Abstract:
A system of microphones, signal processors, and loudspeakers provides enhanced comprehension of speech in noisy social events where the locations of participants are relatively constrained. Speech enhancement comprises both boosting sounds moderately at higher frequencies and delaying them to match the arrival of sounds directly from speakers. Virtual loudspeakers create the illusion for each listener that the boosted sounds come from the vicinity of each talker. Video cameras determine the head positions and facing directions of participants. Subgroups of participants having at least temporary conversational affinities can be identified. Such subgroups may overlap and change. Speech from talking participants is picked up by directional microphones and filtered, sorted, and relayed selectively via loudspeakers to listening participants identified as subgroup members, reinforcing and enhancing the naturally heard speech. More weight can be given to enhancing speech between affined participants. Either conventional or parametric loudspeakers may be used.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional patent application No. 61/205,047 filed 14 Jan. 2009 as “Participant Audio Enhancement System”, which is incorporated herein by reference in its entirety. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
       [0002]    There is and was no federally sponsored research and development. 
       BACKGROUND 
       [0003]    This invention relates to enhancing aural communication and conversational interaction among and between persons in social situations, such as those surrounding a table. 
         [0004]    Participants sitting or arrayed around a table frequently have trouble hearing and conversing with other participants beyond the most nearby neighbors seated to either side. Background noise due to other speakers in the room, and room reverberations and various additional external sources conspire to drown out normal speech. Lower-frequency sounds (or speech components) predominantly mask higher frequency sounds and speech, reducing intelligibility. Older participants are particularly affected as their ability to hear higher frequency sounds is often diminished or lost. One aspect of such an environment is the “cocktail party effect,” wherein a group of talkers instinctively raise their individual acoustic outputs above normal conversational levels in order to overcome the perceived background level, which is often dominated by reverberant sound. Another aspect of the cocktail party effect is the ability of listeners to focus in on a particular talker in the presence of background sound. 
         [0005]    A number of previous techniques have attempted to remedy various shortcomings. One previous technique involves feeding suitably and multiply delayed versions of an electronic acoustic signal to two spaced loudspeakers to produce the illusion to a listener that the sounds emanate from a third, “phantom” location. The listener is assumed to be on a line perpendicular to the midpoint between the spaced loudspeakers to face that midpoint. Another previous technique teaches reproducing a stereophonic signal pair through a pair of spaced loudspeakers that simulate a pair of widely-spaced virtual or phantom loudspeakers placed at or near the optimum locations for conventionally spaced stereo loudspeakers. The spaced loudspeakers may be surprisingly close together, which relaxes accuracy requirements on knowing a listeners head position. Several other previous techniques involve parametric loudspeakers, face and facial feature detection and tracking, and techniques using two cameras to locate and track a single listener&#39;s head using triangulation and skin color for the purpose of enhancing virtual stereophonic sound reproduction. 
         [0006]    None of the previous techniques suitably address the problems discussed above. The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent upon a reading of the specification and a study of the drawings. 
       SUMMARY 
       [0007]    An array of outward facing directional microphones is placed towards the center of the table or around its rim, or placed amongst several conversation participants. Speech sounds collected by those microphones are processed or filtered to selectively or favorably relay them to loudspeakers substantially on opposite sides of the table to enhance, replace or reinforce the natural sounds from all speakers heard directly by each and every listener. In the simplest embodiment, the relayed sounds are merely filtered to enhance higher-frequency components, boosted in strength and time shifted (delayed) to arrive substantially in expected synchronization with the natural sounds or speech heard directly from the speaker. The components of natural speech heard well are the lower frequency components that are also the primary contributors to masking. Thus the filtered sounds replace the masked sounds without substantially increasing overall masking effects. 
         [0008]    However, as human listeners tend to face a speaker to help overcome the “cocktail party effect” and improve hearability, our enhanced preferred embodiment uses virtual sound source synthesis means to create illusions in listeners&#39; minds that the relayed sounds emanate from the vicinities of the talkers being paid attention to. This requires tracking the position and orientation of each participants head. The positions of the heads of participants of a physically constrained social situation such as being seated at a table and the directions they are facing at any particular time are detected by analyzing image frames collected by a video camera substantially located in the center of the table and having a hemispheric or panoramic 360° azimuthal field of view. The analysis comprises conventional face detection and facial feature recognition means, and the dynamic, time-changing affinities of participants for conversational subgroups are calculated. That is, visual clues are used to determine who is mostly talking to whom. 
         [0009]    Both conventional and parametric loudspeakers can be used to create virtual sources. Parametric loudspeakers provide the advantage of low spillover and less addition to the back ground noise. A number of physical embodiments are possible, including a self contained table centerpiece, table rim mounted microphones and loudspeakers, and a suspended “chandelier” unit. In the preferred embodiment separate video and audio digital processors operate in parallel, although the processing functions could be supported by a single physical processor. In some alternative embodiments these processors are physically separated and communicate via wireless links. Personal communications devices are used as remote microphones in another alternative embodiment. In certain alternatives microphones and loudspeakers are “loose” elements whose positions are not predetermined and may change. Their locations are continuously registered using psychoacoustically inaudible sounds transmitted from auxiliary and the primary loudspeakers in the enhancement system. For elongated tables, where a single system would not function well, participant acoustic enhancement system coverage can be expanded by adding additional audio and video control units, in a master-slave configuration. 
       Objects and Advantages 
       [0010]    Unlike the prior art, whose objectives are to enhance the stereophonic sound reproduction, where sounds are emanate from an assembly of spatially distributed sources, the objective of our invention is to enhance the hearability and intelligibility of conversational speech from each single source, or talker, in a group. That is, instead of producing the aural illusion of a spread-out acoustic environment, we want to do the opposite, and narrow the apparent sources of sound. This results in the significant advantage of enhancing to ability of participants in certain social environments to interact with one another even though they may not be in close physical proximity to one another. 
         [0011]    Still further objects and advantages will become apparent from a consideration of the ensuing description and accompanying drawings. 
     
