Abstract:
A sheet metal media free sound attenuator reduces noise from machinery and gas flow in duct systems, that has a solid outer shell box and at least one flow passage made of perforated sheet. The flow passage can have a gap called expansion chamber and supports that divide cavity between a wall of the perforated passage and a wall of the outer shell box. Entrance transition and exit transition of the sound attenuator provides smooth flow into the attenuator, reduces pressure drop, and increases attenuation. In the preferred embodiment, the perforated sheet is a perforated spiral sheet.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application claims benefit of Provisional Appn 60/031,951 filed Nov. 27, 1996. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the attenuation and reduction of machinery and gas flow noise and turbulence in duct systems including, but not restricted to, heating, ventilating and air conditioning air duct systems. 
     2. Background Information 
     Conventional sound attenuators used in duct systems use resistance provided by filler material when sound travels through pores of the filler material. Typically FIBERGLASS, ROCKWOOL, foam and other fibrous materials are used for this purpose. Perforated sheets are used to increase the access of the sound from flow passage to filler material. The filler material that is used to attenuate sound creates some new problems. At higher gas flow velocities the filler material gets eroded into small particles and gets entrained in airflow contaminating indoor air of a facility. The filler materials produce some toxic gases, cause microorganisms to grow or release some hazardous products when they come in contact with some other chemicals. These problems make the use of filler material in sound attenuators dangerous. Pat. No. 4,287,962 Packless Silencer, Ingard et al, Sep. 8, 1981 addresses the above mentioned problems associated with the filler material of fibrous nature. Ingard et al uses sound attenuators with acoustic resistance provided by resistive sheets or perforated face sheets. While sound attenuators having perforated sheets were an improvement, they did not include benefits that round and oval passages inherently have over flat sheets or rectangular shapes. The entrance and exit were not designed optimally and this causes flow noise to increase and often results in turbulence of flow. Turbulent flow increases the energy required to maintain gas flow. These disadvantages led to a less than optimal acoustical and flow performance. As will be seen in the subsequent description, the present invention overcomes these disadvantages of the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention is a media free sound attenuator that reduces machinery and gas flow noise and turbulence in duct systems. The present invention uses acoustic impedance of perforated passages and cavities, shape factors of the round/flat-oval passage elements and transitions to effectively reduce machinery and gas flow noise and turbulence in duct systems instead of using fibrous filler material. The present invention includes a metallic shell and at least one perforated liner element that acts like a flow passage. For optimum performance, the liner element is a spriral element. The shell and the liner elements are separated by divider plates, transitions are placed at the entrance and exit of the media free sound attenuator to reduce pressure drop and increase acoustic performance. The expansion chamber is an area discontinuity inside the sound attenuator that adds to the attenuation by reflecting the noise in the lower frequencies. The shape factor of the round and flat-oval passage elements adds to broad band noise attenuation, also the transitions at the entrance and exit are also effective in noise attenuation. All these factors enhance the performance of the sound attenuator in terms of insertion-loss, pressure drop and gas flow generates noise. Also, the improvements included in the present invention reduces turbulence of flow, which reduces horsepower required to maintain then flow. The present invention includes an optional expansion chamber that adds to the acoustic impedance of the sound attenuator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, closely related figures have the same number but different alphabetical surfaces. 
     FIGS. 1,  1 A,  2 ,  3 ,  4 ,  4 A, and  4 B show Isometric, Plan, Elevation and Side views of an attenuator with two ovoidal form passages with an expansion chamber. 
     FIGS. 5. 5A,  6 ,  7 ,  8 ,  8 A, and  8 B show Isometric, Plan, Elevation and Side views of the attenuator with two ovoidal form passages. 
     FIGS. 9,  9 A,  10 ,  11 ,  12 ,  12 A, and  12 B show Isometric, Plan, Elevation and Side views of the attenuator with a single ovoidal form passage with an expansion chamber in it. 
     FIGS. 13,  13 A,  14 ,  15 ,  16 ,  16 A, and  16 B show Isometric, Plan, Elevation and Side views of the attenuator with the single ovoidal form passage. 
     FIGS. 17,  17 A,  18 ,  19 ,  20 ,  20 A, and  20 B show Isometric, Plan, Elevation and Side views of the attenuator with the single round passage with an expansion chamber. 
