Patent Publication Number: US-11032631-B2

Title: Headphone off-ear detection

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 62/695,674, filed Jul. 9, 2018, entitled “Headphone Off-Ear Detection,” the disclosure of which is incorporated herein by reference in its entirety. 
     This patent application is related to U.S. Non-provisional patent application Ser. No. 16/174,067, filed Oct. 29, 2018, entitled “Headphone Off-Ear Detection,” which is a continuation of U.S. Non-provisional patent application Ser. No. 15/984,068, filed May 18, 2018, entitled “Headphone Off-Ear Detection,” which is a continuation of U.S. Non-provisional patent application Ser. No. 15/792,394, filed Oct. 24, 2017, entitled “Headphone Off-Ear Detection,” which claims the benefit of U.S. Provisional Patent Application No. 62/412,206 filed Oct. 24, 2016, entitled “Headphone Off Ear Detection,” and which claims the benefit of U.S. Provisional Patent Application No. 62/467,731 filed Mar. 6, 2017, entitled “Off Ear Detection,” the disclosures of all of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     Active noise cancellation (ANC) is a method of reducing an amount of undesired noise received by a user listening to audio through headphones. The noise reduction is typically achieved by playing an anti-noise signal through the headphone&#39;s speakers. The anti-noise signal is an approximation of the negative of the undesired noise signal that would be in the ear cavity in the absence of ANC. The undesired noise signal is then neutralized when combined with the anti-noise signal. 
     In a general noise-cancellation process, one or more microphones monitor ambient noise or residual noise in the ear cups of headphones in real-time, then the speaker plays the anti-noise signal generated from the ambient or residual noise. The anti-noise signal may be generated differently depending on factors such as physical shape and size of the headphone, frequency response of the speaker and microphone transducers, latency of the speaker transducer at various frequencies, sensitivity of the microphones, and placement of the speaker and microphone transducers, for example. 
     In feedforward ANC, the microphone senses ambient noise but does not appreciably sense audio played by the speaker. In other words, the feedforward microphone does not monitor the signal directly from the speaker. In feedback ANC, the microphone is placed in a position to sense the total audio signal present in the ear cavity. So, the microphone senses the sum of both the ambient noise as well as the audio played back by the speaker. A combined feedforward and feedback ANC system uses both feedforward and feedback microphones. 
     Typical ANC headphones are powered systems that require a battery or another power source to operate. A commonly encountered problem with powered headphones is that they continue to drain the battery if the user removed the headphones without turning them off. 
     While some headphones detect whether a user is wearing the headphones, these conventional designs rely on mechanical sensors, such as a contact sensor or magnets, to determine whether the headphones are being worn by the user. Those sensors would not otherwise be part of the headphone. Instead, they are an additional component, perhaps increasing the cost or complexity of the headphone. 
     The disclosed examples address these and other issues. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows an example of an off-ear detector integrated into a headphone, which is depicted on-ear. 
         FIG. 1B  shows an example of an off-ear detector integrated into a headphone, which is depicted off-ear. 
         FIG. 2  illustrates an example network for off-ear detection. 
         FIG. 3  illustrates an example network for combined narrowband and wideband off-ear detection. 
         FIG. 4  illustrates an example network for narrowband off-ear detection. 
         FIG. 5  is an example flow diagram illustrating a method of operations for narrowband off-ear detection (OED) signal processing. 
         FIG. 6  illustrates an example network for wideband off-ear detection. 
         FIG. 7  illustrates an example network for transfer function calibration. 
         FIG. 8  is a graph of example transfer functions. 
         FIG. 9  illustrates an example network for wideband OED metric determination. 
         FIG. 10  is an example flow diagram illustrating a method for distortion detection. 
         FIG. 11  is an example flow diagram illustrating a method of OED. 
         FIG. 12  is an example state diagram illustrating modes of operation of OED detection. 
     
    
    
     DETAILED DESCRIPTION 
     Disclosed herein are devices, systems, and/or methods that employ headphone ANC components to perform OED. For example, a narrowband OED system may be employed. In the narrowband OED system, an OED tone is injected into an audio signal at a specified frequency bin. The OED tone is set at a sub-audible frequency so the end user is unaware of the tone. Due to constraints of the speaker when operating at low frequencies, the tone is present when played into the user&#39;s ear, but largely dissipates when the headphone is removed. Accordingly, a narrowband process can determine that a headphone has been removed when a feedback (FB) microphone signal at the specified frequency bin drops below a threshold. The narrowband process can also be determined as a component of a wideband OED system. In either case, a feedforward (FF) microphone may be employed to capture ambient noise. The OED system may determine a noise floor based on the ambient noise and adjust the OED tone to be louder than the noise floor. When the audio signal includes music, the wideband OED system may also be employed. The wideband OED system operates in the frequency domain. The wideband OED system determines a difference metric over a plurality of frequency bins. The difference metric is determined by removing ambient noise coupled between the FF and FB microphones from the FB microphone signal. The FB microphone signal is then compared to an ideal off-ear value based on an audio signal and a transfer function describing an ideal change to the audio signal when the headphone is off-ear. The resulting value may also be normalized according to on an ideal on-ear value based on an audio signal and a transfer function describing an ideal change to the audio signal when the headphone is on-ear. The frequency bins of the difference metric are then weighted, and the weights are employed to generate a confidence metric. The difference metric and the confidence metric are then employed to determine when the earphone has been removed. The difference metric may be averaged over an OED cycle and compared to a threshold. Successive difference metrics may also be compared, with rapid changes in values indicating a state change (e.g. from on-ear to off-ear and vice versa). A distortion metric may also be employed. The distortion metric supports allowing the OED system to distinguish between energy produced by non-linearities in the system from the energy produced by the desired signal. Phase of the signals may also be employed to avoid potential noise floor calculation errors related to wind noise in the FF microphone that is uncorrelated with the FB microphone. 
