048] We introduce an ultra- thin muffler and some applications, therefore.6RECTIFIED SHEET (RULE 91) ISA / EPMuffler Theory

[0049] We describe an ultra-thin muffler and several applications for that muffler, in particular we discuss using the ultra-thin muffler including acoustic tape to quiet a variety of fixed and moving structures such as aircraft wings, helicopter blades, propellers, blowers, and fan blades, gas turbine blades, and ship propellers, potentially including fan blades of turbofan engines, and various vehicles and aircraft.

[0050] Exemplary muffler 100, 200, has an optional back cavity or chamber 110 behind at least two closely-spaced second and first plates 118, 124. Other exemplary embodiments may include more than two plates, such as 3 or more plates.

[0051] Bottom plate 118 has holes 120 between back chamber (if present) or the environment into a space 122 between bottom plate 118 and top plate 124. A top space 122 is top plate 124 having through holes 126 into environment 114. In some, but not all, embodiments, airflow through holes 120, 126 is blocked by athin acoustic membrane 130 that may be below bottom plate 118, above the bottom plate 118 and below top plate 124, or may be above top plate 124, as illustrated in Fig. 1, or a combination thereof. Acoustic membrane 130 is formed of a flexible material that passes sound, such as deformable and bendable polymers, plastics or metallic films and / or foils. Some suitable examples for materials of the acoustic membrane include acoustic diaphragm and drum materials comprising laminates, plastic, aramid fiber such as Kevlar, polypropylene, polyethylene, polycarbonate, glass fiber, carbon fiber, etc.

[0052] Back cavity or chamber 110 is an enclosure configured to prevent a shortcut connection between the fluid, whether the fluid be air, water, oil or another material, before and after the muffler, so that the fluid does not pass directly from an outer surface of the plate 124 to an outer surface of the plate 118 without passing through the space 122 between the plates 124 and 118. A back cavity is essentially a void volume, sealed walls of which may be composed of or include acoustically rigid material. Exemplary embodiments of the present disclosure have a back cavity 110 with a depth 112 between a surface of the plate adjacent to the back cavity and a wall of the back cavity facing the plate adjacent to the back cavity of 50 mm or less.

[0053] To implement an acoustic tape embodying the muffler according to the present disclosure, all elements of the muffler 100, 200, including the top and bottom plates 124 and 118, any additional plates, spacer layer(s), the acoustic membrane 130, and / or the back cavity 110, can be flexible, for example, due to being made from orincluding flexible materials including many plastics, thin metal sheets, as well as textiles. In such embodiments, illustrated in Figs. 18A and 18E, the back cavity 410 can be created by adding a bottom layer 412 with one or more and preferably a plurality of projections 414 extending in the direction of the stack 420, 422 of first and second plates 424, 428, and spacer 430 of the muffler 400. In these embodiments, a thin acoustic membrane 437 transparent to sound but capable of blocking mass airflow may optionally be provided. Preferably, one or more projections 414 are in contact with a side of the stack 420 that faces the back cavity 410. One example of a bottom layer 412 with a plurality of projections 414 is a dimple structure formed by multiple dome-shaped projections, as illustrated in Figs. 18A, 18B and 18C. However, many other types of projections are within the scope of the present disclosure, including pyramidal, rod-shaped, wave-shaped, etc. Such constructions allow the forming of a small back cavity that does not collapse when the acoustic tape embodying the muffler 400 is bent, curved or wound fully or partially about a core 435. In exemplary embodiments of acoustic tapes 400, the depth of such a back cavity 410, defined as a maximum distance between the layer 412 and the stack 420 of plates 424, 428, can range from 0.5 to 5 mm.

[0054] An adhesive layer and optionally a release liner may further be included for the purposes of attaching the muffler 100, 200, 400, to other objects. One or more adhesive layers can also be provided on surfaces of one or more plates 424, 428 and / or one or more spacer layers, such as the spacer layer 430 provided between the first and second plates 424 and 428 as illustrated in Figs. 18A and 18E. Exemplary acoustic tapes described above have the advantage of being suitable for attachment or otherwise being fitted to or around curved surfaces. Another advantage of such embodiments is that they can be provided in the form of a roll 435. For example, as illustrated in Fig. 18H, a roll of acoustic tape 435 can include a core and an exemplary flexible muffler / acoustic tape wound on the core. In other exemplary embodiments, a core can be any desired article that can benefit from a noise reducing acoustic tape application.

[0055] The layer patterns of Fig. 18F-18M can also be used to create soundproof clothing or sonic curtains; in cloth embodiments, layers may be glued together.

[0056] The holes 120, 126 in the bottom and top plates, respectively, may have a variety of shapes, including holes, grooves, slits, rounded slits, squares, rounded squares, triangles, rounded triangles, pentagons, rounded pentagons, hexagons, roundedpentagons or a combination thereof. The shape and / or size of holes 120 in the bottom plate and of the holes 126 in the top plate may be the same or different. In some embodiments, size of the holes ranges from 0.001 mm to 5, 7, 15 mm or more. In some embodiments, the holes of one or both plates 118, 124 are arranged in a regular pattern. In such embodiments, the arrangement of holes can be characterized by a pitch of, e.g., 2 mm to 15 mm or more.

[0057] The top plate 124 and bottom plate 118 are separated by space 122, across the entire perforated areas of the top and bottom plates. A distance 128 between bottom plate 118 and top plate 124 is small enough that thermo-viscous boundary layer effects have a significant impact upon the propagation of sound along the narrow, rigidwalled channel formed between holes by the top plate and bottom plate giving substantial acoustic attenuation within the audio frequency range. This attenuation is a function of 8S / W where 8S= Zn^Zv / jj is Stokes's boundary layer thickness, and v is kinematic viscosity defined as the ratio of the dynamic viscosity (p) over the density (p) of air, water, or another fluid of interest, and W is the channel width or the plate-to-plate distance 128. Viscosity (p) is a function of pressure and temperature, which change with altitude. With rigid-walled channels with widths on the order of Stokes's boundary layer thickness, the attenuation in air can be over 6.86 dB / wavelength; for the channel width of half stokes boundary layer,= 2, would be 16 dB / wavelength and for the channel widthof twice of the stokes boundary layer, = 0.5, would be 3 dB / wavelength.

