Abstract:
Apparatus and methods described herein provide for the management of noise associated with a duct. A sound-dampening apparatus is provided consisting of a duct through which a fluid flows, such as an air duct. A flexuous cord is helically wound around the inner or outer surface of the duct at a pitch corresponding to a selected acoustical frequency range associated with the fluid flow through the duct.

Description:
BACKGROUND 
     Noise in the interior of an aircraft may be generated from a number of sources, including the aircraft engines and other equipment; airframe or fuselage vibrations; airflow over the exterior skin of the fuselage; and fans and airflow in the environmental control systems (ECS). A high level of noise in the interior cabin spaces of the aircraft can have an adverse impact on cabin occupants. Noise may affect passenger comfort and limit communication among the crew and passengers. In addition, prolonged exposure to high noise levels may impair hearing. 
     The ECS in an aircraft may include ram air, cabin pressurization, conditioned air supply, avionics cooling, auxiliary power unit, recirculation, cockpit cooling, and cargo heating systems. Airlines and airplane manufacturers traditionally control noise in the ECS by integrating mufflers and silencers into the systems and by wrapping air ducts in noise-insulating batting. These methods, however, add weight, increase operating costs, reduce performance and range, and increase complexity of the overall aircraft systems. 
     It is with respect to these considerations and others that the disclosure made herein is presented. 
     SUMMARY 
     It should be appreciated that this Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to be used to limit the scope of the claimed subject matter. 
     Apparatus and methods described herein provide for the reduction of noise. According to aspects presented herein, a sound-dampening apparatus is provided consisting of a duct through or around which a fluid flows. In one aspect, the fluid may be air and the duct may be a flexible air duct. A flexuous cord is helically wound around the inner or outer surface of the duct in a continuous length, maintaining a specific pitch to dampen sound in a specific frequency range. According to aspects, the flexuous cord may be made of extruded plastic, metal wire, or synthetic fiber. In one aspect, the flexuous cord has a round cross-section. In another aspect, the flexuous cord has a triangular or other cross-section that has a flat side that abuts the surface of the component. In a further aspect, the flexuous cord is hollow. 
     According to further aspects presented herein, a method for dampening sound associated with an air duct is provided. The method entails determining a range of acoustic frequencies that are desired to be dampened, and then helically wrapping the air duct with a flexuous cord at a specific pitch to dampen sound in the target range of acoustic frequencies. In a further aspect, the duct may consist of several segments, each wrapped by the flexuous cord at different pitch angles to dampen sound in subsets of the target range of acoustic frequencies. 
     The features, functions, and advantages that have been discussed can be achieved independently in various embodiments of the present invention or may be combined in yet other embodiments, further details of which can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a noise-reducing duct used in an aircraft environmental control system, according to embodiments presented herein; 
         FIG. 2  is a series of cross-sections illustrating some examples of profiles of a flexuous cord provided in the embodiments described herein; 
         FIG. 3  is a line graph depicting the relative sound dampening performance of various duct configurations; 
         FIGS. 4A and 4B  are perspective views of noise-reducing ducts illustrating alternative configurations of the flexuous cord, according to further embodiments described herein; 
         FIG. 5  is a line graph depicting the relative sound dampening performance of ducts incorporating various flexuous cord materials; 
         FIG. 6  is a line graph depicting the level of noise produced by air flowing through various duct configurations; 
         FIG. 7  is a perspective view illustrating one method for fabricating a noise-reducing duct according to embodiments described herein; and 
         FIG. 8  is a flow diagram showing one method of reducing noise associated with airflow through a duct, according to embodiments described herein. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is directed to apparatus and methods for reducing noise emanating from the interior or exterior surface of an air duct. As discussed briefly above, the ECS of an aircraft often includes integrated mufflers and silencers as well as air ducts wrapped in insulation batting in order to reduce noise in the aircraft cabin. These elements add weight and complexity to the aircraft, making the aircraft more expensive to manufacture and operate. 
