Patent Publication Number: US-2012039480-A1

Title: Method and apparatus for improved directivity of an acoustic antenna

Description:
The present invention relates to improvements in the perceived directivity of an acoustic antenna, in particular, but not exclusively, to improvements in the directivity of an acoustic antenna as perceived by a user wishing to listen to the main output of such an antenna by reflection e.g. from a wall. 
     It is well known that an acoustic transducer with a given directivity can be produced using an acoustic antenna e.g. in the form of an acoustic array, such as a phased loudspeaker array. 
     A phased loudspeaker array can be thought of as a (linear) array using N driver units that are each fed each with a proprietary signal. 
     For example, a fundamental approach to array processing for generating a directional sound profile is known as ‘delay-and-sum beamforming’. In this approach, a specific target direction is chosen in which it is desired to have maximum output. This can be achieved by adding an individual delay to each loudspeaker signal, such that in the target direction they compensate the phase differences between the sound fields emitted by the individual loudspeakers. 
     The delay values that are needed for this can be determined geometrically from the relative positions of the loudspeakers and the target direction, or position. 
       FIG. 1  shows the geometry for a linear array along the x-axis with constant spacing Δx and a target direction having an angle θ relative to the array normal. 
     It can be seen from  FIG. 1  that the required delay for loudspeaker n (with n=0 for the loudspeaker at the origin and increasing in the positive x-direction) is given by: 
         t   n   =nΔx  sin(θ)  (1)
 
       FIG. 2  shows in detail how an acoustic beam can be generated in one particular direction by an array of N loudspeakers Driver 1 , . . . , DriverN. 
     First, the input signal is replicated N times. Then, each of the N replicas is delayed by an individual delay Integer Delay 1  . . . N, the value of which is determined by the position of the corresponding loudspeaker DriverN and the direction to which the beam is to be steered, according to equation (1) above. Finally, the N delayed signals are fed to their corresponding loudspeakers and an acoustic beam is generated in the desired direction (the gains Gain 1 , . . . , GainN are optional and can be used for amplitude tapering. 
       FIG. 3  shows a typical polar plot of the output of a phased loudspeaker array at a particular frequency. The plot shows a large amplitude main lobe (beam) and side lobes of lesser amplitude. 
     In some applications of loudspeaker arrays, the main listening axis is preferentially not the axis along which the main lobe extends. For example in some applications, the main lobe is intended to be heard by reflection, e.g. from a wall or other surface. This should lead the user to perceive that the sound wave originates from a source other than the loudspeaker array, e.g. behind them and/or to one side. This has beneficial effects for the user&#39;s enjoyment of e.g. films and video games. 
     However, the user may be located at a position which falls on or close to the axis along which one of the side lobes is directly projected. 
     In such an arrangement, the user&#39;s perception of the generated sound will be impaired because not only will he perceive the reflected main lobe by reflection but he will also perceive the undesirable side lobe directly from the loudspeaker array. This can have a detrimental effect on the quality of the sound as perceived by the user and/or on the user&#39;s perception of the location of the origin of the sound. 
     By applying a tapering methodology to the transducer array, the side lobes can be reduced in amplitude, thereby alleviating some of the detriment as perceived by the user. See, Van der Werff “Arrays without side lobes”, Convention paper 5322, Audio Engineering Society. However, such a methodology suffers from drawbacks. For example, the main lobe is often broadened and thus loses a degree of directivity, which is undesirable. 
     An alternative method, which is allegedly capable of eliminating the side lobes completely is described in “Electronically Controlled Loudspeaker Arrays without Side Lobes”, Convention paper 5322, Audio Engineering Soc., Van der Werff, presented at the 110 th  Convention 2001 May 12-15. However, this method also suffers from drawbacks, namely that each loudspeaker element in the loudspeaker array is fed by a digital filter, making the data processing very complex and thus potentially slow and limiting. For example, in the case of e.g. a 16 element array intended to reproduce a multichannel audio system (5 channels to be reproduced), this would require 80 filters, and each filter consists of 512 taps FIR filters. Consequently, the overall digital signal processing (DSP) requirement is large. 
