Patent Publication Number: US-6343127-B1

Title: Active noise control system for closed spaces such as aircraft cabin

Description:
FIELD OF THE INVENTION 
     The present invention is directed to active noise control. More particularly, this invention is an active noise control system for canceling or reducing unwanted noise in a closed space. 
     BACKGROUND OF THE INVENTION 
     Active noise control systems are known which use an inverse-phase sound wave to cancel a disturbance. U.S. Pat. No. 4,562,589 to Warnaka et al. entitled “Active Attenuation of Noise in a Closed Structure” teaches a system for active attenuation of noise within a closed structure such as an aircraft cabin which operates to introduce a canceling sound wave form (anti-noise) into a closed structure which is responsive to an error signal. The system includes an adaptive filter for updating the cancellation signal sent to the transducers (speakers) to produce the canceling wave form. Although this system was a phenomenal advance for its time, it is somewhat inefficient at reducing noise within the closed space. Furthermore, the components are subject to damage upon large impact loads. 
     SUMMARY OF THE INVENTION 
     In light of the advantages and drawbacks to the prior art, the present invention is directed to active noise control system for reducing noise within a closed space caused by a source of disturbance such as from a noise and/or vibration source. More particularly, this invention is an efficient active noise control system comprising a reference sensor for deriving a reference signal indicative of a source of disturbance which causes a disturbing noise to be produced in the closed space, an error sensor for sensing a residual sound pressure level and providing a signal indicative thereof to an electronic controller. The electronic controller includes an adaptive filter for providing a canceling signal to a speaker for generating a canceling wave form. The canceling wave form endeavors to cancel the noise caused in the closed space by the source of disturbance. In the present invention, the speakers are inverted in their enclosures and attached directly to the trim of the closed space, thus, providing for more efficient noise cancellation. Preferably, the enclosures are soft-mounted by elastomer isolators or mounts to protect the speaker components from damage to transient loads applied thereto. Each enclosure assembly and installation preferably performs the function of a planar wave guide and constrains the canceling wave form such that it emanates from the confines of the enclosure in a direction which is substantially parallel to the trim&#39;s surface. Further inventive features of the present invention will be apparent from the following detailed description, claims and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings which form a part of the specification, illustrate several key embodiments of the present invention. The drawings and description together, serve to fully explain the invention. In the drawings: 
     FIG. 1 is a schematic depiction of an embodiment of the active noise control system of the present invention in a propeller-driven aircraft, 
     FIG. 2 is a side view, schematic depiction of an embodiment of the active noise control system illustrating under seat and inverse mounting of the speaker assemblies, 
     FIG. 3 is a frontal view, schematic depiction of another embodiment of the active noise control system illustrating reference sensors adjacent the jet engines and error sensors adjacent the interior trim, 
     FIG. 4 is a schematic depiction of another embodiment of active noise control system using a reference sensor located outside the closed space which receives far-field noise from a source of noise disturbance, 
     FIG. 5 is a schematic depiction of another embodiment of active noise control system using a reference sensor directly adjacent the noise source which is outside the closed space, 
     FIG. 6 is a schematic depiction of another embodiment of active noise control system using a sensor for deriving a reference signal indicative of a vibration emanating from vibration source where the vibration source causes a noise to develop in the closed space, 
     FIG. 7 is a schematic depiction of another embodiment of active noise control system operating in the environment of an automobile passenger compartment, 
     FIG. 8 is a schematic depiction of an inversely-mounted speaker system that includes grommet-type mounts and a wall mounted orientation, 
     FIG. 9 is a schematic depiction of an inversely-mounted speaker system that includes shear-type mounts in a wall mounted orientation, 
     FIG. 10 is a schematic depiction of an inversely-mounted speaker system that includes grommet-type mounts in a floor mounted orientation, and 
     FIG. 11 is a bottom plan view depiction of an inversely-mounted speaker system that includes offset positioning of the speaker and a low-frequency reflex port. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A schematic depiction of an embodiment of the active noise control system of the present invention is shown in FIG. 1 generally at  20   a.  It should be noted that when comparing the various embodiments that like numerals have been used to denote like elements. The system  20   a  is shown with reference to an aircraft application. However, it should be understood that the system  20   a  will operate in any closed space to reduce unwanted noise within. The aircraft shown in this embodiment is a propeller driven aircraft and includes a fuselage  34  having a nose section  21 , an aft section  23 , and interior surface  27  and exterior surface  29 . Interior surface  27  has trim  25  attached thereto by fasteners, adhesive or the like. The trim  25  includes bulkheads  31   a,    31   b,    31   c  and floor  32  (similar to that shown in FIG. 2) and defines and forms the closed space of the aircraft cabin  37   a.  The closed space is generally where the human occupants are resident. It is, therefore, for this reason that a quite environment is desired. 
