Abstract:
The invention is a multi-channel loudspeaker system that provides a compact loudspeaker configuration and filter design methodology that operates in the digital signal processing domain. Further, the loudspeaker system can be designed to include drivers of various physical dimensions and can achieve prescribed constant directivity over a large area in both the vertical and horizontal planes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of and claims priority to U.S. application Ser. No. 10/771,190, filed on Feb. 2, 2004, titled LOUDSPEAKER ARRAY SYSTEM, which application is incorporated by reference in this application in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention generally relates to a multi-way loudspeaker system and in particular to a multi-way loudspeaker system comprised of an array of multiple drivers, capable of achieving high-quality sound. 
         [0004]    2. Related Art 
         [0005]    High-quality loudspeakers for the audio frequency ranges generally employ multiple specialized drivers for dedicated parts of the audio frequency band, such as tweeters (generally 2 kHz-20 kHz), midrange drivers (generally 200 Hz-5 kHz) and woofers (generally 20 Hz-1 kHz). Because of the necessary spacing due to the physical size of the specialized drivers, which is comparable with the wavelength of the radiated sound, the acoustic outputs of the drivers sum up to the intended flat, frequency-independent response only on a single line perpendicular to the loudspeaker, usually at the so-called acoustic center. Outside of that axis, frequency responses are more or less distorted due to interferences caused by different path lengths of sound waves traveling from the drivers to the considered points in space. There have been many attempts in history to build loudspeakers with a controlled sound field over a larger space with smooth out-of-axis responses. 
         [0006]    For example, D&#39;Appolito has presented a geometric approach to eliminate lobing errors in multi-way loudspeakers—a configuration using a center tweeter and two woofers arranged symmetrically along a vertical axis. Several loudspeaker manufacturers have adopted that approach and have even expanded upon it by using arrays of symmetrically arranged midrange drivers and woofers around one or two center tweeters. D&#39;Appolito designs and those of the manufacturers that have adopted D&#39;Appolito&#39;s approach utilize passive or analog crossover circuits or digital filters that emulate analog filters in a digital domain. Analog or passive crossover circuits inevitably introduce phase distortion. Further, with this design, spacing is not optimum and in general too large to completely avoid out-of-axis aberrations from an ideal smooth response. 
         [0007]    In an alternative solution, the basic design concept is to apply very steep, “brick-wall” finite impulse response (FIR) filters to avoid large transition bands, so that the errors become inaudible. However, the individual polar responses of the involved drivers may still be different at the transition point, leaving audible discontinuities. Thus, with this design solution, it may be difficult to achieve a prescribed, smooth polar behavior throughout the whole audible range. 
         [0008]    In yet another alternative, Van der Wal suggests that logarithmically spaced transducer arrays can achieve a very well controlled directivity, approximately constant over a wide frequency range, in one dimension. Some embodiments of this technique are described in U.S. Pat. No. 6,128,395. Like the previously described techniques, this design technique is limited because (i) the logarithmic spacing is prescribed only according to a given formula; (ii) the filter design is only valid for a particular case and (iii) severe errors may occur if the actual spacing deviates from logarithmic spacing, which may be unavoidable due to physical dimensions of the drivers or due to design constraints. Further, the design is restricted to one type of drivers, i.e., full-range drivers, limiting the application to public address systems. Thus, a need still exists for a loudspeaker configuration and filter design that overcomes the limitations of the prior art by providing a loudspeaker system that can contain drivers of various physical dimensions and can achieve prescribed, constant directivity over a large area in both the vertical and horizontal planes. 
       SUMMARY 
       [0009]    The invention is a multi-way loudspeaker speaker system that can produce high-quality sound from a single, compact, line array loudspeaker that can be utilized in a traditional surround sound entertainment system typically having left and right front and rear surround sound channels and a center channel. 
         [0010]    In one embodiment, the line array includes a plurality of tweeters, mid-range drivers and woofers that are arranged in a single housing or assembled as a single unit, having sealed compartments that separate certain drivers from one another to prevent coupling of the drivers. The line array maybe a single channel array having various signal paths from the input to individual loudspeaker drivers or to a plurality of drivers. Each signal path comprises digital input and contains a digital FIR filter and a power D/A converter connected to either a single driver or to multiple drivers. 