     Accordingly, in general, the present invention aims to reduce a users perception of side lobes, without significantly adversely affecting the main lobe and without the need for significant DSP requirements. 
     Therefore, the present invention provides a method of controlling a transducer array to generate an (improved) directional sound profile as set forth in claim  1 . 
     Advantageously, according to the present invention, the amplitude of the auxiliary lobe is reduced, thereby reducing the user&#39;s perception of the auxiliary lobe and thus improving the user&#39;s perception of the quality of the primary sound profile and of the location of the origin of the primary sound profile. 
     The cancelling sound profile may have a cancelling lobe (and possibly a plurality of associated auxiliary lobes), such that the cancelling sound profile substantially matches the profile of the primary sound profile of the transducer array, albeit possibly with a different amplitude and directivity. Indeed, the array is preferably controlled to generate the cancelling sound profile on the basis of the first signal. 
     Preferably, a filter means is provided to generate the cancelling signal on the basis of the first signal, the filter means having a frequency response similar to a known frequency characteristic of a side lobe of a primary sound profile generated by the transducer array in response to a primary signal, which primary signal is generated by a transducer controller on the basis of the first signal, the known frequency characteristic being taken to correspond to the frequency characteristic of the side lobe at the intended position of the user. 
     The filter means may generate the cancelling signal on the basis of the first signal, for example. The filter means may be configured to produce a cancelling signal for controlling the transducer to generate a cancelling sound profile having a main lobe with a frequency characteristic matching that of a known frequency characteristic of a side lobe of a primary sound profile generated by the transducer array in response to a primary signal, generated by a transducer controller on the basis of the first signal, the known frequency characteristic being taken to correspond to the frequency characteristic of the side lobe at the intended position of the user. 
     By effecting modification of the phase of the cancelling lobe of the cancelling sound profile, the undesirable auxiliary lobe of the primary sound profile and the cancelling lobe of the cancelling sound profile can be caused to destructively interfere with one another, preferably to substantially cancel each other out. 
     Therefore, it is desirable for the cancelling lobe of the cancelling beam to exhibit similar, or more preferably identical, frequency dependent characteristics to the (undesirable) auxiliary lobe which is to be cancelled out. A skilled person is capable of providing a suitable means, e.g. a filter means, to produce this effect. 
     A beam canceller according to the present invention is suitable for modifying the output of an acoustic transducer array controllable by a controller to generate a primary sound profile which, when represented in polar coordinates, has a primary lobe extending along a first axis and an auxiliary lobe, associated with the primary lobe, extending along a second axis which is respectively different to the first axis, the controller being configured to control the transducer array on the basis of a received first signal; wherein the beam canceller is configured to provide to the transducer array a cancelling signal, derived from the first signal, suitable for controlling the transducer array to generate a cancelling sound profile which, when represented in polar coordinates, has a cancelling lobe extending along said second axis, but which is modified in phase to interfere destructively with said auxiliary lobe. 
     The beam canceller may be a filter, where the filter gives rise to the cancelling sound profile from the array, the cancelling sound profile having a cancelling lobe which exhibits frequency dependent characteristics similar to the frequency dependent characteristics of the auxiliary lobe of the primary sound profile. 
     In another aspect, the present invention provides an apparatus, e.g. capable of outputting an acoustic sound profile, as set for the in claim  15 . The apparatus may be an audio hi-fi apparatus. The apparatus may be a surround sound apparatus, e.g. for use with a screen such as a television screen and/or a cinema screen. 
     In any aspect, the array of acoustic transducers may be provided as an array of discrete transducers, but alternatively, two or more of the plurality of acoustic transducers may be provided in a single unit. 
