Process for audible acoustic frequency management in gas flow systems   

This application claims the priorities of U.S. Provisional Applications Nos. 61/033,177 and 61/033,198, both filed Mar. 3, 2008, the contents of which are hereby incorporated by reference.

This invention relates to processes for attenuating audible noise resulting from gas flow.

Gas flow systems such as air delivery systems in face masks, buildings, and transportation vehicles can be major, unacceptable sources of noise pollution in commercial, industrial, and residential settings. For example, heating, ventilation, and air conditioning (HVAC) systems in buildings and transportation vehicles comprise air-moving devices (such as fans) in combination with pervasive forced air networks or duct systems. The duct systems are used to distribute conditioned air (source air) throughout the building or vehicle and to ventilate or re-circulate return air. The laminar flow of this forced air, the fan noise, the impingement of the air flow against the duct walls (for example, at turns, bends, and corners), and the vibration modes that are set up on the walls of the ducts (typically sheet metal) are primary sources of audible noise in such systems.

To attenuate such noise, traditional sound proofing materials such as absorbers and reflectors (which attenuate through viscous dissipation and reflection, respectively) have been installed in the gas flow path. Such traditional materials are usually active over a broad range of frequencies without providing frequency selective sound control. Active noise cancellation equipment allows for frequency selective sound attenuation, but it is typically most effective in confined spaces and requires an investment in, and operation of, electronic equipment to provide power and control. Although Bragg scattering has been suggested to provide frequency selective sound control in low velocity flow ducts, its potential benefit has appeared to be limited to relatively low transmission losses.

Traditional sound barriers (for example, dense metal sheets or plates) tend to be relatively heavy and air-tight because the sound transmission loss from a material is generally a function of its mass and stiffness. The so-called “mass law” (applicable to many traditional acoustic barrier materials in certain frequency ranges) dictates that as the weight per unit area of a material is doubled, the transmission loss through the material increases by 6 decibels (dB). The weight per unit area can be increased by using denser materials or by increasing the thickness of the barrier. In at least some gas flow applications, added weight can be undesirable, however, and, more importantly, a noise attenuator for use in such applications generally should not significantly block gas flow or produce an excessive gas pressure drop.

Sound absorbers (for example, fibrous or foam materials) have therefore often been used in gas flow systems, as traditional sound-absorbing materials are generally relatively light in weight and relatively porous. Porous sound absorbers can be less attractive for use in certain environments (for example, HVAC ducts), however, because of the resulting potential for moisture entrapment and bacterial growth in the pores of the porous absorber. This can effectively rule out the use of porous absorbers and barriers as liners within such ducts.

In the U.S., vents and ducts are typically externally insulated, but the primary purpose of this is for thermal insulation. A portion of the duct length is often internally lined with sound absorber material for acoustic control, however, and, where appropriate, mechanical sound attenuators or silencers (for example, boxes comprising vanes or baffles and/or dampers) are placed in the duct work. The mechanical attenuators and silencers can be expensive, can cause significant pressure drop, and can increase energy consumption. Low frequency sound (for example, frequencies below about 1000 hertz (Hz)) can be particularly troublesome, as the absorbers and the mechanical attenuators or silencers (and even second lines of defense including building acoustic barriers such as ceiling tiles) are often inadequate in this range.

SUMMARY

Thus, we recognize that there is a need for processes for managing or controlling noise in gas flow systems that can be at least partially effective in attenuating audible acoustic frequencies (reducing or, preferably, eliminating sound transmission) while preferably utilizing sound barriers that are relatively non-porous (so as to reduce or minimize the likelihood of microbial growth) and/or that do not significantly block gas flow or produce a significant pressure drop. Preferably, the processes can be at least partially effective over a relatively broad range of audible frequencies (preferably, including low frequencies such as those below about 1000 Hz) and/or can be relatively simply and cost-effectively carried out.