     In this embodiment, the propellers  35   a  and  35   a ′ are driven by engines  36   a  and  36   a ′ and cause propeller wash to impinge on the exterior surface  29  of the fuselage  34  along the plane of action indicated by lines L and generate a sound pressure level within the aircraft cabin  37   a.  The system  20   a  includes means for deriving a reference signal indicative of the disturbance which is causing the unwanted noise in the closed space. In this case, two reference signals are used and the reference signals are derived from reference sensors  26   a  and  26   a ′. These sensors  26   a  and  26   a ′ are preferably accelerometers that are placed on or directly adjacent the interior surface  27  of the fuselage  34  in the plane of action of the propeller wash. Alternatively, microphones may be used. Reference sensors  26   a  and  26   a ′ should be placed at a point where the propeller wash disturbance of the fuselage  34  is the greatest. 
     In general, since the predominant tone to be canceled in the closed space in a propeller driven aircraft is the BPF (standing for Blade Pass Frequency) tone caused by the propeller wash impinging on the exterior surface  29  of the fuselage  34 , the BPF tone is what is needed for the reference signal. In other embodiments, other reference signals such as tachometer signals, engine signals indicative of the rotating speed, or other signals indicative of the noise may be required. The key is that the reference signal be indicative of the phase relationship and frequency of the disturbance. Depending on the control method used, the magnitude or frequency of the reference signal may also be important. In this embodiment, the reference signal is directed to electronic controller  22   a  via wire lead  41 . The reference signal may be band-pass filtered, high pass filtered, or low pass filtered, used directly or used to trigger a wave form generator. The conditioning of the signal will depend on the type of filtering and control method used. Power  24   a  is preferably supplied by the aircraft&#39;s resident power supply. 
     The system  20   a  in this embodiment includes a series of speaker assemblies  50 . A description will be detailed as to one assembly  50  only. Other assemblies  50  are preferably similar in makeup. The system  20   a  includes speaker means for generating a canceling wave form for reducing the residual sound pressure level within the aircraft cabin  37   a.  Typically, the control will concentrate on one or more dominant and annoying tones. As a goal, the tonal noise would be completely eliminated, however, usually this is not obtainable, thus, it is realistically desirable to globally reduce the sound pressure level in the aircraft cabin  37   a  to a minimum. 
     In one novel aspect of the present invention, the speaker  30  is rigidly attached to a enclosure  33  by fasteners or the like. The enclosure  33 , which is preferably box like, is then inversely-mounted relative to the trim  25  such that the canceling wave form is primarily and substantially directed at the surface of the trim  25  adjacent the enclosure  33 . This is termed being “inverted” within the enclosure. Prior art active noise control systems for aircraft have directed the canceling noise directly into the cabin. The inversion of the speaker  30  is thought to increase the reverberation of the speaker assembly  50 . This is particularly desired for controlling low-frequency noise such as is experienced in propeller-driven aircraft. Low frequency would be considered in the range of between 20 Hz and 400 Hz. Preferably, the enclosure  33  is attached to the trim  25  such as aft bulk head  31   c,  mid bulkhead  31   b  or to floor  32  (FIG. 2) by mounts  38 . These can be shear-type mounts, sandwich mounts or the like. Preferably, the mounts  38  are elastomeric and act in either shear or compression with preferable stiffness ranges between about 0.5 lb./in. and 15 lb./in. Preferably, four elastomer mounts  38  are used to attach each enclosure  33  to the trim  25 . 
     The enclosure  33 , preferably, includes planar wave guide means in the form of multiple escapeways  40  formed between the trim  25  and the enclosure  33  to direct the escape of canceling wave form as it escapes from the enclosure  33  to be initially in a direction substantially parallel to the surface of trim  25 . Preferably, these escapeways  40  are formed by mounts  38  spacing the enclosure  33  away from the trim  25 . Soft-mounting of the enclosure  33  protects the components in the speaker  30  from shock loads and avoids unwanted vibration from the speaker to be transmitted to the structure. 
     An error sensor  28 , and preferably an array of error sensors are strategically located within the aircraft cabin to allow the control such as least means square (LMS) control to produce a quiet zone adjacent the passengers&#39; heads. The error signal derived from the error sensor  28  is indicative of the sound pressure level at the location of the error sensor. Various averaging schemes can be used when arrays of sensors are used. The error signal is used by an electronic controller  22   a  and produces a canceling wave form in the form of anti-noise (180° out of phase) to reduce the noise at the location of the error sensor  28 . If an array of sensors are used, such as in most aircraft systems, the control will seek to globally reduce and minimize the sound pressure level within the aircraft cabin  37   a.    