     In general, the devices, systems, and/or methods disclosed herein use at least one microphone in an ANC headphone as part of a detection system to acoustically determine if the headphone is positioned on a user&#39;s ear. The detection system does not typically include a separate sensor, such as a mechanical sensor, although in some examples a separate sensor could also be used. If the detection system determines that the headphones are not being worn, steps may be taken to reduce power consumption or implement other convenience features, such as sending a signal to turn off the ANC feature, turn off parts of the headphone, turn off the entire headphone, or pause or stop a connected media player. If the detection system instead determines that the headphones are being worn, such a convenience feature might include sending a signal to start or restart the media player. Other features may also be controlled by the sensed information. 
     The terms “being worn” and “on-ear” as used in this disclosure mean that the headphone is in or near its customary in-use position near the user&#39;s ear or eardrum. Thus, for pad- or cup-style headphones, “on-ear” means that the pad or cup is completely, substantially, or at least partially over the user&#39;s ear. An example of this is shown in  FIG. 1A . For earbud-type headphones and in-ear monitors, “on-ear” means that the earbud is at least partially, substantially, or fully inserted into the user&#39;s ear. Accordingly, the term “off-ear” as used in this disclosure means that the headphone is not in or near its customary in-use position. An example of this is shown in  FIG. 1B , in which the headphones are being worn around the user&#39;s neck. 
     The disclosed apparatus and method are suitable for headphones that are used in just one ear or in both ears. Additionally, the OED apparatus and method may be used for in-ear monitors and earbuds. Indeed, the term “headphone” as used in this disclosure includes earbuds, in-ear monitors, and pad- or cup-style headphones, including those whose pads or cups encompass the user&#39;s ear and those whose pads press against the ear. 
     In general, when the headphones are off-ear, there is not a good acoustic seal between the headphone body and the user&#39;s head or ear. Consequently, the acoustic pressure in the chamber between the ear or eardrum and the headphone speaker is less than the acoustic pressure that exists when the headphone is being worn. In other words, the audio response from an ANC headphone is relatively weak at low frequencies unless the headphone is being worn. Indeed, the difference in audio response between the on-ear and the off-ear conditions can be more than 20 dB at very low frequencies. 
     Additionally, the passive attenuation of ambient noise when the headphone is on-ear, due to the body and physical enclosure of the headphone, is significant at high frequencies, such as those above 1 kHz. But at low frequencies, such as those less than 100 Hz, the passive attenuation may be very low or even negligible. In some headphones, the body and physical enclosure actually amplifies the low ambient noise instead of attenuating it. Also, in the absence of an activated ANC feature, the ambient noise waveform at the FF and FB microphones are: (a) deeply correlated at very low frequencies, which are generally those frequencies below 100 Hz; (b) completely uncorrelated at high frequencies, which are generally those frequencies above 3 kHz; and (c) somewhere in the middle between the very low and the high frequencies. These acoustic features provide bases for determining whether or not a headphone is on-ear. 
       FIG. 1A  shows an example of an off-ear detector  100  integrated into a headphone  102 , which is depicted on-ear. The headphone  102  in  FIG. 1A  is depicted as being worn, or on-ear.  FIG. 1B  shows the off-ear detector  100  of  FIG. 1A , except the headphone  102  is depicted as being off-ear. The off-ear detector  100  may be present in the left ear, the right ear, or both ears. 
       FIG. 2  illustrates an example network  200  for off-ear detection, which may be an example of the off-ear detector  100  of  FIGS. 1A and 1B . An example, such as shown in  FIG. 2 , may include a headphone  202 , an ANC processor  204 , an OED processor  206 , and a tone source, which may be a tone generator  208 . The headphone  202  may further include a speaker  210 , a FF microphone  212 , and a FB microphone  214 . 
     Although likely present for the ANC features of an ANC headphone, the ANC processor  204  and the FF microphone  212  are not absolutely required in some examples of the off-ear detection network  200 . The tone generator  208  is also optional, as discussed below. Examples of the off-ear detection network  200  may be implemented as one or more components integrated into the headphone  202 , one or more components connected to the headphone  202 , or software operating in conjunction with an existing component or components. For example, software driving the ANC processor  204  might be modified to implement examples of the off-ear detection network  200 . 
     The ANC processor  204  receives a headphone audio signal  216  and sends an ANC-compensated audio signal  216  to the headphone  202 . The FF microphone  212  generates a FF microphone signal  220 , which is received by the ANC processor  204  and the OED processor  206 . The FB microphone  214  likewise generates a FB microphone signal  222 , which is received by the ANC processor  204  and the OED processor  206 . Depending on the example, the OED processor  206  may receive the headphone audio signal  216  and/or the compensated audio signal  216 . Preferably, the OED tone generator  208  generates a tone signal  224  that is injected into the headphone audio signal  216  before the headphone audio signal  216  is received by the OED processor  206  and the ANC processor  204 . In some examples, though, the tone signal  224  is injected into the headphone audio signal  216  after the headphone audio signal  216  is received by the OED processor  206  and the ANC processor  204 . The OED processor  206  outputs a decision signal  226  indicating whether or not the headphone  202  is being worn. 
     The headphone audio signal  216  is a signal characteristic of the desired audio to be played through the headphone&#39;s speaker  210  as an audio playback signal. Typically, the headphone audio signal  216  is generated by an audio source such as a media player, a computer, a radio, a mobile phone, a CD player, or a game console during audio play. For example, if a user has the headphone  202  connected to a portable media player playing a song selected by the user, then the headphone audio signal  216  is characteristic of the song being played. The audio playback signal is sometimes referred to in this disclosure as an acoustic signal. 
     Typically, the FF microphone  212  samples an ambient noise level and the FB microphone  214  samples the output of the speaker  210 , that is, the acoustic signal, and at least a portion of the ambient noise at the speaker  210 . The sampled portion includes a portion of ambient noise that is not attenuated by the body and physical enclosure of the headphone  202 . In general, these microphone samples are fed back to the ANC processor  204 , which produces anti-noise signals from the microphone samples and combines them with the headphone audio signal  216  to provide the ANC-compensated audio signal  216  to the headphone  202 . The ANC-compensated audio signal  216 , in turn, allows the speaker  210  to produce a noise-reduced audio output. 
