u . s . patent application ser . no . 13 / 888927 , incorporated here by reference , describes an audio system using near - field speakers located near the heads of the passengers , and a method of configuring that audio system to control the sound stage perceived by each passenger . conventional car audio systems are based around a set of four or more speakers , two on the instrument panel or in the front doors and two generally located on the rear package shelf , in sedans and coupes , or in the rear doors or walls in wagons and hatchbacks . in some cars , however , as shown in fig1 , speakers may be provided in the headrest or other close location rather than in the traditional locations behind the driver . this saves space in the rear of the car , and doesn &# 39 ; t waste energy providing sound to a back seat that , if even present , is unlikely to be used for passengers . the audio system 100 shown in fig1 includes a combined source / processing / amplifying unit 102 . in some examples , the different functions may be divided between multiple components . in particular , the source is often separated from the amplifier , and the processing provided by either the source or the amplifier , though the processing may also be provided by a separate component . the processing may also be provided by software loaded onto a general purpose computer providing functions of the source and / or the amplifier . we refer to signal processing and amplification provided by “ the system ” generally , without specifying any particular system architecture or technology . the audio system shown in fig1 has two sets of speakers 104 , 106 permanently attached to the vehicle structure . we refer to these as “ fixed ” speakers . in the example of fig1 , each set of fixed speakers includes two speaker elements , commonly a tweeter 108 , 110 , and a low - to - mid range speaker element 112 , 114 . in another common arrangement , the smaller speaker is a mid - to - high frequency speaker element and the larger speaker is a woofer , or low - frequency speaker element . the two or more elements may be combined into a single enclosure or may be installed separately . the speaker elements in each set may be driven by a single amplified signal from the amplifier , with a passive crossover network ( which may be embedded in one or both speakers ) distributing signals in different frequency ranges to the appropriate speaker elements . alternatively , the amplifier may provide a band - limited signal directly to each speaker element . in other examples , full range speakers are used , and in still other examples , more than two speakers are used per set . each individual speaker shown may also be implemented as an array of speakers , which may allow more sophisticated shaping of the sound , or simply a more economical use of space and materials to deliver a given sound pressure level . the driver &# 39 ; s headrest 120 in fig1 includes two speakers 122 , 124 , which again are shown abstractly and may in fact each be arrays of speaker elements . the two 122 , 124 speakers ( whether individual speakers or arrays ) may be operated cooperatively as an array themselves to control the distribution of sound to the listener &# 39 ; s ears . the speakers are located close to the listener &# 39 ; s ears , and are referred to as near - field speakers . in some examples , they are located physically inside the headrest . the two speakers may be located at either end of the headrest , roughly corresponding to the expected separation of the driver &# 39 ; s ears , leaving space in between for the cushion of the headrest , which is of course its primary function . in some examples , the speakers are located closer together at the rear of the headrest , with the sound delivered to the front of the headrest through an enclosure surrounding the cushion . the speakers may be oriented relative to each other and to the headrest components in a variety of ways , depending on the mechanical demands of the headrest and the acoustic goals of the system . co - pending application ser . no . 13 / 799 , 703 , incorporated here by reference , describes several designs for packaging the speakers in the headrest without compromising the safety features of the headrest . the near - field speakers are shown in fig1 as connected to the source 102 by cabling 130 going through the seat , though they may also communicate with the source 102 wirelessly , with the cabling providing only power . in another arrangement , a single pair of wires provides both digital data and power for an amplifier embedded in the seat or headrest . fig2 shows two listener &# 39 ; s heads as they are expected to be located relative to the speakers from fig1 . driver 202 has a left ear 204 and right ear 206 , and passenger 208 ′ s ears are labeled 210 and 212 . dashed arrows show various paths sound takes from the speakers to the listeners &# 39 ; ears as described below . we refer to these arrows as “ signals ” or “ paths ,” though in actual practice , we are not assuming that the speakers can control the direction of the sound they radiate , though that may be possible . multiple signals assigned to each speaker are superimposed to create the ultimate output signal , and some of the energy from each speaker may travel omnidirectionally , depending on frequency and the speaker &# 39 ; s acoustic design . the arrows merely show conceptually the different combinations of speaker and ear for easy reference . if arrays or other directional speaker technology is used , the signals may be provided to different combinations of speakers to provide some directional control . these arrays could be in the headrest as shown or in other locations relatively close to the listener including locations in front of the listener . the near - field speakers can be used , with appropriate signal processing , to expand the spaciousness of the sound perceived by the listener , and more precisely control the frontal sound stage . different