Briefly, in one aspect, this invention provides a sound insulation process. The process comprises (a) providing at least one sound barrier comprising a substantially periodic array of structures disposed in a first medium having a first density, the array comprising at least one row of at least two of the structures, the structures being made of a second medium having a second density that is greater than the first density, the second medium being a viscoelastic medium, an elastic medium, or a combination thereof, and the first medium being a gaseous medium (preferably, air); and (b) placing the at least one sound barrier in at least one at least partially enclosed gas stream in a manner such that the row of structures extends in a direction that is perpendicular to the direction of flow of the gas stream (preferably, a gas stream in a gas flow duct). Preferably, the substantially periodic array of structures is a two-dimensional or three-dimensional array (more preferably, a two-dimensional array).

The second medium is preferably an elastic medium; a viscoelastic medium having a speed of propagation of longitudinal sound wave and a speed of propagation of transverse sound wave, the speed of propagation of longitudinal sound wave being at least about 30 times the speed of propagation of transverse sound wave; or a combination thereof. Elastic media can be preferred, for example, for applications requiring durability. Viscoelastic media can be preferred, for example, for applications requiring lower weight and/or cost. Surprisingly, viscoelastic media have been found to be unexpectedly effective in addressing the often troublesome low frequency acoustical range.

It has been discovered that, by forming a sound barrier comprising the above-described spatially periodic array and placing it in a gas flow system, effective noise attenuation in the form of band gaps or at least significant audible acoustic transmission losses (for example, greater than 20 decibels (dB)) can be obtained in at least portions of the audible range (that is, the range of 20 hertz (Hz) to 20 kilohertz (kHz)). In the acoustic industry, attenuation on the order of 20+dB is a very significant loss in transmission, approaching 100 percent reduction in acoustic power.

The sound barriers used in the process of the invention provide noise reduction through Bragg scattering (the substantially periodic array being a phononic crystal structure) and can be, in at least some embodiments, relatively light in weight and relatively small (for example, having external dimensions on the order of a few centimeters or less). By controlling such design parameters as the selection of materials, the number of structures, the shapes of the structures, the type of lattice or arrangement of the structures, the spacing of the structures, and so forth, the number and frequencies of the band gap(s) and their widths can be tuned, or, at a minimum, the transmission loss levels can be adjusted as a function of frequency.

The phononic crystal-based sound barriers can be placed in a gas flow system (for example, so as to extend across at least a portion of the transverse cross-sectional area of the gas stream or flow) to allow only select frequencies to pass through the barrier. The barriers can comprise substantially non-porous materials and can therefore be useful in gas flow systems in which microbial growth is a concern. In addition, the substantially periodic array of structures of the barriers can be effective at sufficiently low fill fractions (relative to the transverse cross-sectional area of the gas stream or flow) that gas flow is not significantly blocked or a significant pressure drop induced.

The barriers can generate acoustic band gaps in a passive, yet frequency selective way. Unlike the most common sound absorbers used in the acoustics industry, phononic crystal-based barriers control sound in transmission mode. Within the range of frequencies of the band gap, there can be essentially no transmission of an incident sound wave through the structure. The band gap is not always absolute (that is, no sound transmission), but, as mentioned above, the sound transmission loss can often be on the order of 20 decibels (dB) or more.

Phononic crystal-based sound barriers can be placed between a sound source and a receiver to allow only select frequencies to pass through the barrier. The receiver thus hears filtered sound, with undesirable frequencies being blocked. By properly configuring the barrier, the transmitted frequencies can be focused at the receiver, or the undesirable frequencies can be reflected back to the sound source (much like a frequency selective mirror). Unlike current acoustic materials, the phononic crystal-based sound barriers can be used to actually manage sound waves, rather than simply to attenuate or reflect them.

Thus, in at least some embodiments, the process of the invention can meet the above-cited need for processes for managing or controlling noise in gas flow systems that can be at least partially effective in attenuating audible acoustic frequencies while preferably utilizing sound barriers that are relatively non-porous (so as to minimize the likelihood of microbial growth) and/or that do not significantly block gas flow or produce a significant pressure drop. The process of the invention can be used to provide sound insulation in a variety of different gas flow systems including HVAC systems in buildings (for example, homes, offices, hospitals, and so forth), HVAC systems in transportation vehicles (for example, automobiles, boats, and airplanes), face masks for gas (for example, air) delivery, fan-containing consumer appliances, and the like, and combinations thereof.