     Utilizing the concepts and technologies described herein, a helically wrapped air duct can be implemented in an aircraft ECS that will reduce noise in the aircraft cabin, eliminating or reducing the need for mufflers or insulation, thus saving weight and cost in the aircraft systems. The noise-reducing air duct may be fabricated from less expensive materials and provide better noise attenuation performance than ducts created from alternative materials. In addition, the frequencies of sound dampened by the noise-reducing air duct can be tuned by changing the pitch of the wrapping. Because the noise-reducing air duct can be fabricated as flexible hoses and made in longer lengths than traditional ducting, it can be installed in the aircraft more easily and inexpensively than rigid ducts made from alternative duct materials. 
     Throughout this disclosure, embodiments are described with respect to air ducts used in the ECS of an aircraft. An aircraft environment provides a useful example for embodiments described herein since there are many sources of unwanted noise and reduction of noise is a relative high priority. However, it should be understood that the concepts presented herein are equally applicable to ECS in other environments, including, but not limited to, commercial and residential buildings, ships, cars, spacecraft, and other vehicles. 
     In the following detailed description, references are made to the accompanying drawings that form a part hereof, and that show by way of illustration specific embodiments or examples. Referring now to the drawings, in which like numerals represent like elements through the several figures, a noise-reducing air duct will be described.  FIG. 1  shows elements of an aircraft ECS  100 , including a noise-reducing air duct  102 , according to embodiments provided herein. Also included in the ECS  100  is a fan  104  that forces conditioned air through the duct  102  to an outlet vent  106  or other air terminal. 
     Unwanted noise may be produced by the components of the depicted ECS  100 , including fan noise and flow-generated noise produced by the flow of air through the duct  102  and the outlet vent  106 . In addition, noise from external sources may be introduced into the ECS, such as noise from the operation of aircraft engines or other equipment, or vibrations and boundary layer noise created by air flowing over the exterior of the fuselage. The noise may be perceived by a passenger  108  in the aircraft cabin at the outlet vent  106  or at a point where the noise is emanated by the duct  102 , known as breakout noise. ECS airflow may further emanate the noise throughout the cabin. 
     According to embodiments, the noise-reducing duct  102  may be a cylindrical and flexible hose, which may be constructed from one or more layers or plies of thin plastic or synthetic fiber materials, including, but not limited to, polyvinylidene fluoride (PVDF) foam, silicone glass-fiber, thermoset composites, or other plastic sheets or resins. It will be appreciated that the noise-reducing ducts may also be manufactured from rigid materials, such as composites, thermoplastics, or metal, according to further embodiments. The noise-reducing duct may also be manufactured in shapes other than cylindrical, such as rectangular ducts. 
     A flexuous cord  110  is helically wound around the internal or external surface of the duct  102  in a continuous length maintaining a specific pitch  112  or distance between subsequent windings. This helical wrapping of flexuous cord  110  creates the desired noise attenuation effect and may also provide structural reinforcement to the duct  102 . The characteristics of the noise attenuating effect depend upon the selected pitch  112  of the helical winding of the flexuous cord  110  as well as the material from which the cord  110  is constructed, as will be discussed in detail below. 
     According to one embodiment, the flexuous cord  110  is constructed from an extruded polyetherimide (PEI), such as ULTEM® from SABIC Innovative Plastics of Pittsfield, Mass. In another embodiment, the flexuous cord  110  is made of KEVLAR® from E. I. du Pont de Nemours and Company of Wilmington, Del. It will be appreciated by one skilled in the art that the flexuous cord  110  may be constructed from a number of materials, including, but not limited to, thermoplastics, metallic wire, carbon fiber, and plastic or elastomer/rubber covered metallic wire. 