effects may be desired for different components of the audio signals — center signals , for example , may be tightly focused , while surround signals may be intentionally diffuse . one way the spaciousness is controlled is by adjusting the signals sent to the near - field speakers to achieve a target binaural response at the listener &# 39 ; s ears . as shown in fig2 and more clearly in fig3 , each of the driver &# 39 ; s ears 204 , 206 hears sound generated by each local near - field speaker 122 and 124 . the passenger similarly hears the speakers near the passenger &# 39 ; s head . in addition to differences due to the distance between each speaker and each ear , what each ear hears from each speaker will vary due to the angle at which the signals arrive and the anatomy of the listener &# 39 ; s outer ear structures ( which may not be the same for their left and right ears ). human perception of the direction and distance of sound sources is based on a combination of arrival time differences between the ears , signal level differences between the ears , and the particular effect that the listener &# 39 ; s anatomy has on sound waves entering the ears from different directions , all of which is also frequency - dependent . we refer to the combination of these factors at both ears , for a source at a given location , as the binaural response for that location . binaural signal filters are used to shape sound that will be reproduced at a speaker at one location to sound like it originated at another location . although a system cannot be designed a priori to account for the unique anatomy of an unknown future user , other aspects of binaural response can be measured and manipulated . fig3 shows two “ virtual ” sound sources 222 and 226 corresponding to locations where surround speakers might ideally be located in a car that had them . in an actual car , however , such speakers would have to be located in the vehicle structure , which is unlikely to allow them to be in the location shown . given these virtual sources &# 39 ; locations , the arrows showing sound paths from those speakers arrive at the user &# 39 ; s ears at slightly different angles than the sound paths from the near - field speakers 122 and 124 . binaural signal filters modify the sound played back at the near - field speakers so that the listener perceives the filtered sound as if it is coming from the virtual sources , rather than from the actual near - field speakers . in some examples , it is desirable for the sound the driver perceives to seem as if it is coming from a diffuse region of space , rather than from a discrete virtual speaker location . appropriate modifications to the binaural filters can provide this effect , as discussed below . the signals intended to be localized from the virtual sources are modified to attain a close approximation to the target binaural response of the virtual source with the inclusion of the response from near - field speakers to ears . mathematically , we can call the frequency - domain binaural response to the virtual sources v ( s ), and the response from the real speakers , directly to the listener &# 39 ; s ears , r ( s ). if a sound s ( s ) were played at the location of the virtual sources , the user would hear s ( s )× v ( s ). for same sound played at the near - field speakers , without correction , the user will hear s ( s )× r ( s ). ideally , by first filtering the signals with a filter having a transfer function equivalent to v ( s )/ r ( s ), the sound s ( s )× v ( s )/ r ( s ) will be played back over the near - field speakers , and the user will hear s ( s )× v ( s )× r ( s )/ r ( s )= s ( s )× v ( s ). there are limits to how far this can be taken — if the virtual source locations are too far from the real near - field speaker locations , for example , it may be impossible to combine the responses in a way that produces a stable filter or it may be very susceptible to head movement . one limiting factor is the cross - talk cancellation filter , which prevents signals meant for one ear from reaching the other ear . one aspect of the audio experience that is controlled by the tuning of the car is the sound stage . “ sound stage ” refers to the listener &# 39 ; s perception of where the sound is coming from . in particular , it is generally desired that a sound stage be wide ( sound comes from both sides of the listener ), deep ( sound comes from both near and far ), and precise ( the listener can identify where a particular sound appears to be coming from ). in an ideal system , someone listening to recorded music can close their eyes , imagine that they are at a live performance , and point out where each musician is located . a related concept is “ envelopment ,” by which we refer to the perception that sound is coming from all directions , including from behind the listener , independently of whether the sound is precisely localizable . perception of sound stage and envelopment ( and sound location generally ) is based on level and arrival - time ( phase ) differences between sounds arriving at both of a listener &# 39 ; s ears , and sound stage can be controlled by manipulating the audio signals produced by the speakers to control these inter - aural level and time differences . as described in u . s . pat . no . 