    
     
       DRAWING FIGURES 
         [0012]      FIG. 1  illustrates the basic layout of an embodiment of the invention. 
           [0013]      FIG. 2  is a block diagram of the preferred embodiment. 
           [0014]      FIG. 3  depicts the primary direct sound paths for a virtual sound source. 
           [0015]      FIG. 4A  is a perspective of the preferred embodiment. 
           [0016]      FIG. 4B  is a perspective view of an alternative embodiment. 
           [0017]      FIG. 5A  presents an alternative embodiment. 
           [0018]      FIG. 5B  presents another alternative embodiment. 
           [0019]      FIG. 5C  is the block diagram of an alternative embodiment. 
           [0020]      FIG. 5D  is the block diagram of an alternative embodiment. 
           [0021]      FIG. 6A  is a perspective view of another alternative embodiment. 
           [0022]      FIG. 6B  shows some details of the system illustrated in  FIG. 6A . 
           [0023]      FIG. 7  is a perspective view of a further embodiment. 
           [0024]      FIG. 8  is a block diagram with personal communications devices. 
           [0025]      FIG. 9  is a block diagram with parametric loudspeakers. 
           [0026]      FIG. 10  depicts a parametric loudspeaker system for virtual sound sources. 
           [0027]      FIG. 11  is a plan view of a table showing multiple control units. 
       
