Patent Publication Number: US-10777184-B2

Title: Correction of a control signal in an active noise control headrest

Description:
TECHNOLOGICAL FIELD 
     The present disclosure relates generally to the field of active noise control (ANC). More specifically the present disclosure relates to the field of correcting signaling used in electronics integrated in an ANC headrest. 
     BACKGROUND 
     Many environments are inherently noisy. Examples of such environments include roadways, vehicle interiors, manufacturing plants, construction sites, and many other environments that include vehicles and/or heavy machinery. To increase personal comfort in such environments, engineers generally incorporate sound suppressing techniques into their designs. Vehicle interiors, in particular, often include noise suppressing design features which give passengers an increased feeling of luxury and comfort. Accordingly, solutions that are designed to suppress noise are often highly-desired. 
     SUMMARY 
     Aspects of the present disclosure are generally directed to active noise control (ANC). Particular aspects are directed to an ANC headrest comprising a center section comprising a longitudinal axis. The headrest further comprises a flange extending away from the center section. The flange is moveable relative to the longitudinal axis of the center section. The headrest further comprises a position sensor configured to detect a position of the flange relative to the center section. The headrest further comprises a speaker mounted to the flange. The speaker is configured to produce antinoise that destructively interferes with frequencies of ambient sound. The headrest further comprises a microphone configured to receive feedback comprising a combination of the antinoise and the ambient sound. The headrest further comprises processing circuitry communicatively coupled to the speaker, the microphone, and the position sensor. The processing circuitry is configured to control the speaker to produce the antinoise based on the feedback and the position detected by the position sensor. 
     In some aspects, the processing circuitry comprises a servo controller communicatively coupled to the microphone. The servo controller is configured to produce a control signal based on the feedback. In such aspects the processing circuitry further comprises filtering circuitry communicatively coupled to the servo controller, the position sensor, and the speaker. The filtering circuitry is configured to generate a corrected control signal based on the control signal from the servo controller and the position detected by the position sensor. To control the speaker to produce the antinoise, the processing circuitry is configured to use the corrected control signal to control the speaker. 
     In some such aspects, to generate the corrected control signal based on the control signal from the servo controller and the position detected by the position sensor, the filtering circuitry is configured to set an attenuation level of the antinoise based on the position detected by the position sensor. In some such aspects, to set the attenuation level of the antinoise based on the position detected by the position sensor, the filtering circuitry is configured to set the attenuation level of the antinoise to one of a plurality of predefined attenuation levels selected based on which of a plurality of predefined position ranges comprises the position detected by the position sensor. Additionally or alternatively, in some aspects, to set the attenuation level of the antinoise based on the position detected by the position sensor, the filtering circuitry is configured to decrease or increase the attenuation level of the antinoise responsive to the flange being moved towards or away from the longitudinal axis, respectively. 
     In some aspects, the headrest further comprises a tuning microphone spaced apart from the microphone. The tuning microphone is configured to receive further feedback comprising a different combination of the ambient sound and the antinoise. In such aspects, the headrest further comprises tuning circuitry communicatively coupled to the tuning microphone and the filtering circuitry. The tuning circuitry is configured to store different values of a configurable filtering parameter in the filtering circuitry over time based on the further feedback from the tuning microphone. To generate the corrected control signal based on the control signal from the servo controller and the position detected by the position sensor, the filtering circuitry is configured to generate the corrected control signal further based on the configurable filtering parameter. In some such aspects, the tuning circuitry is further configured to monitor noise control performance over time based on the further feedback to determine which of the different values of the configurable filtering parameter most reduces a-weighted Root Mean Square (RMS) sound pressure. 
     In some aspects, relative to the antinoise produced by the corrected control signal, the control signal is configured to produce different antinoise having a greater overall a-weighted RMS sound pressure reduction and a peak amplitude at a higher frequency. 
     In some aspects, the headrest further comprises a feedforward microphone configured to provide feedforward input to the processing circuitry, wherein the processing circuitry is further configured to enable or disable feedforward control using the feedforward input based respectively on whether the position of the flange detected by the position sensor is away from the longitudinal axis of the center section by more or less than a threshold amount. 
     Other aspects of the present disclosure are directed to an aircraft. The aircraft comprises a passenger cabin, and a seat disposed within the passenger cabin. The aircraft further comprises a headrest mounted to the seat. The headrest comprises a center section comprising a longitudinal axis. The headrest further comprises a flange extending away from the center section. The flange is moveable relative to the longitudinal axis of the center section. The headrest further comprises a position sensor configured to detect a position of the flange relative to the center section. The headrest further comprises a speaker mounted to the flange. The speaker is configured to produce antinoise that destructively interferes with frequencies of ambient sound. The headrest further comprises a microphone configured to receive feedback comprising a combination of the antinoise and the ambient sound. The headrest further comprises processing circuitry communicatively coupled to the speaker, the microphone, and the position sensor. The processing circuitry is configured to control the speaker to produce the antinoise based on the feedback and the position detected by the position sensor. 
     In some aspects, the processing circuitry comprises a servo controller communicatively coupled to the microphone. The servo controller is configured to produce a control signal based on the feedback. In such aspects, the processing circuitry further comprises filtering circuitry communicatively coupled to the servo controller, the position sensor, and the speaker. The filtering circuitry is configured to generate a corrected control signal based on the control signal from the servo controller and the position detected by the position sensor. To control the speaker to produce the antinoise, the processing circuitry is configured to use the corrected control signal to control the speaker. 
     In some such aspects, to generate the corrected control signal based on the control signal from the servo controller and the position detected by the position sensor, the filtering circuitry is configured to set an attenuation level of the antinoise based on the position detected by the position sensor. In some such aspects, to set the attenuation level of the antinoise based on the position detected by the position sensor, the filtering circuitry is configured to set the attenuation level of the antinoise to one of a plurality of predefined attenuation levels selected based on which of a plurality of predefined position ranges comprises the position detected by the position sensor. In some such additional or alternative aspects, to set the attenuation level of the antinoise based on the position detected by the position sensor, the filtering circuitry is configured to decrease or increase the attenuation level of the antinoise responsive to the flange being moved towards or away from the longitudinal axis, respectively. 