These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawing, wherein:

FIG. 1 is a perspective view of an individual structure or array component that can be used in preparing a sound barrier for use in an embodiment of the process of the invention.

FIG. 2 is a perspective view of a sound barrier, which comprises a substantially periodic array of the structure(s) of FIG. 1, placed in a gas flow duct in carrying out an embodiment of the process of the invention.

These figures, which are idealized, are not drawn to scale and are intended to be merely illustrative and nonlimiting.

FIG. 3 is a plot of sound pressure level (in dBA) versus frequency (in Hz) for the embodiments of the process of the invention described in Examples 1 and 2 and for the process described in Comparative Example 1.

DETAILED DESCRIPTION

Materials that are suitable for use as the above-referenced viscoelastic component or medium of the sound barrier used in the process of the invention include viscoelastic solids and liquids. Useful viscoelastic solids and liquids include those having a steady shear plateau modulus (GoN) of less than or equal to about 5×106 Pascals (Pa) at ambient temperatures (for example, about 20° C.), the steady shear plateau modulus preferably extending from about 30 Kelvin degrees to about 100 Kelvin degrees above the glass transition temperature (Tg) of the material. Preferably, the viscoelastic material has a steady shear plateau modulus of less than or equal to about 1×106 Pa (more preferably, less than or equal to about 1×105 Pa) at ambient temperatures (for example, about 20° C.). Preferred viscoelastic materials have (preferably, at least in the audible range of acoustic frequencies) a speed of propagation of longitudinal sound wave that is at least about 30 times (preferably, at least about 50 times; more preferably, at least about 75 times; most preferably, at least about 100 times) its speed of propagation of transverse sound wave.

Examples of useful viscoelastic materials include rubbery polymer compositions (for example, comprising lightly-crosslinked or semi-crystalline polymers) in various forms including elastomers (including, for example, thermoplastic elastomers and elastomer foams), elastoviscous liquids, and the like, and combinations thereof (preferably, for at least some applications, elastomers and combinations thereof). Useful elastomers include both homopolymers and copolymers (including block, graft, and random copolymers), both inorganic and organic polymers and combinations thereof, and polymers that are linear or branched, and/or that are in the form of interpenetrating or semi-interpenetrating networks or other complex forms (for example, star polymers). Useful elastoviscous liquids include polymer melts, solutions, and gels (including hydrogels and ionic polymer gels).

Preferred viscoelastic solids include silicone rubbers (preferably, having a durometer hardness of about 20 A to about 70 A; more preferably, about 30 A to about 50 A), epichlorohydrin rubbers (preferably, epichlorohydrin closed cell foams), (meth)acrylate (acrylate and/or methacrylate) polymers (preferably, copolymers of isooctylacrylate (IOA) and acrylic acid (AA)), block copolymers (preferably, comprising styrene, ethylene, and butylene), cellulosic polymers (preferably, cork), blends of organic polymer (preferably, a polyurethane) and polydiorganosiloxane polyamide block copolymer (preferably, a silicone polyoxamide block copolymer), neoprene, and combinations thereof. Preferred viscoelastic liquids include mineral oil-modified block copolymers, hydrogels, ionic polymer gels, and combinations thereof.

Such viscoelastic solids and liquids can be prepared by known methods. Many are commercially available.

Materials that are suitable for use as the above-referenced elastic component or medium of the sound barrier used in the process of the invention include essentially all elastic materials. Preferred elastic materials, however, include those having a longitudinal speed of sound that is at least about 2000 meters per second (m/s).

Useful classes of elastic solids include metals (and alloys thereof), inorganic minerals (for example, perlite and aerogels), glassy polymers (for example, cured epoxy resin), and the like, and combinations thereof (including, for example, metal-polymer composites such as a composite of metal powder or metal shavings in a polymeric binder matrix). Preferred classes of elastic solids include metals, metal alloys, glassy polymers, and combinations thereof (more preferably, copper, aluminum, epoxy resin, copper alloys, aluminum alloys, and combinations thereof; even more preferably, copper, aluminum, copper alloys, aluminum alloys, and combinations thereof yet more preferably, copper, copper alloys, and combinations thereof most preferably, copper).