         [0011]    The performance, positioning and arrangement of the loudspeaker drivers in the line array may be determined by a filter design algorithm that establishes the coefficients for each FIR filter in each signal flow path of the loudspeaker. A cost minimization function is applied to prescribed frequency points, using initial driver positions and initial directivity target functions, which establish frequency points on a logarithmic scale within the frequency range of interest. If the obtained results from the application of the cost minimization function do not meet the performance requirements of the system, the position of the drivers may then be modified and the cost minimization function may be reapplied until the obtained results meet the system requirements. Once the obtained results meet the system requirements, the linear phase filter coefficients for each FIR filter in a signal path are computed using the Fourier approximation method or other frequency sampling method. 
         [0012]    The multi-way loudspeakers of the invention may include built-in DSP processing, D/A converters and amplifiers and may be connected to a digital network (e.g. IEEE 1394 standard). Further, the multi-way loudspeaker system of the invention, due to its compact dimensions, may be designed as a wall-mountable surround system. 
         [0013]    The multi-way loudspeaker system may employ drivers of different sizes, producing low distortion, high-power handling because specialized drivers can operate optimal in their dedicated frequency band, as opposed to arrays of identical wide-band drivers. The multi-way speaker design of the invention can also provide better control of in-room responses due to smooth out-of-axis responses. The system is further able to control the frequency response of reflected sound, as well as the total sound power, thereby suppressing floor and ceiling reflections. 
         [0014]    Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0015]    The invention can be better understood with reference to the following figures. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views. 
           [0016]      FIG. 1  illustrates an example of a one-dimensional six-way loudspeaker system mounted along the y-axis symmetrically to origin and a block diagram of signal flow to each of the loudspeaker drivers in the system. 
           [0017]      FIG. 2  illustrates another example implementation of a one-dimensional (ID) four-way loudspeaker system using nine loudspeaker drivers mounted along the y-axis symmetrically to origin. 
           [0018]      FIG. 3  is a flow chart of a filter design algorithm used to design the loudspeaker system. 
           [0019]      FIG. 4  is a graph illustrating the directivity target functions for angle-dependent attenuation. 
           [0020]      FIG. 5  is a graph illustrating the measurement of the amplitude frequency response of one mounted tweeter at various vertical out-of-axis displacement angles. 
           [0021]      FIG. 6  is a graph illustrating acceptable obtained results for a line array similar to the one illustrated in  FIG. 1 , determined along the y-axis. 
           [0022]      FIG. 7  is a graph illustrating the frequency response of the digital filters assigned to signal paths of the line array design illustrated in  FIG. 1  after a cost minimization function has been applied. 
           [0023]      FIG. 8  is a graph illustrating a smoothed frequency response of the third signal path illustrated i n  FIG. 7  together with the frequency response of the linear FIR filter after the FIR filter coefficient has been established and applied. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]      FIG. 1  illustrates an example implementation of a one-dimensional (1D) multi-way loudspeaker  100  of the invention and a block diagram of the signal flow to each of the loudspeaker drivers in the system  100 . As shown in  FIG. 1 , the multi-way loudspeaker  100  may be designed as a six-way loudspeaker having (i) a center tweeter  102  connected to a first power D/A converter  103 , (ii) two additional tweeters  104  and  106  connected to a second power D/A converter  105 , (iii) two midrange drivers  108  and  110  connected to a third power D/A converter  107 , (iv) two midrange drivers  112  and  114  connected to fourth power D/A converter  109 , (v) two woofers  116  and  118  connected to a fifth power D/A converter  111  and (vi) four woofers  120 ,  122 ,  124  and  126  connected to a sixth power D/A converter  113 . The connection between the loudspeakers to each amplifier represents a different way in the multi-way loudspeaker. Thus, the loudspeaker may be designed as a single-channel multi-way loudspeaker. 