     FIGS. 21,  21 A,  22 .  23 .  24 ,  24 A, and  24 B show Isometric, Plan, Elevation and Side views of the attenuator with a single round passage. 
     FIGS. 25,  25 A,  26 , and  27  show Isometric, Elevation, and Plan views of a round attenuator with an annular passage with a bullet inside the attenuator. 
     FIGS. 28,  29 , and  30  show Isometric, Elevation and Plan views of the round attenuator with a round passage without a bullet inside the attenuator. 
     FIGS. 31,  31 A,  32 ,  33 ,  34 ,  34 A, and  34 B show Isomatric, Plan, Elevation, and Side views of an attenuator with a single no-line-of-sight ovoidal form passage. 
     FIGS. 35,  35 A,  36 ,  37 ,  38 ,  38 A, and  38 B show Isometric, Plan, Elevation and Side views of an attenuator with three ovoidal form passages. 
     FIGS. 39,  39 A,  40 ,  41 ,  42 ,  42 A, and  42 B show Isometric, Plan, Elevation, and Side views of an attenuator with two ovoidal form passages converging to a single ovoidal form passage inside the attenuator, thus providing a gas flow passage that has two passages in the beginning which change into one. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention, a fibrous filler material free sound attenuator  101  is illustrated in FIGS. 1,  1 A,  2 ,  3 ,  4 ,  4 A, and  4 B. FIGS. 1,  1 A,  4 A, and  4 B are isometric views. FIG. 2 is a top view. FIG. 3 is an elevation view. FIG. 4 is a side view. The fibrous filler material free sound attenuator  101  has an outer shell box  9 . The outer shell box  9  has entrance transition  5  that directs gas flow from an existing prior art rectangular or square duct (not shown), such as is common to the trade, through two ovoidal form passages  3  of the fibrous filler material free sound attenuator  101 , provided by the ovoidal form perforated spiral sheet  27 . An arrow indicates the direction of gas flow. The shape of the ovoidal form passages  3  adds to broad band noise attenuation. There is a region between the outer shell box  9  and the ovoidal form passages  3  that is divided into cavities using divider plates  7 . The divider plates  7  form cavities containing gas that has seeped through the ovoidal form perforated spiral sheet  27  from the gas flow through the fibrous filler material free sound attenuator  101 . The effect of the perforated spiral sheet  27  permitting gas to seep into these cavities results in sound attenuation. A gap in the ovoidal form passage  3 , serves as an expansion chamber  25  inside the outer shell box  9 . The expansion chamber  25  is an area discontinuity that adds to the sound attenuation by reflecting the noise in the lower frequencies. An exit transition  1  is provided at the other end of the fibrous filler material free sound attenuator  101  for connection to an existing prior art rectangular or square duct (not shown) such as is common to the trade. In the preferred embodiment of the present invention, the ovoidal form passages  3  are provided by the ovoidal form perforated sprial sheet  27 . A ovoidal form perforated sheet results in some sound attenuation but the ovoidal form perforated spiral sheet  27  works better. 
     In FIGS. 5. 5A,  6 ,  7 ,  8 ,  8 A, and  8 B an alternate fibrous filler material free sound attenuator  103  without an expansion chamber  25  is illustrated. It includes the same parts as the fibrous filler material free sound attenuator  101  except there is no expansion chamber  25 . 
     In FIGS. 9,  9 A,  10 ,  11 ,  12 ,  12 A, and  12 B a second alternate fibrous filler material free sound attenuator  105  with the expansion chamber  25  and a single ovoidal form passage  3  is illustrated. As can be seen from the FIG. 9, the ovoidal form passage has a share that essentially has two parallel sides with rounded ends. It includes a single entrance transition  39 , the ovoidal form passage  3 , the ovoidal form perforated sheet  27 , the outer shell box  9 , divider plates  7 , and a single exit transition  37 . 
     In FIGS. 13,  13 A,  14 ,  15 ,  16 ,  16 A, and  16 B, a third alternate fibrous filler material free sound attenuator  107  without the expansion chamber  25  and having one ovoidal form passage  3  is illustrated. It includes an entrance transition  39 , the ovoidal form passage  3 , the ovoidal form perforated spiral sheet  27 , the outer shell box  9 , divider plates  7 , and a single exit transition  37 . 