     FIG. 2 illustrates a side view of another embodiment of active noise control system  20   b  for noise reduction in an aircraft cabin  37   b.  Illustrated are the floor-mounted speaker assemblies  46   a,    46   b,    46   c,  and  46   d  wherein the enclosures  33  are attached, and preferably soft-mounted to the floor  32  beneath the seats  42   a,    42   b,    42   c,  and  42   d  by mounts  38 . The installation is shown with the electronic controller  22   b  positioned behind the rear bulkhead  31   c  in the unpressurized portion of the aircraft. All leads  41   a  through  411  from the speakers  30 , error sensors  28   a,    28   b,    28   c,  and  28   d  and reference sensors  26   a  are collected into a wire bundle  43  which is connected to the electronic controller  22   b.  A sealed connector  47  is used to traverse through the aft bulkhead  31   c.    
     In the FIG. 2 embodiment, the error sensors  28   a,    28   b,    28   c,  and  28   d,  preferably microphones, are installed adjacent the trim  25 , and preferably, directly adjacent the windows  44   a,    44   b,    44   c,  and  44   d.  The trim  25  is directly attached to the fuselage  34 . A wall-mounted speaker assembly  45   a,  which in this case is bulkhead mounted, is illustrated installed in the cockpit  48  of the aircraft and attached to the mid or partition bulkhead  31   b.  Similarly, a wall-mounted speaker assembly  45   c  is mounted on an aft bulkhead  31   c.  In a similar fashion, a wall-mounted speaker assembly could be mounted on the partition bulkhead  31   b  and directed toward the passengers. 
     FIG. 3 illustrates an aft-looking view of another embodiment of active noise control system  20   c  for a jet-engine aircraft which uses floor-mounted speaker assemblies  46   e  and  46   f.  The speakers  30  in the assemblies  46   e  and  46   f  are inversely-mounted in the enclosures  33  underneath the seats  42   e  and  42   f  such that the canceling sound wave form is directed substantially toward the floor  32 . Preferably the enclosures  33  are mounted to the floor by mounts  38 . Error sensors  28   e  and  28   f  are located in the trim adjacent the windows  44   e  and  44   f.  The reference sensors  26   e  and  26   f  are taken from the engines  36   e  and  36   f,  such as turbofan jet engines, to provide reference signals that are indicative of the vibration of the engines  36   e  and  36   f  that imparts noise and vibration to the fuselage  34  through struts  49   e  and  49   f.  The vibration causes unwanted noise in the aircraft cabin  37   c.  The electronic controller  22   e  and power supply  24   e,  in this embodiment, are shown mounted under the floor  32 , but could be mounted at any convenient location 
     FIGS. 4,  5 , and  6  schematically depict various systems  20   g,    20   h,  and  20   j  and closed spaces  37   g,    37   h,  and  37   j  where there is unwanted noise therein to be reduced. Each includes an electronic controller  22   g ,  22   h,  and  22   j  which includes a memory and a digital signal processor (DSP) which is used to execute a control algorithm such as LMS or the like to minimize unwanted noise within the closed spaces  37   g,    37   h,  and  37   j.  Each closed space spaces  37   g,    37   h,  and  37   j  includes a speaker assembly  50   g,    50   h,  and  50   j  which include speakers  30   g,    30   h,  and  30   j  and enclosures  33   g,    33   h,  and  33   j.  The speakers  30   g,    30   h,  and  30   j  are inversely-mounted in the enclosures  33   g,    33   h,  and  33   j  such that the canceling wave form is directed substantially toward the trim  25   g,    25   h,  and  25   j.  In these embodiments, floor mounted versions are shown, but wall mounting is envisioned as well. Further, the speaker enclosures  33   g,    33   h,  and  33   j  are soft-mounted to the trim  25   g,    25   h,  and  25   j  by mounts  38   g,    38   h,  and  38   j.    
     Illustrated are four types of reference sensors  26   g,    26   h,    26   h ′, and  26   j  which are used to derive a signal indicative of the frequency, and/or phase, and/or magnitude of the disturbance noise and/or vibration source. Reference sensor  26   g  picks up noise and generates a signal indicative of the noise in the far-field which is causing unwanted noise in the closed space  37   g.  Reference sensor  26   h  and optionally  26   h ′ pick up noise (and optionally mechanical vibration) generated by a noise source  51   h  and generate a signal indicative of the noise generated by the source  51   h  which is causing an unwanted noise in the closed space  37   h.  The signal may be generated by either an accelerometer or a microphone. Further, a tachometer signal may be used. Similarly, reference sensor  26   j  picks up vibration generated by a vibration source  51   j  such as an engine which is directly attached to the closed space  37   f  by a connecting structure  52   j.  The vibration and noise causes an unwanted noise in the closed space  37   j.  Error sensors  28   g,    28   h,  and  28   j  are used to derive a signal indicative of the residual noise pressure level in the closed spaces  37   g,    37   h,  and  37   j.  Each of these systems  20   g,    20   h,  and  20   j  are efficient systems for reducing unwanted noise, and in particular they are efficient for reducing noise in the frequency range between about 20 Hz and 800 Hz. 