         [0025]    In  FIG. 1 , the drivers, also referred to as transducers, may be mounted in a housing  154  comprised of separate sealed compartments  128 ,  130 ,  132 ,  134 ,  140 ,  142  and  148 , as indicated by separators  136 ,  138 ,  144 ,  146 ,  150  and  152 . By mounting the drivers in separate sealed compartments, coupling of the neighboring drivers is minimized. Although the various compartments are visible in  FIG. 1 , the loudspeaker system may be designed such that the compartments are not visible to the consumer when embodied in a finished product. Compartment  128 , containing woofers  120 ,  122 , may be separated by separator  136  from compartment  132 , which contains woofer  116 . Similarly, compartment  130 , which contains woofers  126  and  124 , may be separated by separator  138  from compartment  134 , which contains woofer  118 . The midrange drivers  112  and  114 , contained in compartments  140  and  142 , respectively, may be separated from compartments  132  and  134  by separators  144  and  146 , respectively. All of the tweeters  102 ,  104 ,  106 , and midrange drivers  110  and  108  may also b e contained in compartment  148  and separated from compartments  140  and  142  by separators  150  and  152 , respectively. 
         [0026]      FIG. 1  illustrates the center tweeter  102 , tweeters  104  and  106 , midrange drivers  110 ,  108 ,  112 ,  114 ,  116  and  118  and low-frequency woofers  120 ,  122 ,  124  and  126  mounted linearly along the y-axis and symmetrically about the center tweeter  102 . A typical arrangement may include tweeters  102 ,  104  and  106  of outer diameters of approximately 40 mm, midrange drivers  110 ,  108 ,  112 ,  114 ,  116  and  118  of outer diameters of approximately 80 mm, and woofers  120 ,  122 ,  124  and  126  of outer diameters of approximately 120 mm. Typically, transducer cone size may differ based on the desired application and desired size of the array. Further, the transducers may utilize neodymium magnets, although it is not necessary for the described application to utilize that particular type of magnet. 
         [0027]    The center tweeter  102  may be mounted on the y-axis at the center point  0  at the intersection between the x and y axis. The tweeters  104  and  106  may be mounted at their centers approximately +/−40 mm from the center point. The midrange drivers  110  and  108  may then be mounted at their centers approximately +/−110 mm from the center point  0 . The midrange drivers  112  and  114  may then be mounted at their centers approximately +/−220 mm from the center point. The low-frequency woofers  116  and  118  may then be mounted at their centers approximately +/−350 mm from the center point. The low frequency woofers  120  and  124  may then be mounted at their centers approximately +/−520 mm from the center point. The low frequency woofers  122  and  126  may then be mounted at their centers approximately +/−860 mm from the center point. 
         [0028]      FIG. 1  also illustrates a block diagram  160  of the signal flow of the multi-way loudspeaker system. While  FIG. 1  illustrates six ways  162 ,  164 ,  166 ,  168 ,  170  and  172  of signal flow, a channel may be divided into two or more ways. The signal flow comprises a digital input  174  that may be implemented using standard interface formats, such as SPDIF or IEEE1394 and their derivatives, and that can be connected to the drivers through various paths or ways, such as those illustrated in  FIG. 1 . Each path or way  162 ,  164 ,  166 ,  168 ,  170  and  172  may contain a digital FIR filter  176  and a power D/A converter  103 ,  105 ,  107 ,  109 ,  111  and  113  connected to either a single or to multiple loudspeaker drivers. The power D/A converters  103 ,  105 ,  107 ,  109 ,  111  and  113  may be realized as cascades of conventional audio D/A converters (not shown) and power amplifiers (not shown), or as class-D power amplifiers (not shown) with direct digital inputs. The FIR filters  176  may be implemented with a digital signal processor (DSP) (not shown). The loudspeaker drivers may be tweeters, midrange drivers or woofers, such as those illustrated. 
         [0029]    In operation, the outputs of each multiple FIR filter  176  are connected to multiple power D/A converters  103 ,  105 ,  107 ,  109 ,  111  and  113 , that are then fed to multiple loudspeaker drivers  102 ,  104 ,  106 ,  108 ,  110 ,  112 ,  114 ,  116 ,  118 ,  120 ,  122 ,  124 , and  126  that are mounted on a baffle of the housing  154 . More than one driver such as  120 ,  122 ,  124 , and  126  may be connected in parallel to a path or way  162  containing a power D/A converter  113 . 
