Patent Publication Number: US-6993141-B2

Title: System for distributing a signal between loudspeaker drivers

Description:
FIELD OF THE INVENTION 
     This invention relates to loudspeakers, and more particularly to a system for distributing a signal or voltage to loudspeaker drivers. 
     BACKGROUND OF THE INVENTION 
     It is well known to provide a loudspeaker unit which includes two or more individual speakers (also known as drivers) to cover different sections of the frequency spectrum. Loudspeakers with multiple drivers are desirable because a single driver large enough to provide adequate response at low frequencies is not capable of providing an adequate response at higher frequencies. Such systems are commonly known as two or three way systems, depending upon whether a separate driver is provided to cover two or three different frequency portions, respectively. 
     Moreover, in some known designs where higher efficiency is a concern, multiple drivers may be provided in each crossover section or for each frequency band. It is not uncommon to have up to three drivers or speakers in a low pass section and even two drivers in a midrange section. 
     A disadvantage to a loudspeaker having multiple drivers is that the drivers occupy more space, and can narrow the spatial characteristics of the system. For example, the sound from multiple speakers or drivers can appear to be more directional than from a single driver. This effect is more pronounced at higher frequencies. 
     One known technique for reducing this disadvantage of multiple drivers is to differentiate the signals fed to the individual drivers in one section. This is achieved by setting different low pass cutoff frequencies for each driver and this is common practice where multiple drivers are provided. The effect of this technique is to reduce the number of drivers participating in sound reproduction at higher frequencies, thereby improving sound dispersion. 
     However, this technique has a number of disadvantages. One of the disadvantages is lower efficiency, since at higher frequencies fewer drivers are radiating the sound. Another disadvantage is that is difficult to achieve a flat frequency response, because of a complex phase relationship between drivers connected to different low pass filters. Even if systems employing low pass filters are designed, using simple mathematical addition, to produce a flat frequency response, in practice, such systems often introduce unwanted and varying phase shifts. At higher frequencies, these phase shifts can be even more pronounced, and, result in a reduced signal level. 
     Accordingly, there is a need for a loudspeaker system to simply and efficiently distribute an input signal between a number of drivers. There is a further need for a system which enables different low pass cut off frequencies to be set for the drivers, while enabling a more flat, total frequency response to be provided. 
     SUMMARY OF THE INVENTION 
     The loudspeaker system according to the present invention utilizes a tapped coil or autotransformer to divide a signal between different drivers. While such autotransformers are known, they have never been used for such a purpose. 
     According to the present invention, a system for distributing a source voltage from a signal source is provided. The system comprises at least one autotransformer for connection to the signal source, and a plurality of drivers electrically connected to the autotransformer. The autotransformer is adapted to distribute the source voltage across each of the plurality of drivers. Preferably, the autotransformer is adapted to produce an output voltage across each of the drivers, wherein the sum of the output voltages is substantially equal to the source voltage multiplied by the number of drivers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made by way of example to the accompanying drawings, which show a preferred embodiment of the present invention and in which: 
         FIG. 1  is a schematic view showing the basic configuration of a center-tap autotransformer, and relationship between input and output voltages; 
         FIG. 2  is a schematic view showing an embodiment of the system according to the present invention for use with two drivers; 
         FIG. 3  is a schematic view showing another embodiment which adds a low pass filter to the embodiment of  FIG. 2 ; 
         FIG. 4  is a graph showing the frequency responses of the voltages across the drivers in the embodiment of  FIG. 3 ; 
         FIG. 5  is a schematic view showing another embodiment which adds another low pass filter to the embodiment of  FIG. 3 ; 
         FIG. 6  is a schematic view showing yet another embodiment for use with three drivers; 
         FIG. 7  is a graph showing the frequency response of the embodiment  FIG. 6 ; 
         FIG. 8  is a schematic view showing yet another embodiment of the present invention for use with four drivers; and 
         FIG. 9  is a schematic view illustrating the relationship between the various elements in the loudspeaker system according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a conventional autotransformer  10 . As is known, the autotransformer  10  is preferably a tapped coil having end connections indicated as a first end connection  12  and a second end connection  14 . The autotransformer  10  also has a tap connection  16 . 
     Continuing to refer to  FIG. 1 , the tap connection  16  is connected to signal source having a source voltage u 1 . The second end connection  14  is connected to ground and the first connection  12  has an output voltage u 2 . As is known, where the tap connection  16  is in the middle of the coil (i.e. the number of windings between the tap connection  16  and first connection  12  is equal to the number of windings between the tap connection  16  and the second end connection  14 ), then the voltages u 1  and u 2  are related to the voltage of the signal source u 1 , as follows:
 
