Patent Publication Number: US-7707711-B2

Title: Acoustic noise reduction in a computer system having a vented cover

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This patent application is a divisional application of U.S. patent application Ser. No. 11/304,132, filed Dec. 15, 2005, entitled “METHOD AND APPARATUS FOR ACOUSTIC NOISE REDUCTION IN A COMPUTER SYSTEM HAVING A VENTED COVER”, which is hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The present invention relates in general to housings for enclosing computer systems and in particular to vented covers with acoustic attenuation for use with such housings. Still more particularly, the present invention relates to a computer system which includes a vented cover having cross-flow ventilation ducts with an acoustic noise reduction lining. 
   2. Background Art 
   Computer systems are using larger amounts of energy, and are generating more heat. Increased heat generation is driven by factors such as increases in processor performance and clock speed, and increases in the number of devices per integrated circuit. Electronic components, such as microprocessors and integrated circuits, must operate within certain specific temperature ranges to perform efficiently. Excessive heat degrades electronic component performance, reliability, life expectancy, and can even cause failure. Air moving devices (AMDs), such as fans and blowers, are widely used for controlling excessive heat. AMDs are often used in combination with heat sinks thermally connected to electronic components to be cooled. Typically, heat sinks are formed with fins to increase the surface area of the heat sink and thereby enhance heat dissipation as air moved by an AMD passes over the heat sink. 
   In many large server applications, the processors of a computer system along with their associated electronics (e.g., memory, disk drives, power supplies, etc.) are packaged in removable drawer configurations stacked within a rack or frame. In other cases, the processors of a computer system along with their associated electronics may be in fixed locations within the rack or frame. Typically, the components are cooled by air moving in parallel air flow paths, usually front-to-back, impelled by one or more AMDs. 
   With the advent of the increased heat generated by computer systems, increased ventilation is required to move cooling air through the computer system. A failure to provide adequate ventilation through a computer system may increase the probability of computer failure due to overheating and may result in damage to the electronic components. Due to the great expense of these electronic components and the concomitant loss of processing time associated with such failures, it is desirable that adequate ventilation be maintained for computer systems. Increased air flow rates are needed to provide adequate ventilation. However, the acoustic noise associated with the increased air flow rates required to provide adequate ventilation, as well as acoustic noise generated by the various components within the computer system, represents a problem that must be overcome. There are limits on the acoustic output of computer systems (e.g., servers and storage products) set by vendors, governments, standards setting bodies, and the like. 
   In order to reduce acoustic noise, it is known to utilize an acoustic noise reduction lining in vented covers of computer systems. An example of such an arrangement is found in U.S. Pat. No. 5,526,228, issued Jun. 11, 1996 to Dickson et al., entitled “COMPUTER SYSTEM UNIT WITH ACOUSTIC DAMPENING COOLING FAN SHROUD PANEL”, which is assigned to the assignee of the present application. As shown in  FIG. 1 , a cooling fan shroud panel  101  includes an acoustic noise reduction lining comprising a side acoustic foam panel  102  and a top acoustic foam panel  104 . The acoustic dampening cooling fan shroud panel  101  is mounted to an intermediate rear panel  106  of a computer system unit  100 . Two cooling fans  108  are mounted within fan mounting apertures of the intermediate rear panel  106 . The cooling fans  108  draw air through computer system unit  100  from an intake ventilation grill (not shown) of a front panel  110  in the direction indicated by the arrows designated with reference numeral  111 . Mounted within computer system unit  100  are a power supply  112  and an electronic component package  114 , which are cooled by the air drawn through computer system unit  100 . Air is directed out an exiting ventilation aperture  116  of cooling fan shroud panel  101  in the direction indicated by the arrow designated with reference numeral  117 . The exiting ventilation aperture  116  is displaced from the mounting position of the cooling fans  108  such that acoustic noise resultant from the cooling fan operation is diminished. Even though acoustic dampening cooling fan shroud  101  is effective in diminishing acoustic noise, it exhibits a number of disadvantages. First, the relatively substantial depth of acoustic dampening cooling fan shroud panel  101  significantly increases the footprint of computer system unit  100 . Second, the small area of exiting ventilation aperture  116  relative to intermediate rear panel  106  reduces the cooling efficiency. 
