Abstract:
A vibration reducing apparatus comprising, a baffle subject to vibration, a first electroacoustical transducer characterized by a first mass seated in the baffle, a second electromechanical transducer mechanically connected to said first transducer or said baffle in the vicinity of the periphery of the first transducer. The rear sides of the diaphragms of the first and the second transducer are not connected to the listening area. The first transducer and the second transducer are constructed and arranged to receive a common electrical signal so that the movable element in the first transducer and the movable element in the second transducer move in phase opposition in response to the common electrical signal to significantly reduce the vibrating force imparted to the baffle.

Description:
The present invention relates in general to baffle vibration reducing and more particularly concerns novel apparatus and techniques for reducing vibration transmitted to structures attached to loudspeaker drivers. 
     BACKGROUND OF THE INVENTION 
     When an electroacoustic transducer, such as a loudspeaker driver, is mounted to a structure, such as a package shelf, the door of a vehicle, the wall of an enclosure, other wall or other baffle, where attachment is usually on the periphery of the transducer frame, an energized transducer motor develops forces in response to an energizing electrical signal. The forces generated by the motor cause the diaphragm of the transducer to move relative to the transducer frame. These forces will also be transmitted through the frame to the structure through the attachment points of the frame. Package shelves and door panels of vehicles are often made of thin material, such as thin sheet metal. Such structures typically have insufficient stiffness to resist vibration and are typically lightly damped. As a result, forces applied to the structure around modal resonance frequencies of the structure may result in excessive vibration of the structure, acoustically perceived as unwanted buzzes and rattles, or degraded frequency response of the radiated sound. 
     It is an important object of the invention to reduce these structurally transmitted vibrations. 
     SUMMARY OF THE INVENTION 
     According to the invention, a first electroacoustical transducer incorporating a movable diaphragm is seated in and structurally coupled to a panel. The transducer is mechanically connected to a device containing a compensating moveable mass driven out of phase with the movement of the diaphragm of the first electroacoustical transducer, to significantly reduce the resultant force applied to the panel. Typically the device with compensating mass is a second electroacoustical transducer identical to the first transducer. According to another feature of the invention, the acoustic output from the first side of the first transducer is directly coupled to a listening environment, such as a vehicle passenger compartment or living room. The acoustic output from the side of the second transducer facing away from the first transducer is also coupled to the listening environment through an acoustical element or elements such as compliant volume and/or port so that the acoustical output into the vehicle compartment from the facing away side of the second transducer is effectively in phase with the output into the vehicle compartment from the first side of the first transducer, over a desired frequency range. The acoustic elements are arranged such that the output from the away facing side of the second transducer is not acoustically coupled to the output from the second side of the first transducer or the output from the first side of the second transducer. Thus, the invention achieves both significant reduction in unwanted mechanical vibration of the supporting structure with enhanced acoustic output from the second transducer. 
     Other features, objects and advantages of the invention will become apparent from the following description when read in connection with the accompanying drawing in which: 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a diagrammatic representation of an embodiment of the invention with the assembly carried by an infinite baffle, such as a vehicle rear deck or door; and 
         FIG. 2  is a graphical representation showing the force exerted on the structure as a function of frequency for various systems; and 
         FIG. 3  is a diagrammatic representation of an alternative embodiment of the invention with the assembly carried by an infinite baffle, such as a vehicle rear deck or door; and 
         FIG. 4  is a diagrammatic representation of an alternative embodiment of the invention with the assembly carried by an infinite baffle, such as a vehicle rear deck or door, incorporating transducers with inverted motor structures. 
     
    
    