[0058] The Stokes's boundary layer thickness for frequency range of 20 Hz to 20kHz would be between 0.09 mm and 3 mm. For instance, at 4000 Hz, (wavelength is 343 / 4000=0.086 m), by selecting a channel width on the order of Stokes’s boundary layer thickness (8S= Zn^v / a) = 2 * n *=0-21 mm), the muffler can have 6.86dB / wavelength means 6.86 / 0.086=80 dB / m attenuation. This number for the case of channel width of half stokes boundary layer, -^ = 2, would be 16 / 0.086=186 dB / m and forthe channel width oftwice ofthe stokes boundary layer,= 0.5, would be 3 / 0.086=34 dB / m. Therefore, shorter sizes of the gap have higher attenuation but also has higher resistance for passing the flow.

[0059] The Stokes boundary layer thickness varies with type of fluid through which sound propagates within the gap between plates 118, 124 (or in openings of spacer plates that separate plates of spacer-plate embodiments) and in holes 120, 126 of plates of the muffler. Example Stokes boundary layer thicknesses are found in table 1 below, Stokes boundary layer thickness can be computed for other materials, such as oils, can be calculated for specific applications. In the table, VIS is kinematic viscosity in M2 / sec, Stokes size is thickness of Stokes boundary layers in millimeters, frequency half Stokes media VIS [Hz] stokes size size air 0.00001516 2 9.759624993 4.879812496 air 0.00001516 20 3.086264409 1.543132204 air 0.00001516 60 1.781855587 0.890927794 air 0.00001516 100 1.380219403 0.690109701 air 0.00001516 500 0.617252882 0.308626441 air 0.00001516 1000 0.436463698 0.218231849 air 0.00001516 4000 0.218231849 0.109115925 air 0.00001516 20000 0.09759625 0.048798125 water 0.0000010035 2 2.510974014 1.255487007 water 0.0000010035 20 0.794039703 0.397019852 water 0.0000010035 60 0.458439036 0.229219518 water 0.0000010035 100 0.355105351 0.177552675 water 0.0000010035 500 0.158807941 0.07940397 water 0.0000010035 1000 0.112294172 0.056147086 water 0.0000010035 4000 0.056147086 0.028073543 water 0.0000010035 20000 0.02510974 0.01255487Table 1, Stokes boundary layer thickness of Air (at 20°C and 1 atm) and Water (at 20°C and 2.3kPa).

[0060] A properly designed muffler requires that the distance 128 between top plate 124 and bottom plate 118 should be no more than twice the Stokes boundary layer thickness estimated at a frequency needing attenuation. The frequencies needing attenuations are typically from 100 to 400 Hz for wind turbines, heat pumps, from 20 to 100 Hz for aircraft, and 400 to 3000 for other vehicles and construction machineries.

[0061] Narrowness of the channel between the entire perforated portions of the top and bottom plates is important in function as this forms a broadband ultra-thin muffler that dampens noise at a wide broadband range of frequency. The muffler can work simultaneously from 2 Hz until 8kHz and in some special designs for even higher frequencies (up to 20kHz). In general, it can perform at ultra-low frequencies (2-100 Hz),at low frequencies (100 Hz to 1000 Hz), and mid frequencies (1000 Hz to 2000 Hz), and higher frequencies (above 2000 Hz). The muffler has been designed, built, and verified by analytic solution and numeric simulation via commercial software and verified by laboratory testing.

[0062] The measured reflection coefficient of the muffler even at ultra-low frequencies is less than 0.05 meaning 99.99% of the acoustic energy is either absorbed or transmitted from the muffler and nearly nothing is reflected back. The muffler, including top and bottom plates with spacer between, can have a thickness of a few millimeters based on the applications. Because of the ultra-thin size of the muffler, it can be easily integrated, for example as an acoustic tape, to other noise reduction systems.

[0063] Depending on application requirements, hole counts, and hole diameters, the gap between top plate and bottom plate of some embodiments is chosen to be between 0.001 and 2 millimeters, inclusive, or between 0.05 and 1.5 millimeters, inclusive, and in some cases 0.3, 0.25, 0.22, 0.2, 0.15, 0.1 or, 0.07 millimeters as given above. Each layer (surface) has multiple opening areas. In an embodiment, the holes of the top plate are shifted from the holes of the first surface such that the majority of the holes of the top plate are not directly aligned with the holes of the bottom plate. In other embodiments, none of the holes of the top plate align directly with holes of the bottom plate. In some embodiments, the gap between top plate and bottom plate is between 0.001 and 1.5 mm, inclusive. In other embodiments, the gap between top plate and bottom plate is 0.05 and 1.5 mm, inclusive, or between 0.1 and 0.3 mm, inclusive.

[0064] The space 122 may be created using spacers disposed between the plates. Examples include a mesh, one or more wires, wedges, rings and / or strips. It could also be a perforated sheet with thickness similar to the needed gap size and with larger opening areas than the holes in the top plate and the bottom plate such that the holes connect the misaligned top and bottom plate opening areas to each other. In one implementation, illustrated in Figs. 18A through 18M, a spacer layer 430 is provided between the second and first plates 424 and 428, 124, which spacer layer 430 includes a plurality of openings or slits 436. The openings can have any suitable shape, such as circular, elliptical, square, rectangular, L-shaped, S-shaped, a combination thereof, etc. Exemplary embodiments of spacer layers 430 having different types of openings 436 are illustrated in Fig. 18C In such exemplary embodiments, the openings of the spacer layer are configured to connect at least two non-overlapping holes of the second and first plates 118,124 or 424, 428. Optionally, the space 122 may contain absorbing or porous material(s), metal foam, and / or fiber.

[0065] As mentioned before, for most of the configurations which are under discussion here, the channel width (spacer gap) is smaller than the doubled Stokes’s boundary layer. Accordingly, it is very plausible that the acoustic attenuation in the interplates spacer gap is the crucial physical phenomena governing the performance of the muffler. This conclusion is also supported by the observation of the strong sensitivity of the reflection coefficient to the inter-plate spacer gap width shown in Fig. 3.

[0066] The dependence of |R| on the viscous boundary layer is also important. To perform a “pure” test, one can play with the value of the gas / air dynamic viscosity in the model keeping other parameters intact. The results of such simulations are given in Fig. 4. The variation of the air viscosity around the nominal value of r = 1.82 * 10-5Ps. s leads to a non-monotonic dependence of the reflection coefficient. For this particular case (D2P7S0.25), the combination of parameters is optimal for the lower reflection at the low end of the frequency. An increase and decrease of the coefficient of dynamic viscosity leads to an increase and decrease of the reflection coefficient at different frequencies.Simulated ExamplesExample 1

[0067] Performance of a muffler made from two perforated sheets having a regular array of circular holes was simulated, see Fig. 5

[0068] As a part of performing the simulation, we modelled the air which passes through the holes plus the air before entering to the muffler and after the outlet.