     FIGS. 2 and 3  show other examples of the utilization of acoustic noise reduction lining in vented covers found in the IBM eServer zSeries 900 server. As shown in  FIG. 2  (Top View), an inlet cover  210  includes an acoustic noise reduction lining comprising two outer acoustic foam panels  212  and central acoustic foam block  214 . An inlet ventilation aperture  216  is defined between outer acoustic foam panels  212 . Similarly, an exhaust cover  220  includes an acoustic noise reduction lining comprising two outer acoustic foam panels  222  and central acoustic foam block  224 . An exhaust ventilation aperture  226  is defined between outer acoustic foam panels  222 . The inlet cover  210  and the exhaust cover  220  are mounted to a computer system frame or rack  200  using hinges (not shown) so that removable drawers (not shown) stacked within computer system frame  200  may be accessed when inlet cover  210  and/or exhaust cover  220  is/are swung open via the hinges. AMDs (not shown) draw air through computer system frame  200  from inlet ventilation aperture  216  and exhaust the air through exhaust ventilation aperture  226 . The air moves in the direction indicated by arrows designated by reference numeral  230 . The removable drawers, which contain processors and their associated electronics, are cooled by the air drawn through computer system frame  200 , as are electronic components fixed within computer system frame  200 . Acoustic noise resultant from the AMD operation is effectively diminished by inlet cover  210  and exhaust cover  220  which have three main attributes: a large amount of acoustic absorbing material; an air/noise path that curves or angles to force sound to impact the acoustic lining; and minimum sharp bends in the air path to minimize airflow resistance. Even though inlet cover  210  and exhaust cover  220  are effective in diminishing acoustic noise, they exhibit a number of disadvantages. First, the relatively substantial depth of inlet cover  210  and exhaust cover  220  significantly increase the footprint of computer system frame  200 . Second, the central acoustic foam block  224  in the exhaust cover  220  reduces cooling efficiency because it acts as a roadblock to exiting air. Third, inlet cover  210  and exhaust cover  220  cannot be made much more efficient without increasing airflow resistance, or increasing the cover depth (i.e., there are practical limits on how deep inlet cover  210  and exhaust cover  220  can be while still allowing the hinges to open). 
     FIG. 3  shows a modification of the configuration of inlet and outlet covers shown in  FIG. 2  to reduce increase in the footprint of the computer system frame. As shown in  FIG. 3 , an inlet cover  310  includes an acoustic noise reduction lining comprising two angled outer acoustic foam panels  312  and central acoustic foam panel  314 . An inlet ventilation aperture  316  is defined between angled outer acoustic foam panels  312 . Similarly, an exhaust cover  320  includes an acoustic noise reduction lining comprising two angled outer acoustic foam panels  322  and central acoustic foam panel  324 . An exhaust ventilation aperture  326  is defined between angled outer acoustic foam panels  322 . The inlet cover  310  and the exhaust cover  320  are mounted to a computer system frame or rack  300  using hinges (not shown) so that removable drawers (not shown) stacked within computer system frame  300  may be accessed when inlet cover  310  and/or exhaust cover  320  is/are swung open via the hinges. AMDs (not shown) draw air through computer system frame  300  from inlet ventilation aperture  316  and exhaust the air through exhaust ventilation aperture  326 . The air moves in the direction indicated by arrows designated by reference numeral  330 . The removable drawers, which contain processors and their associated electronics, are cooled by the air drawn through computer system frame  300 , as are electronic components fixed within computer system frame  300 . As with the configuration shown in  FIG. 2 , acoustic noise resultant from the AMD operation is effectively diminished by inlet cover  310  and exhaust cover  320 , but with a reduced footprint relative to the configuration shown in  FIG. 2 . Even though inlet cover  310  and exhaust cover  320  are effective in diminishing acoustic noise with a reduced footprint, these covers exhibit all of the other of disadvantages of the configuration shown in  FIG. 2 . 
   It should therefore be apparent that a need exists for a computer system enclosure which can both adequately ventilate a computer system housed therein and reduce the amount of acoustic noise, while addressing the disadvantages of the prior art. 