     The tone source or tone generator  208 , introduces or generates the tone signal  224  that is injected into the headphone audio signal  216 . In some versions, the tone generator  208  generates the tone signal  224 . In other versions, the tone source includes a storage location, such as flash memory, that is configured to introduce the tone signal  224  from stored tones or stored tone information. Once the tone signal  224  is injected, the headphone audio signal  216  becomes a combination of the headphone audio signal  216  before the tone signal  224 , plus the tone signal  224 . Thus, processing of the headphone audio signal  216  after injection of the tone signal  224  includes both. Preferably, the resulting tone has a sub-audible frequency so a user is unable to hear the tone when listening to the audio signal. The frequency of the tone should also be high enough that the speaker  210  can reliably produce, and the FB microphone  214  can reliably record, the tone, as many speakers/microphones have limited capabilities at lower frequencies. For example, the tone may have a frequency of between about 15 Hz and about 30 Hz. As another example, the tone may be a 20 Hz tone. In some implementations, a higher or lower frequency tone could be used. Regardless of the frequency, the tone signal  224  may be recorded by the FB microphone  214  and forwarded to the OED processor  206 . The OED processor  206  may, in some cases, detect when the earphone has been removed by the relative strength of the tone signal  224  recorded by the FB microphone  214 . 
     In some examples, the OED processor  206  is configured to adjust the level of the tone signal  224 . Specifically, the accuracy of the OED processor&#39;s  206  ability to perform OED can be negatively impacted when noise levels become significant compared to (e.g. exceeds) the volume of the tone signal. The level of noise experienced by the network  200  is referred to herein as the noise floor. The noise floor may be affected by both the electronic noise and ambient noise. The electronic noise may occur in the speaker  210 , the FF microphone  212 , the FB microphone  214 , signal paths between such components, and signal paths between such components and the OED processor  206 . The ambient noise is the sum of environmental acoustic waves in the vicinity of the user during network  200  operation. The OED processor  206  may be configured to measure the combined noise floor, for example based on the FB microphone signal  222  and the FF microphone signal  220 . The OED processor  206  may then employ a tone control signal  218  to adjust the volume of the tone signal  224  generated by the tone generator  208 . The OED processor  206  may adjust the tone signal  224  to be sufficiently strong compared to (e.g. louder than) the noise floor. For example the OED processor  206  may maintain a margin between the volume of the noise floor and the volume of the tone signal  224 . It should be noted that sudden rapid volume changes in the tone signal  224  may be perceived by some users despite the low frequency of the tone signal  224 . Accordingly, a smoothing function may be employed by the OED processor  206  when changing the volume of the tone signal  224  to gradually change the volume (e.g. over the course of ten milliseconds to five hundred milliseconds). For example, the OED processor may adjust the volume of the tone signal  224 , by employing the tone control signal  218 , according to the following equation: 
                     nextLevel   =     currentLevel   ×     L   0     ⁢       NoiseFloorPowerEstimate   CurrentSignalPower           ,           Equation   ⁢           ⁢   1               
where currentLevel is the current tone signal  224  volume, L 0  is the volume margin between the noise floor and the tone signal  224 , nextLevel is the adjusted tone signal  224  volume, CurrentSignalPower is the current received tone signal  224  power, and NoiseFloorPowerEstimate is an estimate of the total received noise floor including acoustic and electrical noise.
 
     Some examples do not include the tone generator  208  or the tone signal  224 . For example, if there is music playing, especially music with non-negligible bass, there may be sufficient ambient noise for the OED processor  206  to reliably determine whether the headphone  202  is on-ear or off-ear. In some examples, the tone or the tone signal  224  may not, if played by the speaker  210 , result in an actual tone. Rather, the tone or the tone signal  224  may instead correspond to or result in a random noise or a pseudo-random noise, each of which may be bandlimited. 
     As noted above, in some versions of the off-ear detection network  200  it is not necessary to include or operate the speaker  210  and the FF microphone  212 . For example, some examples include the FB microphone  214  and the tone generator  208  without the FF microphone  212 . As another example, some examples include both the FB microphone  214  and the FF microphone  212 . Some of those examples include the tone generator  208 , and some do not. Examples not including the tone generator  208  also may or may not include the speaker  210 . Additionally, note that some examples do not require a measurable headphone audio signal  216 . For example, examples that include the tone signal  224  may effectively determine whether or not the headphone  202  is being worn, even in the absence of a measurable headphone audio signal  216  from an audio source. In such cases, the tone signal  224 , once combined with the headphone audio signal  216 , is essentially the entire headphone audio signal  216 . 
     The OED processor  206  may perform OED in a relatively narrow frequency band, also known as a frequency bin, by injecting the tone signal  224  into the audio signal  216  and measuring the FF microphone signal  220  and FB microphone signal  222  for remnants of the tone signal  224  as modified by the noise floor and known acoustic changes between the speaker  210  and the microphones  212  and  214 , which may be described as a transfer function. When audio data (e.g. music) is included in the audio signal  216  and played by the speaker  210 , an OED processor may also perform a wideband OED process to detect OED based on changes to the audio signal  216  before being recorded by the microphones  212  and  214 . Various examples of such wideband and narrowband OED processes are discussed more fully below. 
     It should be noted that the OED processor  206  may perform OED by computing a frame OED metric, as discussed below. In one example, the OED processor determines a state change (e.g. on-ear to off-ear or vice versa) when the frame OED metric rises above and/or drops below an OED threshold. A confidence value may also be employed so that OED metrics with low confidence are rejected from consideration when performing OED. In another example, the OED processor  206  may also consider a rate of change in the OED metrics. For example, if an OED metric changes faster than a state change margin, the OED processor  206  may determine a state change even when the threshold has not been reached. In effect, the rate of change determination allows for higher effective thresholds and faster determination of state changes when the headphones are well fitted/engaged. 