     In one embodiment, the flexuous cord  110  is bonded to the outer surface of the duct  102  using an adhesive. In other embodiments, the flexuous cord  110  may be incorporated into the duct  102  during fabrication. For example, the flexuous cord  110  may be placed between the plies of a multi-layer or composite duct before the layers of the composite duct are cured. This provides for a single-cycle fabrication process of the noise-reducing duct  102 . It will be appreciated that any number of methods known in the art may be used to bond the flexuous cord  110  to the surface of the duct  102 . In a further embodiment, the flexuous cord  110  is held in contact with the surface of the duct  102  by structural pressure and no bonding between the flexuous cord  110  and the duct surface is required. For example, the flexuous cord  110  may be wound in a helical pattern on a form and the duct material stretched over the outside of the helix, such that the helically wound flexuous cord  110  maintains contact with the inside surface of the duct  102 . 
     The flexuous cord  110  may have a number of profiles  202 A- 202 P, as shown in  FIG. 2 . The profile of the flexuous cord  110  may affect the noise attenuating effect of the duct  102  as well as the ease with which the cord  110  is bonded to the surface of the duct  102 . In one embodiment, the flexuous cord  110  is circular with a flat bottom, as depicted in profiles  202 A- 202 D. The flat bottom provides for ease of attachment of the flexuous cord  110  to the duct material during fabrication of the duct  102 . In other embodiments, the flexuous cord  110  may have a triangular profile  202 E- 202 H, a round profile  202 I- 202 L, or may be a flat, thin strip  202 M- 202 P. Further, the flexuous cord  110  may be solid, as depicted in profiles  202 A,  202 E,  202 I, and  202 M, or it may be hollow with any number of chambers, as shown in profiles  202 B- 202 D,  202 F- 202 H,  202 J- 202 L, and  202 N- 202 P. A hollow flexuous cord  110  provides the advantages of reduced weight, lower cost, and better strength/stiffness, as well as better noise attenuating performance. It will be appreciated that the flexuous cord  110  may also have a profile not depicted in  FIG. 2 . 
     As described briefly above, the sound dampening characteristics of the noise-reducing duct  102  depend upon the selected pitch  112  of the helically wound flexuous cord  110 .  FIG. 3  illustrates the relative noise attenuating performance of a duct  102  with a flexuous cord  110  wound at a relatively wide pitch  112  versus a duct  102  with a flexuous cord  110  wound at a relatively narrow pitch  112 . The line graph  300  depicts the transmission loss or reduction in sound level, measured in decibels (dB), attained by different air ducts at different acoustic frequencies, expressed in Hertz (Hz). 
     The transmission loss was measured in flexible ducts  102  constructed from sheets of PVDF foam and fiberglass fabric, helically wound at various pitches  112  by a hollow PEI flexuous cord  110  with a round profile. For comparison, hard-walled ducts made of KEVLAR® and extruded PEI not helically wound by a flexuous cord were also tested. As shown in the line graph  300 , a duct helically wound at a relatively wide pitch  112  provides greater reduction of noise in the lower frequency range of 400 Hz to 1600 Hz, as depicted by line  302 . A duct helically wound at a relatively narrow pitch  112 , on the other hand, provides greater reduction of noise in the upper frequency range of 2000 Hz to 10000 Hz, as depicted by line  304 . The line graph  300  in  FIG. 3  also shows the reduction in noise provided by the hard-walled ducts made of KEVLAR®, depicted by line  306 , and extruded PEI, depicted by line  308 . 
     The value of the pitch  112  utilized in the fabrication of the noise-reducing duct  102  is determined by ascertaining the acoustic frequency range of noise that is to be reduced. Duct noise attenuation in the target acoustic frequency range can be obtained by applying a specific pitch  112  for the helical winding of the flexuous cord  110  onto ducts  102  of various diameters. For example, the cabin of an aircraft experiences low frequency noise, in the range of 60 Hz to 2000 Hz, generated by the aircraft ECS, other aircraft systems, and boundary layer airflow over the fuselage. TABLE 1 shows pitch values determined from experimentation to address this frequency range for various diameters of duct hoses having an extruded PEI flexuous cord  110  helically wound around the outer surface of the duct, according to one embodiment. 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                   
                   