Such elastic materials can be prepared or obtained by known methods. Many are commercially available.

If desired, the sound barrier used in the process of the invention can optionally comprise other component materials. For example, the sound barrier can include more than one viscoelastic material and/or more than one of the above-described elastic materials. Conventional additive materials can be included (for example, antioxidants can be present to enhance polymer stability at relatively high temperatures), provided that the desired acoustical characteristics of the sound barrier are not unacceptably impacted.

The sound barrier used in the sound insulation process of the invention comprises a substantially periodic (sufficiently periodic that a bandgap can form) array of structures disposed in a first medium having a first density, the array comprising at least one row of at least two of the structures, the structures being made of a second medium having a second density that is greater than the first density, the second medium being a viscoelastic medium, an elastic medium, or a combination thereof, and the first medium being a gaseous medium (preferably, air). For at least some applications, the ratio of the second density to the first density can preferably be at least about 1000. Preferably, the substantially periodic array of structures is a two-dimensional or three-dimensional array (more preferably, a two-dimensional array).

The second medium is preferably an elastic medium; a viscoelastic medium having a speed of propagation of longitudinal sound wave and a speed of propagation of transverse sound wave, the speed of propagation of longitudinal sound wave being at least about 30 times the speed of propagation of transverse sound wave; or a combination thereof. Elastic media can be preferred, for example, for applications requiring durability. Viscoelastic media can be preferred, for example, for applications requiring lower weight and/or cost. As mentioned above (and explained in more detail below, in the section entitled “Use of Sound Barrier”), viscoelastic media have been found to be unexpectedly effective in addressing the often troublesome low frequency acoustical range.

The exposed surfaces of the structures of the array are preferably substantially smooth and/or non-porous (for example, having surface feature and/or pore diameters less than or equal to about one micron in size), so as to reduce or minimize the likelihood of microbial growth. The exposed surfaces preferably comprise material having a density greater than or equal to about 1 gram per cubic centimeter. Thus, the exposed surfaces preferably comprise, consist, or consist essentially of material(s) other than a traditional foam or fibrous absorber material (or, if such a porous material is used, at least the exposed surface of the material is treated in a manner such as sealing or glazing or the like, so as to reduce surface roughness and/or porosity).

Certain material selections can also enhance the antimicrobial characteristics of the structures. For example, copper can be selected for its inherent antimicrobial properties.

In forming the substantially periodic array, the material(s) for the structures (second medium) can be selected from the above-described viscoelastic and elastic materials in accordance with the above-described density characteristics. The structures can be solid or can be at least partially hollow (preferably, at least partially hollow). The structures can further comprise other materials, provided that the specified density, elasticity, and/or viscoelasticity characteristics are maintained. For example, the structures can comprise one or more inviscid fluids (for example, in hollow structures, an inviscid fluid can be present in the form of an air core).

The structures within the array are preferably substantially identical (for example, as to shape, size, composition, and so forth). The number (and size) of the structures in the array can vary widely from array to array, however, depending, for example, upon factors such as the transverse cross-sectional area of the gas flow in a particular gas flow system and/or the desirability of filtering out certain acoustic frequencies.

The shapes or configurations of the structures can also vary widely from array to array and include geometrical solids (for example, spheres, rectangular solids, cylindrical solids, triangular solids, other closed polygonal solids, and so forth) and at least partially hollow versions thereof, annular structures (for example, an inner pipe or rod within an outer pipe), and the like (preferably, spheres, circular cylinders, at least partially hollow versions thereof, and combinations thereof; more preferably, circular cylinders, at least partially hollow circular cylinders, and combinations thereof; most preferably, at least partially hollow circular cylinders). If desired, aerodynamic shapes (for example, airfoils and the like), which can assist in minimizing gas pressure drops in the gas flow system, can be utilized.

