Abstract:
Multiple Helmholtz resonators are combined serially and dynamically to mitigate and/or overcome acoustic noise filtering problems. One Helmholtz resonator is attached to a duct having an acoustic flow path channel containing undesired acoustic signals (noise) and is considered to be an immovable Helmholtz resonator with respect to that flow channel, while at least one movable Helmholtz resonator is movably and acoustically coupled to the immovable Helmholtz resonator. The immovable and movable Helmholtz resonators are acoustically coupled together to adjustably filter two resonant frequencies in the flow path channel with a feedback control system that adjusts the position of the movable Helmholtz resonator in response to the differences in pre-filtered noise versus filtered noise.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to devices for dampening noise, and particularly to multiple Helmholtz resonators that are acoustically coupled together to quickly and adjustably filter out more than one acoustic frequency. 
     2. Description of the Related Art 
     The Helmholtz resonator was first designed by Hermann von Helmholtz in the 1850s. The Helmholtz resonator has a cavity communicating with a main duct through a neck and is used to effectively attenuate narrow-band, low frequency noise. Narrow-band noise in the form of tonal noise is quite common in the case of rotating machinery, and in particular, in applications involving engine breathing systems. For example, an engine exhaust flow path may pass by an opening or throat of a Helmholtz resonator. Beyond the opening is a cavity in the Helmholtz resonator. The dimensions of the throat and cavity, in conjunction with the makeup of the gases involved, will determine the precise resonant frequency absorbed by the Helmholtz resonator. 
     The Helmholtz resonator is often looked at as an acoustic wave equivalent of a spring-mass system, where the spring represents the cavity and the mass represents the neck. Thus, the resonator&#39;s frequency and the transmission loss can be readily determined. 
     While Helmholtz resonators have been used to dampen specific frequencies, and multiple Helmholtz resonators can dampen a corresponding number of frequencies, it is often impractical to employ multiple, separate Helmholtz resonators. Even where the use of multiple Helmholtz resonators is not a problem, their use is ineffective in situations where the ideal frequencies to be filtered are not sufficiently static, especially where those frequencies change quickly. Thus, multiple Helmholtz resonators solving the aforementioned problems are desired. 
     SUMMARY OF THE INVENTION 
     The multiple Helmholtz resonators are combined serially and dynamically to mitigate and/or overcome the aforementioned problems. One Helmholtz resonator is attached to the flow path channel and is considered to be an immovable Helmholtz resonator with respect to that flow channel, while at least one movable Helmholtz resonator is movably coupled adjacent the immovable Helmholtz resonator. The immovable and movable Helmholtz resonators are acoustically coupled together so that they can adjustably filter two frequencies in the flow path channel. 
     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. 1  is a diagram of multiple Helmholtz resonators, illustrating a model identifying variables associated with a dual resonant frequencies formula. 
         FIG. 2  is a perspective view of multiple Helmholtz resonators according to the present invention. 
         FIG. 3  is a diagrammatic side view in section of multiple Helmholtz resonators of the present invention, shown in a first position in which the immovable Helmholtz resonator provides a single Helmholtz resonator operably connected to a duct. 
         FIG. 4  is a diagrammatic front view in section of multiple Helmholtz resonators of the present invention. 
         FIG. 5A  is a diagrammatic single Helmholtz resonator of the present invention and a corresponding mass-spring physical model. 
         FIG. 5B  is a graph of transmission loss (TL) in decibels (dB) versus frequency in Hertz (Hz) corresponding to  FIG. 5A . 
         FIG. 6  is side view in section of the multiple Helmholtz resonators of  FIG. 3 , shown in a second position in which the movable Helmholtz resonator is aligned with the immovable Helmholtz resonator to provide two Helmholtz resonators connected in series operably connected to the duct. 
         FIG. 7A  is a schematic diagram showing aligned multiple Helmholtz resonators and a corresponding mass-spring physical model. 
         FIG. 7B  is a graph of transmission loss (TL) in decibels (dB) versus frequency in Hertz (Hz) corresponding to  FIG. 7A . 