    
    
     
       The present invention will now be described by way of example, in which: 
         FIG. 1  shows a representation useful in understanding the conventional sum and delay technique; 
         FIG. 2  shows an example of an implementation of the conventional sum and delay technique; 
         FIG. 3  shows a typical polar plot representative of the directional output of a loudspeaker array e.g. operated using the conventional sum and delay technique; 
         FIG. 4  shows a schematic of a phased array loudspeaker system to which the present invention is applicable; 
         FIG. 5  shows a typical output of the system of  FIG. 5 , represented on a polar plot, and at a given frequency; 
         FIG. 6  shows a schematic of an apparatus according to the present invention; 
         FIG. 7  shows a typical cancelling output which can be produced in accordance with the present invention; 
         FIG. 8(   a ) shows a representation, to aid understanding, on a polar plot and at a given frequency of the superposition of a cancelling beam, produced in accordance with the present invention, and a conventional beam; 
         FIG. 8(   b ) shows a representation on a polar plot and at a given frequency of the result of the superposition shown in  FIG. 8(   a ); 
         FIG. 9  shows an embodiment of the present invention; 
         FIG. 10A  provides examples of the output of a conventional directional acoustic apparatus at various frequencies in colour; 
         FIG. 10B  provides examples of the output of a conventional directional acoustic apparatus at various frequencies in gray scale; 
         FIG. 11  provides similar information in polar coordinates for the same various frequencies; 
         FIG. 12  shows the desired frequency dependent characteristics of an embodiment of the present invention based on the information provided in  FIGS. 10A ,  10 B and  11 ; 
         FIG. 13A  provides examples of the output of a conventional directional acoustic apparatus at various frequencies in colour; 
         FIG. 13B  provides examples of the output of a conventional directional acoustic apparatus at various frequencies in gray scale; 
         FIG. 14  provides similar information in polar coordinates for the same various frequencies; 
         FIG. 15  shows the desired frequency dependent characteristics of an embodiment of the present invention based on the information provided in  FIGS. 13A ,  13 B and  14 ; 
         FIG. 16  shows an implementation of the present invention using DSP. 
     
    
    
       FIG. 4  shows a simplified phased array loudspeaker system  10  with which the present invention is usable. The system  10  includes a plurality of loudspeaker elements  12 , arranged in an array  14  to produce a directional sound wave. 
     The directivity of the output of the array  14  is controlled by directional means  18 . The directional means is preferably included in a controller  16  for controlling the output sound overall, e.g. the amplitude etc. 
     The means  18  may be configured, for example, to control the array  14  to output sound principally along a direction which is 45° to the array normal. The directional means  18  may be configured to operate in accordance with the sum and delay technique in order to give directivity to the output sound. 
     For example,  FIG. 5  shows a representative polar plot of a main output  21  generatable by such a system  10 . 
     The system  10  may be arranged such that the main beam (lobe)  22  of the output sound is intended to be heard by a user at position X, e.g. located on the normal to the array  14 , by reflection from one or more surfaces  20 , e.g. walls. Due to the path length of the main beam, it may be attenuated before it reaches the ears of the user. Additionally, the reflecting surfaces  20 , e.g. the walls, will attenuate the main beam to a greater or lesser degree depending on the precise characteristics of the surfaces  20 . In some cases, the attenuation may be significant. 
     The origin of the polar plot can be considered to be for example any one of the loudspeaker elements  12 , e.g. the middle element  12 , or an element  12  located at one end of the array  14 . 
     This may be desirable, for example, where the user is intended to perceive that the output sound is generated by a source located behind them (i.e. assuming the user is facing the origin of the polar plot when located at position X). Such an effect may enhance the user&#39;s experience of a film or a video game, for example. 
     However, a characteristic of the system  10  is that side lobes  24  and  26  are also generated. When the system  10  is configured such that the user perceives the main beam  22  by reflection, it is possible that a side lobe  24  is output directly at or very close to the user at position X. The user therefore undesirably experiences the side lobe  24  which interferes with the user&#39;s perception of the reflected main beam  20 . 