 u   2 =2 ·u   1   (1)
 
     This type of connection is known as a “center tap” connection. 
       FIG. 2  shows one embodiment of the present invention. The autotransformer  10  is preferably identical to  FIG. 1  and like parts of the autotransformer  10  have been referred to by like reference numbers. It will be understood by those skilled in the art that any other suitable autotransformer configuration may be used. Two drivers  18  and  20 , are each connected to end connections  12  and  14  and ground. The drivers may be any suitable loudspeaker, such as, for example, 4 ohm drivers (as indicated by the number “4” in  FIG. 3 ). A signal source  22  is connected between the tap connection  16  and ground, as shown. The signal source  22  generates an input signal having a source voltage E, and output voltages V 1  and V 2  are produced across drivers  18 ,  20  by the autotransformer  10 . The signal source may be any conventional element capable of producing a source voltage E, such as a conventional power amplifier, a low pass filter, or the like. It will be understood by those skilled in the art that the audio signal produced by the drivers  18 ,  20  is proportional to the voltage across the drivers (i.e. as the voltage increases, the sound pressure produced by the drivers increases). 
     The inventor has discovered that the sum of the output voltages V 1  and V 2  remain constant, disregarding the load impedances. If the loads are identical, then each of the voltages V 1 , V 2  are identical and equal to the source voltage E. More specifically, if the source voltage is E, then the relationship between input and output voltages is described by the following equation:
 
 V   1 + V   2 =2 E   (2)
 