    
    
     REFERENCE NUMERALS IN DRAWINGS 
       [0028]      
         [0000]    
       
         
               
               
               
               
             
           
               
                   
               
             
             
               
                 2 
                 loudspeaker unit 
                 4 
                 unidirectional microphone 
               
               
                 6 
                 table 
                 8 
                 participants 
               
               
                 10 
                 control unit 
                 12 
                 camera 
               
               
                 14 
                 fish-eye lens 
                 15 
                 ribbon cable 
               
               
                 16 
                 digital video processor 
                 17 
                 contact plate 
               
               
                 18 
                 video frame grabber 
                 20 
                 digital audio processor 
               
               
                 22 
                 multi-channel ADC 
                 24 
                 multi-channel DAC/driver 
               
               
                 26 
                 data path 
                 28 
                 optional data path 
               
               
                 30 
                 centerpiece 
                 31 
                 right ear 
               
               
                 32 
                 talker 
                 33 
                 left ear 
               
               
                 34 
                 listener 
                 36 
                 loudspeaker pair 
               
               
                 38 
                 direct sound paths 
                 40 
                 microphone sound path 
               
               
                 42 
                 true sound paths 
                 44 
                 table rim 
               
               
                 46 
                 table cover 
                 47 
                 wireless data link terminal 
               
               
                 48 
                 audio processing unit 
                 49 
                 wireless data link 
               
               
                 50 
                 video processing unit 
                 51 
                 panoramic camera 
               
               
                 52 
                 wireless drive links 
                 53 
                 wireless drive link terminal 
               
               
                 54 
                 control unit 
                 56 
                 table pad 
               
               
                 58 
                 flush mounted microphone 
                 60 
                 end terminal 
               
               
                 69 
                 wireless network adapter 
                 70 
                 personal comm. device 
               
               
                 71 
                 auxiliary microphone 
                 72 
                 beam steering signals 
               
               
                 74 
                 parametric loudspeaker units 
                 76 
                 ultrasonic beams 
               
               
                 78 
                 master control unit 
                 80 
                 slave unit 
               
               
                   
               
             
          
         
       
     