     It should also be noted that the OED processor  206  may be implemented in various technologies, such as by a general purpose processor, an application specific integrated circuit (ASIC), a digital signal processor (DSP), a field programmable gate array (FPGA), or other processing technologies. For example, the OED processor  206  may include decimators and/or interpolators to modify the sampling rates of corresponding signals. The OED processor  206  may also include analog to digital converters (ADCs) and/or digital to analog converters (DACs) to interact with and/or process corresponding signals. The OED processor  206  may employ various programmable filters, such as bi-quad filters, bandpass filters, etc. to process the relevant signals. The OED processor  206  may also include memory modules, such as a registers, cache, etc., which allow the OED processor  206  to be programmed with relevant functionality. It should be noted that  FIG. 2  includes only the components relevant to the present disclosure for purposes of clarity. Hence, a fully operational system may include additional components, as desired, which are beyond the scope of the particular functionality discussed herein. 
     In summary, network  200  acts as a signal processor for headphone off-ear detection. The network  200  includes an audio output to transmit an audio signal  216  toward a headphone speaker  210  in a headphone cup. The network  200  also employs a FB microphone input to receive a FB signal  222  from a FB microphone  214  in the headphone cup. The network  200  also employs OED processor  206  as an OED signal processor. As discussed in greater detail below, when operating in the frequency domain, the OED processor  206  is configured to determine an audio frequency response of the FB signal  222  over an OED frame as a received frequency response. The OED processor  206  also determines an audio frequency response of the audio signal  216  times an off-ear transfer function between the headphone speaker  210  and the FB microphone  214  as an ideal off-ear response. The OED processor  206  then generates a difference metric (e.g. frame OED metric  620 ) comparing the received frequency response to the ideal off-ear frequency response. Finally, the OED processor  206  employ the difference metric to detect when the headphone cup is disengaged from an ear as shown in  FIG. 1B . Further, the OED processor  206  employs a FF microphone input to receive a FF signal  222  from a FF microphone  212  outside of the headphone cup. The OED processor  206  may remove a correlated frequency response between the FF signal  220  and the FB signal  222  when determining the received frequency response. The OED processor  206  may also determine an audio frequency response of the audio signal  216  times an on-ear transfer function between the headphone speaker  2120  and the FB microphone  214  as an ideal on-ear response. The OED processor  206  may then normalize the difference metric based on the ideal on-ear response. The difference metric may be determined according to equations 2-5 as discussed below. Further, the difference metric may include a plurality of frequency bins, and the OED processor  206  may weight the frequency bins. The OED processor  206  may then determine a difference metric confidence (e.g. confidence  622 ) as a sum of frequency bin weights. The OED processor  206  may employ the difference metric confidence when detecting the headphone cup is disengaged from the ear. In an example, the OED processor  206  may determine the headphone cup is engaged when a difference metric confidence is above a difference metric confidence threshold and the difference metric is above a difference metric threshold. In another example, the OED processor  206  may average difference metrics over an OED cycle, and determine the headphone cup is disengaged when the average difference metric is above a difference metric threshold. In another example, a plurality of difference metrics may be generated over an OED cycle, and the OED signal processor  206  may determine the headphone cup is disengaged when a change between difference metrics is greater than a difference metric change threshold. 
     The network  200  may also include the tone generator  208  to generate the OED tone  224  at a specified frequency bin to support generation of the difference metric when the audio signal drops below a noise floor. Further, the OED processor  206  controls the tone generator  208  to maintain a volume of the OED tone  224  above the noise floor. It should also be noted that the headphones may include two earphone, and hence a pair of FF microphones  212 , speakers  210 , and FB microphones  214  (e.g. left and right). As discussed in more detail below, wind noise may negatively impact the OED process. Accordingly, the OED processor  206  may select a weaker of the FF signals to determine the noise floor when wind noise is detected in a stronger of the FF signals. 
       FIG. 3  illustrates an example network  300  for combined narrowband and wideband off-ear detection. Network  300  may be implemented by circuitry in an OED processor  206 . Network  300  may include a decimator  302 , which may be connected to, but implemented outside of, the OED processor. The OED processor may also include a narrowband OED circuit  310 , a wideband OED circuit  304 , a combination circuit  306 , and a smoothing circuit  308 . 
     The decimator  302  is an optional component that reduces the sampling rate of the audio signal  216 , the FB microphone signal  222 , and the FF microphone signal  220 , referred to collectively as the input signals. Depending on implementation, the input signals may be captured at a higher sampling rate than is supported by the OED processor. Hence, the decimator  302  reduces the sampling rate of the input signals to match the rate supported by the other circuitry. 
     The narrowband OED circuit  310  performs OED on acoustic changes in the frequency bin associated with the OED tone signal  224 . The wideband OED circuit  304  focuses on a set of frequency bins associated with general audio output at the speaker  210 , such as music. As discussed in more detail with respect to  FIG. 8  below, a white noise on-ear transfer function and a white noise off-ear transfer function may be strongly correlated at some frequencies and loosely correlated at other frequencies. Accordingly, the wideband OED circuit  304  is configured to perform OED by focusing on acoustic changes, due to general audio output, in portions of the spectrum where an ideal off-ear transfer function is different from an ideal on-ear transfer function. The transfer functions are specific to the headphone design, and hence the wideband OED circuit  304  may be tuned to focus on different frequency bands for different example implementations. The primary difference is that the narrowband OED circuit  310  operates based on a sub-audible tone, and hence can operate at any time. In contrast, the wideband OED circuit  304  operates on audible frequencies, and hence only operates when the headphones are playing audio content. However, by performing OED across a wider frequency range, the wideband OED circuit  304  may increase the accuracy of the OED process over employing only the narrowband OED circuit  310 . The narrowband OED circuit  310  can be implemented to operate in either time domain or frequency domain. Implementations of both domains are discussed below. The wideband OED circuit  304  is more practical to implement in the frequency domain. As such, in some examples the narrowband OED circuit  310  is implemented as a sub-component of the wideband OED circuit  304  that operates at a particular frequency bin. The narrowband OED circuit  310  and the wideband OED circuit  304  both operate on the input signals (e.g. the decimated audio signal  216 , FB microphone signal  222 , and FF microphone signal  220 ) to perform OED as discussed below. 