     DETAILED DESCRIPTION 
     With reference now to the drawing and more particularly  FIG. 1  thereof, there is shown a diagrammatic representation of an embodiment of the invention with structure carried by infinite baffle  11 , typically a vehicle rear shelf or door panel carrying a first transducer, such as loudspeaker driver  12 , mechanically connected to a second transducer, such as loudspeaker driver  13 , preferably identical to loudspeaker driver  12 , through mechanical link  14 . The two transducers are ideally mounted in substantially parallel planes such that the diaphragms move in the same axial direction. The front side of transducer  12  is directly coupled to listening area  18 . If baffle  11  is the rear package shelf of a vehicle, listening area  18  will be the passenger compartment of the vehicle. The second side of transducer  12  is coupled to volume  30 , which would be the vehicle trunk if baffle  11  is the rear package shelf. The second side of diaphragm  22  of driver  13  is coupled to the listening area  18  through compliant column  15  and port tube  16 . The front of diaphragm  22  of transducer  13  is coupled to volume  30 . Power amplifier  17  energizes first loudspeaker driver  12  and second loudspeaker driver  13  with the same signal but drives them in opposite polarity. The system is arranged such that when diaphragm  21  of first driver  12  is moving up, diaphragm  22  of second driver  13  is moving down, which significantly reduces the forces applied to and unwanted resultant vibrations of baffle  11 . Meanwhile, the output from the rear of second driver  13  is coupled by compliant volume  15  and port tube  16  for emission substantially in phase with the output from the front surface of first driver  12 . The output from the rear of second driver  13  could also be coupled through a conduit of substantially constant or smoothly varying cross section to listening area  18  without loss of generality. 
     The second transducer need not be an identical transducer. All that is required for significant reduction in vibration is for the moving mass and generated motor force of the second transducer to approximately equal the moving mass and generated motor force of the first transducer. Such a component could be made at lower cost than the cost of a transducer identical to the first transducer. It is still desirable for the frames of the two transducers to be similar so that the second transducer maybe attached to the first transducer at the same attachment points where the first transducer is attached to the baffle. An alternative means of assembly would be to rigidly attach the top of the motor structure of the first driver  12  to the rear of the motor structure of the second driver  13 , using a rigid connecting member  19  such as a threaded metal rod shown in dotted outline in FIG.  1 . 
       FIG. 3  shows an alternative construction. Driver  13  is now physically inverted with respect to driver  12 .  FIG. 3  shows the rear of the motor structure of driver  12  rigidly attached to the rear of the motor structure of driver  13  through spacer  20 , although spacer  20  is not required. Structural coupling of the two transducers could also be accomplished through attachment around the periphery of the transducer frames, as shown in the system of FIG.  1 . The arrangement of  FIG. 3  would also work equally well if each of transducers  12  and  13  were inverted compared to what is shown. In this case, structural connection would be more easily accomplished through attachment around the periphery of the transducer frames. 
     Since drivers  12  and  13  are physically inverted with respect to each other, cancellation of vibration will occur when signals of the same relative polarity are applied to each transducer. Each driver is attached to the output of amplifier  17  such that when the signal provided to driver  12  causes diaphragm  21  of driver  12  to move down, signal output from amplifier  17  which is connected to driver  13  causes diaphragm  22  of driver  13  to move up, in the opposite direction to the motion of diaphragm  21 . 
     Above a certain frequency, output from the second transducer will not be in phase with the output from the first transducer. The frequency response of the combined system may exhibit a comb filter behavior with the first null occurring when the path length difference between the front of the first transducer and the listening position and the rear of the second transducer and the listening position is a half wavelength. 
     One approach for reducing the effects of this comb filter behavior is by using a low-pass filter to restrict the spectral components delivered to both drivers to spectral components below the first null and using other transducers for reproducing higher frequency spectral components. The low-pass filters used could be identical for both drivers, or they may have different orders and/or corner frequencies. The output from one of the drivers could be restricted to being below a predetermined cutoff frequency while the other was allowed to operate over a wider frequency range. Preferably, the first transducer operates over a wider frequency range than the second transducer. This result can be achieved by placing a low-pass filter in the signal path of the second transducer only, or by having a low-pass filter in the signal path of the first transducer with a higher corner frequency and/or lower order than a low-pass filter in the second transducer signal path. The result can also be achieved either in combination with or solely by the appropriate design of the acoustic elements connecting the second driver to the listening region such that the acoustic elements, in combination, form a low pass filter. 
     It may also be advantageous to include a low-pass filter in the signal path of the second transducer and a complementary all-pass filter in the signal path of the first transducer. A complementary all-pass filter has the same phase response as a function of frequency as a corresponding low-pass filter. This feature can be accomplished, for example, by using a second order critically damped low-pass filter in the second transducer signal path, and a first order all-pass filter in the first transducer signal path, where the corner frequencies of the low-pass and all-pass filters are substantially identical. 
     According to another embodiment, a fourth order low-pass filter in the second transducer signal path and a second order all-pass filter in the first transducer signal path may be used. Other examples of complementary all-pass filter/low-pass filter combinations will be evident to those skilled in the art. 
     The use of complementary all-pass filters and low-pass filters as described above can be combined with other signal processing as disclosed in U.S. Pat. No. 5,023,914, incorporated by reference herein, to simultaneously achieve improved system frequency response and reduce vibration. 
     Referring to  FIG. 2 , there is shown a graphical representation of force upon a baffle as a function of frequency for various structures. Curve  21  shows the resultant response of using two Bose eight-inch Nd drivers according to the invention in an acoustic system having a low-pass filter. Curve  22  shows the applied force when using only a single eight-inch Nd driver with a low-pass filter. Curve  23  shows the applied force when using just two eight-inch Nd loudspeaker drivers connected according to the invention without the low-pass filter. Curve  24  shows the applied force with just a single eight-inch Nd loudspeaker driver. These graphical representations demonstrate the significant reduction in force applied to the baffle with the invention and the advantage of incorporating the low-pass filter into the system. 
     The embodiments shown in  FIGS. 1 and 3  show use with infinite baffle  11 . Although the arrangements are described having region  18  as the listening area, this is not required. The invention can be adapted to work equally well with volume  30  or region  18  operating as the listening area without loss of generality. 
       FIG. 4  shows use of the invention with transducers of an alternate construction.  FIG. 4  shows transducers  32  and  33  physically inverted with respect to each other with motor structures that are inverted with respect to the motor structures of transducers  12  and  13 . Transducers  32  and  33  have diaphragms  41  and  42 , respectively. Use of inverted motor structure transducers is not limited to the orientation shown. Any of the previous arrangements described for non-inverted motor transducers is also applicable for transducers with inverted motor structures. Use of inverted motor structure transducers in the current invention can significantly reduce the overall thickness of the multiple transducer assembly, which can reduce intrusion into a vehicle trunk or allow a system to fit within a wall space where an arrangement using traditional transducers would not fit. Note also that mechanical links  14  can be made much thinner than links  14  shown in  FIG. 1  in the embodiment using transducers without inverted motor structures. 
     It is evident that those skilled in the art may now make numerous uses and modifications of and departures from the specific apparatus and techniques herein disclosed without departing from the inventive concepts. Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features present in or possessed by the apparatus and techniques herein disclosed and limited only by the spirit and scope of the appended claims.