       DETAILED DESCRIPTION OF THE INVENTION 
       [0029]      FIG. 1  shows the basic layout of one embodiment of our invention. Loudspeaker units  2  and unidirectional microphones  4  are arrayed substantially around the center of table  6 . Participants  8  are normally seated around table  6 , although they may exchange positions or be temporarily or permanently absent. Table  6  can have any shape, including round (as depicted), square, rectangular, oval, polygonal, etc. The locations of participants  8  can be arbitrary, although their individual distances from the center of table  6  are at least relatively well known, as they may be assumed reasonably to be sitting normally at the table. Notably, the use of a table in this detailed description is meant to be illustrative, and not limiting. As such, in various embodiments participants  8  may be seated or standing in a region that does not include a table; i.e. a region that is empty or that contains other things. For example, participants  8  may be seated or standing in an empty area, or may be seated on couches facing each other, etc.  FIG. 1  also illustrates sub-groupings of participants  8 , labeled as “A”, “B”, and “C”. 
         [0030]    Loudspeaker units  2  may contain or comprise single loudspeakers, multiple loudspeakers, or arrays of loudspeakers, or comprise loudspeaker pairs arranged to form virtual or “phantom” loudspeakers. In addition, loudspeaker units  2  may be “parametric” loudspeakers that employ ultrasonic beams to produce audible sounds through nonlinear interactions with air, and may further be arrayed to form parametric virtual loudspeakers. Loudspeaker units  2  are aimed substantially outward from the center or rim of table  6 . 
         [0031]    Unidirectional microphones  4  preferably have cardioid or hypercardioid response patterns to reduce sound pickup from the sides and rear, and are also aimed substantially outward from the center of table  6  so as to selectively sense speech from the nearest participants  8 . 
         [0032]    Loudspeaker units  2  and unidirectional microphones  4  are connected to control unit  10 , which is placed substantially in the center of table  6 . The connections between units  2  and microphones  4  and control unit  10  may be made using wires or electronic cables or, alternatively, may be wireless links. In one embodiment, the system may be permanently installed on table  6 , while in another embodiment the system may be configured for portable use or for temporary use. 
         [0033]      FIG. 2  is a block diagram of the enhanced preferred embodiment of the invented system, which includes the optional video processing subsystem. In this embodiment control unit  10  comprises video digital processor  16 , frame grabber  18 , audio digital processor  20 , multichannel ADC  22 , and multichannel DAC/speaker driver  24 . Control unit  10  also mounts optional video camera  12 . Preferably, camera  12  employs a fish-eye lens  14  and is pointed substantially vertically on order to produce a substantially hemispherical image with 360° azimuthal coverage. Alternatively, camera  12  is a system comprising three or more individual cameras whose individual images may be electronically stitched together to produce a single 360° panoramic image. 
         [0034]    Video processor  16  continuously or substantially continuously extracts participants&#39; head position and orientation and passes this information to audio processor  20  via data path  26 . This information is used to calculate the appropriate time delays required for synchronizing sounds arriving at the listeners with sounds heard directly from the participating speakers. Alternatively, in the simplest embodiment, these time delays may be estimated without knowledge of actual participants&#39; locations, merely by inferring where participants are likely to be. In addition, knowledge of participants&#39; locations may be used to implement advanced signal processing steps, such as those required to synthesize a “phantom” speaker. Audio processor  20  accepts digitized signals picked up by microphones  4 , processes said signals, and directs them as required to DAC/driver  24 . Each output channel from DAC/driver  24  may contain audio information intended for one or more participants  8  or to be directed towards locations where participants may be reasonably expected to be. 
         [0035]    In general, the preferred embodiment depicted in  FIGS. 1 and 2  basically operates by picking up speech sounds from a talking participant on one side of table  6 , filtering the sounds and relaying them with appropriate time shifts or delays and amplification to the participants substantially on the other side of the table from the talker. All participants may be talkers, perhaps simultaneously, and their speech can be directed at subgroups of possible listeners, including individual participants or the entire group. 