     The combination circuit  306  is any circuitry and/or process capable of combining the output of the narrowband OED circuit  310  and the wideband OED circuit  304  into usable decision data. Such outputs may be combined in a variety of ways. For example, the combination circuit  306  may select the output with the lowest OED decision value, which would bias the OED determination toward an off-ear decision. The combination circuit  306  may also select the output with the highest OED decision value, which would bias the OED determination toward an on-ear decision. In yet another approach, the combination circuit  306  employs a confidence value supplied by the wideband OED circuit  304 . When the confidence is above a confidence threshold, the wideband OED circuit  304  OED determination is employed. When the confidence is below the confidence threshold, including when audio output is low volume or non-existent, the narrowband OED circuit  310  OED determination is employed. Further, in the example where the narrowband OED circuit  310  is implemented as a sub-component of the wideband OED circuit  304 , a weighting process may be employed to by and/or in lieu of the combination circuit  306 . 
     The smoothing circuit  308  is any circuit or process that filters the OED decision values to mitigate sudden changes that could result in thrashing. For example, the smoothing circuit  308  may lower or raise individual OED metrics to that the stream of OED metrics are consistent over time. This approach removes erroneous outlier data so that a decision is reached based on multiple OED metrics. The smoothing circuit  308  may employ a forgetting filter, such as a first order infinite impulse response (IIR) low pass filter. 
     It should be noted that both the wideband OED circuit  304  and the narrowband OED circuit  310  are capable of mitigating negative effects associated with wind noise. Specifically, the network  300  may allow an OED signal processor, such as OED processor  206 , to determine an expected phase of the FB signal  222  based on a phase of the audio signal  216 . A corresponding confidence metric (e.g. confidence  622 ) may then be reduced when a difference in phase of a received frequency response associated with the FB signal  222  and the expected phase of the received frequency response associated with the FB signal  222  is greater than a phase margin. 
       FIG. 4  illustrates an example network  400  for narrowband off-ear detection. Specifically, network  400  may implement time domain OED in a narrowband OED circuit  310 . In network  400 , the audio signal  216 , the FB microphone signal  222 , and the FF microphone signal  220  are passed through a bandpass filter  402 . The bandpass filter  402  is tuned to remove all signal data outside of a predetermined frequency range. For example, the network  400  may review the input signals for an OED tone  224  at a specified frequency bin, and hence the bandpass filter  402  may remove all data outside of the specified frequency bin. 
     The transfer function  404  is a valued stored in memory. The transfer function  404  may be determined at time of manufacture based on a calibration process. The transfer function  404  describes an amount of acoustic coupling between the FF microphone signal  220  and the FB microphone signal  222  in an ideal case when the earphone is not engaged to a user&#39;s ear. For example, the transfer function  404  may be determined in the presence of white noise at the audio signal  216 . During OED, the transfer function  404  is multiplied by the FF microphone signal  220  and then subtracted from the FB microphone signal  222 . This serves the subtract the expected acoustic coupling between the FF microphone signal  220  and the FB microphone signal  222  from the FB microphone signal  222 . This process removes the ambient noise recorded by the FF microphone from the FB microphone signal  222 . 
     The variance circuits  406  are provided to measure/determine the level of energy in the audio signal  216 , FF microphone signal  220 , and FB microphone signal  222  at the specified frequency bin. Amplifiers  410  are also employed to modify/weight the gain of the FF microphone signal  220  and the audio microphone signal  216  for accurate comparison with the FB microphone signal  222 . At comparison circuit  408  the FB microphone signal  222  is compared to the combined audio signal  216  and FF microphone signal  220 . When the FB microphone signal  222  is greater than the combined audio signal  216  and FF microphone signal (as weighted) by a value in excess of a predetermined narrowband OED threshold, an OED flag is set to on-ear. When the FB microphone signal  222  is not greater than the combined audio signal  216  and FF microphone signal by a value in excess of the predetermined narrowband OED threshold, the OED flag is set to off-ear. In other words, when the FB microphone signal  222  contains only attenuated audio signals  216  and noise  220 , and does not contain additional energy associated with the acoustic of a user&#39;s ear as described by the narrowband OED threshold, the earphone is considered to be off-ear/disengaged by the time domain narrowband process described by network  400 . 
     It should be noted that network  400  can also be modified to adapt to certain use cases. For example, wind noise may result in uncorrelated noise between the FB microphone signal  222  and the FF microphone signal  220 . Accordingly, in the case of wind noise, removal of the transfer function  404  may result erroneously removing the wind noise from the FB microphone signal  222  as coupled data, which results in fault data. As such, the network  400  may also be modified to review the phase of the FB microphone signal  222  at the comparison circuit  408 . In the event the phase of the FB microphone signal  222  is outside an expected margin, the OED flag may not be changed to avoid false results related to wind noise. It should also be noted that such modifications for wind noise are equally applicable to the wideband network (e.g. wideband OED circuit  304 ) discussed above. 
       FIG. 5  is an example flow diagram illustrating a method  500  of operations for narrowband off-ear detection (OED) signal processing, for example, by the OED processor  206 , the narrowband OED circuit  310 , and/or network  400 . At operation  502 , a tone generator injects a tone signal, and the OED processor receives the FF microphone signal and the FB microphone signal. The tone generator may raise and/or lower the tone signal to make any transient effects inaudible to the listener while maintaining a volume above a noise floor. The headphone audio signal, the FF microphone signal, and the FB microphone signal may be available in bursts, with each burst containing one or more samples of the signals. As noted above, the tone signal and the FF microphone signal are optional, so some examples of the method  500  may not include injecting the tone signal or receiving the FF microphone signal  220 . 
     The time domain ambient noise waveform correlation between the FF microphone signal and FB microphone signal is better for narrowband signals than wideband signals. This is an effect of non-linear phase response of the headphone enclosure. Thus, at operation  504 , a bandpass filter may be applied to the headphone audio signal, the FF microphone signal, and the FB microphone signal. The bandpass filter may include a center frequency of less than about 100 Hz. For example, the bandpass filter may be a 20 Hz bandpass filter. Thus, the lower cutoff frequency for the bandpass filter could be around 15 Hz, and the upper cutoff frequency for the bandpass filter could be around 30 Hz, resulting in a center frequency of about 23 Hz. The bandpass filter may be a digital bandpass filter and may be part of an OED processor. For example, the digital bandpass filter could be four biquadratic filters: two each for the low-pass and the high-pass sections. In some examples, a low-pass filter may be used instead of a bandpass filter. For example, the low-pass filter may attenuate frequencies greater than about 100 Hz or greater than about 30 Hz. Regardless of which filter is used, the filter state is maintained for each signal stream from one burst to the next. 
     At operation  506 , the OED processor updates, for each sample, data related to the sampled data. For example, the data may include cumulative sum and cumulative sum-squares metrics for each of the headphone audio signal, the FF microphone signal, and the FB microphone signal  2 . The sum-squares are the sums of the squares. 
     At operation  508 , operation  504  and operation  506  are repeated until the OED processor processes a preset duration of samples. For example, the preset duration could be one second&#39;s worth of samples. Another duration could also be used. 
     At operation  510 , the OED processor determines a characteristic, such as the power or energy of one or more of the headphone audio signal, the FF microphone signal, and the FB microphone signal, from the metrics computed in the previous operations. 
     At operation  512 , the OED processor computes relevant thresholds. The thresholds may be computed as a function of the audio signal power and the FF microphone signal power. For example, the volume of music in the audio signal and/or the ambient noise recorded in the FF microphone signal may vary significantly over time. Accordingly, the corresponding thresholds and/or margins may be updated based on predefined OED parameters, as desired, to handle such scenarios. At operation  514 , an OED metric is derived based on the threshold(s) determined in operation  512  and the signal power determined at operation  514 . 
     At operation  516 , the OED processor assesses whether the headphone is on-ear or off-ear. For example, the OED processor may compare the power or energy of one or more of the headphone audio signal, the FF microphone signal, and the FB microphone signal to one or more thresholds or parameters. The thresholds or parameters may correspond to one or more of the headphone audio signal, the FF microphone signal, or the FB microphone signal, or the power or energy of those signals, under one or more known conditions. The known conditions may include, for example, when the headphone is already known to be on-ear or off-ear or when the OED tone is playing or not playing. Once the signal values, energy values, and power values are known for the known conditions, those known values may be compared to determined values from an unknown condition to assess whether or not the headphone is off-ear. 
     The operation  516  may also include the OED processor averaging multiple metrics over time and/or outputting a decision signal, such as OED decision signal  226 . The OED decision signal  226  may be based at least in part on whether the headphone is assessed to be off-ear or on-ear. The operation  516  may also include forwarding the outputting the decision signal to a combination circuit  306  for comparison with wideband OED circuit  304  decisions in some examples. 
       FIG. 6  illustrates an example network  600  for wideband off-ear detection. The network  600  may be employed to implement a wideband OED circuit  304  in an OED processor  206 . Network  600  is configured to operate in the frequency domain. Further, network  600  performs both narrowband OED and wideband OED, and hence may also implement narrowband OED circuit  310 . 
     The network  600  includes an initial calibration  602  circuit, which is a circuit or process that performs a calibration at the time of manufacture. Activating the initial calibration  602  may include testing the headphones under various conditions, for example on-ear and off-ear conditions in the presence of a white noise audio signal. The initial calibration  602  determines and stores various transfer functions  604  under known conditions. For example, the transfer functions  604  may include a transfer function between the audio signal  216  and the FB microphone signal  222  when off-ear (T HP   Off ), a transfer function between the audio signal  216  and the FB microphone signal  222  when on-ear (T HP   On ), a transfer function between the FF microphone signal  220  and the FB microphone signal  222  when off-ear (T FF   Off ), and a transfer function between the FF microphone signal  220  and the FB microphone signal  222  when on-ear (T FF   On ). The transfer functions  604  are then used at runtime to perform frequency domain OED by an OED circuit  606 . 
     The OED circuit  606  is a circuit that performs the OED process in the frequency domain. Specifically, the OED circuit  606  produces an OED metric  620 . The OED metric  620  is a normalized weighted value that describes the difference between a measured acoustic response and an ideal off-ear acoustic response over a plurality of frequency bins. The measured acoustic response is determined based on the audio signal  216 , the FB microphone signal  222 , and the FF microphone signal  220 , as discussed in more detail below. The OED metric  620  is normalized by a value that describes the difference between the measured acoustic response and an ideal on-ear acoustic response over the frequency bins. The weights applied to the OED metric  620  can then be aggregated to generate a confidence value  622 . The confidence value  622  can then be employed to determine to what extent the OED metric  620  should be relied upon by the OED processor. The frequency domain OED process is discussed in greater detail with respect to  FIG. 9  below. 
     A time averaging circuit  610  may then be employed to average multiple OED metrics  620  over a specified period, for example based on a forgetting filter, such as a first order infinite impulse response (IIR) low pass filter. The average may be weighted according to the corresponding confidence values  622 . In other words, the time averaging circuit  610  is designed to consider the difference in confidence  622  in various frame OED metrics  620  over time. The frame OED metrics  620  associated with greater confidence  622  are emphasized/trusted in the average while frame OED metrics  620  associated with lower confidence  622  are de-emphasized and/or forgotten. The time averaging circuit  610  may be employed to implement a smoothing filter  308  to mitigate thrashing in the OED decision process. 
     The network  600  may also include an adaptive OED tone level control circuit  608 , which is any circuit or process capable of generating a tone control signal  218  to control a tone generator  208  when generating a tone signal  224 . The adaptive OED tone level control circuit  608  determines an ambient noise floor based on the FF microphone signal  220  and generates the tone control signal  218  to adjust tone signal  224  accordingly. The adaptive OED tone level control circuit  608  may determine an appropriate tone signal  224  volume to maintain the tone signal  224  near to and/or or above the volume of the noise floor, for example according to equation 1 above. The adaptive OED tone level control circuit  608  may also apply a smoothing function, as discussed above, to mitigate sudden changes in tone signal  224  volume that might be perceived by some users. 
