Abstract:
An acoustic attenuator includes an intake air duct having an intake air duct opening leading to an outside environment and a blower fan opening leading to a blower fan. Air is drawn through the intake air duct opening towards the blower fan and a primary reflecting panel disposed in the intake air duct. The primary reflecting panel is configured to reflect sound propagated from the blower fan away from the intake air duct opening.

Description:
This is a Continuation of application Ser. No. 09/875,792, filed Jun. 6, 2001 now U.S. Pat. No. 6,668,970, which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to acoustics. More specifically, reducing sound in an HVAC system is disclosed: 
     BACKGROUND OF THE INVENTION 
     Mechanical air control equipment of a Heating Ventilation and Air Conditioning (HVAC) system can be a major source of sound in a building. The sound generated by the HVAC system travels both upstream and downstream in the intake air duct and exhaust air duct, respectively. Various sound sources within the duct include blower fans, diffusers, airflow regulating valves, etc. Noise generated by the blower fans is of broad band frequency, typically ranging from about 200 Hz through the rest of the frequency range of human hearing. Therefore, noise generated by HVAC system, when travelling upstream through the very short intake air duct, exits the intake air filter and enters the building living area creating a noise pollution problem. 
     A cost-efficient HVAC system often requires placing the HVAC heat exchange system in the middle of a building. This exchange system is often placed in a hallway, which frequently leads to a family room, dining room, bedroom, kitchen, etc. Noise generated by the HVAC blower fan and other mechanical parts travels through the short section of the intake air duct and enter the living space. This noise pollution makes normal conversation and comfortable television listening difficult. 
     Various attempts have been made to minimize the sound generated by an HVAC system. However, these attempts generally address the treatment at the exhaust air duct section, and not the treatment at the intake air duct section. Because the intake air duct section is usually a very short section of the air duct, it leaves very little space for any effective noise treatment. One exhaust air duct treatment system is commonly referred to as a dissipative silencer, which provides a noise attenuating liner either inside or outside the duct. This liner may be mineral wool or fiberglass insulation. These materials moderately attenuate sound over a broad range of frequencies. However, these liners are often not desirable because of large space requirements and the extended length of coverage that is required to produce adequate attenuation. 
     Additionally, reactive silencers have been used to attenuate sound. They typically consist of perforated metal facings that cover a plurality of tuned chambers. Generally, reactive silencers attenuate low frequency noise. Broad band attenuation is more difficult to achieve with reactive silencers, due to the larger area required to achieve a noticeable result. Another way to reduce the noise in an exhaust duct is by employing an acoustic resonator. This technique includes at least one resonating chamber having walls that define a length and a height. The length of the resonating chamber is selected to provide noise attenuation at a predetermined frequency. Generally, acoustic resonators only attenuate a predetermined frequency or frequencies. To achieve broad band frequency attenuation with an acoustic resonator requires a large number of different lengths and sizes of resonating chambers, which, in turn, requires a large area and volume of work space. 
     All of the above mentioned noise-attenuating techniques require a large area and volume of workspace which is only possible in the exhaust duct area. The usually short intake air duct provides too small an area for the previous designs to effectively attenuate broad band HVAC noise. 
     Another attempt to reduce noise is by active noise attenuation. This is accomplished by sound wave interference. Undesirable noise propagating within a duct is attenuated by the introduction of a canceling sound. An input microphone typically measures the undesirable noise up stream in a duct and converts it to an electrical signal. The signal is processed by a digital computer that generates a sound wave of equal amplitude and 180 degrees out of phase (a mirror image of the noise). This secondary noise source destructively interferes with the noise and cancels a significant portion of the unwanted noise. However, the adaptive process that is used to generate the canceling signal can be adversely affected by acoustical reflection from distant elements in the overall duct system. Active attenuation is only useful on low frequency (below about 100 Hz) noise attenuation and is not efficient in attenuating higher frequencies. Additionally, the high cost of this system further limits its use. 
     SUMMARY OF THE INVENTION 