         [0036]    A listener, of course, will hear some speech sounds coming directly from a speaker, especially the lower frequency components. Optimally, the augmented sounds are timed to reinforce the real ones and/or are more intense than the real ones, at least in specified frequency bands, thus enhancing the ability of a listener to hear a speaker in a noisy background environment. Simply filtering and adjusting the time delays between sounds picked up by microphones  4  and radiated by loudspeaker units  2  and modestly boosting (3-10 dB) them will substantially improve the intelligibility of a conversation across or around a table. To enhance speech intelligibility, sound bandwidths should be significantly narrower than those used for hi-fi or stereo music reproduction. FM radio and television sound channels span 30 Hz to 15 kHz, CD audio covers 20 Hz to 20 kHz, professional, and audiophile audio 20 Hz to above 22 kHz. A bandwidth of 300-3400 Hz is often used in telephony, but a reproduced speech bandwidth of 200-7000 Hz can markedly reduce fatigue, improve concentration, and increase intelligibility. Human ear sensitivity peaks at a frequency of about 3300 Hz, and the speech articulation index, which is a measure of intelligibility, increases some 25% when the band is extended from 3.4 to 7 kHz, although most of the improvement occurs below 5 kHz. Most masking of speech by background noise occurs above 500 Hz, so that lower-frequency speech sounds traveling along direct paths  38  of  FIG. 3  are not strongly affected. Furthermore, narrower bandwidths simplify the synthesis of convincing virtual sound sources. In our preferred embodiment, we choose to employ a sound channel band extending from 500 Hz to 5 kHz for enhancing intelligibility. It is not necessary, however, to use this entire band for enhancing the ability to understand speech. Satisfactory performance may be had merely by augmenting speech components above 1-2 kHz. 
         [0037]    However, our invention also goes beyond simply adjusting time delays. In an alternative embodiment, we extend the virtual or phantom source concept to encompass arbitrary listener position and head orientation. Images from optional camera  12  are analyzed using methods well understood by knowledgeable workers in the field of face and facial feature recognition and tracking (computer applications to do this are commercially readily available from a multiplicity of vendors) to extract both the estimated head position of each and every participant  8  relative to table  6  and the approximate direction each and every participant  8  is facing. Such methods typically employ skin detection as a starting point. Because the locations of participants around table  6  are relatively constrained (e.g., by the nature of being seated around a table) and therefore reasonably accurately known, two- or three-dimensional methods such triangulation and trilateration that require two or more well-spaced cameras as taught in some of the prior art are not necessary. The angular position of every participant around table  6  is extracted from the angular position of the images of his or her head in the hemispheric images captured by frame grabber  18  with respect to a predetermined reference axis (not shown). This process is simplified by the fact that participants  8  are unlikely to block one another in the field of view of camera  12 , and if they happen to, there is little or no practical consequence. Non-participants, such as persons not seated reasonably closely around table  6  and in the background scene, may be discriminated against on the basis of relative or expected face and head size. Advanced eye- or pupil-tracking means may also be employed to assist determination of which direction each participant  8  is looking as well as facing. 
         [0038]    For simplicity in the purpose of this description,  FIG. 3  depicts the basic arrangement in which at some instance in time there is one participant talker  32 , one participant listener  34 , and one loudspeaker pair  36 . There are a multiplicity of direct, or primary, sound paths, including paths  38  from talker  32  to the ears of listener  34 , path  40  from talker  32  to a microphone  4 , and paths  42  from loudspeaker pair  36  to the right ear  31  and left ear  33  of listener  34 . Microphone  4  may be selected as the closest to talker  32 , or, optionally, selected using position information from video processor  16  combined with audio data from processor  20  to provide the optimum virtual source illusion. In a further option, sounds received by a multiplicity of microphones  4  may be combined using methods known to workers familiar with that art to synthesize sounds from talker  32  that would be received by a virtual or “phantom” microphone at a known location, which would be used in place of sounds received by a physical microphone  4 . Associated listeners  34  all tend to face talker  32  (in order to hear him or her), while talker  32  may switch frequently between perceived listeners. However, since there is only one primary sound path from the talker to the closest microphone  4 , the orientation of talker&#39;s  32  head is relatively less important than that of listeners  34 . Audio processor  20  will actually process simultaneously sounds from a multiplicity of participants  8  and distribute said sounds to all other participants, although not necessarily with equal strength, by synthesizing a virtual sound source at or in the vicinity of each talker  32  appropriate for every potential listener  34 . Thus each listener  34  will hear the direct sounds along paths  38  plus the sounds passing along paths  42  that are sensed to emanate from talker  32  and also seem to pass along paths  38 . 