     In FIGS. 17,  17 A,  18 ,  19 ,  20 ,  20 A, and  20 B, a fibrous filler material free sound attenuator  109  with the expansion chamber  25  and having one round passage  36  is illustrated. It includes a round entrance transition  35 , the round passage  36 , the round perforated spiral sheet  28 , the outer shell box  9 , divider plates  7 , and a round exit transition  33   
     In FIGS. 21,  21 A,  22 ,  23 ,  24 ,  24 A, and  24 B, a fourth alternate fibrous filler material free sound attenuator  111  without the expansion chamber  25  and having one round passage  36  is illustrated. It includes the round entrance transition  35 , one round passage  36 , the round perforated spiral sheet  28 , the outer box shell  9 , divider plates  7 , and the round exit transition  33 . 
     In FIGS. 25,  25 A,  26 , and  27 , a round fibrous filler material free sound attenuator  113  with a round spiral outer shell  15  and an annular perforated passage spiral duct  17  is illustrated. There is a perforated bullet  21  in the center of the attenuator  113  with z-trims  19  attaching it within the annular perforated passage spiral duct  17 . The bullet  21  has solid nose cones  29  on both ends. The perforated bullet  21  further reduces machinery and gas flow noise in this embodiment of the present invention. 
     In FIGS. 28,  29 , and  32 , an alternate round fibrous filler material free sound attenuator  115  with a round spiral outer shell  15  and an annular perforated passage spiral duct  17  is illustrated. 
     FIGS. 31,  31 A,  32 ,  33 ,  34 ,  34 A, and  34 B depict a rectangular fibrous filler material free sound attenuator  117  without a line of sight. The attenuator  117  has a ovoidal form passage  3  that has a bend  31  inside the attenuator  117  in such a way that the other end of the attenuator  117  can not be seen from one end of the attenuator  117 . The attenuator  117  has the outer shell box  9 , an offset entrance transition  43 , the nvoidal form passage  3 , a ovoidal form perforated spiral sheet  27 , and an offset exit transition  41 . 
     In FIGS. 35,  35 A,  36 ,  37 ,  38 ,  38 A, and  38 B, a fifth alternate fibrous filler material free sound attenuator  119  having three ovoidal form passages  3  is illustrated. It differs from the alternate fibrous filler material free sound attenuator  103  illustrated in FIGS. 5 to  8  in that there are three ovoidal form passages  3 . 
     In FIGS. 39,  39 A,  40 ,  41 ,  42 ,  42 A, and  42 B, a rectangular fibrous filler material free sound attenuator  121  with two ovoidal form passages  3 A at the entrance and converying to a single ovoidal form passage  3 B is illustrated. It includes the entrance transition  1 , an ovoidal form exit transition  47 , a transition  23  that changes from the two passages  3 A to single passage  3 B, divider plates  7 , the outer shell box  9 , and the ovoidal form perforated spiral sheet  27 . This embodiment of the present invention further reduces machinery and gas flow noise. 
     Ovoidal form passages  3 ,  3 A, and  3 B such as are shown in various figures such as FIGS. 10 and 40, as well as the annular perforated passage duct  17  as shown in FIG. 25 are a significant advance in reducing machinery and gas flow noise in duct systems. Aside from sound attenuation, there is also a reduction in flow turbulence. The transitions enumerated in this description, such as the entrance transition  5  and the exit transition  1  as shown in FIGS. 4A and 4B, reduce turbulence of flow by providing a transition from a rectangular duct leading into the present invention to the ovoidal form or round shape of the passage or duct. This reduction in turbulence not only reduces noise, it also reduces pressure drop from flow through various attenuators described in this specification. Reducing pressure drop means less horsepower is required to maintain a given flow of gas, such as air, throuh a a duct system. So, incorporating the present invention in a duct system not only reduces noise, it also saves energy. 
     Although the description above contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. For example, gas flow is mentioned, but as obvious to anyone skilled in the state of the art, this invention applies to air, which is a mixture of gases. Also, while the preferred embodiment of the present invention incorporates perforated spiral sheets to form passageways, flat sheet will produce some sound attenuation, but not as much as the spiral sheets. 
     Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.