     In the cases described above where the main beam is attenuated by the path length of the main beam  22  and/or by the characteristics of the surfaces  20 , the residual sound pressure level (associated with the main beam) at the user&#39;s ear at position X might be equivalent to the sound pressure at the user&#39;s ear at position X. 
     Accordingly, the present invention provides a modified version of the system  10 , in the form of system  100 , as shown in  FIG. 6 . The system  110  includes a plurality of loudspeaker elements  112  forming an array  114 . The array  114  is preferably controlled on the basis of a directional means  118  and a cancelling means  130 . The directional means  118  and the cancelling means  130  may be provided in respective controllers  116  or in a common controller  116  (not shown). 
     The directional means  118  operates substantially as the directional means  18  described above. 
     The output of the array  114  is further controlled by the cancelling means  130  to generate a cancelling sound output  140 , which in the present embodiment preferably resembles the polar representation shown in  FIG. 7 . The cancelling sound output profile, represented on a polar plot, will generally include a main beam (lobe)  142  and side lobes  144  and  146 . 
     In  FIG. 7 , position X is the same as position X shown in  FIG. 5 . Likewise, the origin of the polar representation shown in  FIG. 7  is the same as that shown in  FIG. 5 . 
     The system  110  further includes an inverter means  132 , for inverting the cancelling sound output  140  with respect to the main output  21 . The inverter means  132  may be incorporated in the cancelling means  130 . The inverter means  132  may be incorporated in the controller incorporating the cancelling means  130 , but e.g. not in the cancelling means  130 . 
     With reference to  FIG. 8  ( a ), it can be seen that the directional means  118  and the cancelling means  130  each effectively cause the array  114  to generate a respective output, which superimpose on one another, such that the main cancelling beam (lobe)  142  being superimposed on the main output side lobe  24 . The inclusion of the inverter means  132  results in the superimposed lobes acting to cancel each other out. 
       FIG. 8  ( b ) shows the resulting output of the system  100 , where the side lobe  24  is effectively cancelled, whereas the main beam  22  is substantially unaffected by the cancellation. 
     Accordingly, the user at position X will perceive the main beam  22  by reflection from surface  20 , and there is no lobe or beam incident directly on him to detrimentally affect his perception of the reflected main beam, 
     Depending on the characteristics of the array  114 , and on the directional means  118 , the size of the outputted side lobe  24  can be determined by a skilled person for any given signal input to the system  10 . 
     Therefore, a skilled person will have no difficulty in providing a cancelling means  130  and inverter  132  which effectively mimics the characteristics of the system  10  resulting in the side lobe  24 , thereby producing a cancelling output from the array  114  (similar to array  14 ) having a main (cancelling) beam  142  with substantially the same characteristics, e.g. amplitude, frequency response, etc., as the side lobe  24  so as effectively to match it and substantially to cancel it. 
     An example of a sound system according to the present invention is shown in  FIG. 9 , which shows a system  210  which includes a speaker array  214  of speaker elements  212 , controllable by an apparatus  213  according to the present invention. 
     The apparatus  213  is configured to receive a surround left channel, a centre channel and a surround right channel. 
     The surround left channel is fed to a directional means  218 L for controlling the array to generate an output with a mean beam (lobe) at a (e.g. left-hand) angle to the main direction of output of the array  214 , e.g. the normal to the array  214 . The angle may be 45°. The angle may be another angle between 0° and 90°, preferably non-inclusive. 
     The surround right channel is fed to a directional means  218 R for controlling the array to generate an output with a mean beam (lobe) at a (e.g. right-hand) angle to the main direction of output of the array  214 , e.g. the normal to the array  214 . The angle may be 45°. The angle may be another angle between 0° and 90°, preferably non-inclusive. 