     An acoustical reflective and dissipative attenuation system is disclosed that is used to reduce broad band noise in the intake air duct (also referred to as a return air duct) of an HVAC heat exchange system. In many cases, the intake air duct is a very short section of the air duct system with very limited workspace, generally less than 20 cubic feet in volume. Significant broad band noise reduction is achieved by appropriately placing a noise-reflecting panel (shield) with an appropriate amount of acoustic absorbing padding at a strategic location. The reflecting panel contains the noise in the intake air duct section and greatly increases the noise absorption by the acoustic absorbing padding before this noise can exit the intake air duct filter and enter the living area of a building. 
     It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, a method, or a computer readable medium such as a computer readable storage medium or a computer network wherein program instructions are sent over optical or electronic communication links. Several inventive embodiments of the present invention are described below. 
     In one embodiment, an acoustic attenuator includes an intake air duct having an intake air duct opening leading to an outside environment and a blower fan opening leading to a blower fan wherein air is drawn through the intake air duct opening towards the blower fan and a primary reflecting panel disposed in the intake air duct, the primary reflecting panel being configured to reflect sound propagated from the blower fan away from the intake air duct opening. 
     In one embodiment, an acoustic attenuator module configured to be housed within an intake air duct having an intake air duct opening includes an open end having sides configured to be attached to passageway leading from the intake air duct to a blower fan; primary reflecting side configured to reflect sound propagating from the blower fan through the open end away from the intake air duct opening; and an open side configured to allow from the air intake air duct opening to circulate around the module through the open side and the open end to the blower fan. 
     In one embodiment, an acoustic attenuator module configured to be housed within an intake air duct having an intake air duct opening includes an open end having sides configured to be attached to passageway leading from the intake air duct to a blower fan; an open side configured to allow from the air intake air duct opening to circulate through the module through the open end to the blower fan; and a primary reflecting plate offset from the open side to block noise propagated from the blower fan from propagating through the open side. 
     In one embodiment, sound propagating from a blower fan into a living space through an intake air duct having an intake air duct opening is attenuated by reflecting sound propagated from the blower fan away from the intake air duct opening using a primary reflecting panel in the intake air duct. 
     These and other features and advantages of the present invention will be presented in more detail in the following detailed description and the accompanying figures which illustrate by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: 
       The present invention can be better understood from the detailed description with the accompanying drawings, in which: 
         FIG. 1  is an illustration of an HVAC heat exchange system. 
         FIG. 2A  is a three dimensional view of the intake air duct section with an acoustic absorbing padding lining. 
         FIG. 2B  is a three dimensional view of the intake air duct section with primary and secondary reflecting panels installed. 
         FIG. 3A  is a three dimensional view of a triangular module. 
         FIG. 3B  is a three dimensional view of how a triangular module is installed under the blower fan. 
         FIG. 3C  is a side view of an HVAC heat exchange system with a triangular module installed under the blower fan. 
         FIG. 4A  is a three dimensional view of a square module. 
         FIG. 4B  is a top view of a square module. 
         FIG. 4C  is a side view of a square module. 
         FIG. 4D  is a front view of a square module. 
         FIG. 4E  is a side view of an HVAC heat exchange system with a square module installed under the blower. 
         FIG. 4F  illustrates an alternate arrangement of the primary and secondary panels wherein the primary and secondary reflecting panels have been replaced by a series of small reflecting shields. 
         FIG. 4G  is a top view of the alternate arrangement of the primary and secondary reflecting panels of the square module. 
         FIG. 4H  is a side view of the alternate arrangement of the primary and secondary reflecting panels of the square module. 
         FIG. 4I  is a front view of the alternate arrangement of the primary and secondary reflecting panels of the square module. 
         FIG. 5A  is a side view of an HVAC heat exchange system wherein the air intake is in line with the blower fan and a deflecting panel is inserted in the air intake duct. 
         FIG. 5B  is a side view of an HVAC heat exchange system wherein the air intake is in line with the blower fan and a deflecting panel is inserted in the air intake duct. 
         FIG. 6  is a three dimensional view of an air intake duct section. 
     