The dimensions of the structures (for example, heights and transverse cross-sectional areas and, for hollow structures, thicknesses) can vary widely from array to array (for example, ranging from on the order of millimeters to as large as a meter or more), depending upon the spatial and/or acoustical requirements of a particular gas flow application. If desired, the structures can be multi-layer structures (for example, in the form of concentric annular structures).

The structures can be individually or collectively attached to a gas flow system by any known or hereafter-developed method or manner of attachment or placement. Preferably, however, the sound barrier further comprises an intervening attachment, placement, and/or containment component (for example, a frame comprising a damper, a slider, a spacer, or the like, or a combination thereof) that can effectively decouple the barrier from the vibrations of the gas flow system (for example, the vibrations of a gas flow duct into which the barrier is inserted).

The resulting sound barrier can be a macroscopic construction (for example, having a size scale on the order of centimeters or less). If desired, the barrier can take the form of a spatially periodic lattice with uniformly-sized and uniformly-shaped structures at its lattice sites, surrounded by a gas matrix (for example, air). Design parameters for such barriers include the type of lattice (for example, square, triangular, and so forth), the spacing between the lattice sites (the lattice constant), the make-up and shape of the unit cell (for example, the fractional area of the unit cell that is occupied by the structures—also known as f, the so-called “fill factor”), the physical properties of the materials utilized (for example, density, Poisson ratio, moduli, and so forth), the shape of the structures (for example, rod, sphere, hollow rod, square pillar, and so forth), and the like. By controlling such design parameters, the frequency of the resulting band gap, the number of gaps, and their widths can be tuned, or, at a minimum, the level of transmission loss can be adjusted as a function of frequency.

Preferred arrays include those having a fill factor in the range of about 0.1 to about 0.65 or higher (more preferably, about 0.2 to about 0.6; most preferably, about 0.3 to about 0.55). Preferred types of lattices include those that are relatively “open” (for example, rather than those having staggered rows of structures), so as to minimize any gas pressure drop due to the presence of the sound barrier. Thus, preferred lattices include those other than triangular (more preferably, square lattices, rectangular lattices, and combinations thereof).

Preferably, the sound barrier is a two- or three-dimensional array (more preferably, a two-dimensional array) in the form of cylindrical structures (solid or at least partially hollow) in a square lattice pattern surrounded by a gas matrix. The cylindrical structures are more preferably circular cylindrical structures (solid or at least partially hollow), most preferably hollow circular cylindrical structures comprising at least one metal, at least one viscoelastic material, or a combination thereof.

The total number of component structures of arrays of such sound barriers can range, for example, from as few as two to as high as hundreds or more. Dimensions of the structures can also vary widely (depending upon, for example, the gas stream size and/or the desired acoustic frequencies to be filtered out) but are preferably on the order of centimeters or less. Such dimensions and numbers of structures can provide sound barriers having dimensions on the order of centimeters or less. If desired, the various layers of multi-layer structures can be cleaned (for example, using surfactant compositions or isopropanol) prior to addition of one or more other layers, and/or one or more bonding agents (for example, adhesives or mechanical fasteners) can optionally be utilized (provided that there is no significant interference with the desired acoustics).

A preferred sound barrier comprises from 1 to about 4 rows (more preferably, from 1 to 3 rows; most preferably, 1 or 2 rows) of two or more structures (preferably, hollow structures), which can span at least a portion of the transverse cross-sectional area of the gas stream. (Generally, the smallest numbers of rows and/or columns that can provide the desired acoustical effect for a particular application can be preferred, so as to minimize any resulting gas pressure drop due to the presence of the sound barrier.) The structures can comprise viscoelastic material (preferably, silicone rubber, acrylate polymer, or a combination thereof) and/or elastic material (preferably, copper, copper alloy, aluminum, aluminum alloy, or a combination thereof; more preferably, copper, copper alloy, or a combination thereof).