         FIG. 8  is schematic diagram of a control system for adaptively damping acoustic noise using multiple Helmholtz resonators according to the present invention. 
     
    
    
     Similar reference characters denote corresponding features consistently throughout the attached drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The multiple Helmholtz resonators adaptively and adjustably filter more than one acoustic frequency, often including more than one acoustic resonant frequency. 
       FIG. 1  shows a diagram of multiple Helmholtz resonators, illustrating an analytical model associated with a dual resonant frequencies formula and diagrammatically identifying variables used in the formula. The model is used in combination with the following formula to determine the dual resonant acoustic properties of the multiple Helmholtz resonators that employ two Helmholtz resonators arranged serially: 
               f     1   ,   2       =         c   o       2   ⁢       2   ⁢           ⁢   π           ⁢                 (         A     C   ⁢           ⁢   1           l     C   ⁢           ⁢   1     ′     ⁢     V   1         +       A     C   ⁢           ⁢   2           l     C   ⁢           ⁢   2     ′     ⁢     V   1         +       A     C   ⁢           ⁢   2           l     C   ⁢           ⁢   2     ′     ⁢     V   2           )     ±                     (         A     C   ⁢           ⁢   1           l     C   ⁢           ⁢   1     ′     ⁢     V   1         +       A     C   ⁢           ⁢   2           l     C   ⁢           ⁢   2     ′     ⁢     V   1         +       A     C   ⁢           ⁢   2           l     C   ⁢           ⁢   2     ′     ⁢     V   2           )     2     -     4   ⁢     (         A     C   ⁢           ⁢   1           l     C   ⁢           ⁢   1     ′     ⁢     V   1         ⁢       A     C   ⁢           ⁢   2           l     C   ⁢           ⁢   2     ′     ⁢     V   2           )                                 
where the formula and  FIG. 1  are shown in “Dual Helmholtz resonator,” by M. B. Xu, A. Selamet and H. Kim, as published in  Applied Acoustics , Vol. 71, Issue 9 (September 2010) pp. 822-829. The variables in the formula are as shown in  FIG. 1  and described in the Xu et al. articles, which is incorporated herein by reference.
 
       FIG. 2  shows a perspective view of multiple Helmholtz resonators  200  acoustically coupled to a duct  205  that carries the sounds to be filtered through a gas medium. The duct  205  acts as an acoustic waveguide for transporting undesired sounds for filtering. Although the multiple Helmholtz resonators are described with respect to gaseous media, any media capable of carrying sound could be used, including liquid and solid media. The duct  205  is open to an immovable Helmholtz resonator  210  through a neck aperture that functions as the neck of the immovable Helmholtz resonator  210 . The immovable Helmholtz resonator  210  is not required to be completely immovable, and its designated name is used to bring into contrast one or more other Helmholtz resonators that move by design. The immovable Helmholtz resonator  210  is essentially fixed above the neck aperture in the duct  205 . For example, the immovable Helmholtz resonator  210  may be welded to the duct  205 . The immovable Helmholtz resonator  210  also has an upper aperture, i.e., it does not have a top and can be considered topless. 
     Shown above the immovable Helmholtz resonator  210  in  FIG. 2  is a movable laminate plate  215 . The movable laminate is a rectangular plate in shape and moves along the same longitudinal axis as the duct  205 , as indicated by the arrows. The movable laminate plate  215  has a neck aperture in it for allowing sounds to pass through the plate. In the case of the movable laminate plate  215 , sounds pass through the neck aperture to a movable Helmholtz resonator  220 . The movable Helmholtz resonator  220  is attached to the movable laminate plate  215  so it will move with respect to the immovable Helmholtz resonator  210  as the position of the movable laminate plate  215  is varied. 