    
    
     DETAILED DESCRIPTION 
     A detailed description of a preferred embodiment of the invention is provided below. While the invention is described in conjunction with that preferred embodiment, it should be understood that the invention is not limited to any one embodiment. On the contrary, the scope of the invention is limited only by the appended claims and the invention encompasses numerous alternatives, modifications and equivalents. For the purpose of example, numerous specific details are set forth in the following description in order to provide a thorough understanding of the present invention. The present invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the present invention is not unnecessarily obscured. 
     A general layout of a household HVAC heat exchange system is shown in FIG.  1 . The HVAC system includes an air conditioner cooling coil/heat exchanger  11 , gas furnace heat exchange panels  12 , a blower fan  13 , and an intake air duct section  14 . These four sections form a passageway  17  through which air flows in a direction indicated by arrows  15 . The blower fan sucks in air from the building living area through the intake air filter  23  forcing the air to pass through the gas furnace heat-exchanging panels  12  and the air conditioner cooling coil/heat exchanger  11 . The renewed air  19  is then distributed throughout the building. Noise  18  generated by the blower fan and other mechanical parts can easily travel through the short section of intake air duct  14  and exit through the intake air filter  23  and enter living area with very little of its energy impeded. In one embodiment, the acoustic attenuator is installed in the limited space of intake air duct  14  by attaching it directly under the lower section of the blower fan  13 . 
       FIG. 2A  is a three dimensional view of an intake air duct  14  with acoustical absorbing padding  222  added to the inside walls. Noise  224  generated by a blower fan and other mechanical parts travels through this duct section, exits the intake air filter  223  and enters the living area. The acoustical absorbing padding attenuates the noise somewhat. 
       FIG. 2B  is a three dimensional view of the intake air duct section with a primary reflecting panel  240  and secondary reflecting panels  242  installed. Primary reflecting panel  240  can be made out of plywood or sheet metal and covered with acoustical absorbing padding  244 . When the primary reflecting panel is installed at an appropriate location, it reflects noise  246  away from intake air filter opening  248  and back into the intake air duct. This action augments the noise absorption by acoustical absorbing padding  244 . To augment the effect of primary reflecting panel  240 , secondary reflecting panels  242  are included at the sides and bottom of the air filter opening  248 . The reflecting panels reflect sound back toward the blower fan while allowing air flow from the intake air duct opening to pass around the reflecting panels to the blower fan. 
       FIG. 3A  is a diagram illustrating a triangular acoustic attenuator module that attenuates noise in a similar manner as the primary and secondary reflecting panels attenuate noise. The triangular module  301  may be constructed using a single sheet of galvanized metal. The direction of the airflow is indicated by the arrows  302 . Noise from a blower fan is reflected off of primary panel  304  and secondary side panels  306 . Air from the intake air duct opening circulates around the reflecting panels and through the open side  308  upward to the blower fan. When the triangular module is covered with an acoustical absorbing padding and installed directly under the blower fan as shown in  FIG. 3B , the noise attenuation is significant. 
       FIG. 3B  is a three dimensional view of how a triangular module is installed directly under the blower. Module  301  is bolted to the sides of an opening leading to the blower so that it extends downward into the intake air duct. Noise from the blower fan is reflected off of reflecting panels  304  and  306  and air  302  from the intake air duct opening  314  circulates around the reflecting panels and through open side  308  upward to the blower fan. 
       FIG. 3C  is a side view of an HVAC heat exchange system with triangular module  301  installed under the blower fan. Air flows in the direction indicated by arrows  322 .  FIG. 4A  is a diagram illustrating a square acoustic attenuating module  401 . The square module efficiently utilizes the limited workspace in the intake air duct section, and, as a result, a relatively thick (between about 15 and 40 inches, depending on the height and volume of the air duct) acoustical absorbing padding  402  can be installed. The noise spectrum of the blower fan and other mechanical parts can be empirically determined so that a particular thickness of acoustical absorbing padding can be installed to achieve satisfactory noise attenuation. Thicker acoustical absorbing padding results in broader frequency band absorption, especially in the lower frequency band. Primary reflecting panel  406  is offset between about 3 and 6 inches from the intake air duct opening and is preferably slightly bigger than the intake air duct opening. Reflecting panel  406  is disposed substantially parallel to the intake air duct opening. The arrangement of the primary reflecting panel  406  and secondary reflecting panels  408  greatly improves the confinement of the HVAC noise. The thick acoustical absorbing padding helps to attenuate the HVAC noise. This efficient arrangement of primary and secondary reflecting panels also allows intake air  410  to flow through the duct opening, past the secondary reflecting panels, and around the primary reflecting panel with little obstruction. 