         [0039]    An important aspect of our invention is the dynamic association of individual participants to subgroups. Subgroups may overlap, so an individual participant could be associated by video processor  16  to one or more subgroups. Different demographic groups, for example teenagers and retirees, are liable to have different group dynamics. In addition, group dynamics are subject to change with time. Therefore processor  16  continuously collects information on how long participants appear to pay attention to each other, how often they apparently switch allegiances, and adjusts the distribution and weighting of sounds received by microphones  4  to loudspeaker units  2  accordingly. Participants may be assigned a time-variable “affinity index” that depends on his or her apparent association with one or more subgroups, including no subgroup. 
         [0040]    Referring again to  FIG. 3 , we see that the lengths of sound paths  38 ,  40 , and  42  can be calculated preferably by video processor  16  from the known or estimated positions of talker  32 , listener  34 , loudspeaker pairs  36 , and microphone  4 , plus the facing direction of listener  34 , relative to control unit  10 . Optionally, these lengths can be calculated by audio processor  20  or by processors  16  and  20  together. In a further option, the functions of processors  16  and  20  may be combined into a single physical digital processor (not shown). Note that pairs of loudspeakers  36  need not be contiguous—adjacent or nearby pairs may overlap. Once the lengths of sound paths  38 ,  40 , and  42  are calculated, processor  20  synthesizes a virtual sound source located at or substantially close to the location of talker  32  using methods known to workers in the field, such as a method whereby a series of delayed and diminishing and polarity reversing versions of the digitized outputs of microphone  4  are summed and transmitted to loudspeaker pair  36 . According to one embodiment of the present invention there need be no restrictions on the position and facing direction of listener  34 , and the virtual source can be located at talker  32 . 
         [0041]      FIG. 4A  depicts an alternative embodiment of our invented participant acoustic enhancement system wherein control unit  10 , camera  12 , microphones  4 , and loudspeaker units  36  are all contained in or supported on or by table centerpiece  30 . Optional data link  28 , depicted in  FIG. 2 , passes any auxiliary participant position information extracted by audio processor  20 . Such information may be obtained by correlating the sounds received at two or more microphones  4  and determining relative time-of-arrival offsets and relative sound intensities. This information may be used to assist locating participants  8  and to help verify their actual existences. Additionally, this information may be used to determine the shape of table  6  with respect to control unit  10 . In a further option, correlation or other signal analysis means may be employed to cancel acoustic pickup by microphones  4  from adjacent or nearby loudspeakers  2 . In this embodiment the angular positions of loudspeakers  2  and microphones  4  are well defined and the choice of a reference axis is straightforward and trivial. Preferably, our invented system is battery operated using rechargeable batteries and is controlled only by a simple on-off switch, requiring no other operator interaction. 
         [0042]      FIG. 4B  depicts a further alternative embodiment wherein loudspeaker units  2  (or loudspeaker pairs  36 ) and microphones  4  are mounted or clipped to the rim  44  of table  6  under table cover  46 , which may be a tablecloth or a tablecloth-overlay combination. Because the table cover  46  will attenuate sounds to degree determined by the porosity and other sound transmission and absorption factors of the cloth, so more intense loudspeaker output requirements are expected. However, in this alternative embodiment the relatively shorter sound path lengths between participants  8  and loudspeakers  36  and microphones  4  at least partially compensate for sound attenuation by table coverings. Ribbon cables  15  under table cover  46  connect to contact plate  17  which is also underneath cover  46 . Electrical contacts (not shown) on top of contact plate  17  and underneath the base of centerpiece  30  penetrate cover  46  to complete the electrical circuits between control unit  10  and loudspeaker pairs  36  and microphones  4 . 
         [0043]      FIGS. 5A and 5B  present alternative embodiments wherein loudspeaker units  2  (or loudspeaker pairs  36 ) and microphones  4  are also mounted or clipped to the rim  44  of table  6  under table cover  46 . In these alternative embodiments it is convenient to divide and separate the functions of control unit  10  into its video and audio processing aspects and to connect them via wireless data links. 