   SUMMARY OF THE INVENTION 
   According to the preferred embodiments of the present invention, a vented cover includes a pair of cross-flow ventilation ducts each including an acoustic noise reduction lining. The ventilation ducts are “cross-flow” in that they cross and bypass one another. The vented cover is affixed to an enclosure containing components of a computer system and abuts against a panel of the enclosure having an airflow aperture. An air moving device (AMD) passes air through the enclosure from the cross-flow ventilation ducts in the case where the vented cover is an intake ventilation cover, and/or into the cross-flow ventilation ducts in the case where the vented cover is an exhaust ventilation cover. The cross-flow ventilation ducts increase the air path length, along with the acoustic absorbing surface, thereby increasing acoustic attenuation. Airflow resistance is reduced by reducing surfaces perpendicular and close to the area where air enters and by reducing sharp turns in the ducts. The vented cover has a relatively thin depth because the cross-flow ventilation ducts cross and bypass each other in a very space efficient manner. 
   The foregoing and other features and advantages of the present invention will be apparent from the following more particular description of the preferred embodiments of the present invention, as illustrated in the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred exemplary embodiments of the present invention will hereinafter be described in conjunction with the appended drawings, where like designations denote like elements. 
       FIG. 1  is a sectional, partly schematic side view of a computer system incorporating an acoustic dampening cooling fan shroud panel, as known in the art. 
       FIG. 2  is a sectional, top view of a computer system enclosure incorporating vented covers each with an acoustic noise reduction lining that includes two outer acoustic foam panels and a central acoustic foam block, as known in the art. 
       FIG. 3  is a sectional, top view of a computer system enclosure incorporating vented covers each with an acoustic noise reduction lining that includes two angled outer acoustic foam panels and a central acoustic foam panel, as known in the art. 
       FIG. 4  is a top view of a computer system incorporating vented covers having cross-flow ventilation ducts with an acoustic noise reduction lining according to the preferred embodiments of the present invention. 
       FIG. 5  is a top view of the computer system shown in  FIG. 4 , with its vented covers open. 
       FIG. 6  is a perspective view of the computer system shown in  FIG. 4 , including a front vented cover (intake cover) having cross-flow ventilation ducts with an acoustic noise reduction lining according to the preferred embodiments of the present invention. 
       FIGS. 7   a - 7   d  are respectively a partial front view; a section, top view; a partial rear view; and a partial side view of a rear vented cover (exhaust cover) having cross-flow ventilation ducts with an acoustic noise reduction lining according to the preferred embodiments of the present invention. 
       FIGS. 7   e - 7   g  are respectively partial sectional views of the rear vented cover (exhaust cover) shown in  FIG. 7   c  along sections A-A, B-B, and C-C. 
       FIGS. 8   a - 8   b  are respectively a rear view and a section, top view of a rear vented cover (exhaust cover) of the having cross-flow ventilation ducts with an acoustic noise reduction lining according to the preferred embodiments of the present invention. 
       FIGS. 9   a - 9   b  are respectively a front view and a top view of the rear vented cover (exhaust cover) shown in  FIGS. 8   a - 8   b.    
       FIG. 10  is a partial, front perspective view of the rear vented cover (exhaust cover) shown in  FIGS. 8   a - 8   b.    
       FIG. 11  is a partial, rear perspective view of the rear vented cover (exhaust cover) shown in  FIGS. 8   a - 8   b.    
       FIG. 12  is an enlarged partial, rear perspective view of one of the pair of cross-flow ventilation ducts of the rear vented cover (exhaust) shown in  FIGS. 8   a - 8   b.    
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   1.0 Overview 
   In accordance with the preferred embodiments of the present invention, a vented cover can include a pair of cross-flow ventilation ducts or a series of paired cross-flow ventilation ducts each including an acoustic noise reduction lining. The ventilation ducts are “cross-flow” in that they cross and bypass one another. The vented cover is affixed to an enclosure containing components of a computer system and abuts against a panel of the enclosure having an airflow aperture. An air moving device (AMD) passes air through the enclosure from the cross-flow ventilation ducts in the case where the vented cover is an intake ventilation cover, and/or into the cross-flow ventilation ducts in the case where the vented cover is an exhaust ventilation cover. The cross-flow ventilation ducts increase the air path length, along with the acoustic absorbing surface, thereby increasing acoustic attenuation. Airflow resistance is reduced by reducing surfaces perpendicular and close to the area where air enters and by reducing sharp turns in the ducts. The vented cover has a relatively thin depth because the cross-flow ventilation ducts cross and bypass each other in a very space efficient manner. 
