Abstract:
Electroacoustical apparatus includes at least first and second acoustically coupled electroacoustical drivers. An electrical network couples their inputs so that an electrical drive signal applied to one reduces the effect of acoustic coupling from that one to the other.

Description:
[0001]    This invention relates in general to electroacoustical transducing, and more particularly concerns novel apparatus and techniques for electroacoustical transducing with a plurality of acoustically coupled electroacoustical transducers. 
       BACKGROUND OF THE INVENTION 
       [0002]    For background, reference is made to U.S. Pat. Nos. 4,146,745 and 4,146,744. 
       SUMMARY OF THE INVENTION 
       [0003]    According to the invention, there are at least first and second acoustically coupled electroacoustical drivers having first and second inputs respectively for receiving first and second electrical drive signals respectively. An electrical network intercouples the first and second inputs and is constructed and arranged to provide a first opposition signal on said second input in phase opposition to a first electrical drive signal on said first input to reduce the effect of acoustic coupling from the first electroacoustical driver to the second electroacoustical driver when the first electrical drive signal is applied to the first input. There may be a common cabinet enclosure enclosing the first and second electroacoustical drivers. The first and second electroacoustical drivers may be of the same design or of differing designs. The electrical network may be constructed and arranged to provide a second opposition signal on the first input in phase opposition to a second electrical drive signal on the second input to reduce the effect of acoustic coupling from the second electroacoustical driver to the first electroacoustical driver when the second electrical driver signal is applied to the second input. 
         [0004]    Other features, objects and advantages will become apparent from the following detailed description when read in connection with the accompanying drawing in which: 
     
     
       BRIEF DESCRIPTION OF DRAWING 
         [0005]      FIG. 1  is a combined pictorial-block diagram of an exemplary embodiment of the invention; 
           [0006]      FIG. 2  is a combined pictorial-block diagram of a modification of the embodiment shown in  FIG. 1 ; 
           [0007]      FIG. 3  is a graphical representation of the excursion transfer function X_ 21  with and without an H_ 22  filter; 
           [0008]      FIG. 4  is a graphical representation of net excursion attenuation; 
           [0009]      FIG. 5  is a perspective view of the commercially available Bose Companion 5 satellite speakers; and 
           [0010]      FIG. 6  is a block diagram of an embodiment with filters  16  and  16 ′ and summing circuits  17  and  17 ′. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    With reference now to the drawing and more particularly  FIG. 1 , there is shown a combined pictorial-block diagram of an embodiment of the invention. Electroacoustical transducer  1   11  and electroacoustical transducer  2   12  reside in enclosure  13  and have first and second inputs  14  and  15 , respectively, for receiving first and second electrical drive signals V 1  and V 2 , respectively. The first filter  16 , having a transfer characteristic H 22 , couples input  14  to the −input of summing circuit  17  whose +input receives a second input signal V ii  and provides as an output the electrical drive signal V 2 . 
         [0012]    Having described the physical arrangement of the embodiment, the mode of operation will be described. It is convenient to describe the mechanical excursion of the cone of transducer  2  in response to the electrical drive signal V 1  caused by acoustic coupling from the movement of the cone of transducer  1  as X 21  per unit of V 1  and its mechanical excursion in response to the electrical drive signal V 2  as X 22  per unit of V 2 . The resultant excursion X 2  of the cone of transducer  2  in response to the input signals V i  and V ii  in the absence of circuits  16  and  17  is: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           X 
                           2 
                         
                         = 
                         
                           
                             
                               V 
                               1 
                             
                             * 
                             
                               X 
                               21 
                             
                           
                           + 
                           
                             
                               V 
                               2 
                             
                             * 
                             
                               X 
                               22 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           
                             
                               V 
                               i 
                             
                             * 
                             
                               X 
                               21 
                             
                           
                           + 
                           
                             
                               V 
                               ii 
                             
                             * 
                             
                               X 
                               22 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   
                     
                       
                         ( 
                         1 
                         ) 
                       
                     
                   
                   
                     
                       
                         ( 
                         
                           1 
                            
                           a 
                         
                         ) 
                       
                     
                   
                 
               
             
           
         
       
     
         [0013]    It is convenient to define a filter based on the first two transfer functions as: 
         [0000]        H   22   =X   21   /X   22    (2) 
         [0014]    The output signal from filter  16  with transfer characteristic H 22  corresponds to the input signal V i  multiplied by transfer characteristic H 22 . Applying this output signal with phase reversed through summing circuit  17  creates a component of the electrical drive signal V 2  applied to transducer  2  that cancels the sympathetic vibration of transducer  2  caused by the acoustic coupling from transducer  1  in enclosure  13 . 
         [0015]    The modified excursion of transducer  2 , X 2 ′, is expressed: 
         [0000]        X   2   ′=X   2   −V   i   *H   22   *X   22    (3) 
         [0016]    Substituting equations (1a) and (2) for terms X 2  and H 22  respectively gives: 
         [0000]        X   2   ′=V   i   *X   21   +V   ii   *X   22   −V   i *( X   21   /X   22 )* X   22    (4) 
         [0017]    Note that the first and third terms of equation (4) cancel, leaving: 
         [0000]        X   2   ′=V   ii   *X   22    (5) 
         [0018]    So the mechanical response of the cone of transducer  2  to the input Vi is identically 0. 
         [0019]    Referring to  FIG. 2 , there is shown another embodiment of the invention having a second filter  16 ′ having a transfer characteristic H′ 22  providing an output delivered to the −input of summing circuit  17 ′ that receives the input signal V i  on the +input to provide a signal V i ′ including a component that cancels the sympathetic vibration of transducer  1  in response to the signal V 2 . 
         [0020]    Referring to  FIG. 3 , there is shown a graphical representation of the excursion transfer function X 21  as a function of frequency with and without filter  16 , respectively. 
         [0021]    Referring to  FIG. 4 , there is shown a graphical representation of the net excursion attenuation with filter minus excursion without filter. 
         [0022]    Referring to  FIG. 5 , there is shown a perspective view of a commercial embodiment of the invention in the Bose Companion  5  satellite cabinet enclosure showing transducers  1  and  2  in a sealed enclosure. In the specific embodiment of this invention, transducers  1  and  2  are 50 mm drivers in a sealed cabinet enclosure angled at 51 degrees with the enclosure volume 11.1 inch 3 . 
         [0023]    Referring to  FIG. 6 , there is shown a block diagram of an embodiment showing filter  16  and  16 ′ and summing circuits  17  and  17 ′ combined. 
         [0024]    In a specific form of the invention a Texas Instruments DA708E001RFP250 DSP chip loaded with the ASCII representation of hex code in the appended text file implements the sympathetic vibration cancellation. 
         [0025]    While the invention has been illustrated with two electroacoustical drivers, the principles of the invention may be extended to a larger plurality of drivers. 
         [0026]    The invention has a number of advantages. In a system where a plurality of drivers in a common enclosure each receive different signals, the distortion in the acoustic output generated by any one of the drivers due to the acoustic coupling between it and the other drivers is significantly reduced. It helps maintain the excursion of the driver cones within the linear region of the transducers to facilitate reproducing sound at substantial levels without audible distortion. It is evident that those skilled in the art may now make numerous uses and modifications of and departures from the specific embodiments disclosed herein 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.