Abstract:
The expandable chamber acoustic silencer may be installed at the inlet or outlet of virtually any mechanism that processes air or other gas flow therethrough, to reduce the audible output of the gas flow. The silencer may be used to reduce the noise produced in an air conditioning system, in the inlet or outlet side of an air compressor, or as a muffler for an internal combustion engine, among other applications. The device includes an expansion chamber having adjustable walls driven by actuators installed within the walls to adjust the cross-sectional area of the chamber, with a portion of the walls being formed of a flexible or resilient material to enable such expansion and retraction. One or more microphones are installed at the outlet and/or inlet ends of the silencer and communicate with a controller that operates the actuators in accordance with a predetermined algorithm.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to sound attenuation devices, and particularly to an expandable chamber acoustic silencer having a variable cross-sectional area controlled in accordance with signals received from a controller. 
     2. Description of the Related Art 
     It is well known that air or other gas flow and/or expansion in or from a closed system results in the production of sound. This may be a desirable outcome, and certain devices (e.g., musical instruments, sirens, etc.) are deliberately configured to produce an audible output. However, many other devices produce an audible output(s) as an unintentional side effect of their operation. Examples include air conditioning systems having fan or blower supplied airflow and intake systems for air compressors and internal combustion engines. Internal combustion engines are also well known to produce relatively loud and obtrusive exhaust noise due to the expansion of the heated gases used to produce the power output developed by the engine. 
     In many cases the audible output of the device is quite variable, depending upon the amount of air or gas flow through the device, among other factors. Generally speaking, the greater the air or gas flow through the device, the louder the sound output, although the operating frequency (e.g., the RPM of an internal combustion engine) and system resonance(s) also have a great deal of effect. In many cases, particularly in the field of musical instruments, the audible output may be varied in tone and/or intensity by varying the internal cross-sectional area of the instrument relative to the inlet and/or outlet cross-sectional areas. 
     In the above examples, where audible output is an undesired side effect of the operation of the device, e.g., in air compressors and engines, innumerable devices have been developed in the past to reduce the sound output of such devices. Most such devices are formed of relatively thin sheet metal and have a labyrinthine path therein for the air or gas to follow. Others rely upon some form of porous barrier that may also serve as a filtration system for incoming air to the system. Still others may utilize some form of active control, e.g., generating sound that is out of phase with the undesired sound output so that the generated sound substantially cancels the sound output of the device that produces the unwanted sound. Such systems not only require microphones to receive the sound output of the machine, but also require some form of audible output device (i.e., a speaker or speakers) to produce the out of phase audible signal to cancel the unwanted sound. 
     While such devices are effective to some degree, none have proven entirely satisfactory. Thus, an expandable chamber acoustic silencer solving the aforementioned problems is desired. 
     SUMMARY OF THE INVENTION 
     The expandable chamber acoustic silencer comprises various embodiments of a reactive silencer or muffler that detects the sound input and/or output of a device by means of microphone pickups, and uses those detected sounds to direct mechanisms that adjust the cross-sectional area of an expansion chamber to change its resonant frequency in accordance with the detected sound input, thereby substantially canceling the sound input to the muffler or silencer. Certain embodiments may have a sound detection microphone at only the output end of the silencer device, while other embodiments may include such microphones at both the input and the output of the device. The muffler or silencer may be formed to have virtually any practicable shape and interior gas flow path, but all embodiments include a relatively thick wall with actuators installed therein. The actuators extend or retract in accordance with signals from a controller to drive sections of the wall outward or inward, thereby adjusting the cross-sectional area of the muffler or silencer relative to its inlet and/or outlet cross-sectional areas to change its resonant frequency and substantially cancel the sound being input to the muffler or silencer device. 
     These and other features of the present invention will become readily apparent upon further review of the following specification and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a top perspective view in section of an expandable chamber acoustic silencer according to the present invention, illustrating its internal structure. 
         FIG. 1B  is an elevation view in section of the expandable chamber acoustic silencer of  FIG. 1A  according to the present invention, particularly illustrating its internal cross-sectional area. 
         FIG. 2A  is a top plan view in section of the expandable chamber acoustic silencer of  FIG. 1 , including a schematic representation of an electronic control system having both input and output sound level detection microphones. 