         [0044]    In  FIG. 5A  audio processing unit  48  is placed under table  6  and is hard-wired to loudspeakers  36  and microphones  4 . Video processing unit  50  is contained in centerpiece  30  and communicates with audio processing unit  48  via wireless signal path  49 .  FIG. 5C  is a block diagram of this optional version of this alternative embodiment. Here, wireless link  49  connects processing units  48  and  50 . 
         [0045]    In  FIG. 5B  both the video and audio processing functions are supported by control unit  10 , which is similar to control unit  10  of  FIG. 4B , for example, except that that wireless data links  52  transmit signals to loudspeaker units  36  and from microphones  4 . In this embodiment, wireless data links  52  are preferably radio-frequency links.  FIG. 5D  presents a block diagram corresponding to this optional version of this alternative embodiment. In addition,  FIG. 5D  illustrates the further option wherein the images produced by three cameras  51  are stitched together to produce a single 360° panoramic view. 
         [0046]    Yet another alternative embodiment is illustrated in  FIG. 6A , wherein loudspeaker units  2  and flush mounted microphones  58  are built into table pads  56 . Centerpiece  30  will now contain control unit  54 . Because table pad  56  may extend under centerpiece  30 , end terminals  60  of wireless data links  52  may be spaced closely apart, as illustrated in  FIG. 6B . Links  52  can be implemented optionally as radio-frequency, inductively coupled, capacitive, acoustic, optical/infrared links, or even direct electrical contacts penetrating table cover  46 . Link terminals  60  in close proximity to one another transmit and receive data through table cover  46 . Very small loudspeakers or sound-emitting apertures are required for this alternative embodiment because table pads  56  are desirably very thin, yet acoustic energy must radiated efficiently and significant audio distortion avoided. 
         [0047]    A further alternative embodiment is depicted in  FIG. 7 , which is essentially an “upside down” version of the preferred embodiment. Here, participant acoustic enhancement system  68  comprising control unit  10 , camera  12 , loudspeaker units  2 , and unidirectional microphones  4  are suspended from the ceiling much as a chandelier substantially above the center of table  6 . Participant acoustic enhancement system  68  may be disguised as or configured into an actual chandelier with the addition of lamps and can be utilized for permanent instead of portable or temporary setups as are the previously described preferred and alternative embodiments of our invention. In this embodiment operating power may be supplied from the electrical mains. 
         [0048]    In an even further alternative microphones  4  and  58  are replaced by personal communications devices such as iPhones or Bluetooth enabled mobile telephones that can be configured to join a local or peer-to-peer network established by control unit  10  in the preferred embodiment or the equivalent in the other presented and possible alternative embodiments so as to pick up talkers&#39;  32  speech and transmit each to control unit  10  or equivalent.  FIG. 8  is a block diagram illustrating this alternative, with wireless network adapter  69  and personal communication devices  70 . Not all participants desire or need to use personal communication devices, and a further alternative (not shown) optionally accommodates a mixture of microphones  4  or  58  and personal communication devices  70 . 
         [0049]    It is apparent in the alternative embodiments depicted in  FIGS. 5A-C ,  FIG. 8 , and, to some degree,  FIG. 6A-B , that “loose” elements comprising loudspeaker units  2 , microphones  4 , loudspeaker pairs  36 , personal communication devices  70 , etc., can be placed rather arbitrarily, perhaps by accident, and therefore need to be registered with respect to a reference axis set by control unit  10  or its equivalent. Thus another aspect of our invented participant acoustic enhancement system is the ability to self-register those loose elements. Registration is accomplished by means of producing and radiating, preferably continuously, wideband (or, short effective duration) audio signals that are psychoacoustically inaudible to humans, from loudspeaker units  2 , etc. Such inaudible signals are known to practitioners familiar with that art. Referring back to  FIGS. 5A and 5B , each loudspeaker  2  radiates a distinct psychoaoustically inaudible signal that is received by three or more auxiliary microphones  71  on control unit  10  or  50 . Said distinct inaudible signals may be radiated sequentially or, if encoded, simultaneously. The arrival times of the distinctly encoded signals at auxiliary microphones  71  are extracted by audio processor  20 , from which the azimuth and distance to each loudspeaker  2  can readily be calculated or deduced relative to a pre-established reference axis. Once the placement order and locations of loudspeakers  2  are determined, the locations of the remaining loose elements comprising microphones  4  and personal communication devices  70  can then be deduced by similarly extracting the arrival times of inaudible sounds from loudspeakers  2  at microphones  4  or personal communications devices  70 . Optionally, auxiliary loudspeakers (not shown) mounted on control units  10  in place of auxiliary microphones  71  also radiate inaudible and distinctly encoded signals. Then the azimuths of and distances to microphones  4  and personal communications devices  70  are determined by measuring the arrival times of those sounds by those devices, in an analogous process to that described above. Continuing the analogy, the locations of loudspeakers  2  are then deduced by measuring arrival times of inaudible sounds received by microphones  4  and personal communications devices  70  from loudspeakers  2 . 