[0069] There are four important acoustic parameters to characterize any components as Reflection coefficient (|R|) Transmission coefficient (T) Absorption coefficient (a or Alpha) Transmission Loss (TL)

[0070] The first two are independent (R and T) and the other two (a and TL) are related to (R and T).

[0071] These parameters |R|, T, Alpha, and TL are more fully defined as:

[0072] Reflection coefficient (|R|): a ratio of the reflected wave to that of the incident wave.

[0073] Transmission coefficient (T): a ratio of the transmitted wave to that of the incident wave.

[0074] Absorption coefficient (a or Alpha): the fraction of the incident wave's energy that is absorbed by the muffler or material. It is calculated as 1-|R|A2-|T|A2

[0075] Transmission Loss (TL): a logarithmic ratio of the power of incident wave coming towards the muffler or material to that of the power of transmitted wave going away from the material.

[0076] For the muffler with the dimensions of- sheet thicknesses: l[mm]; DI, D2 (holes diameters): 2 [mm]; pitch 1, pitch2 (distance between origin of two circles): 7[mm]; spacer: 0.25 [mm], the result of mentioned four acoustic parameters for frequency range of 5Hz- 20kHz are shown in Fig. 6.

[0077] For another muffler with the dimensions of-sheet thicknesses: l[mm]; DI, D2 (holes diameters): 2[mm]; pitchl, pitch2 (distance between origin of two circles): 7[mm]; spacer: 0.25 [mm], the result of mentioned four acoustic parameters for frequency range of 5Hz- 20kHz are shown in Fig. 7.

[0078] The results of Fig. 7 show that with a two layers’ muffler, we can have absorption of 70% at 1500 Hz with only a 2 cm. deep back cavity. The value of the reflection coefficient for frequencies below 100 Hz is less than 0.15 and for frequencies below 1000 Hz is less than 0.55. Moreover, the transmission loss can be more than 6 dB while transmission coefficient is 0.7 at 1000 Hz. The transmission loss would be more than 15 dB at 4000 Hz and 20 dB at 10kHz while transmission coefficient is less than 0.15.

[0079] The following configurations were found to have acceptable performance: a. sheet thicknesses: l[mm]; DI, D2 (holes diameters): 2[mm]; pitchl, pitch2 (distance between origin of two circles): 7[mm]; spacer: 0.2 [mm] which has |R|=0.38, a =0.47 @ 50 Hz. b. Sheet thicknesses: l[mm]; DI, D2 (holes diameters): 2[mm]; pitchl, pitch2 (distance between origin of two circles): 6[mm]; spacer: 0.15 [mm] which has |R|=0.65, a =0.38 @ 1000 Hz.c. Sheet thicknesses: 1 [mm]; DI, D2 (holes diameters): 1 [mm]; pitch 1, pitch2 (distance between origin of two circles): 4[mm]; spacer: 0.15 [mm] which has |R|=0.6, a =0.35 @ 1000 Hz. d. Sheet thicknesses: 1 [mm]; DI, D2 (holes diameters): 1 [mm]; pitch 1, pitch2 (distance between origin of two circles): 2[mm]; spacer: 0.07 [mm] which has |R|=0.57, a =0.36 @ 4000 Hz, a =0.30 @ 6000 Hz. e. Sheet thicknesses: l[mm]; DI, D2 (holes diameters): 2[mm]; pitchl, pitch2 (distance between origin of two circles): 3[mm]; spacer: 0.07 [mm]; back cavity 3 [cm] which has |R|=0.03, a =0.99 @ 1800 Hz. f. Sheet thicknesses: l[mm]; DI, D2 (holes diameters): 3[mm]; pitchl, pitch2 (distance between origin of two circles): 4.5[mm]; spacer: 0.07 [mm] which has |R|=0.46, a =0.48 @ 1000 Hz, a =0.38 @ 4000 Hz and a =0.30 @ 6000 Hz. g. Sheet thicknesses: l[mm]; DI, D2 (holes diameters): 3[mm]; pitchl, pitch2 (distance between origin of two circles): 7[mm]; spacer: 0.15 [mm] which has |R|=0.62, a =0.40 @ 1000 Hz, a =0. 10 @ 4000 Hz. h. Sheet thicknesses: l[mm]; DI, D2 (holes diameters): 2[mm]; pitchl, pitch2 (distance between origin of two circles): 3[mm]; spacer: 0.07 [mm] which has |R|=0.31 a =0.44 @ 5 Hz without back cavity, |R|=0.34 a =0.42 @ 1000 Hz without back cavity, and |R|=0.05 a =0.99 @ 5500 Hz with 5mm back cavity as shown in Fig. 8.Example 2

[0080] Another simulated example is using slit configurations instead of round holes is shown in Fig. 9A and 9B.

[0081] Simulation results of some slit configurations with various spacer gap are shown in Fig. 10A.

[0082] The following configurations were found to have acceptable performance: a. Sheet thickness 0.6 mm; slit_width: 0.5mm; slit_height: 1.5mm; slit_pitch: 1.3mm, spacer 0.07mm which has |R|= 0.1 @ 1000 Hz, |R|= 0.4 @ 4000 Hz. b. Sheet thickness 1 mm; slit_width: 0.7mm; slit_height: 2mm; slit pitch: 2mm, spacer 1.5mm which has |R|= 0.3 @ 1000 Hz. c. Sheet thickness 1 mm; slit_width: 0.5mm; slit_height: 1.5mm; slit_pitch: 2mm, spacer 0.1mm which has |R|= 0.2 @ 1000 Hz, |R|= 0.6 @ 4000 Hz.d. Sheet thickness 1 mm; slit_width: 0.5mm; slit_height: 2.5mm; slit_pitch: 1.5mm, spacer 0.08mm which has |R|= 0.15 @ 1000 Hz, |R|= 0.45 @ 4000 Hz. e. Sheet thickness 1 mm; slit_width: 0.6mm; slit_height: 2mm; slit_pitch: 1.5mm, spacer 0.08mm which has |R|= 0.2 @ 1000 Hz, |R|= 0.42 @ 4000 Hz. f. Sheet thickness 1 mm; slit_width: 0.7mm; slit_height: 2.5mm; slit_pitch: 2mm, spacer 0.08mm which has |R|= 0.12 @ 1000 Hz, |R|= 0.5 @ 4000 Hz.Example 3