     FIG. 7 illustrates the present invention active noise control system  20   k  used in the environment of a vehicle such as an automobile. The vehicle  53  includes an engine  36   k,  and a transmission  54  for driving wheels  55  or the like. The active noise control system  20   k  operates to reduce interior noise due to the engine  36   k  which causes unwanted noise in the passenger compartment  37   k.  Speaker assemblies  45   k,    46   k,  and  50   k  mount to the trim  25   k  such as underneath seats  42   k,  on the window platform, or in the front of the rear seat  42   k ′ or the like. Each speaker assembly is mounted to the trim  25   k  by mounts  38  and speakers  30  inversely-mounted in the enclosure  33 . At least one error sensor  28   k  is included in the closed space  37   k.  Preferably, multiple sensors such as  28   k  and  28   k ′ are used in the areas where localized quiet zones are desired. 
     FIG. 8 illustrates a wall-mounted speaker assembly  451  including acoustic speaker  301  which is rigidly attached to an enclosure  331  by fasteners  561  or the like. The enclosure preferably includes an interior volume  571  and a low-frequency reflex port  581 . Speaker  301  is preferably offset to one corner of the enclosure  331  to reduce the acoustic loading on the speaker  301 . The enclosure  331  attaches to the trim  251  by way of mounts  381 . In this embodiment, grommet-type mounts are used. The mounts  381  include means for attaching to the enclosure  331  such as a first bracket  591 , bolt  621  and nut  631 . The mounts  381  also include means for attaching to the trim  251  such as second bracket  601  and screw  641 . Flexing elements  611  and  611 ′ such as grommets are compressed between first bracket  591  and second bracket  601 , and similarly, between first bracket  591  and washer  651  by torqueing fastener  661 . Grommets are compressed enough such that they allow for flexible relative movement between the enclosure  331  and the trim  251  without slippage. Preferably, the grommets are loaded in compression under vertical gravity loading. 
     FIG. 9 depicts another type of mount  38   m  for flexibly mounting the enclosure  33   m  to the trim  25   m.  The mounts  38   m  are bonded compression mounts. Each includes a first bracket  59   m  for attachment to the enclosure  33   m  and a second bracket  60   m  for attachment to the trim  25   m  and a flexing element  61   m  bonded therebetween. For this wall-mounted assembly, it is desired that the flexing element  61   m  be elastomer such as natural rubber and be loaded in direct compression. 
     FIG. 10 depicts floor-mounting the enclosure  33   n  of the speaker assembly  45   n  with grommet-type mounts  38   n  for flexibly mounting the enclosure  33   m  to the trim  25   m.  Each mount  38   n  includes a bracket  60   n  a washer  65   n,  and flexing elements  61   n  and  61   n ′. Torqueing fastener  66   n  properly precompresses flexing elements  61   n  and  61   n′.    
     FIG. 11 depicts bottom view of the speaker assembly  45   p  with the enclosure  33   p  soft-mounted with grommet-type mounts  38   p  for flexibly mounting the enclosure  33   p  to the trim (not shown). Preferably, four mounts  38   p  are used with one at each corner. The enclosure  33   p  preferably includes a low-frequency reflex port  58   p.  Further, the speaker  30   p  is preferably offset towards one corner to reduce the acoustic loading on the speaker  30   p  when it is actuated. 
     In summary, the present invention is directed to an efficient active noise control system for use in a closed structure. The system comprises a reference sensor for deriving a reference signal indicative of a source of disturbance, an error sensor for sensing a residual sound pressure level and providing a signal indicative thereof to an electronic, the electronic controller includes an adaptive filter for providing a canceling signal to a speaker for generating a canceling wave form. In the present invention, the speakers are inversely-mounted in their enclosures and attached directly to the trim of the closed space, thus, providing for more efficient noise cancellation within the space. Preferably, the enclosures are soft-mounted by mounts to protect the speaker components from damage to transient loads applied thereto and to prevent transmission of unwanted vibration to the supporting structure. In another aspect, each speaker assembly and installation preferably performs the function of a planar wave guide and constrains the canceling wave form such that it emanates from the confines of the enclosure in a direction which is substantially parallel to the trim&#39;s surface. 
     Various changes, alternatives and modifications will become apparent to one of ordinary skill in the art following a reading of the foregoing specification. It is intended that all such changes, alternatives, and modifications come within the spirit and scope of the appended claims are to be considered part of the present invention.