   2.0 Detailed Description 
   With reference to the figures and in particular  FIG. 4 , there is depicted a top view of a computer system  400  having a computer system enclosure  401  which incorporates two vented covers with acoustic noise reduction according to the preferred embodiments of the present invention. Although the preferred embodiments of the present invention are described herein within the context of an enclosure for containing a computer system, those skilled in the art will appreciate that the present invention may be practiced with an enclosure for containing any type of system. For example, the present invention may be practiced with an enclosure for an air treatment system, such as an air filter, air cleaner, dehumidifier, air conditioner, heater, or the like. Likewise, the present invention may be practiced with an enclosure for containing a computer system different than that shown in  FIG. 4 . For example, the present invention can be applied to enclosures containing computer systems, including personal computers, servers and data storage systems, of various sizes such as small towers (e.g., desktop computer systems), individual rack units and large rack frames (e.g., receiving multiple server units). 
   As illustrated in  FIG. 4 , computer system enclosure  401  preferably includes a front panel  402 , a rear panel  404 , a top panel  406 , a bottom panel (not shown), and two side panels  408 ,  410 . However, those skilled in the art will appreciate that computer system enclosure  401  may have any number and configuration of panels. One or more vented covers  412 ,  414  are also provided according to the preferred embodiments of the present invention. As illustrated, vented covers  412 ,  414  are respectively mounted at the front and rear of computer system enclosure  401 . The vented covers  412 ,  414  include one or more pair of cross-flow ventilation ducts, which are described in detail below. Vented cover  412  is affixed to computer system enclosure  401  and abuts against front panel  402 , which has an airflow aperture  403  therein. Similarly, vented cover  414  is affixed to computer system enclosure  401  and abuts against rear panel  404 , which has an airflow aperture  405  therein. The configuration of vented covers  412 ,  414  shown in  FIG. 4  is illustrative, and the present invention is not limited thereto. The vented covers  412 ,  414  may abut against any panels of the computer system enclosure  401  having airflow apertures therein. For example, vented covers  412 ,  414  may be respectively mounted at the side and top of computer system enclosure  401 . Also, a single vented cover may be used (e.g., an exhaust cover, an intake cover, or a combination exhaust/intake cover), or more than two vented covers may be used. 
   The vented covers  412 ,  414  are preferably affixed to computer system enclosure  401  in a movable or removable manner to allow access to computer system components within computer system enclosure  401 . Referring now to  FIG. 5 , vented covers  412 ,  414  are preferably hingedly affixed to computer system enclosure  401  to permit access to removable drawers when vented covers  412 ,  414  are swung open. Alternatively, vented covers  412 ,  414  may be removably mounted to computer system  401 , using fasteners such as bolts, screws, clamps or hangers. 
   With reference now to both  FIGS. 4 and 5 , computer system enclosure  401  provides mechanical support for one or more electronic component packages, such as electronics drawers  420 . The electronics drawers  420  are used to package processors of computer system  400 , along with their associated electronics (e.g., memory, disk drives, power supplies, etc.). Alternatively, the processors of computer system  400  and their associated electronics may be mounted in computer system enclosure  401  without being packaged in electronics drawers. Computer system enclosure  401  further includes at least one air moving device, such as device  422 . In computer system enclosures having multiple electronics drawers, one or more moving device  422  may be associated with each electronics drawer  420 . Each air moving device  422  may be physically attached to the electronics drawer  420  with which it is associated. Alternatively, air moving devices may be physically attached to computer system enclosure  401 . Preferably, electronics drawers  420  are slidably mounted within computer system enclosure  401 , providing easy access to the contents of electronics drawers  420  for repair, maintenance, and upgrades. Alternatively, electronics drawers  420  may be permanently mounted within computer system enclosure  401 , using fasteners such as bolts, screws or clamps. 
   Referring to  FIG. 4 , air moving devices  422  cause ambient air to enter computer system enclosure  401  through one or more pair of intake apertures (not shown in  FIG. 4 ) in front vented cover  412  in the directions shown by the intake arrows designated with reference numeral  432 . In other words, in the embodiment shown in  FIG. 4  front vented cover  412  is an intake cover. Air then flows over or through electronics drawers  420 , where heat is transferred to the air from heat generating components within electronics drawers  420 , thereby increasing the temperature of the air as it passes over or through electronics drawers  420 . Heated air then exits computer system enclosure  401  through one or more pair of exhaust apertures (not shown in  FIG. 4 ) in rear vented cover  414  in the directions shown by the exhaust arrows designated by reference numeral  434 , where it returns to and mixes with room ambient air. In other words, in the embodiment shown in  FIG. 4  rear vented cover  414  is an exhaust cover. 
     FIG. 6  is a perspective view of computer system enclosure  401  incorporating the front vented cover  412 . Air is drawn into computer system enclosure  401  through one or more pair of intake apertures in front vented cover  412 . These intake apertures are preferably generally triangular intake apertures, such as the right-side intake apertures  602  shown in  FIG. 6 . Each right-side intake aperture  602  shown in  FIG. 6  defines the intake of one member of a pair of cross-flow ventilation ducts. The left-side intake aperture that defines the intake of the other member of each pair of cross-flow ventilation ducts cannot be seen from the right-side perspective shown in  FIG. 6 , but is a generally triangular intake aperture on the left side of front vented cover  412  that is substantially identical to and in a complimentary orientation with respect to its mate (i.e., right-side intake aperture  602 ). In addition, front vented cover  412  is preferably shaped to include right-side and right-side chamfered surfaces where right-side intake aperture  602  and the left-side intake aperture respectively reside at an angle (e.g., 90° or greater) with respect to one another. This allows the cover to hinge open without being impeded by a similar adjacent cover. 
   The configuration and the relative orientation of each pair of intake apertures of front vented cover  412  (i.e., intake cover) can be seen in  FIGS. 7   a ,  9   a  and  10 , which show analogous (preferably identical) exhaust apertures of rear vented cover  414  (i.e., exhaust cover). Although the front vented cover  412  shown in  FIG. 6  includes eight pair of cross-flow ventilation ducts (as indicated by the eight right-side intake apertures  602  shown therein), any number of pair of cross-flow ventilation ducts may be used therein according to the preferred embodiments of the present invention. Likewise, although the rear vented cover  414  shown in  FIG. 9   a  includes eight pair of cross-flow ventilation ducts (as indicated by the eight pair of exhaust apertures shown therein), any number of pair of cross-flow ventilation ducts may be used therein according to the preferred embodiments of the present invention. 
   Preferably, rear vented cover  414  is identical to front vented cover  412  to reduce the number of unique parts used in computer system  400 , and hence reduce the cost of producing and maintaining computer system  400 . In that case, rear vented cover  414  would include cross-flow ventilations ducts identical to those of front vented cover  412 , and include generally triangular exhaust apertures identical to the generally triangular intake apertures in front vented cover  412 . 
     FIGS. 7   a - 7   d  are respectively a partial front view; a section, top view; a partial rear view; and a partial side view of rear vented cover  414  (exhaust cover).  FIGS. 7   e - 7   g  are respectively partial sectional views of rear vented cover  414  along sections A-A, B-B, and C-C in  FIG. 7   c .  FIG. 7   b  shows rear cover  414  attached to rear panel  404  of computer system  401 , while  FIGS. 7   a  and  7   c - 7   g  show rear cover  414  isolated from computer system enclosure  401  for the sake of clarity.  FIGS. 7   a - 7   g  illustrate a single pair of cross-flow ventilation ducts  702 / 704 . As noted above, rear vented cover  414  is preferably identical to front vented cover  412 , and hence the description of the rear vented cover  414  (exhaust cover) below also applies to front vented cover  412  (intake cover) although the direction of airflow would be reversed. 
     FIGS. 8   a - 8   b ,  9   a - 9   b , and  10 - 12  provide additional views of rear vented cover  414  (exhaust cover).  FIGS. 8   a - 8   b  are respectively a rear view and a section, top view of rear vented cover  414 .  FIGS. 9   a - 9   b  are respectively a front view and a top view of rear vented cover  414 .  FIG. 10  is a partial, front perspective view of rear vented cover  414 .  FIG. 11  is a partial, rear perspective view of the rear vented cover  414 .  FIG. 12  is an enlarged partial, rear perspective view of a single pair of cross-flow ventilation ducts of rear vented cover  414 . 
   Referring now to  FIGS. 7   a - 7   d , a lower duct  702  is formed in rear vented cover  414  (exhaust cover) between a generally triangular exhaust aperture  712  and an intake port  722 . The intake port  722  occupies substantially one-half (right side from the perspective shown in  FIG. 7   c ) surface area of rear cover  414  where rear vented cover  414  abuts against rear panel  404  of computer system enclosure  401 . Intake port  722  is substantially rectangular except for a relatively small, generally triangularly shaped surface  723  (shown in  FIG. 7   c ) at one corner thereof. An upper duct  704  is formed in rear vented cover  414  between a generally triangular exhaust aperture  714  and an intake port  724 . The intake port  724 , which is coplanar with intake port  722 , occupies substantially one-half (left side from the perspective shown in  FIG. 7   c ) surface area of rear cover  414  where rear vented cover  414  abuts against rear panel  404  of computer system enclosure  401 . Intake port  724  is substantially rectangular except for a relatively small, generally triangularly shaped surface  725  (shown in  FIG. 7   c ) at one corner thereof. It is important to note that only relatively small, triangular surfaces  723  and  725  block the flow of air near where rear vented cover  414  abuts against rear panel  404  of computer system enclosure  401 . Accordingly, airflow is improved relative to conventional vented covers due to reduced surfaces blocking airflow close to the airflow aperture  405  in rear panel  404  through which air exits computer system enclosure  401 . 
   Lower duct  702  and upper duct  704  are “cross-flow” in that they cross and bypass one another. Accordingly, the direction of airflow in lower duct  702  (shown in  FIGS. 7   a - 7   g  by arrows designated with reference numeral  732 ) crosses the direction of airflow in upper duct  704  (shown in  FIGS. 7   a - 7   g  by arrows designated with reference numeral  734 ). Both lower duct  702  and upper duct  704  include an acoustic noise reduction lining, such as an acoustic foam lining. Examples of acoustic noise reduction lining include open and closed cell, flexible polyurethane, polyimide, melamine and other foams available from Soundcoat Company of Deer Park, N.Y. (These examples are representative of a class of products serving similar functions and do not imply any particular requirement for the specific characteristics of these products.) Preferably, both lower duct  702  and upper duct  704  are lined with an acoustic noise reduction lining in their entirety, i.e., from their intake ports  722 ,  724  to their exhaust apertures  712 ,  714 . Alternatively, selected portions of lower duct  702  and upper duct  704  may be lined with an acoustic noise reduction lining. 
   The acoustic noise reduction lining may be, for example, flat, self-adhesive panels that are cut to conform to the surface of the cross-flow ventilation ducts  702 ,  704  and applied (in tile-like fashion) thereto. Alternatively, the acoustic noise reduction lining may be provided by any other technique known in the art (e.g., cutting, molding, spraying, etc.). 
   As mentioned above,  FIGS. 7   e - 7   g  respectively show three parallel partial sectional views of rear vented cover  414  along sections A-A, B-B, and C-C in  FIG. 7   c . As shown in  FIG. 7   e , at cross-section A-A lower duct  702  and upper duct  704  are preferably each generally triangular. As shown in  FIG. 7   f , at cross-section B-B lower duct  702  and upper duct  704  are preferably each generally rectangular. The generally rectangular shape of lower duct  702  and upper duct  704  at cross-section B-B is also shown (as hidden lines) in  FIG. 12 . As shown in  FIG. 7   g , at cross-section C-C lower duct  702  and upper duct  704  are preferably each generally triangular. 
   To reduce airflow resistance, it is preferable to maintain the same cross-sectional area throughout the cross-flow ventilation ducts  702 ,  704 . The area of lower duct  702  remains substantially constant between cross-sections A-A and B-B and between cross-sections B-B and C-C, as does the area of upper duct  704 . Additionally, the cross-sectional area of lower duct  702  preferably remains substantially constant (or increases) from cross-section A-A to exhaust aperture  712 , while the cross-sectional shape remains generally the same. Likewise, the cross-sectional area of upper duct  704  preferably remains substantially constant (or increases) from cross-section C-C to exhaust aperture  714 , while the cross-sectional shape remains generally the same. Similarly, the cross-sectional area of intake port  722  is preferably at least as large as the cross-sectional area of lower duct  702  at cross-section C-C, and the cross-sectional area of intake port  724  is preferably at least as large as the cross-sectional area of upper duct  704  at cross-section A-A. Accordingly, airflow is improved relative to conventional vented covers because air flows substantially unrestricted along the entire path of the cross-flow ventilation ducts  702 ,  704 . 
   As mentioned above, exhaust apertures  712 ,  714  are preferably generally triangular. This is preferable to maintain the cross-sectional shape of lower duct  702  from cross-section A-A to exhaust aperture  712 , and maintain the cross-sectional shape of upper duct  704  from cross-section C-C to exhaust aperture  714 . 
   In general, it is desirable to avoid tight bends in the cross-flow ventilation ducts to reduce airflow resistance and increase air moving efficiency (otherwise, a larger and/or additional air moving devices may be necessitated). Consequently, the cross-flow ventilation ducts  702 ,  704  preferably present gently curved surfaces. 
   Airflow resistance is reduced by reducing surfaces perpendicular and close to where air initially exits through the airflow aperture  405  in rear panel  404 . In this regard, only relatively small, triangular surfaces  723  and  725  block airflow where the air initial initially exits through the airflow aperture  405  in rear panel  404 , i.e., intake ports  722 ,  724  occupy substantially the entire area of the airflow aperture  405  in the rear panel  404  where rear vented cover  414  abuts against rear panel  404 . Accordingly, airflow is improved relative to conventional vented covers due to reduced surfaces blocking airflow close to the airflow aperture  405  in rear panel  404  through which air exits computer system enclosure  401 . For example, in the conventional exhaust covers  220 ,  320  shown in  FIGS. 2 and 3 , central acoustic foam block  224  and central acoustic foam panel  324  disadvantageously present large roadblocks to exiting air. 
   It is also generally desirable for air/noise to stay in the cross-flow ventilation ducts  702 ,  704  for as long as possible to increase attenuation efficiency. Consequently, the cross-flow ventilation ducts  702 ,  704  each preferably provide a relatively long air path. This long air path allows the surface area of the sound absorbing material (i.e., the acoustic noise reduction lining) to be increased relative to conventional vented covers, thereby improving acoustic attenuation. The long air path also reduces the “line of sight” to noise sources, which further improves acoustic attenuation. In this case, “line of sight” means that if you can easily see through the ducts to the other side of the cover, then noise has a similar, easy way out of the cover. Reducing “line of sight” reduces the level of noise that can pass through. 
   Preferably, in large sizes, rear vented cover  414  includes a metal outer shell into which is/are inserted one or more plastic inserts that provide cross-flow ventilation ducts  702 ,  704 . The outer metal shell could provide electromagnetic interference protection. For smaller applications such as PC tower sizes, the outer shell could be plastic or other nonmetal material. The plastic inserts are preferably made from thermally formed plastic. For example, a flat sheet of plastic may be heated to form the bends (shown in  FIG. 12 ) that define the gently curved surface of cross-flow ventilation ducts  702 ,  704 . Alternatively, one or more metal inserts may be used in lieu of plastic inserts. For example, metal inserts may be formed by working sheet metal using any technique known in the art (e.g., bending, welding, riveting, adhering, etc.). Advantageously, plastic inserts reduce the weight of rear vented cover  414  relative to using metal inserts. Also, it may be desirable to form the insert from an acoustic noise reduction material. 
   It is important to note that the vented cover according to the preferred embodiments of the present invention has a relatively thin depth because the cross-flow ventilation ducts  702 ,  704  cross and bypass each other in a very space efficient manner. This contrasts with conventional vented covers, which typically are relatively thick and thereby significantly and disadvantageously increase the footprint of the computer system enclosure to which they are attached. 
   One skilled in the art will appreciate that many variations are possible within the scope of the present invention. Although the preferred embodiments of the present invention is described herein within the context of an enclosure for containing a computer system, those skilled in the art will appreciate that the present invention may be practiced with an enclosure for containing any type of system. For example, the present invention may be practiced with an enclosure for an air treatment system, such as an air filter, air cleaner, dehumidifier, air conditioner, heater, or the like in lieu of an enclosure containing a computer system. Thus, while the present invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that these and other changes in form and details may be made therein without departing from the spirit and scope of the present invention.