       FIG. 4B  shows a top view of the square module. The primary reflecting panel  406  is spaced apart from sides of the duct opening by a pair of supports  423 . Secondary reflecting panels  408  extend across the duct opening and are angled inward between about 45 and 90 degrees so that the secondary reflecting panels allow air to pass into the duct from outside and reflect sound that would otherwise leave the duct back into the intake air duct. 
       FIG. 4C  shows a side view of the square module. The square module efficiently uses the limited space available in the intake air duct. Therefore, it allows relatively thick acoustical absorbing padding  402  to be installed. Noise propagating down from the blower fan is absorbed by the padding and reflected back by primary reflecting panel  406 . Noise that propagates past the primary reflecting panel to the duct opening is reflected back by secondary reflecting panels  408 . 
       FIG. 4D  shows a front view of the square module. The large opening  430  located on the front of the square module allows air to flow through and past the secondary reflecting panels with minimum obstruction. 
       FIG. 4E  is a side view of an HVAC heat exchange system with square module  401  installed directly under blower  462 . Air flows in the direction indicated by the arrows  464 . 
       FIG. 4F  illustrates an alternative arrangement of a square module acoustic reflecting module. A series of small reflecting panels  470  fulfil the noise reflecting function of both the primary and secondary reflecting panels. This design allows air to flow through the duct opening with minimum obstruction. 
       FIG. 4G  shows a top view of the alternate square module. Reflecting panels  470  extends inward from the sides of the duct opening. Acoustical absorbing material  473  is disposed inside the module. 
       FIG. 4H  shows a side view of the alternate square module. Reflecting panels  470  are shown extending inward at an angle between about 30 and 60 degrees relative to the front side of the square module. Acoustical absorbing material  473  is also shown inside the module. 
       FIG. 4I  shows a front view of the alternate square module. A series of openings  472  are shown that readily allow air to enter the module. 
       FIG. 5A  illustrates another embodiment of the invention wherein the intake air filter opening is in line with the blower fan and the intake air duct workspace is sufficiently large to accommodate a large primary reflecting panel. Primary reflecting panel  502  is placed between blower fan  504  and intake air filter  506 . This arrangement serves the purpose of preventing the HVAC noise from escaping directly into the living area unimpeded. Acoustical absorbing padding  508  is positioned at both ends of the module. 
       FIG. 5B  illustrates another embodiment of the invention wherein the intake air filter opening is in line with the blower fan and the intake air duct workspace is sufficiently large to accommodate a large primary reflecting panel. Primary reflecting panel  512  is placed between blower fan  514  and intake air filter  516 . This arrangement serves the purpose of preventing the HVAC noise from escaping directly into the living area unimpeded. Acoustical absorbing padding  518  is positioned at the end of the module. 
     In order to effectively attenuate the noise generated by a blower fan, which tends to be generated at about 200 Hz and above, the thickness of the acoustic absorbing padding is preferably selected so that the cutoff frequency for attenuation is lower than the lowest frequency of the noise generated. In one embodiment, the thickness of acoustical absorbing padding can be calculated as provided below. 
     The wavelength of the cutoff frequency for attenuation can be calculated by the relation: 
      λ=C/f 
     
         
         
           
             where C is the speed of sound in air (approximately around 1100 feet per second); f is the frequency in Hz, and λ is the wavelength. Since C is approximately 1100 feet per second, a 200 Hz frequency will have a wavelength of approximately five and a half feet, which is equal to about 66 inches. Given the wavelength of the cutoff frequency, the preferred thickness of the acoustical absorbing padding can be determined. In one embodiment, a acoustical absorbing padding with a thickness equal to or greater than ¼ the wavelength of the cutoff frequency is used. For example, a ¼ wavelength padding thickness to achieve a 200 Hz frequency cutoff would equal about 16.5 inches. Depending on the noise spectrum of a given system with a given blower, the desired cutoff frequency and therefore the desired padding thickness may change. 
           
         
       
    
       FIG. 6  is a three dimensional view of an air intake duct section. 
     Techniques and devices have been described that provide broad band frequency noise attenuation. Noise generated by a blower fan that propagates to living space through the blower fan air intake duct is attenuated. An important advantage of the described designs is that while the propagation of noise is reduced, air flow is not substantially restricted. 
     Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. It should be noted that there are many alternative ways of implementing both the process and apparatus of the present invention. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.