         [0030]      FIG. 2  is another one-dimensional multi-way loudspeaker, similar to the loudspeaker of  FIG. 1 , except that it contains two rather than four mid-range drivers and four rather than six woofers. In particular,  FIG. 2  illustrates a single channel, one-dimensional, four-way loudspeaker  200  having a center tweeter  202  encircled by two additional tweeters  204  and  206 . Additionally, the loudspeaker  200  contains two midrange drivers  208  and  210  and four woofers  214 ,  216 ,  218  and  220 . Tweeters  202 ,  204  and  206 , the midrange drivers  208  and  210 , and the four woofers  214 ,  216 ,  218  and  220  are all aligned linearly along the y-axis symmetrically about the center tweeter  202 . 
         [0031]    Three signal paths (not shown) may be fed into compartment  226 . A first path may be fed to center tweeter  202 ; a second path may be fed to tweeters  204  and  206 ; and a third path may be fed to midrange drivers  208  and  210 . Just above and below compartment  226 , divided by separators represented by lines  228  and  230 , respectively, are compartments  222  and  224  containing woofers  214  and  218  and woofers  216  and  220  respectively. Woofers  214 ,  218 ,  216  and  220  may all be fed by a fourth path. 
         [0032]    A typical arrangement of the multi-way loudspeaker illustrated in  FIG. 2  may include tweeters  202 ,  204  and  206  of outer diameters of approximately 40 mm, midrange drivers  208  and  210  of outer diameters of approximately 80 mm, and woofers  214 ,  216 ,  218  and  220  of outer diameters of approximately 160 mm. As previously mentioned, transducer cone size may differ based on the desired application and desired size of the array. The number of signal paths and number of any particular type of driver may also vary. 
         [0033]    The center tweeter  202  may be mounted on the y-axis at the center point  0 , which is illustrated in  FIG. 2  at the intersection between the x and y axis. The tweeters  204  and  206  may then be mounted at their centers approximately +/−40 mm from the center point. 
         [0034]    The midrange drivers  208  and  210  may then be mounted at their centers approximately +/−110 mm from the center point  0 . The low frequency woofers  214  and  216  may then be mounted at their centers approximately +/−240 mm from the center point. The low frequency woofers  218  and  220  may then be mounted at their centers approximately +/−380 mm from the center point. 
         [0035]      FIG. 3  is a flow chart of a filter design algorithm  300  used to design the loudspeaker system of the invention. The purpose of the filter design algorithm  300  is to determine the coefficients for each FIR filter for each signal flow path of the loudspeaker. As illustrated in further detail below, the initial driver positions and initial directivity target functions are first determined  310 . The initial positions or design configuration of the speaker and drivers may be designed in accordance with a number of different variables, depending upon the application, such as the desired size of the speaker, intended application or use, manufacturing constraints, aesthetics or other product design aspects. Driver coordinates are then prescribed for each driver along the main axis. Initial guesses for directivity target functions are then set, which includes establishing frequency points on a logarithmic scale within an interval of interest. The cost function is then minimized at the prescribed frequency points  312 . If the results do not meet the performance requirements of the system, step  314 , the position of the drivers are then modified and the cost minimization function is applied again  316 . This cycle may be repeated until the results meet the requirements. Once the results meet the requirements, the linear phase filter coefficients are computed  318 . Additionally computations  320  may also be made to equalize the drivers and to compensate for phase shifts and to modify beam steering. 
         [0036]    In the first step  310 , the initial driver positions and initial directivity target functions are established. As previously mentioned, the number, position, size and orientation of the drivers are primarily determined by product design aspects. Once orientated, initial coordinate values may then be prescribed for initial driver coordinates p(n), n=1 . . . N for N drivers on the main axis. For example, in a one-dimensional (1D) array as illustrated in  FIG. 1 , N= 13 : p(n)=[−0.86, −0.52, −0.35, −0.22, −0.11, −0.04, 0, 0.04, 0.11, 0.22, 0.35, 0.52, 0.86] m (meters). 