[0083] For noise reduction purposes at ultra-low frequencies, a noise absorption system is considered including the ABC muffler with a large volume as the back cavity. This combination allows the muffler to work across a wide frequency range from 1 Hz to 20 kHz with absorption up to 99%. It means 90 dB noise reduction. In the lab tests with a small back cavity (V: 0.005 m3), the muffler with total area of 0.0015 m2 absorbed noise from 20 Hz to 2000 Hz with 0.96 absorption coefficient at all frequencies below 2000Hz simultaneously, means almost all the acoustic noise which enters into the ABC muffler is fully absorbed. For bigger size of the back cavity, the numerical simulation has been done. The area of the muffler is constant and each time the size of the back cavity is changed and the absorption coefficient at each frequency below 500 Hz is calculated which is illustrated on Fig. 10B.Example 4

[0084] To illustrate the muffler's performance across higher frequency ranges, we have prepared an additional example. In this instance, we have considered multi plate configurations, varying from 2 to 10 plates. Figure 1 IB displays the absorption coefficient of the muffler at each frequency between 2Hz and 20,000 Hz. The plot reveals that when noise at two or more frequencies need to be effectively suppressed, it becomes feasible to design an ABC muffler with multiple plates, resulting in peaks at the desired frequencies. Furthermore, the plot demonstrates that as the number of plates increases, the overall absorption decreases, but the number of distinct peak points also increases. Fig. 11C shows four important acoustic parameters of the muffler with 4 plates with design of D: 2mm, Pitch: 7 mm, spacer gap: 0.25 mm.Muffler Applications

[0085] The ultra-low frequency broadband ultra-thin muffler is useful at a broadband range of frequencies. The muffler can work simultaneously from 2 Hz until 2kHz and in some exemplary embodiments for even higher frequencies (up to 20kHz). The muffler can also be tuned to work at a specific region of frequencies. The muffler has been designed, built, and verified by analytical solutions and numerical simulations and it is verified by tests of prototypes in the lab for the specific range of frequency which was possible to measure but performance is not limited to the measured frequency range.

[0086] The muffler can have a thickness of some microns to some centimeters and the other sizes (length and width of the muffler) can be on order of microns to hundreds of meters as needed for the wind turbine application discussed below. Because of the ultra-thin size of the muffler, it can be easily integrated to any kind of geometries that generate noise. The performance of the muffler can reach up to and more than 99% of energy being damped. The measured reflection coefficient of exemplary mufflers even at ultra-low frequencies is less than 0.05 which means 99.99% of the acoustic energy is either absorbed or transmitted from the muffler and actually almost nothing is reflected back.Wind Turbine Blade Applications

[0087] Blade tip velocities of typical wind turbines may exceed 8 times the wind velocities the wind turbine is exposed to, thus reaching tip speeds of hundreds of miles per hour in light to moderate winds, while many wind turbines vary blade pitch to reduce tip-to-wind speed ratios at moderate to high wind speeds, high blade-tip velocities can still be expected. Blade tips at such high velocities can cause significant noise.

[0088] A common wind turbine blade of length about 50 meters and NACA 0018 airfoil is illustrated in Fig. 12. As blade tips generate much of wind-turbine noise, we simulated our muffler as used on a portion of the blade near the tip as shown in Fig. 12. While simulations focused on the portion near the tip, our muffler can be applied anywhere along the blade .

[0089] Near the blade tip, we simulate the blade airfoil with a portion along the trailing edge formed according to our two-plate muffler with closely-spaced top and bottom plates including 2 mm holes distancing 6 mm from one another. The top plate is shifted from the bottom plate by 2 mm so that the air acoustic wave is compelled to follow a tortuous path as it passes through these two layers of holes (narrow channel). Inside thesimulated blade is a void and air is trapped in this internal volume cavity. Thickness of the blade wall is 1cm and this stems from the void or back cavity volume V to the total area of the openings of the muffler A (number of holes times area of the holes) part fraction. In this context V / A should be more than 5 for ultra-low frequencies (blow 100Hz) to achieve a 99% absorption, where V and A denote back cavity volume and muffler area, respectively, as illustrated in Fig. 13.

[0090] As can be seen in Figs. 14-16, adding the muffler at the upwind trailing edge of the blade can reduce the maximum Sound Pressure Level area from that generated by an unmodified airfoil (Fig. 14) at 100 Hz to a much smaller area (Fig. 15A and 15B).

[0091] Further reductions in sound pressure levels can be achieved by adding the muffler at both upwind and downwind edges as shown in Fig. 16A. In an embodiment, a muffler according to the present disclosure is located in a region of the blade or airfoil that is closest to the acoustic source and / or on the suction side of the blade. A plurality of mufflers according to the present disclosure may be incorporated in a blade or airfoil at locations where the acoustic field has the maximum absolute value of the acoustic pressure. In one implementation, a muffler according to the present disclosure may span at least a substantial portion of the surface of the blade or airfoil. In embodiments, these mufflers have an acoustic membrane across their surface to prevent disruption of airflow. The membrane can be disposed over the outer surface of the muffler and / or another location as described above.

[0092] Other exemplary applications containing mufflers according to the present disclosure are illustrated in Figs. 16A - 16B and Fig 17. In an alternative embodiment, muffler surfaces are provided on both leading and trailing edges of the blade.

[0093] Helicopter rotors, airplane wings, and wind turbine blades are all airfoils, airplane wings and helicopter rotors may be quieted with the muffler positioned in surfaces of the wing in a manner similar to quieting a wind turbine blade. Further, some additional fan blades can also be quieted in similar manner using two layers of perforated material, the layers closely spaced, holes of both layer nonoverlapping each other, as part of their trailing edges. Additionally, or alternatively, mufflers according to the present disclosure may be incorporated into aerodynamically shaped surfaces for converting mechanical torque into flow velocity, or converting flow velocity into mechanical torque, or converting flow velocity into lift force.