     In some aspects, the aircraft further comprises a tuning microphone spaced apart from the microphone. The tuning microphone is configured to receive further feedback comprising a different combination of the ambient sound and the antinoise. In such aspects, the aircraft further comprises tuning circuitry communicatively coupled to the tuning microphone and the filtering circuitry. The tuning circuitry is configured to store different values of a configurable filtering parameter in the filtering circuitry over time based on the further feedback from the tuning microphone. To generate the corrected control signal based on the control signal from the servo controller and the position detected by the position sensor, the filtering circuitry is configured to generate the corrected control signal further based on the configurable filtering parameter. In some such aspects, the tuning circuitry is further configured to monitor noise control performance over time based on the further feedback to determine which of the different values of the configurable filtering parameter most reduces a-weighted Root Mean Square (RMS) sound pressure. 
     In some aspects, relative to the antinoise produced by the corrected control signal, the control signal is configured to produce different antinoise having a greater overall a-weighted RMS sound pressure reduction and a peak amplitude at a higher frequency. 
     In some aspects, the aircraft further comprises a feedforward microphone configured to provide feedforward input to the processing circuitry, wherein the processing circuitry is further configured to enable or disable feedforward control using the feedforward input based respectively on whether the position of the flange detected by the position sensor is away from the longitudinal axis of the center section by more or less than a threshold amount. 
     Other aspects are directed to a method implemented by an ANC headrest. The method comprises producing antinoise from a speaker of the headrest. The antinoise destructively interferes with frequencies of ambient sound and the speaker is mounted to a flange of the headrest that extends away from a center section of the headrest and is movable relative to a longitudinal axis of the center section. The method further comprises receiving feedback comprising a combination of the antinoise and the ambient sound, and detecting a position of the flange relative to the center section. The method further comprises controlling the speaker to produce the antinoise based on the feedback and the detected position of the flange relative to the center section. 
     In some aspects, the method further comprises using a servo controller to produce a control signal based on the feedback, and generating a corrected control signal based on the control signal from the servo controller and the detected position of the flange relative to the center section. Controlling the speaker to produce the antinoise comprises using the corrected control signal to control the speaker. In some such aspects, generating the corrected control signal based on the control signal from the servo controller and the detected position of the flange relative to the center section comprises setting an attenuation level of the antinoise to one of a plurality of predefined attenuation levels selected based on which of a plurality of predefined position ranges comprises the detected position. In some additional or alternative aspects, generating the corrected control signal based on the control signal from the servo controller and the detected position of the flange relative to the center section comprises decreasing or increasing an attenuation level of the antinoise responsive to the flange being moved towards or away from the longitudinal axis, respectively. 
     In some additional or alternative aspects, the method further comprises using a tuning microphone spaced apart from the microphone to receive further feedback comprising a different combination of the ambient sound and the antinoise, and using different values of a configurable filtering parameter to modify the control signal differently over time based on the further feedback from the tuning microphone. Generating the corrected control signal based on the control signal from the servo controller and the detected position of the flange relative to the center section comprises generating the corrected control signal further based on the configurable filtering parameter. In some such aspects, the method further comprises monitoring noise control performance over time based on the further feedback to determine which of the different values of the configurable filtering parameter most reduces a-weighted Root Mean Square (RMS) sound pressure. 
     In some aspects, the method further comprises enabling or disabling feedforward control to produce the antinoise based respectively on whether the detected position of the flange is away from the longitudinal axis of the center section by more or less than a threshold amount. 
     The features, functions and advantages that have been discussed can be achieved independently in various aspects or may be combined in yet other aspects, further details of which can be seen with reference to the following description and the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Having thus described variations of the disclosure in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale. Indeed, aspects of the present disclosure are illustrated by way of example and are not limited by the accompanying figures with like references indicating like elements. In general, the use of a reference numeral should be regarded as referring to the depicted subject matter according to one or more aspects, whereas discussion of a specific instance of an illustrated element will append a letter designation thereto (e.g., discussion of a speaker  210 , generally, as opposed to discussion of particular instances of speakers  210   a ,  210   b ). 
         FIG. 1  is a side-view schematic illustrating a portion of an example vehicle interior, according to aspects of the present disclosure. 
         FIG. 2  is a front-view schematic illustrating an example seat assembly, according to aspects of the present disclosure. 
         FIG. 3  is a side-view schematic illustrating an example headrest, according to aspects of the present disclosure. 
         FIG. 4A  is a top-view schematic illustrating an example headrest, according to aspects of the present disclosure. 
         FIG. 4B  is a top-view schematic illustrating an example headrest, according to aspects of the present disclosure. 
         FIG. 4C  is a top-view schematic illustrating an example headrest, according to aspects of the present disclosure. 
         FIG. 4D  is a top-view schematic illustrating an example headrest to which a tuning microphone is mounted via a flexible boom, according to aspects of the present disclosure. 
         FIG. 4E  is a top-view schematic illustrating an example headrest in an over-the-ear arrangement, according to aspects of the present disclosure. 
         FIG. 5  is a top-view schematic illustrating an example headrest comprising a hinge, according to aspects of the present disclosure. 
         FIG. 6  is a block diagram illustrating an example ANC system, according to aspects of the present disclosure. 
         FIG. 7  is a block diagram illustrating an example servo controller, according to aspects of the present disclosure. 
         FIGS. 8-11  are flow diagrams illustrating an example methods, according to aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Aspects of the present disclosure are generally directed to active noise control (ANC). Particular aspects are suitable for use in vehicles, such as aircraft, spacecraft, rotorcraft, satellites, rockets, terrestrial vehicles, water-borne surface vehicles, water-borne sub-surface vehicles, subterranean vehicles, or any combination thereof. Particular aspects are suitable for commercial, transport, and/or industrial purposes. Different vehicles often present different noise control challenges. 
     Indeed, techniques that may be effective for noise control in one type of vehicle may be unsuitable for noise control in another type of vehicle. Consider, for example, noise control in a turboprop aircraft as compared to a jet aircraft. In a turboprop aircraft, the majority of the interior sound field is typically related to the propellers, such that noise at one location in the cabin has a coherent relationship to the noise at other locations in the cabin, even at relatively large distances. In such a vehicle, a cancelling field can be effectively produced at one location based on sound input received at a relatively distant location. As long as the complexity of the sound field can be reproduced (which increases with increasing frequency), good noise cancellation can be achieved. Also, since the noise is generally periodic and changes over a relatively slow time scale, adaptation of the control law to cancel the sound is generally not computationally intensive. 