         [0037]    To determine the initial directivity target functions, one must define initial guesses for directivity target functions T(f,q), which are determined based upon the desired performance of the drivers at specific angles q.  FIG. 4  is a graph illustrating an example set of target functions for angle-dependent attenuation at five specific angles q. The directivity target functions specify the intended sound level attenuation in dB (y-axis) that can be measured at various frequencies at sufficiently large distance from the speaker (larger than the dimensions of the speaker) in an anechoic environment, at an angle q degrees apart from a line perpendicular to the origin (center tweeter). Frequency vector f specifies a set of frequency points, e.g. 100, on a logarithmic scale within the interval of interest, e.g. 100 Hz . . . 20 kHz. 
         [0038]    Angle vector q(i),i=1, . . . ,Nq specifies a set of angles for which the optimization will be performed. While  FIG. 4 , illustrates the initial guess for directivity at five set angles: 
         [0000]      (Nq=5): q=[0,10,20,30,40]°
 
         [0000]    in most cases it may be sufficient to prescribe directivity at only two angles, i.e., Nq=2. In this instance, targeted directivity may be specified at an outer angle, for example 40 degrees, and at 0 degrees, the prescribed zero directivity on axis, i.e., q=[0,40]°. 
         [0039]    Except for the on-axis target function, the target functions at each angle, are linearly descending on a double logarithmic scale from T=0 dB at f=0 until a value T&lt;0 dB at a specified frequency fc (e.g. fc=350 Hz), then remain constant. The on-axis target function  402  remains constant at 0 db across the entire frequency range. The target directivity functions at ten (10) degrees  404 , twenty (20) degrees  410 , thirty (30) degrees  412  and forty (40) degrees  414 , all begin at T=0 dB and descend on a double logarithmic scale until the functions reach fc, which is represented by 350 Hz in  FIG. 4 , and then remain constant across the remaining frequency range of interest. 
         [0040]    After the initial driver positions and initial directivity target functions are determined, the next step  312  is to minimize the cost function F(f) at the prescribed frequency vector points f, starting with the lowest frequency increment stepwise, e.g. 100 Hz, using the obtained solution as the initial solution for the next step, respectively, by using the following equations: 
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         [0000]    where H m (n, f, q) is a set of measured amplitude frequency responses for the considered driver n, frequency f, and angle q, normalized to the response obtained on axis (angle zero), an example of which is illustrated in  FIG. 5 .  FIG. 5  illustrates the measured frequency responses  500  of one mounted tweeter at various vertical displacement angles normalized to on axis. In  FIG. 5 , line  502  represents the on-axis response, line  504  is the measured frequency response at ten degrees, line  506  is the response at twenty degrees, line  508  is the response at thirty degrees and line  510  is the measured frequency response at forty degrees, all measured at frequencies ranging between 1 kHz and 20 kHz. 
         [0041]    Further, the minimization is performed by varying real-valued frequency points of the channel filters C opt(n,f), where n is the driver index and f is frequency, within the interval [0,1]. In addition, the constraint 
         [0000]        C   opt ( n, f )=0,  f&gt;f   o   , f&lt;f   u    
         [0000]    must be fulfilled, depending on properties of particular driver n. For example, in case of a woofer, the upper operating limit is fo=1 kHz, for a tweeter, the lower limit is fu=2 kHz, for a midrange driver it could be fu=300 Hz, fo=3 kHz 
         [0042]    The above described procedure for minimizing the cost function may be performed by a function “fminsearch,” that is part of the Matlab® software package, owned and distributed by The MathWorks, Inc. The “fminsearch” function in the Matlab software packages uses the Nelder-Mead simplex algorithm or their derivatives. Alternatively, an exhaustive search over a predefined grid on the constrained parameter range may be applied. Other methodologies may also be used to minimize the cost function. 