[0094] Another application for mufflers according to the present disclosure with a surface covered with an acoustic membrane may be hoods and other exterior surfaces of motor vehicles, where it is desirable to dampen both engine noises, tire noises, and airflow noises. Another example is absorbing aeroacoustics noise of an automobiles or trucks. The example is a hood of a moving vehicle (or body of any moving device). It is possible to consider the internal volume of the engine side as a back cavity because it has a different acoustic pressure respect to the air surrounding the vehicle. Therefore, by applying the ultra-thin muffler to the hood, a connection between surrounding air and the air inside the engine part is prepared and the closest way to connect these two fluids is passing through the muffler which causes noise reduction. The muffler in these vehicle applications is covered by an acoustic membrane (cover car wrap) so there are no rough edges to provoke vortexes on the surface and allowing a smooth paintable surface. Figure 17 shows a vehicle that has a muffler incorporated in its hood and in a rear tire cover. Additionally or alternatively, mufflers according to the present disclosure may be incorporated into doors or other external and / or internal surfaces of vehicles. Vehicles in which the mufflers according to the present disclosure may be used include, without limitation, helicopters, airplanes, drones, construction equipment, rotary machines, cars, and trucks. In general, the mufflers according to the present disclosures are designed for incorporation in fixed parts and parts that move at the speed of 1 m per second or more. When made from electrically conductive materials, it can also be used to reduce electromagnetic noise and shield from electromagnetic pulse or other electromagnetic interference, such as when the muffler is arranged to high-voltage electricity cables or electronic boards, as well as dampening fan noises of computers, laptops, and other electronic equipment. The perforations in the mufflers allow heat to dissipate, preventing any significant temperature increase inside the muffler.ExamplesAcoustic Tape Applications

[0095] We have developed over 50 configurations of Acoustic Tapes that can be applied to various surfaces or suspended in the air to reduce noise. These tapes have been tested using the impedance tube setup at TU / e, with some results presented in Fig. 18A through 18N. Fig. 18N illustrates the absorption coefficient of 10 samples of acoustic tapes, which are depicted in Fig. 18A through Fig. 18M. As shown in Fig. 18N, most of these samples achieve 60% absorption at frequencies between 50 Hz and 400 Hz with athickness of less than 1 mm. The primary variations between the acoustic tapes in Fig. 18N stem from differences in hole patterns, gap spacing, and the number of layers used. Increasing the number of layers can improve absorption up to 90%. To maximize absorption performance, multiple sets of mufflers can be arranged with short distances between them, optimizing overall sound absorption. The spacing between each pair of mufflers can range from 1 mm to 5 cm, depending on the desired absorption characteristics. When utilizing materials such as textiles that inherently possess acoustic absorption properties, the absorption percentages may increase even further. For applications involving soft textile materials, such as cotton, polyester, or blends, the layers should be glued, sewn, or stitched together to ensure the holes and micro channels remain securely in place. Moreover, the material should be sufficiently stretched to prevent the layers moving or bending. While glueing is effective without any stitching, in the cases of stitching or sewing, it's important to stretch the material slightly to prevent the layers from shifting, which could increase the distance between them. If the layers move, it would disrupt the microchannels' functionality, thus reducing the efficiency of the muffler. When glued, this risk is eliminated, ensuring the microchannels remain intact and perform as expected.

[0096] The micro-gaps in the fabric can absorb noise effectively, but there is a trade-off when the gaps become too small. Smaller gaps increase the pressure drop, causing more air to reflect instead of flowing through. The fabric material and the manufacturing process are essential for optimizing this balance between absorption and airflow. As mentioned before, using gaps around half the Stokes boundary layer thickness (approximately 0.07 to 0.25 mm) allows good absorption with manageable pressure drop. Reducing the gap size further requires more precise manufacturing, which can increase noise absorption but also raises resistance to airflow. This can cause the air to avoid flowing through the channel, impacting the noise reduction efficiency. To achieve a higher absorption rate while balancing pressure drop, we combine the fabric material's inherent absorption properties with perforated patterns. The perforations help reduce pressure drop compared to fabric alone, allowing us to benefit from both the fabric's noise absorption and the perforations' airflow efficiency. This approach enables us to leverage the advantages of fabric absorption, which tends to absorb higher frequencies, and the benefits of perforations, which improve airflow and target lower frequencies. Byoptimizing the pattern and gap size, we aim to create a system with higher overall absorption across a broader frequency range.

[0097] To ensure that fabric or plastic materials cease acting as an acoustic membrane, they need to be stiffened by sewing, gluing, or stitching the layers together, and secured to reduce movement. Typically, thin, flexible materials used in acoustic membranes vibrate easily, allowing sound waves to pass through with minimal absorption and reflection. However, for muffling purposes, the material must be tightly secured and immobilized to prevent this movement and enhance absorption.

[0098] For example, when fabric or nylon sheets are fixed between semi-rigid, semi-flexible grids, their movement is restricted, allowing the material to function as a muffler that absorbs sound more effectively. On the other hand, if the fabric is not stretched or secured properly, low-frequency noise can cause the material to move, leading to less than 20% absorption. Tightening the material ensures improved sound absorption, particularly at lower frequencies. For most of our work, we sew the layers of material together first and then glue the muffler to the desired surface. Fig. 24 illustrates a muffler formed of three layers of fabric, including a first and a second perforated layers with misaligned holes, and a spacer layer between the first and second perforated layers, the spacer layer having larger holes that communicate between holes of the first and second perforated layers. The muffler of Fig. 24 is attached, such as by gluing, to a semirigid frame shown in the Fig. 24 as a black grid.Acoustic Jacket Application

[0099] In our initial test, we integrated a 1 [mm] thick ABC muffler configured as illustrated in Fig. 1 into the traditional acoustic compressor jacket of a heat pump to form a modified acoustic jacket Fig. 19C and installed it as shown in Fig. 19A and 19B on the compressor. We then compared the noise reduction results with a traditional acoustic jacket. Our upgraded Acoustic Jacket achieved a 20 dB peak noise reduction at 160 Hz, corresponding to a 6 dB reduction in overall heat pump noise, as shown in Fig. 19D.Acoustic Panel Application

[0100] We have created various configurations of acoustic panels using the ABC muffler configured as illustrated in Fig. 1. Impedance tube tests indicate that these panels can achieve Class A acoustic panel status, with significantly higher absorptionthan standard Class A panels when both have the same size back cavity (plenum). Notably, some of our acoustic panels with a 5 cm back cavity outperform Class A panels with a 20 cm back cavity, as demonstrated in Fig. 20A or Fig. 20B. In Fig. 20A, the absorption coefficient of some of the acoustic panels with different pattern and back cavity of 5 cm are shown. As can be seen, with this short back cavity, the panel has 50% at 100 Hz and between 50% to 99.9% absorption at frequencies between 100 Hz and 4000 Hz. In addition, the absorption coefficient of some other Acoustic Jacket (AJ) samples with total thickness between 1cm to 4 cm and Acoustic Panels (AP) total thickness including the back cavity between 2.5 cm to 6 cm are shown in Fig. 20B which shows it is possible to absorb 65% at ultra-low frequencies and absorb up to 99% at low frequencies between 100 Hz and 1000 Hz.Blower Noise Reduction Application