     In contrast, on a jet aircraft, a significant (if not a majority) of the noise is caused by turbulent flow of air over aircraft surfaces. The typical resulting sound field does not display good coherence (even over small distances) and also changes rapidly over time. Thus, noise sampled from a relatively distant location is often inadequate for producing an effective noise cancelling field elsewhere. This is just one example in which the same approach that works on one vehicle may not be as effective (or may be ineffective) in another vehicle. 
     There are numerous similar challenges and difficulties in implementing effective noise control solutions in different environments. Various aspects of the present disclosure are suitable for a variety of such environments. At least some of the aspects discussed herein are particularly useful for noise control in vehicles of various types, though other aspects may be useful in other environments in which noise control may be desired.  FIG. 1  illustrates an example of an environment in which aspects of the present disclosure may be advantageous.  FIG. 1  is a schematic side-view of a portion of an aircraft  100  with a cut-away revealing the interior of a passenger cabin  140 . Positioned within the passenger cabin  140  is a seat assembly  110 . The seat assembly  110  comprises a seat  130 , a headrest  200 , and a sound-suppressing enclosure  120 . 
     The sound-suppressing enclosure  120  is disposed within, and spaced from, the interior walls of the aircraft  100 . As shown in more detail in the schematic of  FIG. 2 , the sound-suppressing enclosure  120  has an interior cavity  250  and (as will be explained further below) is configured to produce suppressed sound by suppressing frequencies of ambient sound that enter the interior cavity  250 . In some aspects, the sound-suppressing enclosure  120  has a geometry and/or comprises materials such that the suppressed frequencies are above a threshold frequency. The headrest  200  is disposed within the interior cavity  250  of the sound-suppressing enclosure  120 , and is mounted to the seat  130 . 
     The headrest  200  comprises a center section  230 , which may (in some aspects) be padded and/or molded to comfortably accommodate the head of a passenger (not shown in  FIG. 2 ). One or more speakers  210  are mounted to the headrest  200 . In the particular example of  FIG. 2 , the headrest  200  comprises flanges  220   a ,  220   b  extending away from the center section  230  on opposing lateral sides of the center section  230 , and a speaker  210   a ,  210   b  is mounted to each of the flanges  220   a ,  220   b , respectively. The speakers  210   a ,  210   b  are configured to produce antinoise that destructively interferes with frequencies of the suppressed sound. In some aspects, the speakers  210   a ,  210   b  are configured to produce the antinoise such that the frequencies that are destructively interfered with are below the aforementioned threshold frequency. 
     In some aspects, the sound-suppressing enclosure  120  and the antinoise output from the speakers  210   a ,  210   b  in the headrest  200  work jointly to actively control noise across a broad band of frequencies. For example, in some aspects, the sound-suppressing enclosure  120  is configured to suppress frequencies of ambient sound above the threshold frequency, but as a practical consequence of its design, may (in some aspects) amplify sound frequencies below the threshold frequency. This amplification induced by the sound-suppressing enclosure may, for example, be due to resonance within the interior cavity  250 . In some such aspects, the antinoise output from the speakers  210   a ,  210   b  in the headrest  200  is configured to counteract the amplification caused by the sound-suppressing enclosure  120  by destructively interfering with frequencies of the suppressed sound below the threshold frequency. In particular, to destructively interfere with the frequencies below the threshold frequency, the antinoise may be configured to, at a given listening position (e.g., the ear of a listener), destructively interfere by amounts respectively greater than any respective amplification of the frequencies below the threshold frequency induced by the sound-suppressing enclosure  120 . 
     Additionally or alternatively, in some aspects, the antinoise output from the speakers  210   a ,  210   b  is configured to destructively interfere with frequencies below the threshold frequency, but as a practical consequence of its design, may (in some aspects) amplify sound frequencies above the threshold frequency. This amplification induced by the antinoise may, for example, be due to dynamic ambient sound conditions that cause the antinoise to misalign such that some constructive interference occurs. In some such aspects, the sound-suppressing enclosure  120  is configured to counteract the amplification caused by the antinoise output from the speakers  210   a ,  210   b . In particular, to suppress the frequencies above the threshold frequency, the sound-suppressing enclosure  120  may be configured to, at a given listening position (e.g., the ear of a listener) suppress the frequencies above the threshold frequency by amounts respectively greater than any respective constructive interference of the frequencies above the threshold frequency induced by the antinoise. 
     Thus, in view of the above, the antinoise and/or sound-suppressing enclosure  120  may jointly contribute to the efficacy of the overall ANC system, e.g., in a complimentary fashion. In some particular aspects, the suppressing (provided by the sound-suppressing enclosure  120 ) and the destructive interference (provided by the antinoise) jointly provide a peak power reduction of sound energy at a frequency below 200 Hz. 
     In particular aspects, practical considerations may limit the magnitude on overall sound pressure provided by the sound-suppressing enclosure  120  on a jet aircraft. For example, it may be impractical to seal the sound-suppressing enclosure  120  or otherwise limit a passenger of the aircraft  100  from freely getting in and out of their seat  130 . Notwithstanding, the sound-suppressing enclosure  120  may, in some aspects, alter the power spectrum of the ambient noise such that the predominant sound frequency (i.e., the frequency having the most sound energy) is lowered. This may be accomplished with a sound-suppressing enclosure  120  as illustrated schematically in  FIG. 2 , for example, while still allowing easy ingress and egress (e.g., by having a partially- or fully-open side to the sound-suppressing enclosure  120 ). 
     A shift of peak amplitude in the sound power spectrum from high frequencies to low frequencies caused by the sound-suppressing enclosure  120  may provide significant benefit to the overall reduction in sound power, even in aspects in which the overall sound pressure is the same with and without the sound-suppressing enclosure  120 . For example, the sound-suppressing enclosure may synergize with the noise controlling effect of antinoise that is more effective at reducing sound at low frequencies, and less effective at high frequencies. 