                 Polyetherimide 
                   
               
               
                   
                   
                 Cord 
               
               
                 Duct 
                 Duct 
                 Diameter (in.) 
                 Pitch (in.) 
               
             
          
           
               
                 Construction 
                 Diameter (in.) 
                 Hollow 
                 Solid 
                 Hollow 
                 Solid 
               
               
                   
               
               
                 1-Ply 
                 0.75 
                   
                 0.06 
                   
                 0.37 
               
               
                   
                 1.00-1.25 
                   
                 0.06 
                   
                 0.43 
               
               
                   
                 1.50 
                 0.08 
                 0.08 
                 0.50 
                 0.50 
               
               
                   
                 1.75-2.50 
                 0.09 
                 0.09 
                 0.62 
                 0.62 
               
               
                   
                 3.00 
                 0.09 
                 0.09 
                 0.75 
                 0.75 
               
               
                   
                 3.50-4.00 
                 0.10 
                 0.10 
                 0.75 
                 0.75 
               
               
                   
                 4.50-5.50 
                 0.10 
                 0.10 
                 0.75 
                 0.75 
               
               
                 2-Ply 
                 5.50-7.50 
                 0.12 
                 0.12 
                 0.87 
                 0.87 
               
               
                   
                 8.00-9.00 
                 0.15 
                 0.15 
                 0.87 
                 0.87 
               
               
                   
                  9.50-12.00 
                 0.18 
                 0.18 
                 0.95 
                 0.95 
               
               
                   
               
             
          
         
       
     