         [0043]    If the deviation between the obtained result and the target is sufficiently small, or acceptable as determined by one skilled in the art for the particular design application, the FIR filter coefficients for each signal path in the line array are then obtained.  FIG. 6  is a graph  600  of acceptable obtained results for a line array similar to the one illustrated in  FIG. 1 , determined along the y-axis. The graph shows the obtained filter frequency responses V(f,q) after passing step  314  in  FIG. 3 . Passing means that the result met the requirements. In  FIG. 6 , line  602  represents the on-axis response V(f,q(1)), line  604  the frequency response at ten degrees V(f,q(2)), line  606  is the response at twenty degrees V(f,q(3)), line  608  is the response at thirty degrees V(f,q(4)) and line  610  is the measured frequency response at forty degrees V(f,q(5)), all shown at frequencies ranging between 50 Hz and 20 kHz. 
         [0044]      FIG. 7  is graph  700  illustrating the resulting frequency responses Copt(n,f) of each of the six signal paths in the line array loudspeakers system illustrated in  FIG. 1  once the cost minimization function has been applied and the obtained results have been found to be sufficiently small or within the acceptable range for the desired application. The line represented by L 1  or  702  is the frequency response of the first signal path which feeds the center channel tweeter  102  ( FIG. 1 ); L 2  or  704  is the frequency response of the second signal path which feeds the tweeters  104  and  106  ( FIG. 1 ); L 3  or  706  is the frequency response of the third signal path which feeds the mid-range drivers  110  and  108  ( FIG. 1 ); L 4  or  708  is the frequency response of the forth signal path which feeds mid-range drivers  114  and  116  ( FIG. 1 ); L 5  or  710  is the frequency response of the fifth signal path which feeds woofers  116  and  118  and L 6  or  812  is the frequency response of the sixth signal path which feeds woofers  120 ,  122 ,  124  and  126 . 
         [0045]    If the deviation between the obtained results and the target are not acceptable for the particular design application, i.e. or are too large, the driver positions or geometry, and/or parameters q(i) and fc o f the target function T(f,g) (see  FIG. 3 ) should then be modified. Once modified, the cost minimization function should again be applied and the process should be repeated until obtained results and the target are sufficiently small or with an acceptable range for the application. 
         [0046]    Once the driver positions and driver geometry are positioned such that the algorithm as shown in  FIG. 3  yields results within an acceptable range of the target function, the FIR filter coefficients for each signal path n=1 . . . N must then be determined, depicted as step  318  in  FIG. 3 . One method for determining the FIR coefficients is to use a Fourier approximation (frequency sampling method), to obtain linear phase filters of given degree. When applying the Fourier approximation, or other frequency sampling method, a degree should be chosen such that the approximation becomes sufficiently accurate. 
         [0047]    The Fourier approximation method may be performed by a function “firls,” that is part of the Matlab® software package, owned and distributed by The MathWorks, Inc. Similar methodologies may be used to minimize the cost function by implementing in other software systems. 
         [0048]      FIG. 8  is a graph  800  illustrating a frequency response of one signal path  802  which is identical to L 4  or  708  of  FIG. 7 , together with the frequency response of the linear phase FIR filter  804  after the FIR filter coefficients have been obtained in accordance with the method described above. 
         [0049]    Additionally, modifications can be made to the FIR filters to equalize the measured frequency response of one or more drivers (in particular tweeters, midranges). The impulse response of such a filter can be obtained by well-known methods, and must be convolved with the impulse response of the linear phase channel filter when determining the FIR filter coefficients, as described above. Further, the voice coils (acoustic centers of the drivers) may not be aligned. To compensate for this, appropriate delays can be incorporated into the filters by adding leading zeros to the FIR impulse response. 
         [0050]    Further, delays may be added to each channel in accordance with the following equation: 
         [0000]      Δ t=p/c ·sin α, ( p =driver coordinates, c=345 m/sec)
 
         [0000]    where the main sound beam, which is otherwise perpendicular to the main axis, can be steered to a desired direction with angle α. 
         [0051]    Further, the geometry of the one-dimensional layout maybe modified such that the design process can be carried out in two dimensions, i.e., along both the x and y-axis, as described above by making the geometry symmetrical. Due to the symmetry, the same directivity characteristics will result along the y-axis (vertical), except of a higher corner frequency. 
         [0052]    While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible within the scope of this invention. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.