     One or more microphones  340  are also disposed within the interior cavity  250  of the sound-suppressing enclosure  120 . In the example of  FIG. 2 , microphones  340   a ,  340   b  are mounted to the front grills of the speakers  210   a ,  210   b , respectively. The microphones  340   a ,  340   b  are configured to receive feedback comprising a combination of the suppressed sound produced by the sound-suppressing enclosure  120  and the antinoise produced by the speakers  210   a ,  210   b . Each microphone  340   a ,  340   b  is connected via a respective input line  350   a ,  350   b  to processing circuitry  330 , as shown in  FIG. 3 . 
       FIG. 3  is schematic of the headrest  200  as viewed from the side, cutaway to reveal example details of the interior of the headrest  200 . In this particular example, processing circuitry  330  is disposed within the headrest  200 , and is communicatively coupled to the speaker  210  via an output line  360 . The processing circuitry  330  is also communicatively coupled to the microphone  340  via an input line  350   a . The processing circuitry  330  is also connected to a power source (not shown), such as a battery or electrical outlet via power line  390 . The processing circuitry  330  is configured to control the speaker  210  to produce the antinoise based on the feedback received by the microphone  340 . 
     The speaker  210 , which is mounted to the headrest  200 , comprises (among other things) a front grill  320 , a mounting bracket  380 , a housing  370 , and a diaphragm  310 . The front grill  320  is disposed over the diaphragm  310  and is mounted to the mounting bracket  380  which mates with the headrest  200  (e.g., using retention clips or screws, not shown). The diaphragm  310  in this example is substantially flat and disposed within the housing  370 . The housing  370  is connected to (and retained within the headrest  200  by) the mounting bracket  380 . 
     Although the diaphragm  310  in this example is substantially flat, other aspects of the present disclosure include a diaphragm  310  having any suitable geometry to produce the antinoise (e.g., cone-shaped). In some aspects, a substantially flat diaphragm  310  advantageously provides a smaller distance between the diaphragm  310  and the microphone  340  mounted to the front grill  320  as compared to geometries that use a diaphragm  310  that is concave within the housing  370 . In some such aspects, this relatively smaller distance reduces the delay in the transfer function between the speaker  210  and the microphone  340 , which results in a higher bandwidth error rejection and increased performance. Indeed, aspects that include small distances between the diaphragm  310 , the microphone  340 , and the ear of a listener may keep differences in sound energy at those respective locations small so that benefits in error rejection are similar. 
     In some aspects, the headrest  200  may include one or more feedforward microphones  355 . For example, as shown in  FIG. 3 , the headrest  200  may comprise a feedforward microphone  355  that is communicatively connected to the processing circuitry  330  via an input line  350   b . In this example, the feedforward microphone  355  is mounted to the headrest  200  at a location opposing the front grill  380 . In other aspects, the feedforward microphone  355  may be positioned anywhere else on the headrest  200 , e.g., perpendicular to a longitudinal axis of the headrest  200  (shown in  FIGS. 4A-E  and discussed below). In some aspects, the processing circuitry  330  uses microphone  340  for feedback control and feedforward microphone  355  for feedforward control. In some such aspects, the processing circuitry  330  may be configured to switch between feedback and feedforward modes by respectively switching between using microphone  340  and feedforward microphone  355  to produce a control signal used as a basis for controlling the speaker  210 . In other such aspects, the processing circuitry  330  may use microphone  340   a  and microphone  340   c  to perform both feedback and feedforward control. 
     A speaker  210  that acts as a uniform source is generally preferable over a speaker that produces significant diffraction, or in which diffraction occurs at frequencies in which noise control is less effective. In some aspects, the speaker  210  is of a relatively small diameter (e.g., 2.5 inches), which may serve to reduce diffraction that undermines the efficacy of the emitted antinoise. Although a single, larger speaker (e.g., 8 inches in diameter) mounted to the center section  230  may, in some aspects, serve a similar purpose in reducing diffraction (as compared to smaller speakers  210   a ,  210   b  mounted to the flanges  220   a ,  220   b , respectively), the diffraction caused by a relatively larger speaker  210  may occur at a lower frequency where noise control is generally less effective. If diffraction occurs at a given frequency, variation of phase and/or amplitude in the sound field may spatially decrease the desirable effects of ANC. 
       FIGS. 4A, 4B, 4C, 4D, and 4E  are top-down schematic views of the headrest  200  according to various aspects. In  FIG. 4A , the flanges  220   a ,  220   b  are canted inward (e.g., towards the head  400  of a listener, if present), such that projection axes  420   a ,  420   b  extending in the direction in which the antinoise is projected from the center of each of the speakers  210   a ,  210   b , respectively, intersect at an angle θ. In this example, the angle θ of intersection between the projection axes  420   a ,  420   b  is 50 degrees, as each flange  220   a ,  220   b  is canted at an angle α of 25 degrees relative to a longitudinal axis  430  of the center section  230 . In this particular example, the proportions of the headrest  200 , mounting positions of the speakers  210   a ,  210   b , and angle α of the flanges  220   a ,  220   b  relative to the longitudinal axis  430  of the center section  230  are such that the projection axes  420   a ,  420   b  advantageously pass through the ears  410   a ,  410   b  of the listener. 
     In some aspects, placement of speakers  210   a ,  210   b  in the headrest  200  at angle α toward the ears  410   a ,  410   b  of the listener as shown in  FIG. 4A  reduces the latency between the speakers  210   a ,  210   b  and the listener as compared to the headrest  200  illustrated in  FIG. 4B , while also reducing the passive amplification impact of the speakers  210   a ,  210   b , as compared to placement at an angle of 90 degrees as shown in  FIG. 4C . Indeed, in some aspects, the perpendicular orientation of the speakers  210   a ,  210   b  relative to the center section  230  may cause a local resonant amplification of sound frequencies in the range from 500 to 1000 Hz. Since this is a range where feedback control of sound may be less effective in some aspects, passive amplification of this kind has the potential to negatively impact overall closed-loop performance. Thus, although aspects of the present disclosure may include an arrangement as shown in  FIG. 4C , particular aspects which use the smaller angle α depicted in  FIG. 4A , which may result in relatively little passive amplification of the sound field (or indeed, none whatsoever, in some aspects). 
     Other aspects of the present disclosure include a headrest  200  in which the flanges  220   a ,  220   b  are not angled inward, as shown in  FIG. 4B , such that the projection axes  420   a ,  420   b  do not intersect. While this configuration avoids some or all of the passive resonant amplification of the speakers  210   a ,  210   b  discussed above with respect to the arrangement illustrated in  FIG. 4C , the speakers  210   a ,  210   b  are placed at positions further away from the ears  410   a ,  410   b  of the listener, which may introduce more error between the antinoise generated by the ANC system and the sound energy at the listener&#39;s ears  410   a ,  410   b  relative to the arrangement illustrated in, e.g.,  FIG. 4A . 
     Of course, an additional design concern for the headrest  200  is the comfort of the person whose head  400  rests in it, which is often a matter of personal taste. For example, a person may find the headrest  200  arrangement illustrated in  FIG. 4C  preferable to those in  FIGS. 4A and 4B  when trying to sleep because it may prevent the head  400  from jostling around during turbulent flight conditions. As another example, a person may find the headrest  200  arrangement illustrated in  FIG. 4A or 4B  preferable to that illustrated in  FIG. 4C  while eating due to the increased freedom of head  400  movement available. 
     In view of the above, the headrest  200  may, in some aspects, be flexible and/or jointed such that the headrest  200  is able to be selectively positioned in accordance with  FIGS. 4A, 4B , and/or  4 C. For example, as shown in the example schematic of  FIG. 5 , the headrest  200  may comprise one or more hinges  500  between the center section  230  and any or all of the flanges  220  to permit the flange(s)  220  to be positioned to any angle α as may be desired. Although in some aspects of the present disclosure, the speakers  230   a ,  230   b  mounted to the flanges  220   a ,  220   b  are configured to project the antinoise at respective projection axes  420   a ,  420   b  that intersect at an optimum angle that minimizes latency and avoids passive amplification at a given listening position, in some aspects, a user may be able to move the flanges  220   a ,  220   b  such that the headrest  200  is arranged in accordance with any of  FIG. 4A, 4B , or  4 C, as desired. This may, in some aspects, allow a user to balance physical comfort concerns with noise control efficacy according to their own preferences, for example. 
     In particular, the headrest  200  may be arranged as depicted in the example schematic shown in  FIG. 4E . The example headrest  200  illustrated in  FIG. 4E  comprises cushions  440   a ,  440   b  attached to respective flanges  220   a ,  220   b . The cushions  440   a ,  440   b  are respectively configured to enclose and/or mate with ears  420   a ,  420   b  of a listener when respective flanges  220   a ,  220   b , are positioned away from the longitudinal axis  430  (e.g., sufficiently away from the longitudinal axis  430  depending on the size of the listener&#39;s head  400 ). Such an example may allow the listener to use the headrest  200  as over-the-ear or on-ear headphones when the flanges  220   a ,  220   b  are positioned as illustrated in  FIG. 4E , and as stereo speakers when the flanges  220   a ,  220   b  are positioned as illustrated in  FIG. 4B , for example. 
     In some particular aspects, the headrest  200  further comprises position sensors configured to detect the positions the flanges  220   a ,  220   b  relative to the center section  230 . For example, a position sensor may be configured to detect the angle α at which flange  220   a  is positioned away from the longitudinal axis  430  of the center section  230 , and another position sensor may be configured to detect the angle α at which flange  220   b  is positioned away from the longitudinal axis  430  relative to the center section  230 . As will be discussed in greater detail below, the position sensors are communicatively coupled to the processing circuitry  330 , and the processing circuitry  330  may control the speakers  220   a ,  220   b  to produce the antinoise based, in whole or in part, on the positions detected by the position sensors. In some aspects, the processing circuitry  330  may control the speakers  220   a ,  220   b  based on input from the position sensor(s) in addition to the above-discussed feedback received by the microphone  340 . 
     According to one such example, the processing circuitry  330  is configured to operate according to different control configurations based on which of a plurality of positions is detected by a position sensor. For example, the processing circuitry  330  may be configured to set an attenuation level of the antinoise based on the position detected by the position sensor. In some such aspects, the processing circuitry  330  may (for example) provide more attenuation to the antinoise when a flange  220  is positioned away from the longitudinal axis  430  (e.g.,  FIG. 4E ) as compared to when the flange  220  is positioned toward the longitudinal axis  430  (e.g.,  FIGS. 4A and/or 4B ). Additionally or alternatively, the processing circuitry  330  may (for example) provide less gain to the antinoise when a flange  220  is positioned away from the longitudinal axis  430  (e.g.,  FIG. 4E ) as compared to when the flange  220  is positioned toward the longitudinal axis  430  (e.g.,  FIGS. 4A and/or 4B ). 
     In particular, feedforward control may be possible in some aspects (e.g., due to the simplified acoustic space when the ears  410   a ,  410   b  are in proximity to the microphones  340   a ,  340   b  and enclosed by cushions  440   a ,  440   b , respectively). Accordingly, the processing circuitry  330  may, in some aspects, be configured to commence feedforward control responsive to flanges  220   a ,  220   b  being positioned away from the longitudinal axis  430  (e.g.,  FIG. 4E ) and cease feedforward control responsive to the flanges  220   a ,  220   b  being positioned toward the longitudinal axis  430  (e.g.,  FIGS. 4A and/or 4B ). 
     Additionally or alternatively, as will be explained further below, aspects of the present disclosure allow the processing circuitry  330  to be tuned through the use of a feedback loop.  FIG. 4D  is a top-view schematic illustrating an example headrest  200  to which a tuning microphone  640  is mounted via a boom  490 . In some aspects, the boom  490  is flexible to permit the tuning microphone  640  to be positioned to a listening position  480 , such as the likely location of one or the other of a typical listener&#39;s ears  410   a ,  410   b . In some aspects, the tuning microphone  640  may be freely coupled and decoupled to the processing circuitry  330  (not shown) as needed in order to tune the ANC system (e.g., via a tuning port  485  that provides tuning input to the processing circuitry  330 ). According to various aspects, this tuning may provide a baseline configuration for producing the antinoise, which is adjusted based on feedback received via the microphone  340  and/or input from the position sensor(s), resulting in improved sound suppressing performance of the antinoise. 
     In view of the above,  FIG. 6  illustrates an example ANC system  600  which, according to various aspects of the present disclosure, is useful in whole or in part with a headrest  200  in accordance with at least some of the aspects described above. The ANC system  600  comprises a microphone  340 , processing circuitry  330 , and a speaker  210 . In general, the processing circuitry  330  is configured to control the speaker  210  to produce antinoise that destructively interferes with ambient sound to produce feedback. The microphone  340  is configured to receive the feedback (which comprises a combination of the ambient sound and antinoise), and provide that feedback to the processing circuitry  330  for further use in performing ANC. In this regard, the processing circuitry  330  may (in some aspects) be configured to control the speaker  210  to produce the antinoise without feedforward control. 
     According to other aspects, the processing circuitry  330  may be configured to control the speaker  210  to produce the antinoise with feedforward control. In such aspects, the ANC system  600  may comprise a feedforward microphone  355  communicatively connected to the processing circuitry  330 , as discussed above. The feedforward microphone  355  is configured to receive ambient sound and provide feedforward input to the processing circuitry  330  for further use in performing ANC. In such aspects, the feedforward microphone  355  may be mounted to the headrest  200  such that the feedforward microphone  355  is insulated from detecting the antinoise. According to various aspects, the processing circuitry  330  may produce the antinoise based on the feedforward input, the feedback from the microphone  340 , or both. In particular, aspects of the processing circuitry  330  may switch between using the feedforward input from the feedforward microphone  355 , the feedback from the microphone  340 , and/or both (e.g., based on a position detected by a position sensor  660 , as discussed above). 
     In some aspects, the ANC system  600  further comprises the above-discussed sound-suppressing enclosure  120 . In such aspects, the ambient sound enters an interior cavity  250  of the sound-suppressing enclosure  120  and is suppressed as discussed above to produce suppressed sound. In such aspects, the antinoise destructively interferes with the suppressed sound to produce feedback that is received by the microphone  340 . In such aspects that also include a feedforward microphone  355 , the feedforward microphone  355  receives this suppressed sound to provide the above-discussed feedforward input to the processing circuitry  330 . 
     The microphone  340  is located at a first position (e.g., mounted to the front grill  320  of the speaker  210 ). The microphone  340  sends the feedback received at the first position to the processing circuitry  330 . The processing circuitry  330  comprises a servo controller  610  and filtering circuitry  620 , which are communicatively connected to each other. Based on the feedback received at the first position by the microphone  340 , the servo controller  610  generates a control signal which the filtering circuitry  620  uses to generate a corrected control signal. In some particular aspects, the filtering circuitry  620  generates the corrected control signal based on the control signal from the servo controller  610  and one or more filtering parameters. In various aspects of the present disclosure, one, some, or all of these filtering parameters are configurable, as will be further discussed below. The filtering circuitry  620  sends the corrected control signal to the speaker  210  to produce the antinoise, which (as discussed above) combines with the ambient or suppressed sound to provide feedback to the servo controller  610  via the microphone  340 . Thus, the ANC system  600  comprises a feedback loop by which effective noise control is achieved. 
     Some aspects of the present disclosure additionally or alternatively comprise a feedforward loop by which effective noise control is achieved. In at least some such aspects, based on the feedforward input received from the feedforward microphone  355  (e.g., in addition to, or instead of, the feedback received from the microphone  340 ), the servo controller  610  generates the control signal which the filtering circuitry  620  uses to generate the corrected control signal. Whether the servo controller  610  may determine which of the feedback and feedforward input to use for generating the control signal based on a position of the headrest  200 , e.g., as detected by position sensor  660  communicatively coupled to the servo controller  610 . 
     Although the control signal produced by the servo controller  610  may be effective at controlling the speaker  610  to produce antinoise without the correction performed by the filtering circuitry  620 , such a servo controller  610  may be designed to provide high overall control performance which, in some aspects, may actually amplify certain frequencies (e.g., one or more frequencies above the threshold frequency). Accordingly, in some aspects, the filtering circuitry  620  tailors the control signal so that the antinoise destructively interferes with the ambient or suppressed sound such that this amplification is suppressed. 
     The correction introduced by the filtering circuitry  620  may, in some aspects, be tuned through the use of a tuning microphone  640  and tuning circuitry  630 , which (in some aspects) may be pluggable into, and removable from, the ANC system  600  as desired. The tuning microphone  640  is placed at a second position, spaced apart from the microphone  340 . In aspects that include the sound-suppressing enclosure  120 , the tuning microphone  640  may also be disposed within the interior cavity  250 . In particular aspects, the tuning microphone  640  may be positioned closer to where a listener&#39;s ear  410  is expected to be, e.g., by suspending the tuning microphone  640  on the end of a boom  490  mounted to the center section  230  of the headrest  200 , or by other means. 
     The tuning microphone  640  is communicatively coupled to the tuning circuitry  630 , and is configured to receive further feedback comprising a different combination of the ambient (or suppressed) sound and the antinoise (i.e., a combination as observed from the second position rather than from the first position where the microphone  340  is located). The tuning microphone  640  is further configured to provide the further feedback to the tuning circuitry  630 . The tuning circuitry  630  is configured to receive the further feedback from the tuning microphone  640 , and based on the further feedback, store different values of the configurable filtering parameter(s) in the filtering circuitry  620  over time. 
     In one particular example, while the ANC system  600  is being tuned (e.g., at a manufacturer or installer of the ANC system  600 ), simulated or prerecorded noise may be used as the ambient sound, and the tuning circuitry  630  may use a genetic algorithm in which values of various filtering parameters are provided to the filtering circuitry  620  over time while resultant noise control performance is monitored. Over multiple feedback loop iterations and over time, the best performing filtering parameters (e.g., the filtering parameter(s) that most reduce the a-weighted Root Mean Square (RMS) sound pressure) may be then be stored in the filtering circuitry  620  (e.g., in a memory  650 ) for subsequent use (e.g., during actual operation of the vehicle). 
     The correction introduced by the filtering circuitry  620  may additionally or alternatively be adjusted, in some aspects, based on a position of a flange  220  of the headrest  200  (e.g., relative to the center section  230  or longitudinal axis  430  of the headrest  200 ). According to such aspects, the position is detected by a position sensor  660 , and sent to the processing circuitry  330 , which is configured to control a speaker  210  to produce the antinoise based on the feedback and the detected position (e.g., by incorporating feedforward control). Thus, in some aspects, different antinoise may be produced as appropriate depending on how the flange  220  is positioned. In particular, the attenuation and/or gain of the antinoise may be adjusted based on the position of flange  220 . 
     For example, such adjustments may be made based on a position of the flange  220  to counteract passive amplification resulting from particular configurations of the headrest  200 , such as that illustrated in  FIG. 4C  above and/or to more aggressively suppress sound using the antinoise in other particular configurations of the headrest  200  that do not experience passive amplification to as great a degree (or at all), such as the configuration illustrated in  FIG. 4B . In another example, such adjustments may be made based on a position of the flange  220  to add feedforward control in response to the acoustic space around a listener&#39;s ear  410  being simplified (e.g., by positioning the flange  220  away from the longitudinal axis  430  of the headrest as shown in  FIG. 4E  and discussed above) and to cease feedforward control in response to the acoustic space around the listener&#39;s ear  410  being complicated (e.g., by positioning the flange  220  towards the longitudinal axis  430  of the headrest as shown in  FIG. 4A  and/or  FIG. 4B  and discussed above). 
     Moreover, the headrest  200  may comprise multiple flanges  220   a ,  220   b  that are movable independently from each other. Accordingly, in some aspects, the headrest  200  comprises, for each flange  220 , a respective position sensor  660  configured to detect the position (e.g., the angle) of the flange  220  relative to the center section  230 . Correspondingly, the processing circuitry  330  may control the speaker  210  mounted to each flange  220  based on the feedback and the position detected by the corresponding position sensor  660 . 
     The processing circuitry  330  may control the speaker in a variety of ways, according to various aspects of the present disclosure. For example, to set the attenuation level of the antinoise based on the position detected by the position sensor  660 , the filtering circuitry  620  may be communicatively connected to the position sensor  660  and configured to decrease or increase the attenuation level of the antinoise responsive to the flange  220  being moved towards or away from the longitudinal axis  430 , respectively. Thus, responsive to the flange  220  being moved away from the longitudinal axis  430  (and towards the head  400  of a listener), for example, the filtering circuitry  620  may increase the attenuation level. Correspondingly, responsive to the flange  220  being moved towards the longitudinal axis  430  (and away from the head  400  of the listener), the filtering circuitry  620  may decrease the attenuation level. 
     Additionally or alternatively, to set the gain level of the antinoise based on the position detected by the position sensor  660 , the filtering circuitry  620  may, in some aspects, be configured to increase or decrease the gain level of the antinoise responsive to the flange  220  being moved towards or away from the longitudinal axis  430 , respectively. Thus, responsive to the flange  220  being moved away from the longitudinal axis  430  (and towards the head  400  of a listener), for example, the filtering circuitry  620  may decrease the gain level. Correspondingly, responsive to the flange  220  being moved towards the longitudinal axis  430  (and away from the head  400  of the listener), the filtering circuitry  620  may increase the gain level. 
     To set the attenuation and/or gain level of the antinoise based on the position detected by the position sensor  660 , the filtering circuitry  620  may, in some aspects, be configured to set the attenuation and/or gain level of the antinoise to one of a plurality of predefined attenuation and/or gain levels selected based on which of a plurality of predefined position ranges comprises the position detected by the position sensor  660 . For example, responsive to the position sensor  660  detecting that the flange  220  is positioned at an angle α of less than twenty-five degrees away from the longitudinal axis  430 , the filtering circuitry  620  may set the attenuation level to a predefined minimum attenuation level and/or set the gain level to a predefined maximum gain level. Responsive to the position sensor  660  detecting that the flange  220  is positioned at an angle α of more than eighty degrees away from the longitudinal axis  430  (for example), the filtering circuitry  620  may set the attenuation level to a predefined maximum attenuation level and/or set the gain level to a predefined minimum gain level. Further, responsive to the position sensor  660  detecting that the flange  220  is positioned at an angle α between twenty-five and eighty degrees away from the longitudinal axis  430  (for example), the filtering circuitry  620  may set the attenuation level and/or gain level to a level between the minimum and maximum attenuation and/or gain levels. Indeed, aspects of the present disclosure may include any number of predefined attenuation and/or gain levels and corresponding position ranges, e.g., as may be appropriate to provide accurate gain control in view of the particular design of the headrest  200  and/or environment in which the headrest  200  will be installed (e.g., in the aircraft  100 ). 
     Additionally or alternatively, responsive to the position sensor  660  detecting that the flange  220  is positioned at an angle α of more than a given threshold away from the longitudinal axis  430 , the servo controller  610  may produce the control signal using feedforward control. Correspondingly, responsive to the position sensor  660  detecting that the flange  220  is positioned at an angle α of less than the given threshold away from the longitudinal axis  430 , the servo controller  610  may refrain from and/or cease producing the control signal using feedforward control. 
     In some aspects, the servo controller  610  performs one or more proportional (P), integral (I), and/or derivative (D) control functions based on the feedback to produce a control signal that is useful for controlling the speaker  210  to produce antinoise. Thus, in some aspects, the servo controller  610  is a P controller, a PI controller, a PID controller, or a PD controller. 
       FIG. 7  illustrates an example servo controller  610 , according to particular aspects of the present disclosure. The servo controller  610  comprises proportional control circuitry  710 . In some aspects, the servo controller  610  further comprises integral control circuitry  720  and/or derivative control circuitry  730 . 
     In particular, the servo controller  610  may be a P controller in which the proportional control circuitry  710  produces a control signal for outputting antinoise from the speaker  210  in proportion to the feedback received at the microphone  340 . In other aspects, the servo controller  610  may be a PI controller that further comprises the integral control circuitry  720 . In such aspects, the proportional control circuitry  710  may contribute predominantly to the control signal, and the integral control circuitry  720  may be configured to take an integral of the antinoise over time, which is combined with the output from the proportional control circuitry  710  to smooth out error or deviance between the feedback and the sound to be controlled. 
     Alternatively, the servo controller  610  may be a PD controller or a PID controller that comprises the derivative control circuitry  730 . The derivative control circuitry  730  is configured to produce an output that shapes the output of the proportional control circuitry  710  (and intergral control circuitry  720 , if present) based on a rate of change to the input to the servo controller  610 . By factoring in the rate of change, the servo controller  610  attempts to predict and compensate for future errors between the antinoise and sound to be controlled. Thus, the derivative control circuitry  730  may be included in the servo controller  610  when the servo controller  610  will be used to control noise in a stable, predictable, and/or uniform sound environment (e.g., in a turboprop aircraft). Correspondingly, the derivative control circuitry  730  may be omitted from the servo controller  610  when the servo controller  610  will be used in a highly-complex and/or unpredictable sound environment (e.g., in a jet aircraft). 
     In view of all of the above,  FIG. 8  illustrates an example method  800  of performing ANC within a vehicle, according to various aspects of the present disclosure. The method  800  comprises producing suppressed sound by suppressing frequencies of ambient sound above a threshold frequency that enter an interior cavity of a sound-suppressing enclosure  120  disposed within, and spaced from, interior walls of the vehicle (block  810 ). The method  800  further comprises receiving, by a microphone  340  disposed within the interior cavity  250  of the sound-suppressing enclosure  120 , feedback comprising a combination of the suppressed sound produced by the sound-suppressing enclosure  120  and antinoise produced by one or more speakers  210  mounted to a headrest  200  disposed within the interior cavity  250  of the sound-suppressing enclosure  120  (block  820 ). The method  800  further comprises controlling the speakers  210  to produce the antinoise based on the feedback, such that the antinoise destructively interferes with frequencies of the suppressed sound that are above the threshold frequency (block  830 ). 
       FIG. 9  illustrates a more detailed example method  900  of performing ANC within a vehicle. The method  900  comprises producing suppressed sound by suppressing frequencies of ambient sound according to aspects discussed above (e.g., using a sound-suppressing enclosure  120 ) (block  910 ). The method  900  further comprises receiving the suppressed sound using a microphone  340  (block  920 ) and producing a control signal (e.g., using a servo controller  610 ), according to aspects discussed above (block  930 ). The method  900  further comprises generating a corrected control signal (e.g., using filtering circuitry based on the suppressed sound) (block  940 ), and controlling the speakers to produce antinoise (block  950 ) in accordance with aspects discussed above. The method  900  further comprises receiving feedback comprising suppressed sound and the antinoise (block  960 ) and again producing a control signal (block  930 ), and so on, as discussed above. 
       FIG. 10  illustrates another method  1000  implemented by an ANC system  600 . The method  1000  comprises using a PI controller to produce a control signal based on feedback that comprises a combination of ambient sound and antinoise (block  1010 ). The method  1000  further comprises generating a corrected control signal based on the control signal and a configurable filtering parameter (block  1020 ). The method  1000  further comprises producing the antinoise under control of the corrected control signal such that the antinoise destructively interferes with frequencies of the ambient sound to produce the feedback (block  1030 ). The method further comprises using a microphone to receive the feedback and provide the feedback to the PI controller (block  1040 ). 
       FIG. 11  illustrates yet another method  1200  implemented by an ANC headrest  200 . The method  1200  comprises producing antinoise, from a speaker  210  of the headrest  200 , that destructively interferes with frequencies of ambient sound (block  1210 ). The speaker  210  is mounted to a flange  220  of the headrest  200  that extends away from a center section  230  of the headrest  200  and is movable relative to a longitudinal axis  430  of the center section  230 . The method  1200  further comprises receiving feedback comprising a combination of the antinoise and the ambient sound (block  1220 ), and detecting a position of the flange  220  relative to the center section  230  (block  1230 ). The method further comprises controlling the speaker  210  to produce the antinoise based on the feedback and the detected position of the flange  220  relative to the center section  230  (block  1240 ). 
       FIG. 12  illustrates a more detailed method  1100  implemented by an ANC system  600  and/or ANC headrest  200 . The method  1100  comprises producing a control signal (e.g., using a servo controller  610 , such as a PI controller), in accordance with aspects discussed above (block  1110 ). The method  1100  further comprises generating a corrected control signal (e.g., based on the control signal, a configurable filtering parameter, and/or a position of a flange  220  of the headrest  200  relative to the center section  230 ) in accordance with aspects discussed above (block  1120 ). The method  1100  further comprises producing antinoise in accordance with aspects discussed above (e.g., by controlling a speaker  210  using the corrected control signal) (block  1130 ). The method  1100  further comprises receiving feedback (e.g., using a microphone  340  as discussed above) (block  1140 ) and sending the feedback to the servo controller (block  1160 ) for continued production of the control signal (block  1110 ). 
     In some aspects, the method  1100  further comprises receiving further feedback (e.g., using a tuning microphone  640 ) (block  1150 ), and storing a filtering parameter (e.g., in filtering circuitry  620 ) for use in further generating the corrected control signal (block  1120 ), in accordance with aspects discussed above. 
     In some additional or alternative aspects, the method  1100  further comprises detecting a position of a flange  220  of the headset  200  relative to the center section  230  (block  1180 ), and increasing (block  1190 ) or decreasing (block  1195 ) a gain level of the antinoise responsive to the flange being moved towards or away from the longitudinal axis, respectively (e.g., by using the increased or decreased gain level in further generating the corrected control signal (block  1120 )). As discussed above, the increased or decreased gain level may, in some aspects, be set to one of a plurality of predefined gain levels selected based on which of a plurality of predefined position ranges comprises the detected position. 
     Those skilled in the art will appreciate that the various methods and processes described herein may be implemented using various hardware configurations that generally, but not necessarily, include the use of one or more microprocessors, microcontrollers, digital signal processors, or the like, coupled to memory storing software instructions or data for carrying out the techniques described herein. In particular, those skilled in the art will appreciate that the circuits of various aspects may be configured in ways that vary in certain details from the broad descriptions given above. For instance, one or more of the processing functionalities discussed above may be implemented using dedicated hardware, rather than a microprocessor configured with program instructions. Such variations, and the engineering tradeoffs associated with each, will be readily appreciated by the skilled practitioner. Since the design and cost tradeoffs for the various hardware approaches, which may depend on system-level requirements that are outside the scope of the present disclosure, are well known to those of ordinary skill in the art, further details of specific hardware implementations are not provided herein. 
     Aspects of the present disclosure may additionally or alternatively include one or more aspects of the claims enumerated below, and/or any compatible combination of features described herein. The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present aspects are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein. Although steps of various processes or methods described herein may be shown and described as being in a sequence or temporal order, the steps of any such processes or methods are not limited to being carried out in any particular sequence or order, absent an indication otherwise. Indeed, the steps in such processes or methods generally may be carried out in various different sequences and orders while still falling within the scope of the present invention.