         FIG. 2B  is a top plan view in section of the expandable chamber acoustic silencer of  FIG. 1 , including a schematic representation of an electronic control system having only an output sound level detection microphone. 
         FIG. 3  is a partial top detail view in section of one corner of an expandable chamber acoustic silencer according to the present invention, illustrating a first embodiment of an expandable wall section incorporating a resilient material. 
         FIG. 4  is a partial top detail view in section of one corner of an expandable chamber acoustic silencer according to the present invention, illustrating a second embodiment of an expandable wall section incorporating a metal bellows material. 
         FIG. 5  is a perspective view of another embodiment of an expandable chamber acoustic silencer according to the present invention, wherein the device has a generally cylindrical configuration. 
         FIG. 6  is a diametric cross-sectional view of the expandable chamber acoustic silencer of  FIG. 5 , illustrating its general internal structure. 
     
    
    
     Similar reference characters denote corresponding features consistently throughout the attached drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The expandable chamber acoustic silencer comprises various embodiments of a reactive-type silencer of the expansion chamber variety that serves to reduce the audible output of various mechanical devices, such as air compressors, air conditioning systems, internal combustion engines, and other devices that process or transfer air or other gases therethrough during their operation. Silencers are classified into two categories: (a) the passive type, and (b) the active type. 
     The passive type includes silencers where the sound is attenuated by absorption or reflection of the acoustic energy within the silencer. Two subcategories of the passive type are: (i) dissipative silencers, which contain sound absorptive material capable of converting sound energy into heat; and (ii) reactive silencers (such as expansion chambers and side branch resonators), which depend on the reflection or expansion of sound waves with corresponding self-destruction as a noise reduction mechanism. A combination of dissipative and reactive silencers is noted in practice, the automotive muffler being a common example. 
     The active-type silencer is a silencer in which the noise is cancelled by electronically generating an “anti-noise” field, which is superimposed on the noise field. With careful matching of amplitude and phase using feedforward and feedback control techniques, a cancellation process results, with lower noise levels. 
     The expandable chamber acoustic silencer is a reactive-type silencer, based on reflective self-destruction of unwanted acoustic waves. The silencer associated with these devices may be known by the term “silencer,” “muffler,” or similar term indicating its sound attenuation properties, the term “muffler” commonly being used for such devices used with internal combustion engines. Each of the embodiments of the silencer includes at least one movable wall that adjusts the cross-sectional area of the expansion chamber using a controller and at least one actuator controlling the positioning of the movable wall(s). 
       FIG. 1A  of the drawings provides an illustration in section through the center of a first embodiment of an expandable chamber acoustic silencer, designated as silencer or muffler  10 . It will be understood that the silencer  10  of  FIG. 1A  would normally be closed by additional structure that is a mirror image to the portion illustrated in  FIG. 1A , thereby forming a substantially closed container  12 . A lateral elevation view in section of such a closed silencer container  12  illustrating the variable internal cross-sectional area of the device is provided by  FIG. 1B . An alternative embodiment differing primarily in external shape is illustrated in  FIG. 5  of the drawings, which clearly shows the substantially closed configuration, except for the inlet and outlet. The silencer  10  of  FIGS. 1A and 1B  has a configuration suitable for use as a muffler for an internal combustion engine, having an inlet passage  14  and outlet passage  16 . Conventional sound suppression means, (not shown) may be disposed within the closed container  12 , e.g., dissipative material, loose glass fiber material, baffles, and/or other pipe or duct patterns or configurations, etc., but is omitted from the drawing for clarity. 
     The container  12  formed by the silencer  10  is defined by a fixed central wall portion  20  and by first and second movable wall portions  22  and  24  extending outwardly therefrom. Each wall portion, i.e., the single fixed wall portion  20  and the two movable wall portions  22  and  24 , may be formed of an inner panel and an outer panel. The wall portion  20  of  FIGS. 1A and 1B  thus includes outer panel  20   a  and inner panel  20   b . The first movable wall portion  22  comprises movable first outer panel  22   a  and movable first inner panel  22   b , and the second movable wall portion  24  comprises movable second outer panel  24   a  and movable second inner panel  24   b . First outer and inner flexibly expandable portions or members  26   a  and  26   b  join the respective first outer and inner panels  22   a ,  22   b  to the respective outer and inner panels  20   a  and  20   b , and second outer and inner flexibly expandable portions or members  28   a  and  28   b  connect the respective second outer and inner panels  24   a ,  24   b  to the respective outer and inner panels  20   a  and  20   b . The flexibly expandable material may be any suitable material, including resilient elastomers for silencer applications where high temperatures are not a concern. As the flexibly expandable material stretches and retracts in accordance with a mechanism described further below, the internal cross-sectional area of the device is varied or adjusted accordingly, as shown in  FIGS. 2A through 4 . 
     The spans between the various outer and inner panels, and particularly the outer and inner panels  20   a  and  20   b , define an actuator housing  30  therebetween, for housing or containing the actuators of the system, as discussed further below. The wall portion  20 , movable wall portions  22  and  24 , and flexibly expandable portions  26   a ,  26   b ,  28   a , and  28   b  define the variable internal width  32   a  of the device, with the internal height of the container  12  being indicated by the vertical dimension  32   b  in  FIGS. 1A and 1B , (The lower limit of the vertical dimension  32   b  in  FIG. 1A  is the inner panel  20   b , the upper limit being designated by a line representing the opposite upper surface of the inner panel  20   b , not shown in  FIG. 1A .) These two mutually orthogonal dimensions  32   a  and  32   b , when taken together, define the variable internal cross-sectional area of the silencer  10 . As the movable wall portions  22  and  24  are moved outwardly and inwardly by the mechanism described further below, the internal width  32   a , and accordingly the internal cross-sectional area, is varied accordingly to adjust the resonance cavity of the device according to the sound input to the device, thereby reducing the audible output from the silencer  10 . 
       FIGS. 2A and 2B  illustrate two embodiments of the silencer  10 , which differ only in the location(s) of the microphone or microphones used to detect the sound level being processed by the device. The basic physical structure of the silencer  10  of  FIGS. 2A and 2B  is the same as that illustrated in  FIGS. 1A and 1B , corresponding designations being used to designate corresponding components. The actuator housings  30  between the inner and outer panels of the container walls have actuators  34  installed therein. The actuators  34  may comprise any practicable type of actuator, e.g., electrically powered, pneumatic, hydraulic, etc. Preferably, the actuators  34  comprise conventional electromechanical linear actuators, in which a shaft selectively extends linearly from the actuator body when an appropriate electrical signal is received to drive the actuator  34 . The actuators  34  may be located within cavities or housings  30  within the fixed wall structure  20  of the device, their shafts being connected to structure within the two movable wall structures  22  and  24  to selectively expand those walls outwardly or retract them inwardly. 
     An actuator controller  36  is provided with the system, the controller  36  being connected to and communicating electrically with the actuators  34 . The controller  36 , in turn, receives and processes acoustic signals from one or more microphones associated with the system. In  FIG. 2A , a first microphone  38   a  is located at the inlet  14  of the silencer  10 , and a second microphone  38   b  is located at the outlet  18  of the silencer  10  (i.e., a feedforward system). These two microphones  38   a  and  38   b  pick up acoustic signals from the inlet  14  and from the outlet  18 , respectively, of the silencer  10 , and transmit those signals electronically to the controller  36 . The controller  36  processes those signals and transmits appropriate commands to the actuators  30 . The controller  36  processes the signals received from the microphone(s), and commands the actuators  30  correspondingly. The actuators  34  then extend or retract to drive the movable walls  22  and  24  outwardly or inwardly, thereby adjusting the interior cross-sectional area  32  of the silencer  10  to control the sound output from the silencer  10 . 
     The controller  36  processes the signals received from the microphone(s)  38   a ,  38   b  in accordance with the algorithm: 
             TL   =         10   ⁢           ⁢     log   10       |     1   +       1   4     ⁢       (     m   -     1   ⁢     /     ⁢   m       )     2     ⁢     sin   2     ⁢   kL       ⁢     |   2     ⁢           ⁢     where   ⁢           ⁢   k       =         2   ⁢   π     λ     =       2   ⁢   π   ⁢           ⁢   f     c               
and m=A 2 /A 1  in which TL is the Transmission Loss, A 2  is the cross sectional area of the container  12  as defined by the variable internal width  32   a  and the internal height  32   b , A 1  is the cross sectional area of inlet passage  14 , f is the sound frequency, c is the velocity of sound in the working medium (e.g., air), and L is the length of the container  12 , λ is the wavelength of the sound wave, and k is the wavenumber. It will be readily apparent that the Transmission Loss TL depends upon the ratio m between the cross-sectional area A 2  of the container  12  and the cross-sectional area A 1  of the inlet, so that automatically adjusting the cross-sectional area of the container  12  while the cross-sectional area of the inlet  14  remains fixed permits the reactance of the expansion chamber to be adjusted so that the transmission loss cancels the noise when the noise level changes.
 
       FIG. 2B  is nearly identical to  FIG. 2A , with the exception that the silencer  10  and control system of  FIG. 2B  includes only a single microphone  38   b  (i.e., a feedback only system) located at the outlet  18  of the device. It should be noted that the electrical wiring illustrated in  FIGS. 2A and 2B  is intended as a schematic illustration only, and is shown to the exterior of the internal cavity or space between the inner and outer panels of the device for clarity in the drawings. It is anticipated that the actual wiring could be placed between the inner and outer panels to produce a neater installation, depending upon the temperature to which the silencer device is to be subjected. 
       FIGS. 3 and 4  provide details of an exemplary actuator installation and alternative materials that may be used to form the expandable sections of the device. The basic structure of the silencer illustrated in both  FIGS. 3 and 4  is substantially identical to that illustrated in  FIGS. 1A ,  1 B,  2 A, and  2 B, with only the material of the flexibly expandable portions differing between the two Figures. In  FIG. 3 , a resilient and flexible elastomer material  40  is used to form the flexibly expandable portions  28   a  and  28   b  of the device, it being understood that the silencer device of  FIG. 3  would not be used in high temperature installations due to the relatively low melting point of such elastomer material  40 . However, the silencer device of  FIG. 4  incorporates a flexible corrugated metal bellows  42  for the flexibly expandable portions  28   a ,  28   b . Such material  42  is not as subject to deterioration at high temperatures as the elastomer material  40  used in the silencer of  FIG. 3 , and therefore the silencer of  FIG. 4  might be incorporated in the exhaust system of an internal combustion engine. 
       FIGS. 5 and 6  of the drawings illustrate another embodiment of the silencer, designated as silencer  110 . The silencer  110  of  FIGS. 5 and 6  has a generally cylindrical configuration for its container  112 , as opposed to the generally rectangular parallelepiped (with rounded corners) configuration of the silencer  10 . However, the same basic principles of operation apply to both silencers  10  and  110 . The silencer  110  includes a fixed central inner wall or panel  120  that may also serve as at least a portion of the sound suppression structure of the device. A plurality of rigid but movable arcuate outer wall panel sections  122 ,  124 , and  126  extend circumferentially about the central inner wall  120 , the arcuate sections  122  through  126  being joined by corresponding flexible sections  128 ,  130 , and  132  to form a complete cylindrical shape. The ends of the cylinder are closed about the inlet  114  and outlet  116  by first and second flexible panels  134  and  136  to close the container  112 . The flexible materials used for the sections or panels  128  through  136  may comprise resilient elastomer materials for relatively low temperature use, or flexible metal for high temperature use, with the end panels  134  and  136  each having a series of circumferential corrugations in such a construction. 
     The actuators  34  may be identical to the actuators  34  of the embodiments of  FIGS. 1A through 4 , but are arranged radially within the variable internal volume of the cylindrical silencer  110 . The actuators  34  are anchored at their inboard ends to the fixed inner wall panel  120 , and each of the actuators extends radially outward to connect to and drive one of the rigid arcuate outer wall panel sections  122  through  126 . The control system, comprising one or more microphones and a controller, is substantially the same as that provided for the embodiments illustrated in  FIGS. 2A and 2B . Operation of the cylindrical silencer embodiment  110  of  FIGS. 5 and 6  is substantially the same as that of the rectangular parallelepiped embodiments of  FIGS. 1A through 4 . However, the function of each of the embodiments described herein is substantially the same, the actuators being adjusted inward and outward by the controller in accordance with signals received from the microphone(s) to adjust the variable internal cross-sectional area of the expansion chamber silencer device, thereby reducing the sound output level of the silencer. 
     It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.