8 , 325 , 936 , incorporated here by reference , not only the near - field speakers but also the fixed speakers may be used cooperatively to control spatial perception . if a near - field speaker - based system is used alone , the sound will be perceived as coming from behind the listener , since that is indeed where the speakers are . binaural filtering can bring the sound somewhat forward , but it isn &# 39 ; t sufficient to reproduce the binaural response of a sound truly coming from in front of the listener . however , when properly combined with speakers in front of the driver , such as in the traditional fixed locations on the instrument panel or in the doors , the near - field speakers can be used to improve the staging of the sound coming from the front speakers . that is , in addition to replacing the rear - seat speakers to provide “ rear ” sound , the near - field speaker are used to focus and control the listener &# 39 ; s perception of the sound coming from the front of the car . this can provide a wider or deeper , and more controlled , sound stage than the front speakers alone could provide . the near - field speakers can also be used to provide different effects for different portions of the source audio . for example , the near - field speakers can be used to tighten the center image , providing a more precise center image than the fixed left and right speakers alone can provide , while at the same time providing more diffuse and enveloping surround signals than conventional rear speakers . in some examples , the audio source provides only two channels , i . e ., left and right stereo audio . two other common options are four channels , i . e ., left and right for both front and rear , and five channels for surround sound sources ( usually with a sixth “ point one ” channel for low - frequency effects ). four channels are normally found when a standard automotive head unit is used , in which case the two front and two rear channels will usually have the same content , but may be at different levels due to “ fader ” settings in the head unit . to properly mix sounds for a system as described herein , the two or more channels of input audio are up - mixed into an intermediate number of components corresponding to different directions from which the sound may appear to come , and then re - mixed into output channels meant for each specific speaker in the system , as described with reference to fig4 and 5 . one example of such up - mixing and re - mixing is described in u . s . pat . no . 7 , 630 , 500 , incorporated here by reference . an advantage of the present system is that the component signals up - mixed from the source material can each be distributed to different virtual speakers for rendering by the audio system . as explained with regard to fig3 , the near - field speakers can be used to make sound seem to be coming from virtual speakers at different locations . as shown in fig4 , an array of virtual speakers 224 i can be created surrounding the listener &# 39 ; s rear hemisphere . five speakers , 224 - 1 , 224 - d , 224 - m , 224 - n , and 224 - p are labeled for convenience only . the actual number of virtual speakers may depend on the processing power of the system used to generate them , or the acoustic needs of the system . although the virtual speakers are shown as a number of virtual speakers on the left ( e . g ., 224 - 1 and 224 - d ) and right ( e . g ., 224 - n and 224 - p ) and one in the center ( 224 - m ), there may also be multiple virtual center speakers , and the virtual speakers may be distributed in height as well as left , right , front , and back . a given up - mixed component signal may be distributed to any one or more of the virtual speakers , which not only allows repositioning of the component signal &# 39 ; s perceived location , but also provides the ability to render a given component as either a tightly focused sound , from one of the virtual speakers , or as a diffuse sound , coming from several of the virtual speakers simultaneously . to achieve these effects , a portion of each component is mixed into each output channel ( though that portion may be zero for some component - output channel combinations ). for example , the audio signal for a right component will be mostly distributed to the right fixed speaker fr 106 , but to position each virtual image 224 - i on the right side of the headrest , such as 224 - n and 224 - p , portions of the right component signal are also distributed to the right near - field speaker and left near - field speaker , due to both the target binaural response of the virtual image and for cross - talk cancellation . the audio signal for the center component will be distributed to the corresponding right and left fixed speakers 104 and 106 , with some portion also distributed to both the right and left near - field speakers 122 and 124 , controlling the location , e . g ., 224 - m , from which the listener perceives the virtual center component to originate . note that the listener won &# 39 ; t actually perceive the center component as coming from behind if the system is tuned properly — the center component content coming from the front fixed speakers will pull the perceived location forward , the virtual center simply helps to control how tight or diffuse , and how far forward , the center component image is perceived . the particular distribution of component content to the output channels will vary based on how many and which near - field speakers are installed . mixing the component signals for the near - field speakers includes altering the signals to account for the difference between the binaural response to the components , if they were coming from real speakers , and the binaural response of the near - field speakers , as described above with reference to fig3 . fig4 also shows the layout of the real speakers , from fig1 . the real speakers are labeled with notations for the signals they reproduce , i . e ., left front ( lf ), right front ( fr ), left driver headrest ( h 0 l ), and right driver headrest ( h 0 r ). while the output signals fl and fr will ultimately be balanced for both the driver and passenger seats , the near - field speakers allow the driver and passenger to perceive the left and right peripheral components and the center component closer to the ideal locations . if the near - field speakers cannot on their own generate a forward - staged component , they can be used in combination with the front fixed speakers to move the left and right components outboard and to control where the user perceives the center components . an additional array of speakers close to but forward of the listener &# 39 ; s head would allow the creation of a second hemisphere of virtual locations in front of the listener . we use “ component ” to refer to each of the intermediate directional assignments to which the original source material is up - mixed . as shown in fig5 , a stereo signal is up - mixed into an arbitrary number n of component signals . for one example , there may be a total of five : front and surround for each of left and right , plus a center component . in such an example , the main left and right components may be derived from signals which are found only in the corresponding original left or right stereo signals . the center components may be made up of signals that are correlated in both the left and right stereo signals , and in - phase with each other . the surround components may be correlated but out of phase between the left and right stereo signals . any number of up - mixed components may be possible , depending on the processing power used and the content of the source material . various algorithms can be used to up - mix two or more signals into any number of component signals . one example of such up - mixing is described in u . s . pat . no . 7 , 630 , 500 , incorporated here by reference . another example is the pro logic iiz algorithm , from dolby ®, which separates an input audio stream into as many as nine components , including height channels . in general , we treat components as being associated with left , right , or center . left components are preferably associated with the left side of the vehicle , but may be located front , back , high , or low . similarly right components are preferably associated with the right side of the vehicle , and may be located front , back , high , or low . center components are preferably associated with the centerline of the vehicle , but may also be located front , back , high , or low . fig5 shows an arbitrary number n of up - mixed components . the relationship between component signals , generally c 1 through cn , virtual image signals , v 1 through vp , and output signals fl , fr , h 0 l , and h 0 r is shown in fig5 . a source 402 provides two or more original channels , shown as l and r . an up - mixing module 404 converts the input signals l and r into a number , n , of component signals c 1 through cn . there may not be a discrete center component , but center may be provided a combination of one or more left and right components . binaural filters 406 - 1 through 406 - p then convert weighted sums of the up - mixed component signals into a binaural signal corresponding to sound coming from the virtual image locations v 1 through vp , corresponding to the virtual speakers 224 - i shown in fig4 . while fig5 shows each of the binaural filters receiving all of the component signals , in practice , each virtual speaker location will likely reproduce sounds from only a subset of the component signals , such as those signals associated with the corresponding side of the vehicle . as with the component signals , a virtual center signal may actually be a combination of left and right virtual images . re - mixing stages 418 ( only one shown ) recombine the up - mixed component signals to generate the fl and fr output signals for delivery to the front fixed speakers , and a binaural mixing stage 420 combines the binaural virtual image signals to generate the two headrest output channels h 0 l and h 0 r . the same process is used to generate output signals for the passenger headrest and any additional headrest or other near - field binaural speaker arrays , and additional re - mixing stages are used to generate output signals for any additional fixed speakers . various topologies of when component signals are combined and when they are converted into binaural signals are possible , and may be selected based on the processing capabilities of the system used to implement the filters , or on the processes used to define the tuning of the vehicle , for example . the u . s . patent application ser . no . 13 / 888 , 927 mentioned above describes the signal flows within the near - field mixing stage 420 and peripheral speaker re - mixing stage 418 . another particular feature that can be provided with the system described above is a replacement for the traditional “ fader ” control . in typical car audio systems , with a set of stereo speakers in the front and another set of stereo speakers in the rear playing a scaled version of the same signal , a fader control adjusts the balance of sound energy between the front and rear speakers . for a full front setting , only the front speakers receive signal , and for a full rear setting , only the rear signals receive a signal . in the system described above , this would not be desirable , assuming the headrest speakers would be substituted for the rear speakers , as the signals going to the front and to the headrest speakers do not contain the same content , and don &# 39 ; t play sound in the same bandwidths . instead , a new interpretation of the fader is provided , which manipulates the mixing of component content into virtual image locations and fixed speaker signals . as discussed above , a binaural filter is designed that adjusts each virtual signal to account for the difference in binaural perception between signals coming from the virtual locations and the real speaker locations . each virtual signal receives a mix of weighted component signals , which determines the location from which the listener perceives each component signal to originate . rather than simply shifting sound energy between front and rear , this mixing can be varied for each virtual image location to change the precision and location of each component and the amount of envelopment provided by the virtual images . to provide a sound stage control instead of a traditional fader function , two different sets of component mixing weights are designed , based on two different sound stage presentations . in some examples , as shown in fig6 a and 6b , different types of