The sound barrier used in the process of the invention can optionally further comprise one or more conventional or hereafter-developed sound insulators (for example, conventional absorbers, conventional barriers, local resonance structures, and the like, and combinations thereof; preferably, local resonance structures) and/or can further comprise one or more components that address other aspects of gas flow (for example, filtration, thermal management, and so forth). If desired, such conventional sound insulators can be layered, for example, to broaden the frequency effectiveness range of the sound barrier.

The above-described sound barriers can be used in carrying out the sound insulation process of the invention by placing one or more of the sound barriers in an at least partially enclosed gas stream in a gas flow system (preferably, in a gas flow duct). Such gas flow systems include HVAC duct systems in buildings and transportation vehicles, exhaust lines, and the like. Preferably, the sound barrier can be placed in the gas stream in a manner such that the barrier spans at least a portion of the transverse cross-sectional area of the gas stream.

In general terms, such usage can include interposing or placing the sound barrier between an acoustic source (preferably, a source of audible acoustic frequencies) and an acoustic receiver (preferably, a receiver of audible acoustic frequencies). Common acoustic sources in gas flow systems include noises due to gas flow, fan noises, and the like (preferably, noises or other sounds having an audible component; more preferably, noises or other sounds having a frequency component in the range of about 250 Hz to about 10,000 Hz). The acoustic receiver can be, for example, a human ear, any of various recording devices, and the like (preferably, the human ear).

The above-described sound barriers can be particularly effective in addressing a relatively broad range of audible frequencies. If desired, however, the barriers can be used in combination with one or more local resonance structures to provide broader filtering action (particularly at low frequencies such as, for example, below about 1000 Hz).

The process of the invention can be carried out by placing one or more of the sound barriers in essentially any appropriate locations in the gas stream of the gas flow system. For example, referring to FIGS. 1 and 2, in an embodiment of the process of the invention sound barrier 20 (shown in FIG. 2) comprising a substantially periodic array of hollow structures 10 (shown in FIG. 1) comprising an air core 12 and an elastic (for example, metal) or viscoelastic shell 14 is placed in gas stream 30 flowing through gas duct 40 (shown in FIG. 2). The sound barrier can preferably be placed relatively close to the acoustic source of the gas flow system (for example, relatively close to a furnace of an HVAC system), so as to reduce the need for multiple sound barriers in the gas flow system.

The process of the invention can be used to achieve transmission loss across a relatively large portion of the audible range (with preferred embodiments providing a transmission loss that is greater than or equal to about 20 dB across the range of about 800 Hz to about 10,000 Hz; with more preferred embodiments providing a transmission loss that is greater than or equal to about 20 dB across the range of about 500 Hz to about 10,000 Hz; with even more preferred embodiments providing a transmission loss that is greater than or equal to about 20 dB across the range of about 250 Hz to about 10,000 Hz; and with most preferred embodiments providing substantially total transmission loss across at least a portion of the range of about 500 Hz to about 10,000 Hz). Such transmission losses can be achieved without the use of porous absorber materials and/or without significant gas pressure drops (for example, less than about 25 percent; preferably, less than about 10 percent).

Surprisingly, viscoelastic media have been found to be unexpectedly effective in addressing the often troublesome low frequency acoustical range. It has been discovered that, when a viscoelastic medium is used as the second medium in forming a substantially periodic array of hollow structures, the resulting sound barrier can exhibit not only a phononic bandgap but also surprisingly can exhibit a narrower local resonance bandgap at low frequency (below 1500 Hz; for example, about two orders of magnitude below the Bragg frequency of the phononic bandgap). Such sound barriers can thus be useful in targeting discrete annoying low frequencies in addition to providing sound insulation at the higher frequencies of the phononic bandgap.

Objects and advantages of this invention are further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention. All parts, percentages, ratios, and the like in the examples are by weight, unless noted otherwise. Solvents and other reagents were obtained from Sigma-Aldrich Chemical Company, St. Louis, Mo. unless otherwise noted.