       FIG. 7  illustrates an example network  700  for transfer function  604  calibration. The network  700  may be employed at the time of manufacture, and the determined transfer functions  604  may be stored in memory for use at run time in network  600 . A sample of white noise  702  may be applied to a stimulus emphasis filter  704 . White noise  702  is a random/pseudorandom signal that contains roughly equal energy/intensity (e.g. constant power spectral density) across a relevant frequency band. For example, the white noise  702  may contain approximately equal energy across an audible and sub-audible frequency range employed by the headphones. Due to physical constraints related to design of the headphones, the microphones  212  and  214  may receive different levels of energy at different frequency. Accordingly, the stimulus emphasis filter  704  is one or more filters that modify the white noise  702  when played from the speaker  210  so that energy received by the relevant microphones  212  and  214  is approximately constant at each frequency bin. The network  700  then employs a transfer function determination circuit  706  to determine the transfer functions  604 . Specifically, the transfer function determination circuit  706  determines the change in signal strength between the speaker  210  and the FF microphone  212  and the change in signal strength between the speaker  210  and the FB microphone  214  in both an ideal off-ear configuration and an acoustically sealed ideal on-ear configuration. In other words, the transfer function determination circuit  706  determines and saves T HP   Off , T HP   On , T FF   Off , and T FF   On  as the transfer function  604  for use in network  600  at run time. 
       FIG. 8  is a graph  800  of example transfer functions, for example between a speaker  210  and a FB microphone  214  in a headphone. Graph  800  illustrates an example on-ear transfer function  804  and off-ear transfer function  802 . The transfer functions  802  and  804  are depicted in terms of magnitude in decibels (dBs) versus frequency in hertz (Hz) on an exponential scale. In this example, the transfer functions  802  and  804  are highly correlated above about 500 Hz. However, the transfer functions  802  and  804  are different between about 5 Hz and about 500 Hz. As such, the wideband OED circuit, such as wideband OED circuit  304  may operate on a band from about 5 Hz to about 500 Hz for headphones with transfer functions depicted by graph  800 . 
     For purposes of discussion, an OED line  806  has been depicted half way between the transfer functions  802  and  804 . Graphically, when a measured signal is graphed between the transfer functions  802  and  804 , OED is determined relative to the OED line  806 . Each frequency bin can be compared to the OED line  806 . When a measured signal has a magnitude below the OED line  806  for a particular frequency bin, that frequency is considered off-ear. When a measured signal has a magnitude above the OED line  806  for a particular frequency bin, that frequency is considered on-ear. The distance above or below the OED line  806  informs the confidence in such a decision. Hence, the distance between the measured signal at a frequency bin and the OED line  806  is employed to generate a weight for that frequency bin. As such, decisions near the OED line  806  are given little weight and decisions near the on-ear transfer function  804  or off-ear transfer function  802  are given significant weight. As the distance between the transfer functions  802  and  804  vary at different frequencies, the OED metric is normalized, for example so small fluctuations where the transfer function difference is small are given as much consideration as larger fluctuations at frequencies where the transfer function difference is larger. An example equation for determining the weighted and normalized OED metric is discussed below. 
       FIG. 9  illustrates an example network  900  for wideband OED metric determination. For example, network  900  may be employed to implement OED circuit  206 , wideband OED circuit  304 , narrowband OED circuit  310 , combination circuit  306 , smoothing circuit  308 , OED circuit  606 , and/or combinations thereof. The network  900  includes a Fast Fourier Transform (FFT) circuit  902 . The FFT circuit  902  is any circuit or process capable of converting input signal(s) into the frequency domain for further computation. The FFT circuit  902  converts the audio signal  216 , the FB microphone signal  222 , and the FF microphone signal  224  into the frequency domain. For example, the FFT circuit  902  may apply a five hundred twelve point FFT to the input signals with windowing. The FFT circuit  902  forwards the converted input signals to a determine audio value circuit  904 . 
     The determine audio value circuit  904  receives the transfer functions  604  and the input signals and determines the uncorrelated frequency of the audio signal  216  received in the FB microphone signal  222 . Such value may be determined according to equation 2:
 
Received=FB−FF( T   FF   Off ),  Equation 2
 
where received is the uncorrelated frequency response of the audio signal at the FB microphone, FB is the frequency response of the FB microphone, FF is the frequency response of the FF microphone, and T FF   Off  is the transfer function between the audio signal and the FF microphone signal  222  when off-ear. In other words, received includes the audio signal as received at the FB microphone without noise components recorded by the FF microphone. The determine audio value circuit  904  also determines the ideal off-ear and ideal on-ear frequency responses that would be expected at the FB microphone based on the audio signal, which can be determined according to equations 3-4, respectively:
 
Ideal_off_ear=HP( T   HP   Off ),
 
Ideal_on_ear=HP( T   HP   On ),  Equations 3-4
 
where Ideal_off_ear is an ideal off-ear frequency response at the FB microphone based on the audio signal, HP is the frequency response of the audio signal, T HP   Off  is the ideal transfer function between the audio speaker and the FB microphone when off-ear, Ideal_on_ear is an ideal on-ear frequency response at the FB microphone based on the audio signal, and T HP   On  is the ideal correlation between the audio speaker and the FB microphone when on-ear.
 
     The determine audio value circuit  904  may forward these values to an optional transient removal circuit  908  (or directly to a smoothing circuit  910  in some examples). The transient removal circuit  908  is any circuit or process capable of removing transient timing mismatches at the leading and trailing edges of the frequency response window. The transient removal circuit  908  may remove such transients by windowing in some examples. In other examples, the transient removal circuit  908  may remove transients by computing an inverse FFT (IFFT), applying the IFFT to the values to convert them to the time domain, zero a portion of the values equal to an expected transient length, and applying another FFT to return the values to the frequency domain. The determine audio value circuit  904  then forwards the values to a smoothing circuit  910 , which may smooth the values with a forgetting filter as discussed above with respect to smoothing circuit  306 . 