     The centre channel is fed to a directional means  218 C for controlling the array to generate an output with a mean beam (lobe) substantially along the main direction of output of the array  214 , e.g. the normal to the array  214 . Directional means  218 C may be a 0° beamformer for projecting the centre channel signal. 
     As described above, each of the directional means  218 R and  218 L (at least) end up generating side lobes in the output sound wave. The amplitude of those sidelobes varies with frequency. 
     The apparatus  213  further includes cancelling inverters  230 L and  230 R. The cancelling inverters  230 L,  230 R can be thought of as filters which act to mimic the frequency response of the sidelobes described above, but the filter response is in an anti-phase relationship relative to the frequency response of the side lobes themselves—in other words the filter response is inverted relative to the side lobes. 
     Each signal processed by the cancelling inverter  230 L and/or the cancelling inverter  230 R is then scaled and mixed with the centre channel signal at means  215  in accordance with the skilled person&#39;s common general knowledge, for example. 
     Thus, the signal(s) generated by each or either of the cancelling inverters  230 L,  230 R acts to cause a side lobe(s) associated with each of the surround left and right channels to be cancelled out. 
     The present invention lies in the provision of one or more of the cancelling inverts  230 L,  230 R, e.g. the cancelling means  130  and inverting means  132 , and it is not necessary to include the speaker array in an embodiment according to the invention. It is sufficient for a skilled person to know about the frequency response of an array to which the present invention is to be applied, for example. 
     Indeed,  FIG. 16  shows an example of how the present invention can be implemented using less complex digital signal processing (DSP). In  FIG. 16 , the delays t 1 -tn can be implemented by DSP based on filter coefficients e.g. stored in a memory  301 , which are referred to by the filter  303 . The filter coefficients can be modified to account for the characteristics of any given array, and the preferred effect of the filter  302 . The necessary gain can then be applied by the gain means  304 , to provide the preferred gain corresponding to the respective channel gains gain( 1 )−gain(n). As shown in  FIG. 9 , an inverter  306  may be required to invert a given channel. 
     EXAMPLE 1 
     In an example of a way in which the required frequency dependent characteristics of the cancelling means (cancelling inverter) can be determined, if there is provided a system  10  having ten loudspeaker elements  12 , where the main beam  22  (lobe) is directed at 60° to the main axis of the array  14  (e.g. to the normal of the array  14 ), the frequency response in terms of amplitude against spatial distribution can be represented as shown in  FIGS. 10A and 10B  for frequencies 0.519 kHz, 1.038 kHz, 2.075 kHz, 2.940 kHz, 3.978 kHz and 5.015 kHz. Position X marks the intended location of a user. 
       FIG. 11  shows a set of polar representations of the information shown in  FIGS. 10A and 10B  for particular amplitude at the various selected frequencies. Point X lies along the 0° axis. 
       FIG. 12  shows the frequency response of the example system at position X. Accordingly, this is the preferred frequency response of a cancelling means should exhibit in an apparatus or system according to the present invention if a user at position X is not to perceive the side lobe(s) output by the array. 
     EXAMPLE 2 
     In another example there is provided a system  10  having ten loudspeaker elements  12 , but the main beam  22  (lobe) is directed at 40° to the main axis of the array  14  (e.g. to the normal of the array  14 ). The frequency response in terms of amplitude against spatial distribution can be represented as shown in  FIGS. 13A and 13B  for frequencies 0.519 kHz, 1.038 kHz, 2.075 kHz, 2.940 kHz, 3.978 kHz and 5.015 kHz. Position X marks the intended location of a user. 
       FIG. 14  shows a set of polar representations of the information shown in  FIGS. 10A and 10B  for a particular amplitude at the various selected frequencies. Point X lies along the 0° axis. 
       FIG. 15  shows the frequency response of the example system at position X. Accordingly, this is the preferred frequency response of a cancelling means should exhibit in an apparatus or system according to the present invention if a user at position X is not to perceive the side lobe(s) output by the array.