     The primary purpose of the movable laminate plate  215  is to movably position the neck aperture of the movable Helmholtz resonator  220  into alignment above the upper (topless) aperture of the immovable Helmholtz resonator  210  to bring the Helmholtz resonators  210 ,  220  into various phases of acoustic alignment. A lower surface of the movable laminate can also completely cover the upper aperture of the immovable Helmholtz cavity to cause the immovable Helmholtz resonator to function as a single Helmholtz resonator, if desired. If the movable laminate slides further, the movable Helmholtz resonator  220  can be positioned directly above the immovable Helmholtz resonator  210 . In this position, the Helmholtz resonators  210 ,  220  can be considered to form a “neck-cavity-neck-cavity”acoustic filtering system having two Helmholtz resonators  210 ,  220  connected in series. This arrangement of Helmholtz resonators  210 ,  220  is capable of attenuating two narrow-band resonant frequency noises, as opposed to a single narrow-band resonant frequency for a single Helmholtz resonator. The formula and model for this is shown above with regard to  FIG. 1 . 
     Alternatively, if desired, the immovable Helmholtz resonator  210  and a plurality (n) of movable resonators  220  can be acoustically coupled to form a stack or series of Helmholtz resonators  210 ,  220  to attenuate (n) narrow-band noises. Partial alignment of Helmholtz resonators may also be desirable in some acoustic filtering cases. 
       FIG. 3  shows a side view in section of the multiple Helmholtz resonators  200  in a first position. Sound, i.e., pressure waves, is shown moving from left to right in the duct  205 . The volume of the sound is indicated by the large arrow inside the duct  205  on the left and it is reduced in volume by the multiple Helmholtz resonators  200 , as indicated by the smaller arrow inside the duct  205  on the right. The neck aperture in the duct  205  leading to the immovable Helmholtz resonator  210  can be easily seen here. The multiple Helmholtz resonators  200  use motorized wheels  325 , each connected to an anchor  330 , to adjust the position of the movable laminate plate  215  and the movable Helmholtz resonator  220  relative to the duct  205  and the immovable Helmholtz resonator  210 . The motorized wheels  325  move in response to a control signal. The multiple Helmholtz resonators  200  are not restricted to motorized wheels  325 . Rollers, linear motors, linear actuators, and other apparatus for moving the movable laminate are envisioned and compatible with the multiple Helmholtz resonators  200 . 
     The upper aperture (topless portion) in the immovable Helmholtz resonator  210  is completely covered by the movable laminate plate  215  in  FIG. 3 . The movable laminate plate  215  has been positioned so that the neck aperture in the movable laminate plate  215  leading to the movable Helmholtz resonator  220  does not overlap at all with the upper aperture of the immovable Helmholtz resonator  210 . Thus,  FIG. 3  illustrates the immovable Helmholtz resonator  210  acting as single Helmholtz resonator, acoustically separated from the movable Helmholtz resonator  220 . This situation is modeled in  FIGS. 5A and 5B , as described herein. 
       FIG. 4  shows a front view in section of the multiple Helmholtz resonators  200 . The motorized wheels  325  are shown in contract with the movable laminate plate  215  in order to position the movable laminate plate  215 , as described herein. The movable laminate plate  215  is in contact with an L-channel  435 , as shown. The L-channel  435  is shaped like the letter “L” and presents a low-friction surface to the movable laminate plate  215  to reduce the load experienced by the motorized wheels  325 . The movable Helmholtz resonator  220  and movable laminate plate  215  are moved by the motorized wheels  325  relative to the immovable Helmholtz resonator  210  and duet  205 , as described herein. 
       FIG. 5A  shows a single Helmholtz resonator and a corresponding mass-spring physical model. A single Helmholtz resonator represents the immovable Helmholtz resonator  210  being completely covered by the movable laminate plate  215  (as shown in  FIG. 3 ). The neck aperture in the duct  205  is modeled as a mass  540 . The immovable Helmholtz resonator  210  has dimensions giving rise to a volume comparable to a spring  545 . The spring  545  is attached to both the mass  540  and a relatively immovable object  550  for modeling purposes and to model the frequency properties of the immovable Helmholtz resonator  210 , as shown. The formula for the resonant frequency of a single Helmholtz resonator is: 
               f   r     =         c   o       2   ⁢           ⁢   π       ⁢       (       A     C   ⁢           ⁢   1           l     C   ⁢           ⁢   1     ′     ⁢     V   1         )               
where A C1  is the area of the neck, V 1  is the volume of the resonator, C o  is the velocity of sound in air, and l′ C1  is the length of the neck.
 
       FIG. 5B  is a graph of transmission loss (TL) in decibels (dB) versus frequency in Hertz (Hz) corresponding to  FIG. 5A . As shown in  FIG. 5A , a frequency response  555  associated with a single Helmholtz resonator has a resonant frequency f r  where the attenuation of sound is greatest. Importantly, the single Helmholtz resonator modeled in  FIG. 5A  corresponds to a single resonant frequency f r  as shown in  FIG. 5B . 
       FIG. 6A  shows a side view in section of the multiple Helmholtz resonators  200  in a second position.  FIG. 6A  corresponds to  FIG. 3 , except that the motorized wheels  325  have repositioned the movable laminate plate  215  so that the movable Helmholtz resonator  220  is positioned directly above the immovable Helmholtz resonator  210 . In this arrangement the immovable Helmholtz resonator  210  is acoustically coupled to the movable Helmholtz resonator  220 , thereby producing a combined frequency response, as described with regard to  FIGS. 7A and 7B . In short, the arrangement shown in  FIG. 6  enables two primary resonant frequencies to be filtered out of the noise in the duet  205 . Additional resonant frequencies can be filtered with additional movable Helmholtz resonators stacked atop the movable Helmholtz resonator  220 . 
       FIG. 7A  shows aligned multiple Helmholtz resonators and a corresponding mass-spring physical model. The aligned multiple Helmholtz resonators correspond to the aligned multiple Helmholtz resonators  210 ,  220  shown in  FIG. 6 . As shown before in  FIG. 5A , in FIG.  7 A the neck aperture in the duet  205  is modeled as a mass  540 . The immovable Helmholtz resonator  210  has dimensions giving rise to a volume comparable to a spring  545 . However, the spring  545  is shown here attached to a mass  760  corresponding to the neck aperture in the movable laminate plate  215 . The mass  760  is connected to a spring  765 . The movable Helmholtz resonator  220  has dimensions giving rise to a volume comparable to the spring  765 . The spring  765  is attached to the relatively immovable object  550  and the mass  760  to model the frequency properties of the combined immovable Helmholtz resonator  210  and movable Helmholtz resonator  220 , as shown. 
       FIG. 7B  is a graph of transmission loss (TL) in decibels (dB) versus frequency in Hertz (Hz) corresponding to  FIG. 7A . As shown in  FIG. 7A , a frequency response  767  associated with a dual Helmholtz resonator has a first resonant frequency f r1  and a second resonant frequency f r2  where the attenuation of sound is greatest. Importantly, the dual Helmholtz resonator modeled in  FIG. 7A  corresponds to dual resonant frequencies f r1  and f r2  as shown in  FIG. 7B  and acoustic filtering is improved as compared to the single Helmholtz resonator model in  FIG. 5B . 
       FIG. 8  is diagram of a control system for adaptively damping noise using multiple Helmholtz resonators of the present invention. The starting arrangement shown in  FIG. 8  corresponds to that shown in  FIG. 3 , but is adjusted by a control system  800  to an arrangement such as shown in  FIG. 6 . Intermediate positions may also be desirable. The control system  800  uses an input microphone  870  to detect sound before filtering by the multiple Helmholtz resonators  200  and produces corresponding input signals. An error microphone  875  detects sound after filtering by the multiple Helmholtz resonators  200  and produces corresponding error signals. Signals from the input microphone  870  and error microphone  875  are transmitted to a controller  880  that includes a microprocessor. The controller  880  processes information from the microphones  870 ,  875  to produce and transmit control signals to the motorized wheels  325 , which slide the movable laminate plate  215  in response to those signals. Adjustments in the positioning of the movable Helmholtz resonator  220  on the movable laminate plate  215  by the controller  880  enables the multiple Helmholtz resonators  200  to generate the desired transmission loss spectrum. The controller  880  uses a feedback mechanism to control the positioning of the movable Helmholtz resonator  220  by analyzing differences between input signals from the input microphone  870 , representing pre-filtered noise, and error signals from the error microphone  875 , representing filtered noise, to obtain the desired or best acoustic filtering. 
     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.