       FIGS. 4A and 4B  illustrate noise-reducing ducts  102 A and  102 B having flexuous cords  110 A,  110 B, and  110 C helically wound around the surface of the ducts at various pitches  112 A- 112 C. According to one embodiment, a noise-reducing duct  102 A having a flexuous cord  110 A wound at a relatively narrow pitch  112 A, as depicted in  FIG. 4A , may be combined in an aircraft ECS with ducts  102  having a flexuous cord  110  wound at a relatively wide pitch  112 , as illustrated in  FIG. 1 . The combined ducts  102 A and  102  provide noise reduction in both the high and low frequency ranges, respectively, thus providing a broader range of noise reduction in the overall system in order to address the desired target range. 
     In another embodiment, the ducts  102 B of the aircraft ECS are helically wound with two, separate flexuous cords  110 B and  110 C, one wound at a wide pitch  112 B and the other wound at a narrow pitch  112 C, as depicted in  FIG. 4B . The two flexuous cords  110 B,  110 C may be wound in the same direction or opposite directions from each other. In this way, a broader range of noise reduction may be attained, as described above, with only one duct  102 B having to be fabricated. 
     In further embodiments, the pitch  112  of the helical winding may be varied along a single duct  102 . The pitch  112  may be narrower at points where higher frequency dampening is desired or where required structurally by bends in the duct at installation, and wider along the straight segments of the duct or where lower frequency dampening is preferred. By utilizing different pitches  112  at various points along the same duct  102 , a broad range of noise reduction may be achieved in the overall system, while incorporating reduction of specific frequencies of breakout noise at various points along the duct  102  as required. It will be appreciated by one skilled in the art that many more combinations of ducts and flexuous cords of various pitches than those described herein may be imagined to address a particular acoustic frequency range of noise, and this application is intended to cover all such combinations. 
     As described briefly above, the noise attenuating characteristics of the noise-reducing duct  102  also varies depending on the type of material used in the flexuous cord  110 .  FIG. 5  illustrates the relative noise attenuation performance of two PVDF foam ducts, each helically wound by a flexuous cord  110  of similar size and pitch, one fabricated from plastic (PEI) and one from KEVLAR®. As in  FIG. 3 , the line graph  500  shown in  FIG. 5  depicts the transmission loss or reduction in sound level, measured in decibels (dB), attained by different air ducts at different acoustic frequencies, expressed in Hertz (Hz). 
     As shown in the line graph  500 , a duct having a KEVLAR® flexuous cord  110  provides greater noise reduction in the lower frequency range of 65 Hz to 1600 Hz, as depicted by line  502 , while duct having a plastic flexuous cord  110  provides greater reduction of noise in the upper frequency range of 1600 Hz to 8000 Hz, as depicted by line  504 . For comparison, the line graph  500  in  FIG. 4  also shows the noise attenuation performance of a PVDF foam duct having no helically wound flexuous cord  110 , depicted by line  506 . 
     In addition to attenuating noise in the desired frequency range, the noise-reducing duct  102  described herein also produces less noise from airflow through the ECS than ducts made of alternative materials. The line graph  600  shown in  FIG. 6  illustrates the level of noise produced, measured is A-weighted decibels (dBA), for several types of ducts at various airflow rates, expressed in feet-per-minute. As shown in the line graph  600 , noise-reducing ducts  102  constructed of silicone glass-fiber having flexuous cords  110  wound at both wide and narrow pitches  112 , depicted by lines  602  and  604  respectively, produce less airflow noise at all flow rates than ducts made of KEVLAR® or extruded PEI (ULTEM®), depicted by lines  606  and  608 . 
       FIG. 7  shows one method for fabricating a noise-reducing duct  102  described in the embodiments above. The flexuous cord  110  is helically wound around the outer surface of the duct  102  by mounting the duct  102  on a spindle  704  and rotating the spindle  704  to draw the flexuous cord  110  from a spool  706  onto the surface of the duct  102 . The flexuous cord  110  is applied to the surface of the duct in a continuous length, maintaining the desired pitch  112 . The desired pitch  112  may be achieved by having the spool  706  move at a specific speed along the length of the spindle  704  as the spindle is rotated. In one embodiment, an adhesive is applied to the flexuous cord  110  before it contacts the surface of the duct  102  in order to bond the flexuous cord  110  to the surface. In another embodiment, the duct  102  is coated with a thin layer of adhesive before the flexuous cord  110  is applied. 
     As described above, the noise attenuating effects of the helically wound component can be tuned to dampen the desired acoustic frequency range by varying the pitch  112  of the helical winding or by providing multiple helically wound flexuous cords  110  of differing pitches.  FIG. 8  provides a method for reducing noise in an air duct over a target frequency range, according to embodiments. It should be appreciated that more or fewer operations may be performed than shown in  FIG. 8  and described herein. These operations may also be performed in a different order than those described herein. 
     The routine  800  begins at operation  802 , where a target range of acoustic frequencies to be dampened is determined. The target range will depend upon the environment where the duct  102  is being utilized and the desired noise attenuation effect. For example, as described above, the cabin of an aircraft experiences low frequency noise in the range of 60 Hz to 2000 Hz, generated by the aircraft engines, ECS, other aircraft systems, and boundary layer airflow over the fuselage. The target range of acoustic frequencies for an aircraft cabin, therefore, may be this 60 Hz to 2000 Hz range. 
     Once the target range of acoustic frequencies to be dampened is determined at operation  802 , the routine  800  continues to operation  804 , where the optimum pitch  112  of a helically wound flexuous cord  110  is determined to attenuate the target range of frequencies in the duct  102 . The pitch  112  may be determined through experimentation with the types and sizes of ducts being used, or may be calculated based on a formula describing the noise attenuating effect of the noise-reducing duct  102 . The pitch value may also depend on the material and profile of the flexuous cord  110  being wound around the duct  102 . 
     From operation  804 , the routine  800  continues to operation  806 , where the flexuous cord  110  is helically wound around the duct  102  at the determined pitch  112 . This may be performed by a method similar to that described above in regard to  FIG. 7 . As further described above, the noise attenuation characteristics of the noise-reducing duct  102  depend upon the pitch  112  of the helical winding of the flexuous cord  110  as well as the material from which the cord  110  is constructed. If it is determined at operation  808  that the noise-reducing duct  102  wound with a flexuous cord  110  at the determined optimal pitch  112  does not adequately dampen noise across the entire target range of acoustic frequencies, then the routine  800  continues to operation  810 . 
     At operation  810 , an additional helically wound flexuous cord  110  or an additional duct segment is added to the noise-reducing duct  102 , targeting the portion of the range of acoustic frequencies not adequately dampened by the first flexuous cord  110  or duct segment. From operation  810 , the routine  800  returns to operation  804 , where an optimum pitch for the helical winding for the additional flexuous cord  110  or duct segment is determined according to the portion of the target range of frequencies. Once it is determined, at operation  808 , that sufficient noise reduction is achieved across the entire target range by the combination of flexuous cord(s)  110  and duct segment(s), the routine  800  ends. 
     Based on the foregoing, it should be appreciated that technologies for the reduction of noise are provided herein. The subject matter described above is provided by way of illustration only and should not be construed as limiting. Various modifications and changes may be made to the subject matter described herein without following the example embodiments and applications illustrated and described, and without departing from the true spirit and scope of the present invention, which is set forth in the following claims.