[0101] We applied acoustic tape 435 illustrated in Fig. 18H with thicknesses between 1 to 3 mm to the casings of ventilation and industrial blowers, achieving a noise reduction of 3 to 6 dB, as shown in Fig. 21A and Fig. 21C.Fan Noise Reduction Application

[0102] We attached acoustic tape illustrated in Fig. 18 to the blades of an aluminium fan with a diameter of 25 cm, achieving a noise reduction of 4.3 dB without any loss in efficiency, as shown in Fig. 22.Exhaust Duct Muffler or Silent Pipe Application

[0103] We have developed various configurations of open or closed duct mufflers illustrated in Fig. 1, with or without a back cavity but with a membrane, that can either minimize reflection or transmission coefficients of the duct muffler to be below 0.1 value. In both scenarios, the mufflers achieve 50 to 90% absorption, particularly at low frequencies, with lengths ranging from 30 to 50 cm. In specific designs and simulations, we achieved |R|<0.3, |Tr|<0.1, and alpha>0.9 for frequencies above 500 Hz with a muffler diameter of 12 cm and a length of 50 cm as shown in Fig. 23A and 23B. These mufflers have no obstacles or baffles in the airflow path along the length of the muffler, ensuring there is low or no pressure drop, which is a significant advantage compared to current mufflers which usually have baffles through which exhaust must flow. Additionally, they offer high absorption within a limited length and size. In the embodiments of Fig. 23A and Fig. 23B, gas, which may be exhaust gas carrying significant sound, enters at one end of an inner, perforated tube of ABC muffler formed from at least an inner and a second concentric, closely-spaced, perforated tubes, and exitsat another end of the same inner tube with sound reduced; the flowing gas passing by but not through perforations of the inner tube. If the tubes are spaced by providing a spacer plate or tube, the inner tube forms the first plate 424 of Fig. 18A and second tube the second plate 428 of Fig. 18A. In some embodiments, such as the embodiment of Fig. 23B, the ABC muffler has a back cavity formed by surrounding the inner and second tubes with a third tube spaced an appropriate distance from the second tube.

[0104] In case of making a short (below 5 cm length) closed ducted muffler, it can achieve a reflection coefficient below 0.05 across all frequencies between 20 Hz and 1000 Hz. In specific designs, this effective frequency range can be extended to 4000 Hz. Vessel Noise Reduction

[0105] While the most common use of blades as illustrated in Fig. 22 is for use with air in fans, including fan blades of turbofan engines, these blade assemblies are similar in shape to aquatic propellers as commonly used for propulsion of ships and submarines. To quiet aquatic propellers, a surface of each blade of the propeller may be fitted with a muffler as described with reference to Fig. 18A, and shown in Fig. 22. In order to prevent cavitation, avoid clogging from debris, and optimize fluid flow over each blade of the propeller, acoustic membrane 437 is provided while back cavity 410 together with space between plates 424 and 428 and within holes of plates 424 and 428 is filled with a fluid. While the fluid may be air in some surface-vessel applications, when used in submarines, some other surface vessel applications, and other applications where pressures may change, the fluid is a noncompressible fluid such as an aqueous fluid or an oil. In these applications plate spacing is determined according to the Stokes boundary layer thickness of the noncompressible fluid within the back cavity 410 and between plates 424 and 428.

[0106] The propellers described in the preceding paragraph may be in both unducted and ducted configurations; in a ducted configuration, in addition to forming the ABC muffler on all or a portion of the propeller surfaces, the duct around the propeller may in some embodiments also incorporate the muffler tape herein described to further reduce sound emitted.

[0107] Similarly, muffler tape as previously described, equipped with acoustic membrane 437 and with the back cavity 410 and space between plates 410 and 428 filled with noncompressible fluid may be positioned on an exterior surface of machinery spacesand other portions of a ship, submarine, or other vessel. The noncompressible fluid may be chosen from an aqueous or non-aqueous fluid as required for a particular application.Combinations

[0108] The presently disclosed concepts can be combined in multiple ways. Among combinations anticipated are:

[0109] A muffler designated A including a first plate having a plurality of holes; a second plate having a plurality of holes; the first plate mounted at a distance across an entire area having holes of first and second plate that is less than or equal to two millimeters above the second plate; and an additional feature selected from an acoustic membrane over holes of the first plate and a back cavity.

[0110] A muffler designated A2 including the muffler designated A, where at least some of the holes of the first plate are not aligned with the holes of the second plate.

[0111] A muffler designated A3 including the muffler designated A2, where none of the holes of the first plate are directly aligned with holes of the second plate.

[0112] A muffler designated A4 including the muffler designated A, A2, or A3 wherein the distance is between 0.001 and 2 mm.

[0113] A muffler designated A5 including the muffler designated A, A2, A3, or A4, where the first plate is mounted at a distance above the second plate that is no more than twice the Stokes boundary layer thickness estimated at a first predetermined frequency.

[0114] A muffler designated A6 including the muffler designated A5, where the first predetermined frequency is between 2 Hz and 20KHz.

[0115] A muffler designated A7 including the muffler designated A, A2, A3, A4, A5 or A6 where the firstplate is mounted at a distance from 0.001 and 0.3 millimeters above the second plate.

[0116] A muffler designated A8 including the muffler designated A, A2, A3, A4, A5, A6 or A7 where the first plate is mounted at a distance greater than 0.001 millimeters above the second plate.

[0117] A muffler designated A9 including the muffler designated A, A2, A3, A4, A5, A6, A7, or A8 wherein the first and second plates each have a plurality of holes having a diameter from 1 mm to 3 mm and a pitch from 1.5 mm to 8 mm; and wherein the first plate is mounted at a distance from 0.05 to 0.25 mm above the second plate.

[0118] A muffler designated A10 including the muffler designated A, A2, A3, A4, A5, A6, A7, or A8 wherein the pluralities of holes in the first and second plates each including a plurality of slits having a slit width of 0.05 mm to 3 mm, a slit height of 0.05 to 100 mm, and a slit pitch of 0.2 mm to 5 mm; and wherein the first plate is mounted at a distance from 0.05 to 0.25 mm above the second plate.

[0119] A muffler designated Al l including the muffler designated A, A2, A3, A4, A5, A6, A7, or A8, A9 or A 10 further including a third plate having a plurality of holes, the third plate mounted at a second distance across an entire area having holes of the first or second plate above the first or second plate.

[0120] A muffler designated A 12 including the muffler designated A, A2, A3, A4, A5, A6, A7, A8, A9, A10, or Al l where the additional feature is an acoustic membrane, such as polymers, plastics or metallic films and / or foils, laminates, plastic, cellophane, aramid fiber, Kevlar, polypropylene, polyethylene, polycarbonate, glass fiber, carbon fiber, etc.