changes are made to different components . for the first set of mixing weights , associated with the sound stage control being at a first limit of its range and illustrated in fig6 a , the virtual center image is tightly focused at a point 502 in front of the driver , while virtual surround images 504 - 1 through 504 - n are also tightly focused but are close to the driver , and left and right images 506 and 508 are close to the center , so the sound stage is narrow . appropriate mixing weights are created for each set of virtual images . for the second set of mixing weights , associated with the sound stage control being at the other limit of its range , a center image 522 that is still centered , but is larger in width and possibly height or depth is combined with surround images 524 - 1 through 524 - n that are more enveloping and farther away from the driver . the left and right images 526 and 528 are moved farther from center , and also rearward , due to the lack of actual width available in the car , to provide a wider sound stage . other choices in mapping sound stage to control position are possible , depending on the desires of the system designer and the actual number of speakers used . in addition to the components input to the binaural filters that create the binaural virtual image signals , the weights of the components in the re - mixing stages 418 for the front fixed speakers are also modified , changing the mix of components into the front speakers . to effect a transition between the two sound stage configurations as the user adjusts the control , both sets of weights are applied simultaneously , with the relative contribution of each set of weights set based on the position of the sound stage control , as shown in fig7 . fig7 shows two curves 602 and 604 representing the contribution of the two sets of weights as functions of the sound stage control position . the horizontal axis 606 is the control position , ranging a start position 608 to an end position 610 . the start and end positions of the control may be labeled various things in a given application , such as narrow to wide , front to rear ( e . g ., if a traditional “ fader ” control is repurposed ), or solo to orchestra , to name a few examples . the vertical axis 612 is the contribution of each set of weights , ranging from zero to one . note that this graph is entirely abstract — the actual values may be other than zero and one , depending , for example , on the types of filters used to actually implement this control scheme . if the sound stage control is all the way at the start position 608 , the contribution of the first set of weights ( curve 602 ) is set to one and the contribution of the second set of weights ( curve 604 ) is zero . as the fader is moved to the middle and then all the way to the ending position 610 , the contribution of the first set is decreased and the contribution of the second set is increased until , at the full end position , the first set has a contribution of zero and the second set has a contribution of one . the curves are labeled as “ narrow ” and “ wide ”, but this is just a notation for convenience , as the actual description of the effect of the weights will vary in a given application , much like the control position labels mentioned above . thus , the user can adjust the size of the sound stage from narrow and forward to wide and enveloping , or between whatever alternative a given system offers . these settings may also be applied automatically based on the content of the source audio signal , for example , talk radio may be played using the first set of weights with a narrow , forward sound stage , while music may be played using the second set of weights with a wider , more enveloping overall sound stage . the shape of the curves shown is merely for illustration purposes — other curves , including straight lines , could be used , depending on the desires of the system designer and the capabilities of the audio system . in another embodiment , rather than or in addition to changing the mixing weights of the component signals , the binaural filters can be changed to move the virtual image locations . two sets of binaural filters can be combined , based on a weight derived from the fader input control , such that the fader control determines which binaural filters are dominant and therefore where the virtual images are positioned . the fixed speakers may still be varied by changing the weights of the component signals mixed to form the output signals . embodiments of the systems and methods described above may comprise computer components and computer - implemented steps that will be apparent to those skilled in the art . for example , it should be understood by one of skill in the art that the computer - implemented steps may be stored as computer - executable instructions on a computer - readable medium such as , for example , floppy disks , hard disks , optical disks , flash roms , nonvolatile rom , and ram . furthermore , it should be understood by one of skill in the art that the computer - executable instructions may be executed on a variety of processors such as , for example , microprocessors , digital signal processors , gate arrays , etc . for ease of exposition , not every step or element of the systems and methods described above is described herein as part of a computer system , but those skilled in the art will recognize that each step or element may have a corresponding computer system or software component . such computer system and / or software components are therefore enabled by describing their corresponding steps or elements ( that is , their functionality ), and are within the scope of the disclosure . a number of implementations have been described . nevertheless , it will be understood that additional modifications may be made without departing from the scope of the inventive concepts described herein , and , accordingly , other embodiments are within the scope of the following claims .