         [0050]    In the preferred embodiments disclosed above in connection with  FIGS. 2 and 4  and the alternative embodiment presented in  FIG. 7 , pairs of loudspeakers in units  2  are employed to synthesize virtual or phantom sound sources coincident with the locations of participants  8 . In both of these embodiments, parametric speakers that use ultrasonic carrier beams can replace the conventional loudspeakers in units  2 . Furthermore, a pair of parametric speakers can also produce simultaneously multiple virtual sources, and a multiplicity of parametric speakers can produce an even larger set of virtual sources. Parametric loudspeakers take advantage of inherent nonlinearities in sound propagation in air wherein powerful ultrasonic beams, typically in the 100-200 kHz frequency band, act as carriers of the desired audio-frequency sounds, which are modulated by means analogous to those employed by radio-wave transmission and broadcast, including amplitude (AM), single sideband (SSB), phase (PM), and frequency (FM) modulation means. Nonlinear phenomena associated with the response of air to pressure fluctuations act to self-demodulate the desired sounds. Ultrasonic beams can be narrowly focused and readily steered, and the demodulated audio also has those narrow beam features, allowing sounds to be beamed narrowly towards individual chosen participants  8 . This provides the advantage of reducing sound spillover and hence minimizing any increase in background sound level. 
         [0051]      FIG. 9  is a block diagram of an alternative system to that of  FIG. 2  that employs parametric loudspeakers. In this embodiment, video processor  16  also produces a multiplicity of beam steering signals  72  sent to parametric loudspeaker units  74 , which each produce one or more appropriately modulated beams directed towards participants  8 . Parametric loudspeakers typically employ a multiplicity of individual radiating elements (not shown) that can be phased in a manner or pattern analogous to that used for phased array radars to steer the beam. Furthermore, multiple beams may be produced simultaneously merely by superimposing more the one set of driving signals with different phasing patterns. An aspect and novel feature of our invention is the beaming of one or more beams from a parametric loudspeaker towards participants  8 . Each set of driving signals can further carry a distinct sound modulation, intended for specific participants  8 . 
         [0052]    Referring now to  FIG. 10 , in a further option parametric loudspeaker techniques are combined with the virtual or phantom source synthesis techniques described above in connection with  FIG. 3 . Ultrasonic beams  76  emanating from parametric loudspeaker units  74  are directed toward listener  34 , who would normally sense the demodulated sounds as coming from units  74 . By modifying and filtering the audio signals as described in connection with  FIG. 3  that are used to modulate beams  76 , the illusion is produced whereby the source of the demodulated sounds is coincident with talker  32 , which enhances intelligibility. 
         [0053]    Our invented participant acoustic enhancement system performs optimally when table  6  is not greatly elongated and has length-to-width ratios less than about 2.  FIG. 11  depicts a means for expanding the system to accommodate longer tables, by using one master control unit  78  and one or more slave units  80 . Notably, and as stated previously, the use of a table in this detailed description is meant to be illustrative, and not limiting. Units  78  and  80  communicate by wired or wireless means and can be any of the embodiments described above. Furthermore, the locations of slave units  80  may be registered also as described above by means of auxiliary microphones  71  or the analogous use of auxiliary loudspeakers. Because units  78  and  80  are in view of each other, optical registration means may be used as well. Such optical means include temporal, spatial, wavelength (color) encoding, or any combination of these. 
       CONCLUSIONS, RAMIFICATIONS, AND SCOPE 
       [0054]    Accordingly, it can be seen that we have provided a method and apparatus for enhancing the ability of participants in a social environment such as around as table to participate in conversations around and across the table in the presence of background noise that would render conversation beyond immediate neighbors difficult or impossible. 
         [0055]    Although the description above contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Various other embodiments and ramifications are possible within its scope. 
         [0056]    Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.