[0121] A muffler designated A13 including the muffler designated A, A2, A3, A4, A5, A6, A7, A8, A9, A10, or Al l where the additional feature is a back cavity and V / A is more than 5.

[0122] A muffler designated A 14 including the muffler designated A, A2, A3, A4, A5, A6, A7, A8, A9, A10, Al l, A12, or A13 where the additional feature is a back cavity having a depth of 50 mm or less.

[0123] A muffler designated A15 including the muffler designated A, A2, A3, A4, A5, A6, A7, A8, A9, A10, Al 1, A12, A13, or A14, further including a spacer layer having a plurality of openings therethrough disposed between the first and second plates, wherein at least one opening of the spacer layer connects at least one hole of the first plate with at least one non-overlapping hole of the second plate.

[0124] A muffler designated A16 including the muffler designated A, A2, A3, A4, A5, A6, A7, A8, A9, A10, Al 1, A13, A14, or A15, where the additional feature is a back cavity and the back cavity includes a bottom layer with one or more projections extending in the direction of the plates.

[0125] A muffler designated A17 including the muffler designated A, A2, A3, A4, A5, A6, A7, A8, A9, A10, Al l, A12, A13, A14, A15, or A16 further including an adhesive layer and a release liner on an outer surface thereof.

[0126] A muffler designated A17 including the muffler designated A, A2, A3, A4, A5, A6, A7, A8, A9, A10, Al l, A12, A13, A14, A15, or A16 having at least one adhesive layer disposed on one or more of the first plate, the second plate and the spacer layer if present.

[0127] A muffler designated Al 7a including the muffler designated A, A2, A3, A4, A5, A6, A7, A8, A9, A10, Al l, A12, A13, A14, A15, or A16 having at least one adhesive layer disposed on one or more of the first plate, the second plate and the spacer layer if present.

[0128] An article of manufacture designated Al 8 including a core and a muffler of the type designated A, A2, A3, A4, A5, A6, A7, A8, A9, A10, Al 1, A12, A13, A14, A15, A16 or A17 curved about the core.

[0129] A muffler designated A19 including the muffler or article of manufacture designated A, A2, A3, A4, A5, A6, A7, A8, A9, A10, Al l, A12, A13, A14, A15, A16, A17, or A18 wherein the distance between the first and second plate is greater than 0.7 millimeter.

[0130] A muffler designated A20 including the muffler or article of manufacture designated A, A2, A3, A4, A5, A6, A7, A8, A9, A10, Al l, A12, A13, A14, A15, A16, A17, or A18 wherein the distance between the first and second plate is less than 0.6 millimeter.

[0131] A muffler designated A21 including the muffler or article of manufacture designated A, A2, A3, A4, A5, A6, A7, A8, A9, A10, Al l, A12, A13, A14, A15, A16, A17, A18, A19 or A20 wherein the muffler is configured to attenuate sounds in a gas passing past but not through the first plate.

[0132] A muffler designated A22 including the muffler designated A21 where the first and second plates are formed as concentric tubes, and where the gas flowing past the first plate flows through the tube formed of the first plate.

[0133] A muffler designated A23 including the muffler designated A 15, where the first and second plates, and the spacer layer, are formed of a fabric, and where the muffler is attached to a frame that may be a grid restricting movement of the first and second plates.

[0134] A muffler designated B includes a first plate having a plurality of holes; a second plate having a plurality of holes; the first plate mounted at a distance across an entire area having holes of first and second plate that is less than or equal to twomillimeters above the second plate by providing a spacer layer comprising a plurality of openings therethrough disposed between the first and second plates; wherein at least one opening of the spacer layer connects at least one hole of the first plate with at least one non-overlapping hole of the second plate; and wherein the first and second plates and the spacer layer are flexible.

[0135] A muffler designated BA including the muffler designated B further including a flexible back cavity, where the flexible back cavity comprises a bottom layer with one or more projections extending in the direction of the plates.

[0136] A muffler designated BB including the muffler designated B or BA, further including a flexible acoustic membrane.

[0137] A muffler designated BC including the muffler designated B, BA, or BB, further including at least one adhesive layer disposed on one or more of the first plate, the second plate and the spacer layer if present.

[0138] A muffler designated BD including the muffler designated B, BA, BB, or BC, further comprising an adhesive layer on an outer surface thereof and a release liner disposed over said adhesive layer.

[0139] A muffler designated BE including the muffler designated B, BA, BB, BD, BC, or BE, further comprising a back cavity having a depth of 0.5 to 5 mm.

[0140] An article designated C including a core and a muffler of the type designated B, BA, BB, BC, BD, BC, or BE, curved about the core.

[0141] An article designated Cl including a core and a muffler of the type designated B, BA, BB, BC, BD, BC, or BE, wound about the core, preferably to form a roll.

[0142] A blade designated D including one or more mufflers designated A, A2, A3, A4, A5, A6, A7, A8, A9, A10, Al l, A12, A13, A14, A15, or A16 disposed on a surface of the blade.

[0143] A blade designated DA including one or more mufflers designated A, A2, A3, A4, A5, A6, A7, A8, A9, A10, Al l, A12, A13, A14, A15, or A16, wherein the muffler is disposed in a trailing edge surface of the blade, where at least some space within the blade serves as a back cavity for the muffler.

[0144] A blade designated DB including a plurality of mufflers designated A, A2, A3, A4, A5, A6, A7, A8, A9, A10, Al l, A12, A13, A14, A15, or A16.

[0145] A blade designated DC including the blade designated D, DA, or DB, configured for use in air.

[0146] A blade designated DD including the blade designated D, DA, or DB, configured for use in water.

[0147] A blade designated DE including the blade designated DD configured for propelling a vessel in water.

[0148] A wind turbine designated E comprising a plurality of blades of the type designated D, DA, or DB.

[0149] A vehicle designated F including a surface, the surface further including one or more mufflers designated A, A2, A3, A4, A5, A6, A7, A8, A9, A10, Al l, A12, A13, A14, A15, or A16 or the muffler designated as B, BA, BB, BC, BD, BC, or BE.

[0150] A vehicle designated FA including one or more mufflers designated A, A2, A3, A4, A5, A6, A7, A8, A9, A10, Al l, A12, A13, A14, A15, or A16 or the muffler designated as B, BA, BB, BC, BD, BC, or BE, wherein the surface is an outer surface of the vehicle.

[0151] A vehicle designated FB, wherein the surface including one or more mufflers designated A, A2, A3, A4, A5, A6, A7, A8, A9, A10, Al l, A12, A13, A14, A 15, or A16 or the muffler designated as B, BA, BB, BC, BD, BC, or BE, is a part of a hood, wheel cover or door panel.

[0152] A method of fabricating a blade designated G includes forming a first plate with a plurality of holes; forming a second plate with a plurality of holes; mounting the first plate with a gap from the second plate to form a perforated structure; assembling the perforated structure as at least part of a trailing surface of the blade.

[0153] A method designated GA including the method designated G, further including applying an acoustic membrane to the perforated structure .

[0154] A method designated GB including the method designated GA, further comprising filling the gap between first and second plate with a fluid, the fluid being a noncompressible fluid.

[0155] Changes may be made in the above methods and systems without departing from the scope hereof. It should thus be noted that the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. For example, the terms top and bottom are used to illustrate the respective relationships of elements of exemplary embodiments of thepresent disclosure with respect to each other and not their absolute positions in space. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system, which, as a matter of language, might be said to fall there between.

Claims

ClaimsWhat is claimed is:

1. A muffler comprising: a first plate having a plurality of holes; a second plate having a plurality of holes; the first plate mounted at a distance across an entire area having holes of first and second plate that is less than or equal to two millimeters above the second plate; and an additional feature selected from an acoustic membrane over holes of the first plate and a back cavity.

2. The muffler of claim 1, where at least some of the holes of the first plate are not aligned with the holes of the second plate.

3. The muffler of claim 1, where none of the holes of the first plate are directly aligned with holes of the second plate.

4. The muffler of claim 3 wherein the distance is between 0.001 and 2 mm.

5. The muffler of claim 3, where the first plate is mounted at a distance above the second plate that is no more than twice a Stokes boundary layer thickness estimated at a first predetermined frequency.

6. The muffler of Claim 5, where the first predetermined frequency is between 2 Hz and 20KHz.

7. The muffler of claim 6 where the first plate is mounted at a distance from 0.001 and 0.3 millimeters above the second plate.

8. The muffler of claim 1 where the first plate is mounted at a distance greater than 0.001 millimeters above the second plate.

9. The muffler of Claim 7 wherein the first and second plates each have a plurality of holes having a diameter from 1 mm to 3 mm and a pitch from 1.5 mm to 8 mm; and wherein the first plate is mounted at a distance from 0.05 to 0.25 mm above the second plate.

10. The muffler of Claim 7 wherein the pluralities of holes in the first and second plates each comprise a plurality of slits having a slit width of 0.05 mm to 3 mm, a slit height of 0.05 to 100 mm, and a slit pitch of 0.2 mm to 5 mm; and wherein the first plate is mounted at a distance from 0.05 to 0.25 mm above the second plate.

11. The muffler of Claim 10 further comprising a third plate having a plurality of holes, the third plate mounted at a second distance across an entire area having holes of the first or second plate above the first or second plate.

12. The muffler of Claim 10 further comprising a plurality of plates, each plate having a plurality of holes and mounted at a second distance across an entire area having holes of an adjacent plate.

13. The muffler of claim 1, where the additional feature is an acoustic membrane, as the acoustic membrane comprising at least one of polymers, plastics or metallic films and / or foils, laminates, plastic, cellophane, aramid fiber, Kevlar, polypropylene, polyethylene, polycarbonate, glass fiber, or carbon fiber.

14. The muffler of any of claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12 where the additional feature is a back cavity and V / A is more than 5.

15. The muffler of claim 1, where the additional feature is a back cavity having a depth of 50 mm or less.

16. The muffler of claim 1, further comprising a spacer layer comprising a plurality of openings therethrough disposed between the first and second plates, wherein at least one opening of the spacer layer connects at least one hole of the first plate with at least one nonoverlapping hole of the second plate.

17. The muffler of any claim 1, where the additional feature is a back cavity and where the back cavity comprises a bottom layer with one or more projections extending in a direction of the plates.

18. The muffler of claim 1, further comprising an adhesive layer and a release liner on an outer surface thereof.

19. The muffler claim 18 further comprising at least one adhesive layer disposed on one or more of the first plate, the second plate and the spacer layer if present.

20. An article comprising a core and a muffler of claim 1 curved about the core.

21. A muffler comprising : a first plate having a plurality of holes; a second plate having a plurality of holes; the first plate mounted at a distance across an entire area having holes of first and second plate that is less than or equal to two millimeters above the second plate by providing a spacer layer comprising a plurality of openings therethrough disposed between the first and second plates; wherein at least one opening of the spacer layer connects at least one hole of the first plate with at least one non-overlapping hole of the second plate; and wherein the first and second plates and the spacer layer are flexible.

22. The muffler of claim 21 further comprising a flexible back cavity, where the flexible back cavity comprises a bottom layer with one or more projections extending in a direction of the plates.

23. The muffler of claim 21 further comprising a flexible acoustic membrane.

24. The muffler of claim 21, further comprising at least one adhesive layer disposed on one or more of the first plate, the second plate and the spacer layer.

25. The muffler of any of claim 21, further comprising an adhesive layer on an outer surface thereof and a release liner disposed over said adhesive layer.

26. The muffler of claim 21, further comprising a back cavity having a depth of 0.5 to 5 mm.

27. An article comprising a core and a muffler of any of claims 21 to 26 curved or wound about the core.

28. A blade comprising the muffler of any of claims 1 to 13.

29. The blade of claim 28, wherein the muffler is disposed in a trailing edge surface, where at least some space within the blade serves as a back cavity for the muffler.

30. The blade of claim 28, configured for use in air.

31. The blade of claim 29, configured for use in water.

32. The blade of claim 31 configured for propelling a vessel in water.

33. A blade comprising a plurality of mufflers according to any of claims 1 to 13.

34. A wind turbine comprising a plurality of blades incorporating the blade of claim 28.

35. A vehicle comprising a surface, the surface further comprising the muffler of any of claims 1 to 13 or 21 to 26.

36. The vehicle of claim 35, wherein the surface is an outer surface of the vehicle.

37. The vehicle of claim 35, wherein the surface is a part of a hood, wheel cover or door panel.

38. A method of fabricating a blade comprising forming a first plate with a plurality of holes; forming a second plate with a plurality of holes; mounting the first plate with a gap from the second plate to form a perforated structure; assembling the perforated structure as at least part of a trailing surface of the blade.

39. The method of claim 38, further comprising applying an acoustic membrane to the perforated structure.

40. The method of claim 39, further comprising filling the gap between the first and second plate with a noncompressible fluid.

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