     This relationship between the input and output voltages remains constant, even if the loads are varied. Thus, if the impedance is varied so that one voltage, e.g., V 1 , decreases, the other voltage V 2  increases to maintain the relationship indicated by the equation (2) above. 
       FIG. 3  shows a second embodiment of the present invention which utilizes the above relationship. This embodiment is similar to the embodiment illustrated in  FIG. 2 . For simplicity and brevity, like parts are given like reference numbers, and will not be described again. 
     Referring to  FIG. 3 , the output voltages supplied to the first and second drivers  18 ,  20  from the autotransformer  10  are indicated as V 1 , V 2 , respectively. Additionally, the second driver  20  is connected to a filter means, such as a first capacitor  24  with a value, for example, with 100 mircrofarads. The first capacitor  24  is connected to the system in parallel with the second driver  20 . 
     The first capacitor  24  provides a cutoff frequency for the second driver  20 . In effect, as the frequency increases, the combined impedance of the driver  20  and the first capacitor  24  drops, and a greater portion of the current passes through the first capacitor  24 . Consequently, the output voltage across the driver  20  is reduced. In accordance with equation 2 above, the output voltage across driver  18  increases to compensate for the voltage reduction across driver  20 . 
     As the sound level generated by each driver  18 ,  20  corresponds to the voltage across it, the total sound level remains the same (because the sum of the voltages is constant). 
     This relationship is illustrated in  FIG. 4 , which shows the frequency response for the embodiment of  FIG. 3 . The frequency response of the voltage across second driver  20  falls off at higher frequencies. Correspondingly, the frequency response of the voltage across first driver  18  increases. The sum of the voltages V 1 , V 2  across drivers  18 ,  20 , respectively, remains constant (also referred to as flat) and is represented by the straight line at +6 dB (6=20×log(2)). This result also demonstrates that the total efficiency of the system, as a function of frequency, remains the same. 
       FIG. 5  shows another embodiment of the system according to the present invention. Again, parts common with the embodiment of  FIG. 3  are assigned like reference numbers and will not be further described. 
     Referring to  FIG. 5 , a filter means such, as a low pass filter, generally indicated at  28  is provided between the signal source  22  and the autotransformer  10 . It will be understood by those skilled in the art that the filter means may be any other type of filter, such as a band pass filter, high pass filter, all pass filter, or a combination thereof. Each of these filters may comprise one or more coils, capacitors, resistors, or transformers, or a combination thereof. 
     The low pass filter  28  may be any known low pass filter, such as, an inductor  30  and a second capacitor  32  having values selected to give a desired low cut off frequency. For example, for a desired cut-off frequency of 2 kHz, the inductor  30  would have a value of 0.5 mH and second capacitor  32  would have a value of 12.6 uF. This embodiment is particularly suited for driving a pair of drivers  18 ,  20  which are low frequency speakers or woofers. Thus, at a desired cutoff frequency the low pass filter  28  cuts off or reduces the output voltage across the drivers  18 ,  20 . Otherwise, the operation of this embodiment is similar to that described for  FIG. 3  above. 
     It is to be noted that while the low pass filter  28  is located before the autotransformer  10  in  FIG. 5 , the low pass filter  28  may instead be replaced by individual low pass filters for each driver  18 ,  20 , after the autotransformer  10 . Such a configuration would advantageously influence the overall system impedance, which in turn, reduces the likelihood of overloading the amplifier. 
     While the first capacitor  24  shown in  FIG. 5  provides the cutoff frequency for the driver  20 , it will be understood by those skilled in the art that various other elements may be included. For example, any suitable combination of resistors, inductors, and capacitors may be provided to achieve the desired frequency characteristics. 
       FIG. 6  shows yet another embodiment of the loudspeaker system according to the present invention. This embodiment provides a further development of the embodiments previously described, and accordingly like components are assigned like reference numbers and their description is not repeated. 
     Referring to  FIG. 6 , a third driver  40  is added to the system. To distribute the source voltage E from the signal source  22  accordingly, a second autotransformer  42  with end connections  44  and  46  is provided. The third driver  40  is connected to end connection  46  and the tap connection  16  of the first autotransformer  10  is connected to end connection  44  to receive an input voltage therefrom. The signal source  22  is now connected to a tap connection  48  of the second transformer  44 . This tap connection  48  is positioned such that the number of turns of the winding between tap connection  48  and each of the end connections  46 ,  44  is in a ratio of 2:1, respectively (i.e., the number of turns between the connections  46 ,  48  is the twice the number of turns between the connections  44 ,  48 ). 
     Continuing to refer to  FIG. 6 , output voltages V 1 , V 2 , and V 3  are produced across drivers  18 ,  20 , and  40 . The relationships between these voltages and the source voltage E is described by the equation:
 
 V   1 + V   2 + V   3 =3 E   (3)
 