An experimental HVAC duct system was constructed. The system comprised a 0.91 m (3 feet) long sheet metal (aluminum) duct of rectangular cross section with dimensions of 0.09 m by 0.26 m, externally wrapped in 3M™ Thinsulate™ Acoustic Insulation and placed inside a 0.019 m (¾ inch) thick, open-ended plywood box. A diffuser was attached to one end of the box to aid in evenly distributing air into the duct.

Two sound barriers (one termed a “small tube assembly” and the other a “large tube assembly”) were constructed of hollow copper pipes (circular cylinders available from McMaster-Carr Inc., Elmhurst, Ill.). The “small tube assembly” was constructed out of hollow copper pipes that were 0.028575 m (1⅛ inch) outside diameter (O.D.) by 0.026949 m (1.061 inch) inside diameter (I.D.), Mc-Master-Carr, Part number 8967K111, 0.0008128 m (0.032 inch) wall thickness, 0.914 m (3 ft) length (as ordered from McMaster-Carr) copper tubes (straight). The “large tube assembly” was constructed out of hollow copper pipes that were 0.050800 m (2 inch) O.D. by 0.049174 m (1.936 inch) I.D., Mc-Master-Carr, Part number 8967K77, 0.0008128 m (0.032 inch) wall thickness, 0.914 m (3 ft) length (as ordered from McMaster-Carr) copper tubes (straight).

In both assemblies, the copper tubes were arranged in two-dimensional substantially periodic arrays held together with edge bolts by two 0.003175 m (0.125 inch) thick aluminum plates at the top and bottom of the pipes. The plates had circular grooves of 0.002362 m (0.093 inch) thickness cut into them to hold the individual pipes in a desired lattice structure. The barriers had a width slightly less than 0.26 m and a height slightly less than 0.09 m so as to fit relatively snugly inside the duct.

The small tube assembly (Example 1) had three rows of pipes (each row consisting of 7 whole pipes along the center and 2 partial pipes at the two edges to completely span the duct width) arranged in a square lattice pattern having a lattice parameter of 0.029997 m (1.181 inches). The large tube assembly (Example 2) had three rows of pipes (each row consisting of 2 whole pipes along the center and 2 partial pipes at the two edges to completely span the duct width) arranged in a square lattice pattern having a lattice parameter of 0.0599948 m (2.362 inches).

To test each sound barrier, the sound barrier was placed 0.3048 m (12 inches) from the non-diffuser end of the duct. 3M™ Thinsulate™ Acoustic Insulation was wrapped around the sound barrier to ensure that essentially all of the air was traveling through the sound barrier, rather than leaking around it. A Larson Davis™ System 824 Sound Level Meter (SLM) connected to a Larson Davis™ Model 2900B Real Time Analyzer (RTA) (both available from Larson Davis, Inc., a PCB Electronics Div., 3425 Walden Avenue, Depew, N.Y. 14043-2495, USA) was inserted 0.1143 m (4.5 inches) from the end of the duct on the same end (the non-diffuser end) as the sound barrier. 3M™ Thinsulate™ Acoustic Insulation was draped over the box and the SLM to reduce ambient noise as much as possible.

A compressed air hose was used to introduce a stream of air into the duct. The hose was inserted 0.0508 m (2 inches) into the diffuser at the bottom of the entrance to the duct. The compressed air was turned on and kept on continuously while all sound level measurements were taken. Each measurement consisted of a 10 second recording on the SLM. This recording procedure was repeated for a total of 5 measurements each for the small tube assembly (Example 1), the large tube assembly (Example 2), and a control (the duct with no sound barrier; Comparative Example 1). The resulting sound pressure level measurements (in dbA; that is, the A-weighted decibel scale) as a function of frequency (in Hz) were recorded, averaged, and plotted, and the resulting graphical data is shown in FIG. 3.

The referenced descriptions contained in the patents, patent documents, and publications cited herein are incorporated by reference in their entirety as if each were individually incorporated. Various unforeseeable modifications and alterations to this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention. It should be understood that this invention is not intended to be unduly limited by the illustrative embodiments and examples set forth herein and that such examples and embodiments are presented by way of example only, with the scope of the invention intended to be limited only by the claims set forth herein as follows: