Patent Description:
HVAC (heating, ventilating, and air conditioning) systems typically use a series of ducts through which hot or cold air is passed in order to heat or cool a building. Traditionally, HVAC ductwork is made of sheet metal which is installed first and then wrapped with insulation as a secondary operation. Galvanized mild steel is the standard and most commonly used material in fabricating ductwork. The steel sheets are supplied conventionally in rolls of continuous metal sheets, with a standard width of <NUM> to <NUM> meters. The rolls are unrolled manually and cut in desired lengths. Then the lengths are bent together into a rectangular shape and locked together. Currently available flexible ducts, known as flex have a variety of configurations, but for HVAC applications, they are typically flexible plastic over a metal wire coil to make round, flexible ducts. However, such flex ducts have poor noise and thermal insulation characteristics. Light weight, superior noise attenuation and installation speed are among the main desired features of HVAC ducting.

In lightweight composite HVAC ducting, preserving lightweight and flexibility, while increasing acoustic resistance, is a difficult task. Sound can easily propagate through thin composite duct walls. As such, such systems tend to be noisy and disrupt the quality of life in a building while distracting the occupants. HVAC systems may use any one or more of pumps, compressors, chillers, air handlers, and generators which have moving or other mechanical components causing noise to emanate from the mechanical system itself as well as by way of the ducts. The ducts themselves generate additional noise due to air flow turbulence.

The most commonly known acoustic attenuation method for HVAC duct systems is a silencer / muffler. A silencer attenuates sound when it is directly inserted in the ducted path by using a series of perforated sheet metal baffles (rectangular silencers) or bullets (circular silencers) placed inside a silencer single or double wall outer solid shell. An absorptive silencer is the most commonly known type of silencer. It uses absorptive fibrous material within sound baffles or a sound bullet cavity with perforated sheet metal facings that allow sound energy to pass through and be absorbed by the fibrous fill. On the contrary, a reactive muffler uses the phenomenon of destructive interference and/or reflections to reduce noise. A reactive muffler generally consists of a series of expansion and resonating chambers that are designed to reduce sound at certain frequencies.

In either of the above types of mufflers, perforated tubing is used and quite beneficial when large flow velocities are seen inside the muffler. When an exhaust stream exits out of a tube within the muffler, a flow jet typically forms. In order to mitigate this effect, perforated tubing is used to steady the flow and force the flow to expand into the entire chamber. Perforated tubing can also be considered a dissipative element.

Perforated panels have also been used to attenuate sound in various noise control applications, such as ducts, exhaust systems and aircraft engines. One of the advantages of such acoustical materials is that their achieve. When the perforations are reduced to millimeter or sub-millimeter (micro-perforation) size, these materials can afford very interesting sound absorption without any additional classical absorbing material. <CIT> shows a metamaterial muffler forming an acoustic metamaterial noise control system. <CIT> shows a device for reducing sound from an air treatment unit, which includes an air treatment system, including a wall mount HVAC unit with a supply air opening and a return air opening.

What is needed is a way to improve upon present technology mufflers used in HVAC duct systems, in order to better effectuate noise flow reduction while causing as little disruption to the flow of air through the ducts as possible.

The invention is defined by the appended claim <NUM>. Further embodiments are defined in dependent claims <NUM>-<NUM>.

The disclosed technology reduces the aforementioned problems by providing a metamaterial block which is in line with an air duct of an HVAC system to reduce noise. A stack of at least three perforated sheets of acoustically hard material is placed between an ambient medium forming anisotropic air flow from or to an air duct and through each of the at least three perforated sheets. The ambient medium can be air. Each perforated sheet is less than, or equal to, <NUM> thick, in embodiments of the disclosed technology. A diameter of each perforation of each said perforated sheet is between <NUM> and <NUM>, in an embodiment of the disclosed technology. Each perforated sheet of the at least three perforated sheets is spaced apart from at least one other perforated sheet between <NUM> to <NUM>, in an embodiment of the disclosed technology. The spaced-apart distance of the at least three perforated sheets and the diameter of each perforation can be determined based on a Jacobian transformation defined by the formulae listed in the detailed description.

"Substantially" and "substantially shown," for purposes of this specification, are defined as "at least <NUM>%," or as otherwise indicated. Any device may "comprise" or "consist of" the devices mentioned there-in, as limited by the claims.

It should be understood that the use of "and/or" is defined inclusively such that the term "a and/or b" should be read to include the sets: "a and b," "a or b," "a," "b.

An acoustic metamaterial noise control system of embodiments of the disclosed technology combines absorptive materials with acoustic metamaterial principles, with a result of a significant reduction in sound radiation within, or emanating from, an HVAC duct. Sound waves that hit the noise control system placed at the end of the duct cause the sound waves to reflect back to the start of the noise control system and to be absorbed by sound waves within the absorptive core. This is accomplished by way of the use of micro-perforated panels (MPPs) for sound absorption. For purposes of this disclosure, an MPP is defined as a device used to absorb sound and reduce sound intensity comprised of, or consisting of, a thin flat plate less than, or equal to, <NUM> thick, with a hole diameter between <NUM> and <NUM>.

Perforations in the acoustic metamaterial provide acoustic metamaterial anisotropic (directionally dependent) characteristics of the core of the material. Using acoustic metamaterial principles, the noise control system can operate at lower frequencies and also over a broader frequency range than known in the prior art. Acoustic metamaterials are engineered material systems containing embedded periodic resonant or non-resonant elements which modify the acoustic properties of the material either by added dynamics or by wave scattering. Typical prior art ranges of frequencies are <NUM>, with a lowest range of <NUM>,<NUM>, similar to the frequency range for the present technology with a lowest range of <NUM>. However, present technology, based on conventional isotropic acoustics theory, has severe limitations in the lower frequency region (<<NUM>) which can only be solved by increasing thickness and or other parameters of the absorptive material, making it costly, heavy, and thus prohibitive.

The acoustic metamaterial noise control system can be positioned or placed at the beginning or end of the ducting to reduce the noise radiating out of the end of the HVAC ducting. Absorptive lining (defined as a sheet of material with a thickness between <NUM> and <NUM>) periodically placed inside the metamaterial noise control system around the interior spaces further enhances noise reduction over broadband frequency range.

The following principles are used in conjunction with embodiments of the disclosed technology. Transformation acoustics is a mathematical tool which completely specifies the material parameters needed to control the wave propagation through the material. It allows control over a two-dimensional acoustic space with anisotropic characteristics. A transformation from the real (r) space described by the (x, y, z) coordinates to the desired, virtual (v) space specified by the (u, v, w) coordinates is shown below. <MAT>
<MAT>
<MAT> as, <MAT>.

Here, ρ is fluid mass density and K is fluid bulk modulus, r and v superscripts denote the real and virtual spaces, and J is Jacobian transformation.

<FIG> shows a diagram of acoustic metamaterial with anisotropic inertia, used in embodiments of the disclosed technology. Using the transformation acoustics (TA) approach, the densities and bulk modulus in two dimensions on a structure can be engineered to be anisotropic. In <FIG>, <NUM> indicates a two-dimensional metamaterial block having anisotropic characteristics with two different densities, ρ<NUM>, ρ<NUM> along two directions <NUM> (x-axis) and <NUM> (y-axis). In conventional, isotropic acoustics, these densities are assumed to be the same in two directions. <NUM> and <NUM> show layered media, with <NUM> being one fluid medium (e.g., air) whereas the layer <NUM> is made of a different material, such as aluminum, or plastic usually having a greatly different acoustic impedance than <NUM>.

<FIG> shows a diagram of an acoustic metamaterial noise control system, with a rectangular muffler placed at the end of a duct to reduce noise, in embodiments of the disclosed technology. <FIG> shows a cross-section of the rectangular area of the muffler of <FIG>. A noise source <NUM>, such as a fan, motor, impeller, or other moving or rotating part of an HVAC system propagates sound waves <NUM> through a duct <NUM> into a metamaterial structure <NUM>. The metamaterial design comprises a stack of perforated sheets <NUM> made of an acoustically hard material, defined as a surface having almost infinite acoustic impedance (greater than <NUM> * <NUM>^<NUM>/ (m2s) ) compared to the characteristic impedance of the ambient medium, separated by a sound-supporting fluid (e.g., air). The elementary constituent parts of the stack of plates is a 2D rigid hole array, shielding sound near the onset of diffraction. Such a structure thus can be made practical by fabricating it out of micro-perforated panels (MPP) which allow anisotropic variables to be achieved.

<FIG> shows the diagram of <FIG> with a circular muffler placed at the end of a duct to reduce noise, in embodiments of the disclosed technology. <FIG> shows a cross-section of the circular area of the muffler of <FIG>. Here, elements of <FIG> have been incremented by <NUM>. Thus, the noise-producing region <NUM> causes sound waves <NUM> to flow through an HVAC duct <NUM> into the muffler <NUM>. The muffler <NUM> has a curricular cross-section, in this embodiment, with a series of perforated sheets <NUM>.

<FIG> shows an acoustic metameterial block formed by a periodic stack of micro-perforated panels, used in embodiments of the disclosed technology. It has been shown that these metamaterial blocks with perforated stacks exhibit broad-angle negative refraction, unlike fishnet electromagnetic metamaterials, which operate within narrow angular ranges. The proposed metamaterials also do not rely on diffraction to achieve negative refraction, in contrast to phonon crystals. Each perforated layer in this figure indicates a layer made of a hard material or surface, having much higher acoustic impedance (defined as "greater than <NUM> times") than the adjoining layer, which is usually the ambient medium, such as air. In this layer, <NUM> indicates a hole of a certain diameter and spacing from the next hole, whereas <NUM> denotes the hard material or unperforated part of the layer.

<FIG> shows an acoustic metamaterial muffler configuration formed by micro-perforated sheets. A face sheet <NUM> has a plurality of perforations, as do the plurality of perforated sheets <NUM> extending parallel and perpendicular to each other in a lattice formation between the face sheet <NUM> and a back sheet <NUM>.

Since the material parameters for the metamaterial panel are given by the first partial derivatives of the transformation functions, in order to obtain a homogeneous perforated MPP panel, the transformation functions are linear. One such choice suitable for the rectangular object considered here is: <MAT> <MAT> <MAT> It is to be noted that the expression of v may not be linear inside the whole transformation domain; however, it is linear inside each one of the x < <NUM> and x > <NUM> domains. This translates into same material parameters in each half of the metamaterial panel, but different directions of the principal axis, defined as the directions along which the material parameter tensors are diagonal. The constant wz represents a degree of freedom that allows for a tradeoff in performance for fabrication simplicity.

The material parameters inside the metamaterial MPP panel, i.e., mass density pseudotensor and bulk modulus, are given by. >>>(Equation. below)
<MAT>
where ρ<NUM> = <NUM>/ m<NUM> and B<NUM> = <NUM> MPa are the parameters of air, and J is the transformation Jacobian: <MAT>.

According to the coordinate transformation theory, the mapping functions given by the above translate to the following material parameters: <MAT>.

Here K<NUM>,K<NUM>,K<NUM> are constants. To obtain anisotropic metamaterial, perforated plastic plates are used. The size and shape of the perforation determines the momentum in the rigid plate produced by a wave propagating perpendicular on the plate, and, therefore, can be used to control the corresponding mass density component seen by this wave. This property is used to obtain the higher density component. If, on the other hand, the wave propagates parallel to the plate, it will have a very small influence on it, and, consequently, the wave will see a density close to that of the background fluid. The compressibility of the cell, quantified by the second effective parameter, the bulk modulus, is controlled by the fractional volume occupied by the plastic plate.

Expressed in another way, using perforated sheets with acoustically absorbent layers and air gaps in anisotripic metamaterial systems is manipulated by the size and shape of the perforations of the perforated sheets. The spacing between sheets is <NUM> to <NUM>, with a sheet thickness between. <NUM> and <NUM>. The percentage open areas for perforated sheets are between <NUM> and <NUM>% open. An absorptive layer whose thickness is between <NUM> and <NUM> can also be used. This determines the momentum of air particles in the sheets, produced by a wave-propagating perpendicular on the sheets as designed and optimized. The thickness and number of acoustically absorbent layers are also optimized, using metamaterial principles as follows: The perforated anisotropic metamaterial layers and absorptive layers of a particular thickness are arranged in a periodic manner, as shown in <FIG>, to achieve anisotropic properties of the fluid in the area directly next to the face sheet (see <FIG> and <FIG>). In this manner, the sound in air can be fully and effectively manipulated, using realizable transformation acoustics devices. All the geometric parameters of perforated layers and absorptive layers are determined, using numerical simulation based on equations above. This approach can be used to design a duct noise control system to control and manipulate sound waves for the purpose of enhancing noise attenuation, although the required material parameters are highly anisotropic.

Claim 1:
A heating, air-conditioning, and ventilation (HVAC) system comprising:
a heating, air-conditioning, and ventilation air duct; and
a metamaterial muffler forming an acoustic metamaterial noise control system for use in the heating, air-conditioning, and ventilation air duct, said metamaterial muffler comprising a stack of micro-perforated panels disposed at an end of said air duct, said stack of micro-perforated panels positioned in line with said air duct and including at least three perforated sheets (<NUM>, <NUM>) of acoustically hard material between an ambient medium, said stack of micro-perforated panels forming anisotropic air flow from or to said air duct (<NUM>, <NUM>) through each of said at least three perforated sheets (<NUM>, <NUM>), characterized in that each perforated sheet of said at least three perforated sheets (<NUM>, <NUM>) is less than, or equal to, <NUM> thick.