     As before, the second driver  20  is provided with a first capacitor  24 , with a value of for example 50 microfarads, to give a low cutoff frequency. A second capacitor  50  is connected to the third driver  40 . The second capacitor may be configured for any suitable cutoff frequency, such as, for example 100 mircrofarads to give an even lower cutoff frequency. 
     The frequency response of this embodiment is illustrated in  FIG. 7 , where the voltages of the three drivers are indicated by the reference numerals  18 ,  20 , and  40 . The horizontal line  52  illustrates the flat frequency response of the sum of the voltages across each of the three drivers (measured in dBs) in accordance with equation (3) above. The third driver  40  has a relatively low cutoff frequency, as shown. The second driver  20  has a slightly higher cutoff frequency. At high frequencies, the signal illustrated by line  52  is made up of voltage V 1  across the first driver  18 .  FIG. 7  shows line  52  having a total signal level of 9.54 dB (9.54=20×log(3)). 
     Yet another embodiment of the present invention is shown in  FIG. 8 . This embodiment includes the three drivers  18 ,  20 ,  40  and first and second autotransformers  10 ,  42  of  FIG. 6 . A fourth driver  60  is connected to a third autotransformer  62 . The third autotransformer  62  has a turn ratio of 3:1 and is connected between the signal source  22  and second autotransformer  42 . A combination of resistor  64  and third capacitor  66  connected in parallel to the system as shown provide a low pass filter for drivers  18 ,  20 , and  40 . In this embodiment, driver  60  has the widest frequency range. In the manner shown in  FIG. 8 , any driver can be selected as the driver with the widest frequency range. As discussed above, this configuration does not alter the relationship described by the following equation:
 
 V   1 + V   2 + V   3 + V   4 =4 E   (4)
 
     It will be understood by those skilled in the art that the relationship described by equations (2), (3), and (4) above and the system according to the present invention may be extended to any number of drivers.  FIG. 9  illustrates this relationship. Any suitable number of drivers, D 1 –D n  may be provided. Source voltage E from signal source  22  is distributed to drivers D 1 –D n  by autotransformers A 1 –A n−1 . As illustrated, the number of autotransformers is preferably one less than the number of drivers. The autotransformers A 1 –A n−1  produce output voltages V 1 –V n  across each of the drivers D 1 –D n , respectively. The relationship is described by the following equation:
 
 V   1   +V   2   +V   3   + . . . V   n   =nE   (4)
 
where n is the total number of drivers connected to signal source  22 .
 
     Continuing to refer to  FIG. 9 , the first end connection and second end connection of each autotransformer are referred to in this  FIG. 9  as a and d, respectively. The end connection d of each autotransformer A is connected to the corresponding driver D, and the end connection a is connected to the adjacent autotransformer (except end connection a n−1  which is connected to driver D n ). The winding ratio between: (i) the tap connection tc to d; and (ii) tap connection tc to a of a particular autotransformer A x  is: (n−x):1, where n is the number of drivers and x is the position of the autotransformer (such that x=1 for the autotransformer A 1  connected directly to the signal source  22 , x=2 for the autotransformer A 2  connected to A 1 , and so on). 
     Various elements and networks may be added to the system shown in  FIG. 9  to adjust the responses of individual or groups of drivers, as shown in  FIGS. 5 ,  6 , and  8 . Some examples of the elements and networks are capacitors, resistors, and inductors in various combinations, as illustrated in  FIGS. 5 ,  6 , and  8 . These elements and networks may be connected in parallel to the system without affecting the relationship described in Equation 4. If such elements or networks are connected in series with one or more of the drivers, such configurations would disrupt the relationship described by equation 4. However, certain configurations may provide other advantages for the system and only have a small impact on the relationship described in equation 4, such that the advantages would outweigh the impact. It will be understood by those skilled in the art that such variations are within the scope of the present invention. 
     The loudspeaker system according to the present invention utilizes one or more autotransformers, such as a tap coil, to distribute the input signal received by a number drivers. The use of one or more autotransformers to distribute the input signal or voltage provides the advantage of a more flat frequency response from the drivers. Specifically, the sum of the voltages across each driver is constant, regardless whether one or all of the drivers are producing sound. This sum is equal to the source voltage multiplied by the number of drivers. The present invention is particularly useful for loudspeaker systems which are designed such that only a portion of the drivers produce an acoustic signal in a particular frequency range, such as at high frequencies. In such systems, the voltages across the drivers in use increase to preserve the acoustic level of the system. 
     While the above description constitutes the preferred embodiments, it will be appreciated that the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof.