Patent Publication Number: US-8971544-B2

Title: Signal compression based on transducer displacement

Description:
FIELD 
     This disclosure relates to adjusting the performance of an electroacoustic transducer in response to detecting relative motion among components of the transducer. 
     BACKGROUND 
     Conventional active noise reduction (ANR) headsets utilize acoustic output generated by the headsets&#39; electroacoustic transducers to minimize the user&#39;s perception of ambient noise. For example, a conventional ANR headset has a noise cancelling assembly associated with each electroacoustic transducer. The noise cancelling assembly can include a microphone mounted in proximity to the electroacoustic transducer within each of the headset&#39;s ear cups. During operation, the microphones receive an audio input as heard by the user and associated electronics filter the resulting audio signal, based on the principle of feedback control, to generate a noise cancelling signal. The noise cancelling assembly feeds the noise-cancelling signal to the electroacoustic transducer amplifier, which, in turn, combines the noise-cancelling signal with desired audio from an audio source, if one is present. The noise-cancelling signal creates destructive interference with ambient noise, such as noise generated external to the headset, as the ambient noise and the combined audio signal arrive at a user&#39;s ear. The noise cancelling assembly thus minimizes the ambient noise perceived by the user and allows the user to experience a substantially clear audio input from the audio source. 
     In use, when placed over the user&#39;s ears, the ear cups associated with the headset form a seal between the user&#39;s head and a volume containing each of the electroacoustic transducers. The air captured between the user&#39;s ears and each electroacoustic transducer acts as a spring having a relatively high spring constant such that the air reduces displacement or excursion of each electroacoustic transducer during operation (i.e., as compared to the effect of the air captured between the user&#39;s ears and the electroacoustic transducer if the ear cup were not sealed to the head). However, in certain cases, one or both of the ear cups may not be completely sealed against the user&#39;s head. In such a case, because the volume within the ear cup is exposed to external, atmospheric air pressure, the air captured between the user&#39;s ear and the electroacoustic transducer acts as a spring having a relatively low spring constant. In certain cases an associated ANR headset amplifier is designed to provide high amplitude signals to the electroacoustic transducer in order to cancel high levels of noise under normal wearing conditions, such as when the ear cups form a relatively tight seal to the user&#39;s head. During conditions where the seal is not as tight (e.g., is leaky) the resulting decreased spring constant of the air captured between the user&#39;s ear and the electroacoustic transducer can allow the transducer to be over-extended even with a normal voltage drive signal level. 
     To minimize clipping or distortion when using a relatively large voltage drive signal, conventional ANR headsets utilize a compressor to reduce ANR loop gain when the drive signal voltage crosses a threshold that approaches the amplifier clipping limit. Typically, for conventional ANR headsets utilized in relatively high-noise environments, the voltage threshold is based upon the maximum drive signal voltage that the electroacoustic transducer or driver can safely tolerate at low frequencies in free air or unloaded conditions. This is done to protect the electroacoustic transducer or driver from potentially damaging levels of displacement. By reducing ANR loop gain when clipping is approached, the driver is protected and clipping, and the unpleasant audio artifacts therefore, can also be prevented. Accordingly, by reducing the loop gain to the electroacoustic transducer amplifier based upon the drive signal voltage the headset minimizes both clipping of the audio signal output and potential damage to the electroacoustic transducer components as caused by over actuation. 
     SUMMARY 
     Conventional reduction of ANR loop gain compression based upon a drive signal voltage can over-constrain the drive signal to the electroacoustic transducer in typical use cases. For example, the reduction in loop gain is based on the drive signal voltage that corresponds to a maximum safe free-air displacement of the electroacoustic transducer, independent of frequency. However, in the case where the ear cup is not completely sealed against the user&#39;s head, even though the air captured between the electroacoustic transducer and the user&#39;s head has a relatively low spring constant, the air does generate a load on the electroacoustic transducer to decrease the relative displacement of the electroacoustic transducer components. Accordingly, while the compressor in a conventional ANR headset reduces the gain provided by the ANR headset amplifier to the electroacoustic transducer, such reduction is substantially below the relative displacement limits of the components of the transducer at any given degree of partial seal of the ear cup to the head, which reduces the amount of sound pressure that can be generated. 
     By contrast to conventional approaches, embodiments of the present innovation relate to signal compression based upon electroacoustic transducer displacement. In an ANR headset, such as a high-noise headset, a compressor or gain adjustment component is configured to adjust ANR loop gain, such as via feedback compression, based upon components of an electroacoustic transducer approaching their fundamental displacement limits. With such a configuration, when the headset is worn normally by a user (i.e., well sealed to the user&#39;s head), the electroacoustic transducer amplifier can provide an increased amount of power to the electroacoustic transducer during operation, thereby allowing the electroacoustic transducer to generate relatively higher sound pressure levels in the ANR headset, such as sound pressure levels of about 20 dB greater than the levels provided by conventional headsets, before reaching a maximum displacement limit. Additionally, because operation of the signal compression circuitry is dependent upon the displacement of the electroacoustic transducer, the circuitry can adapt its operation based upon any seal condition present between the user&#39;s head and the headset. For example, if the headset is leaky or removed from the user&#39;s head such that the electroacoustic transducer is effectively unloaded by the relatively low spring constant that results from such conditions, the transducer&#39;s displacement will increase, relative to a normally loaded transducer, for a given voltage. Accordingly, the displacement-sensing compressor will trigger at driver voltages akin to the limits in conventional voltage-limiting compressors. 
     In general, one aspect of the disclosure features an acoustic assembly, having an electroacoustic transducer, a microphone transducer disposed in proximity to the electroacoustic transducer, and a gain adjustment circuit disposed in electrical communication with at least one of the magnetic structure and the voice coil. The gain adjustment circuit is configured to receive a displacement signal corresponding to a relative motion between a magnetic structure of the electroacoustic transducer and a voice coil of the electroacoustic transducer, detect a displacement signal value of the displacement signal as one of meeting or exceeding a displacement signal threshold and modify a loop gain of an active noise reduction loop associated with the electroacoustic transducer when the displacement signal value of the displacement signal one of meets or exceeds the displacement signal threshold. 
     Various additional implementations may include one or more of the following features. The acoustic assembly may include a threshold detector, configured to detect, as a displacement signal value, an absolute value of the displacement signal. The gain adjustment circuit can also include a current limited source and an integrator component. In response to detecting the absolute value of the displacement signal value of the displacement signal as one of meeting or exceeding a displacement signal threshold the threshold detector is configured to activate the current limited source to generate a current. Additionally, the current limited source is configured to provide the current to the integrator component of the gain adjustment circuit and the integrator component is configured to provide a compressor control signal to a compressor component of the gain adjustment circuit based upon the output of the integrator component. 
     In one implementation, when modifying the loop gain of the active noise reduction loop associated with the electroacoustic transducer the compressor component of the gain adjustment circuit can be configured to modify the loop gain of the active noise reduction loop associated with the electroacoustic transducer based upon the received compressor control signal. Also, when receiving the displacement signal, the gain adjustment circuit can be configured to receive a displacement signal associated with a change in capacitance within the electroacoustic transducer as created by relative motion between the magnetic structure of the electroacoustic transducer and the voice coil of the electroacoustic transducer. 
     In one implementation, the gain adjustment circuit can be further configured to receive a driving signal associated with the electroacoustic transducer, the driving signal configured to generate relative motion between the magnetic structure of the electroacoustic transducer and the voice coil of the electroacoustic transducer and detect an absolute value of a driving signal value of the driving signal as one of meeting or exceeding a driving signal threshold. When modifying the loop gain of the active noise reduction loop associated with the electroacoustic transducer, the gain adjustment is operable to modify the loop gain of the active noise reduction loop associated with the electroacoustic transducer when at least one of the displacement signal one of meets or exceeds the displacement signal threshold and the absolute value of driving signal value one of meets or exceeds the driving signal threshold. When detecting the displacement signal value of the displacement signal as one of meeting or exceeding a displacement signal threshold, the threshold detector can be configured to detect, as the displacement signal value, an absolute value of the displacement signal. When detecting the absolute value of the driving signal value of the driving signal as one of meeting or exceeding a driving signal threshold, the threshold detector can be configured to detect the absolute value of the driving signal value as one of meeting or exceeding a driving signal threshold. 
     In one implementation, when modifying the loop gain of the active noise reduction loop associated with the electroacoustic transducer, the gain adjustment circuit is configured to reduce the loop gain of the active noise reduction loop associated with the electroacoustic transducer when the displacement signal value of the displacement signal one of meets or exceeds the displacement signal threshold. 
     In general another aspect of the disclosure features a method for adjusting the performance of an electroacoustic transducer. The method includes receiving, by gain adjustment circuit, a displacement signal corresponding to a relative motion between a magnetic structure of the electroacoustic transducer and a voice coil of the electroacoustic transducer. The method includes detecting, by the gain adjustment circuit, a displacement signal value of the displacement signal as one of meeting or exceeding a displacement signal threshold. The method includes modifying, by the gain adjustment circuit, a loop gain of an active noise reduction loop associated with the electroacoustic transducer when the displacement signal value of the displacement signal one of meets or exceeds the displacement signal threshold. 
     In general another aspect of the disclosure features an acoustic assembly, having an active noise reduction assembly having electroacoustic transducer, a microphone transducer disposed in proximity to the electroacoustic transducer, and an amplifier stage disposed in electrical communication with the microphone and the electroacoustic transducer, the active noise reduction assembly defining an active noise reduction loop having a loop gain. The acoustic assembly includes a displacement sensing circuit disposed in electrical communication with at least one of a magnetic structure of the electroacoustic transducer and a voice coil of the electroacoustic transducer. The acoustic assembly also includes a gain adjustment circuit disposed in electrical communication with the active noise reduction loop of the active noise reduction assembly and in electrical communication with the displacement sensing circuit and being operable to modify the loop gain of the active noise reduction loop when a displacement signal value of a displacement signal generated by the displacement sensing circuit one of meets or exceeds a displacement signal threshold. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The foregoing and other objects, features and advantages will be apparent from the following description of particular embodiments of the innovation, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of various embodiments of the innovation. 
         FIG. 1  is a schematic representation of a partial sectional view of a headset, according to one arrangement. 
         FIG. 2  is a schematic representation of a gain adjustment circuit of the headset of  FIG. 1 , according to one arrangement. 
         FIG. 3  is a flowchart illustrating a method for adjusting the performance of an electroacoustic transducer. 
         FIG. 4  is a schematic representation of an adjustment circuit of the electroacoustic transducer assembly of  FIG. 1 , according to an alternate arrangement. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present innovation relate to signal compression based upon electroacoustic transducer displacement. In an ANR headset, such as a high-noise headset, a compressor or gain adjustment component is configured to adjust ANR loop gain with an electroacoustic transducer, such as via feedback compression, based upon components of the electroacoustic transducer approaching their fundamental displacement limits. With such a configuration, when the headset is worn normally by a user (i.e., well sealed to the user&#39;s head), the electroacoustic transducer amplifier can provide an increased amount of power to the electroacoustic transducer during operation, thereby allowing the electroacoustic transducer to generate relatively higher sound pressure levels in the ANR headset, such as sound pressure levels of about 20 dB greater than the levels provided by conventional headsets, before reaching a maximum displacement limit. Additionally, because operation of the signal compression circuitry is dependent upon the displacement of the electroacoustic transducer, the circuitry can adapt its operation based upon any seal condition present between the user&#39;s head and the headset. For example, if the headset is leaky or removed from the user&#39;s head such that the electroacoustic transducer is effectively unloaded by the relatively low spring constant that results from such conditions, the transducer&#39;s displacement will increase, relative to a normally loaded transducer, for a given voltage. Accordingly, the displacement-sensing compressor will trigger at driver voltages akin to the limits in conventional voltage-limiting compressors. 
       FIG. 1  is an example schematic representation of a headset  20 , such as an on-ear active noise reduction (ANR) headset. As illustrated, the headset  20  includes a support apparatus  22  that carries a first housing assembly  24  and a second housing assembly  26  at opposing ends of the apparatus  22 . In the example headset  20  shown, while the support apparatus  22  is illustrated as a head strap the support apparatus  22  can be configured in a variety of ways. For example, the support apparatus  22  can be configured as a nape band or under-helmet support. In one arrangement, each of the first and second housing assemblies  24 ,  26  are configured to be held against a user&#39;s head via the head support apparatus  22  to form respective seals with the user&#39;s head about each of the user&#39;s ears. During operation, each of the first and second housing assemblies  22 ,  24  is configured to deliver noise-reduced audio to the user based upon known noise cancellation techniques. 
     As illustrated, each of the first and second housing assemblies  24 ,  26  includes a corresponding housing  28 - 1 ,  28 - 2  that carries an acoustic assembly  30 - 1 ,  30 - 2  that includes an active noise reduction assembly  35 , a displacement sensing circuit  34 , and a gain adjustment circuit  36 . In one arrangement, the configuration of the components of the acoustic assembly  30 - 1  is substantially similar to the acoustic assembly  30 - 2  of the second housing assembly  26 . For convenience, a description of the components of the first housing assembly  24  is provided below. 
     As illustrated, the displacement sensing circuit  34  is disposed in electrical communication with both the active noise reduction assembly  35  and the gain adjustment circuit  36  while the gain adjustment circuit  36  is disposed in electrical communication with the displacement sensing circuit  34  and with the active noise reduction assembly  35 . In use, and as will be described in detail below, the gain adjustment circuit  36  is configured to adjust a loop gain associated with the active noise reduction assembly  35  based upon the relative displacement of components of an electroacoustic transducer of the active noise reduction assembly  35 , as detected by the displacement sensing circuit  34 . 
     With reference to  FIG. 2 , the active noise reduction (ANR) assembly  35  includes an electroacoustic transducer  32 , a microphone transducer  50  and circuitry (not shown), a compensator  33 , and an amplifier stage  38  arranged to form an ANR loop. For example, the microphone transducer  50  is disposed in proximity to (i.e., in front of) the electroacoustic transducer  32  and is disposed in electrical communication with the compensator  33  via DC blocking capacitor  51 , and resistors  37  and  64 . The compensator  33 , in turn, is disposed in electrical communication with the amplifier stage  38  which is disposed in electrical communication with the electroacoustic transducer  32 . 
     As shown, the electroacoustic transducer  32  includes a diaphragm  39  secured to the housing  28 - 1  by a basket  44  and connected to a voice coil  40  which may be self-supporting or may be wound around a coil-former or bobbin (not shown). The electroacoustic transducer  32  also includes a magnetic assembly  42  disposed in electromagnetic communication with the voice coil  40 . In some examples, the voice coil  40  and at least part of the magnetic assembly  42  are reversed, such that the magnetic assembly  42  moves the diaphragm  39  and the voice coil  40  remains stationary relative to the basket  44 . 
     During operation, the microphone transducer  50  receives an audio input as heard by a user. The microphone transducer circuitry filters the corresponding audio signal generated by the microphone transducer  50 , based on the principle of feedback control, to generate a noise cancelling signal and feeds the noise-cancelling signal through resistors  37  and  65  and then to the amplifier stage  38  through the compensator  33 . The amplifier stage  38 , in turn, can combine the noise-cancelling signal with a desired audio signal from the audio source  46  and feeds the combined audio signal to the electroacoustic transducer  32 . 
     As a result of the amplifier stage  38  providing the combined audio signal to the electroacoustic transducer  32 , the voice coil  40  interacts with a magnetic field of the magnetic assembly  42  to produce forces that move the voice coil  40  and diaphragm  39  relative to the magnetic assembly  42  and basket  44  to acoustically radiate the combined audio signal, as audio input, to a user&#39;s ear. Accordingly, as ambient noise and the combined audio signal arrive at a user&#39;s ear the noise-cancelling portion of the combined audio signal creates destructive interference with the ambient noise to minimize the user&#39;s perceived presence of ambient noise and allow the user to experience a substantially clear audio from the audio source  46 . 
     The displacement sensing circuit  34  is configured to detect the relative displacement of components of the electroacoustic transducer  32 . While the displacement sensing circuit  34  can be configured in a variety of ways, in one arrangement, the displacement sensing circuit  34  measures the displacement based upon a change in the capacitance between certain components of the electroacoustic transducer  32  and converts the capacitance into a signal representative of the displacement. 
     With respect to the capacitance associated with electroacoustic transducer  32 , in one arrangement, a capacitance exists between the voice coil  40  and the side walls of the magnetic assembly  42 . As the voice coil  40  moves in and out of the magnetic assembly  42  along direction  52 , the overlap of the surface area between the voice coil  40  and the side walls of the magnetic assembly  42 , and the resulting capacitance between them, varies. Accordingly, the capacitance between the voice coil  40  and the magnetic assembly  42  is proportional to the relative positioning between the voice coil  40  and the magnetic assembly  42 . In response to receiving a varying signal affected by a change in capacitance between the voice coil  40  and the magnetic assembly  42 , the displacement sensing circuit  34  generates a corresponding displacement signal  54 . Additional description of the capacitive coupling of the voice coil  40  and the magnetic assembly  42  is provided in U.S. patent application Ser. No. 13/075,899, filed Mar. 30, 2011, and entitled “Measuring Transducer Displacement,” the contents and teachings of which are hereby incorporated by reference in their entirety. 
     In another arrangement, the diaphragm  39  of the electroacoustic transducer  32  is coated with a layer of metal. Additionally a corresponding metalized limiter (not shown) is disposed in proximity to the diaphragm  39 . The layer of metal on the limiter forms a back plate and the layer of metal on the diaphragm  39  forms a front plate of a two plate capacitor. The capacitance between the front and back plates is proportional to the relative positioning between the voice coil  40  and the magnetic assembly  42 . For example, in operation, a voltage source with a series resistor, the value of which is selected to maintain a substantially constant charge across the plates, imposes a constant charge condition. As the diaphragm  39  moves relative to the metalized limiter, a capacitance between the plates changes in proportion to the relative change in distance between the diaphragm  39  and the limiter. The change in capacitance changes the corresponding voltage across the plates. In response to receiving the changed voltage signal, the displacement sensing circuit  34  generates a corresponding displacement signal  54 . Additional description of the capacitive coupling of the diaphragm  39  and the limiter is provided in U.S. patent application Ser. No. 12/969,685, filed Jan. 9, 2011, and entitled “Transducer with Integrated Sensor,” the contents and teachings of which are hereby incorporated by reference in their entirety. 
     The displacement sensing circuit  34  utilizes a change in the capacitance between components of the electroacoustic transducer  32  to detect the relative displacement of components of the electroacoustic transducer  32 . With such a configuration, the headset  20  does not require the integration of separate displacement sensing elements, such as an optical encoder or laser interferometer. 
     The gain adjustment circuit  36  is configured to receive a displacement signal  54  from the displacement sensing circuit  34  and adjust the loop gain associated with the ANR assembly  35  based upon the displacement signal  54 . While the gain adjustment circuit  36  can be configured in a variety of ways, in the example configuration illustrated in  FIG. 2 , the gain adjustment circuit  36  includes a threshold detector  58 , a current limited sources  59 , an integrator component  62 , and a compressor component  64 . 
     The threshold detector  58 , such as a diode, transistor, or full wave precision rectifier circuit combined with a comparator, is configured to detect both the positive and negative portions of the displacement signal  54  and compare the positive and negative portions to a displacement signal threshold  60 . For example, during operation, as the threshold detector  58  receives the displacement signal  54 , the threshold detector  58  takes the absolute value of the displacement signal voltages or values that constitute the displacement signal  54  and compares the resulting displacement signal value to the displacement signal threshold  60 . This threshold  60 , in one arrangement, is a voltage level corresponding to a voltage associated with the displacement signal  54  when the voice coil  40  and the magnetic structure  42  experience a relative displacement that can cause clipping of a resulting audio signal or damage to the electroacoustic transducer  32 . For example, assume the case where the relative excursion of voice coil  40  and the magnetic structure  42  of a distance of 1.0 mm will begin to cause clipping of a resulting audio signal and will cause the displacement sensing circuit  34  to include a corresponding displacement signal value of 4V as part of the displacement signal  54 . In such a case, a manufacturer can configure the threshold detector  58  with a displacement signal threshold  60  of slightly below 4V, to minimize the occurrence of clipping. 
     In the case where the displacement signal value meets or exceeds the threshold  60 , the threshold detector  58  enters an operational state and activates the current limited source to provide the current  66  to the integrator component  62 . 
     The integrator component  62  is configured to receive the current  66  from the current limited source  59  and convert the current  66  to a voltage or compressor control signal  70 , V i , that is proportional to the accumulation of current  66 . For example, as illustrated the integrator component  62  is configured as a capacitor. With such a configuration, as soon as the integrator component  62  receives the current  66  from the current limited source  59 , the integrator component  62  increases the compressor control signal  70  (V i ) with a relatively rapid attack rate. Over time, as a displacement signal value of the displacement signal  54  falls below the threshold  60 , the voltage on integrator component  62  decays through resistor  76  with a relatively slow release rate back towards a steady state, such as that occurring at low level or quiet conditions thus returning the compressor control signal  70  to its steady state value. In one arrangement, a discharge resistor  76  combined with the value of the integrating capacitor  62  sets the time constant for the discharge rate. Accordingly, the discharge resistor  76 , as selected by a manufacturer or designer, is utilized in conjunction with the integrator component  62  to drain the charge from the integrator component  62  at a relatively slow rate. 
     In response to the change in the current  66 , the integrator component  62  provides the proportional output  70  (V i ) to the compressor component  64  via a buffer  77 . The compressor component  64  is configured to adjust the loop gain associated with the ANR assembly  35  in response to receiving the integrator component output  70  (V i ). While the compressor component  64  and buffer  77  can be configured a variety of ways, in one arrangement, the compressor component  64  and buffer  77  are configured as a field effect transistor (FET) that operates as a buffer and variable resistor. In use, as will be described in detail below, based upon the integrator component output  70  (V i ), the compressor component  64  attenuates the loop gain feedback signal to minimize clipping of the audio signal produced by the electroacoustic transducer  32  and to minimize potential damage to the components of the electroacoustic transducer  32  caused by relative over-excursion of the components. 
       FIG. 3  is a flowchart  100  illustrating a method performed by the gain adjustment circuit  36  for adjusting the performance of the electroacoustic transducer  32 . 
     In step  102 , the gain adjustment circuit  36  receives a displacement signal  54  corresponding to a relative motion between a magnetic structure  42  of the electroacoustic transducer  32  and a voice coil  40  of the electroacoustic transducer  32 . As indicated above, and with reference to  FIG. 2 , during operation the displacement sensing circuit  34  detects a change in capacitance between the electroacoustic transducer  32  components that is proportional to the relative displacement of the voice coil  40  and the magnetic structure  42 . In response, the displacement sensing circuit  34 , generates a corresponding displacement signal  54  having a voltage that is proportional to the capacitance and, therefore, the relative positioning between the voice coil  40  and the magnetic structure  42 . The displacement sensing circuit  34  provides the displacement signal  54  to the threshold detector  58  in a substantially continuous manner. 
     Returning to  FIG. 3 , in step  104 , the gain adjustment circuit  36  detects a displacement signal value of the displacement signal  54  as one of meeting or exceeding a displacement signal threshold  60 . For example, with reference to  FIG. 2 , assume the case where the displacement signal threshold  60  is set to a value of 4V. As the threshold detector  58  receives the displacement signal  54  during operation, the threshold detector  58  takes the absolute value of the displacement signal  54  and compares the absolute value of the displacement signal  54  to the displacement signal threshold  60 . In the case where the displacement signal  54  includes a displacement signal voltage value of +/−4V, the threshold detector  58  detects the displacement signal value as meeting the threshold  60  and activates a current limited source  59  to generate a current  66 . The current limited source  59  provides the current  66  to the integrator  62  which, in turn, provides a corresponding compressor control signal (V i )  70 , having a voltage proportional to the accumulation of current  66 , to the compressor component  64 . 
     Returning to  FIG. 3 , in step  106 , the gain adjustment circuit  36  modifies a loop gain of an active noise reduction loop  35  associated with the electroacoustic transducer  32  when the displacement signal value of the displacement signal  54  one of meets or exceeds the displacement signal threshold  60 . For example, with reference to  FIG. 2 , the compressor component  64  initiates feedback compression with respect to the loop gain of the ANR assembly  35  based upon the compressor control signal  70  received from the integrator  62 . In one arrangement, the voltage V in  of the ANR assembly  35  is based upon the relationship: V in =(R comp /(R comp +R i ))*V mic  where R comp  is the resistance of the compressor component  64 , R i  is the resistance of the resistor  37  and V mic  is the voltage associated with the microphone transducer  50 . As the compressor component  64  receives the compressor control signal V i    70  from the integrator  62 , the compressor component  64  reduces its resistance R comp  to a value inversely proportional to the compressor control signal  70 . Accordingly, based upon the above relationship, as R comp  decreases, V in  also decreases where the reduction of V in  relates to a reduction in the loop gain of the ANR assembly  35 . It should be noted that in the case where the value of the compressor control signal V i    70  decreases over time, as caused by the relatively slow release rate of the integrator  62 , the compressor component  64  increases its resistance R comp  to increase the loop gain of the ANR assembly  35  at the correspondingly slow rate. 
     For a headset  20 , such as a high-noise headset, feedback compression or gain reduction based upon the relative displacement of the electroacoustic transducer  32  components limits the audio signal provided to the electroacoustic transducer  32  to minimize transducer excursion clipping and potential damage to the electroacoustic transducer  32  components. Accordingly, when the headset  20  is worn by a user, the gain adjustment circuit  36  allows the electroacoustic transducer  32  to receive an increased amount of power, compared to conventional headsets. With such an increase in the amount of power, the headset  20  can generate higher cancelling pressures, such as an increase by about 20 dB before the electroacoustic transducer reaches the displacement limit for the voice coil  40  and the magnetic assembly  42 . Additionally, when the headset  20  is removed from the user&#39;s head, a front portion of the electroacoustic transducer  32  becomes unloaded and a relative displacement between the voice coil  40  and the magnetic assembly  42  can increase for a given audio source signal voltage. In such a case, the gain adjustment circuit  36  can be activated to reduce electroacoustic transducer driver voltages, such as at the voltage levels found in conventional voltage-limiting headsets. 
     While various embodiments of the innovation have been particularly shown and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the innovation as defined by the appended claims. 
     For example, the gain adjustment circuit  36  can work in conjunction with other components to adjust the gain of the amplifier stage  38 . With reference to  FIG. 4 , the gain adjustment circuit  36  can be configured to operate based upon receipt of a driving signal  94  from the electroacoustic transducer  32 , in addition to receipt of the displacement signal  54 . As illustrated, the threshold detector  58  is configured with a driving signal threshold  92  that corresponds to an absolute value of voltage of an audio or driving signal associated with a clipping limit of the amplifier stage  38 . During operation, the threshold detector  58  receives the driving signal  94  from the electroacoustic transducer  32  and receives the displacement signal  54  from the displacement sensing circuit  34 . When the threshold detector  58  detects either an absolute value of a driving signal value associated with the driving signal  94  as meeting or exceeding the driving signal threshold  92  or the displacement signal value, such as the absolute value of the displacement signal value  90  of the displacement signal  54  as meeting or exceeding the displacement signal threshold  58 , the threshold detector  58  causes the compressor component  64  to reduce a loop gain of the ANR assembly  35 , such as described above. 
     Utilization of both the driving signal  94  and the displacement signal  54  provides the ANR assembly  35  with a level of operational flexibility. For example, operation of threshold detector  58  based upon the driving signal  94  can be appropriate to protect the electroacoustic transducer  32  from a thermal perspective or if amplifier clipping is impending. 
     Furthermore, as indicated above, the gain adjustment circuit  36  includes a threshold detector  58 , an integrator component  62 , and a compressor component  64 . It should be noted that each component  58 ,  62 ,  64  of the gain adjustment circuit  36  can be configured as individual analog components or as a single discrete analog component. Additionally, the gain adjustment circuit  36  can be configured as a computerized device having a controller, such as a processor and a memory, or a digital signal processor operable to perform the functions of the gain adjustment circuit  36  as described herein. When configured as a computerized device or a digital signal processor, the integrator component  62  and compressor component  64  can be configured as a counter operable to adjust ANR loop gain. In use, when the threshold detector  58  detects the absolute value of the displacement signal  54  as exceeding the threshold  60 , then the counter is decremented at a relatively fast rate (e.g., relatively large steps in value) to rapidly reduce ANR loop gain. When the absolute value of the displacement signal  54  subsequently falls below the threshold  60 , the counter is then incremented at a relatively slow rate (e.g., relatively small steps in value) to slowly restore ANR loop gain. 
     In another example, and as indicated above,  FIG. 1  is a schematic representation of an over ear or on-ear headset  20 . Such representation is by way of example only. In one arrangement, the headset is configured as an in-ear headset where a user places at least a portion of the housings  24 ,  26  within his ear and the friction between the housings  24 ,  26  and the user&#39;s ears maintain the headset  20  on the user&#39;s head. Alternately, the headset can be configured as a circum-aural or as a supra-aural headset. 
     In another example, the gain adjustment circuit  36  is described as modifying the loop gain of the ANR loop between the microphone transducer  50  and the compensator  33 . Such description is by way of example only. The gain adjustment circuit  36  is operable to modify the loop gain anywhere in the ANR loop signal path from the microphone transducer  50  to the amplifier stage  38 . 
     As indicated above, the threshold detector  58  is configured to detect both the positive and negative portions of the displacement signal  54  and compare the positive and negative portions to a displacement signal threshold  60 . During operation, as the threshold detector  58  receives the displacement signal  54 , the threshold detector  58  takes the absolute value of the displacement signal voltages or values that constitute the displacement signal  54  and compares the resulting displacement signal value to the displacement signal threshold  60 . As described, in the case where the absolute value of the displacement signal value meets or exceeds the threshold  60 , the threshold detector  58  enters an operational state and activates the current limiting source  59  to provide the current  66  to the integrator component  62 . Such description is by way of example only. In one arrangement, the displacement signal threshold  60  includes a first displacement signal threshold  60 - 1 , corresponding to a positive threshold, and a second displacement signal threshold  60 - 2 , corresponding to a negative threshold, and the current limited source  59  is configured as a first current limited source  59 - 1  and a second current limited source  59 - 2 . 
     In use, when the threshold detector  58  detects the displacement signal  54  as being above the first displacement signal threshold  60 - 1 , the threshold detector  58  activates the first current limited source  59 - 1  to deliver a positive current  66  to the integrator component  62 . Additionally, when the threshold detector  58  detects the displacement signal  54  as being below the second displacement signal threshold  60 - 2 , the threshold detector  58  activates the second current limited source  59 - 2  to deliver a positive current  66  to the integrator component  62 . In such an arrangement, the threshold detector  58  is configured to activate a current limited source, either the first or second current limited source  59 - 1 ,  59 - 2 , to cause the compressor component to adjust ANR loop gain in response to either the positive and negative portions of the displacement signal  54  meeting or crossing the respective thresholds  60 - 1 ,  60 - 2 . 
     As described above, the threshold detector  58  is configured to detect both the positive and negative portions of the displacement signal  54  and compare the positive and negative portions to a displacement signal threshold  60 . Such description is by way of example only. In one arrangement, the threshold detector  58  is configured to detect either the positive or negative portions of the displacement signal  54  and compare the respective positive and negative portions to a displacement signal threshold  60 .