     A normalized difference metric circuit  910  then computes a frame OED metric  620 . Specifically, the normalized difference metric circuit  910  compares the estimated off-ear frequency response and actual received response to quantify how different they are. The results is then normalized based on the estimated on-ear response. In other words, the frame OED metric  620  includes a measure of deviation of the received signal from the ideal off-ear signal, which may also be normalized by the deviation of the ideal on-ear signal from the ideal off-ear signal at the frequency bin. For example, the frame OED metric  620  may be determined according to equation 5 below: 
                       normalized_difference   ⁢   _metric     =       log   ⁢           ⁢          Received               Idea_off   ⁢   _ear                log   ⁢            Ideal_on   ⁢   _ear                 Ideal_off   ⁢   _ear                  ,           Equation   ⁢           ⁢   5               
where normalized_difference_metric is the frame OED metric  620  and the other values are as discussed in equations 3-4.
 
     The frame OED metric  620  is then forwarded to a weighting circuit  914 . The weighting circuit  914  is any circuit or process capable of weighting frequency bins in the frame OED metric  620 . The weighting circuit  914  may weight the frequency bins in the frame OED metric  620  based on multiple rules selected to emphasize accurate values and deemphasize suspect values. The following are example rules that may be used to weight a frame OED metric  620 . First, selected frequency bins may be weighted to zero in order to remove extraneous information. For example, the frequency bin for the tone and a relevant audio band of frequency bins (e.g. 20 Hz and 100 Hz-500 Hz) may be given a weight of one and other bins weighted to zero. Second, bins with a signal below the noise floor may also be weighted to zero to mitigate the influence of noise on the determination. Third, frequency bins may be compared to each other, such that bins containing power that is negligible compared to the most powerful bin (e.g. below a power difference threshold) may be weighted down. This de-emphasizes the frequency bins that are least likely to have useful information. Fourth, bins with the highest difference between the ideal on-ear/off-ear values and the measured value are weighted up. This emphasizes the frequency bins that are most likely to be determinative. Fifth, bins with an insignificant difference (e.g. below a power difference threshold) between the ideal on-ear/off-ear values and the measured value are weighted down. This de-emphasizes frequency bins near the OED line  806  as discussed above, because such bins are more likely to give false results due to random measurement variance. Six, bins that act as local maxima (e.g. greater than both neighbors) are weighted up to one, as such bins are most likely to be determinative. A sum of the weights may then be determined by a sum circuit  916  to determine a Frame OED confidence  622  value. In other words, a significant number of high weights indicates the Frame OED metric  620  is likely accurate, while no high weights indicates the Frame OED metric  620  is likely in-accurate (e.g. noisy sample, bins near the OED line  806  that could indicate either on or off ear, etc.) A dot product circuit  912  applies a dot product of the weights to the Frame OED metric  620  to apply the weights to the Frame OED metric  620 . The Frame OED metric  620  may then act as a determination based on a plurality of frequency bin decisions. 
     The Frame OED metric  620  and the Frame OED confidence  622  value may also be forwarded through a distortion rejection circuit  918 . The distortion rejection circuit  918  is a circuit or process capable of determining the presence of significant distortion and reducing the Frame OED confidence  622  value to zero in the event distortion is greater than a distortion threshold. Specifically, the design of network  900  presumes that the audio signal  216  flows to the FB microphone in a relatively linear fashion. However, in some cases, the audio signal  216  saturates the FB microphone causing clipping. This may occur, for example, when a user listens to high volume music and removes the headphones. In such a case, the signal received at the FB microphone is very different from the ideal off ear transfer function due to the distortion, which may result in an on-ear determination. Accordingly, the distortion rejection circuit  918  computes a distortion metric whenever the Frame OED metric  620  indicates an on-ear determination. The distortion metric may be defined as the variance of the detrended normalized difference metric over the bins with non-zero weight (e.g. excluding the OED tone bin). Another interpretation for distortion metric is the minimum mean square error for a straight-line fit. The distortion metric may only be applied when more than one bin has a non-zero weight. Distortion rejection is discussed more below. In summary, the distortion rejection circuit  918  generates a distortion metric when the determination is on-ear, and weights the Frame OED confidence  622  (causing the system to ignore the Frame OED metric  620 ) when distortion is above a threshold. 
       FIG. 10  is an example flow diagram illustrating a method  1000  for distortion detection, for example by a distortion rejection circuit  918  operating in an OED circuit  606  in a wideband OED circuit  304  of an OED processor  206 , and/or combinations thereof. At block  1002 , a frame OED metric  620  and a frame OED confidence  622  are computed, for example according to the processes described with respect to network  900 . At block  1004 , the frame OED metric is compared to an OED threshold to determine if the headphones are considered on ear. As noted above, the distortion detection method  1000  focuses on the case where a headphone is improperly considered on-ear. Accordingly, when the frame OED metric is not greater than the OED threshold, the determination is the headphones are off-ear and distortion is not a concern. Hence, when the frame OED metric is not greater than the OED threshold, the method  1000  proceeds to block  1016  and ends by moving to a next OED frame. When the frame OED metric is greater than the OED threshold, the determination is on-ear and distortion may be an issue. Hence, the method proceeds to block  1006  when the frame OED metric is greater than the OED threshold. 
     At block  1006 , a distortion metric is computed. Computing a distortion metric involves computing a best fit line in between the frequency bin points in the frame OED metric. The distortion metric is the mean squared error for an approximation of the line slope. In other words, block  1006  computes a linear fit to detect distortion in frequency domain sample. At block  1008 , the distortion metric is compared to a distortion threshold. The distortion threshold is a mean square error value, and hence if the mean square error of the distortion metric is higher than the acceptable mean square error specified by the distortion threshold, distortion may be a concern. As an example, the distortion threshold may be set at about two percent. As such, when the distortion metric is not greater than the distortion threshold, the method  1000  proceeds to block  1016  and ends. When the distortion metric is greater than the distortion threshold, the method  1000  proceeds to block  1010 . 
     Effects of distortion may be more extreme at low frequency bins because, generally less signal energy is received by the FB microphone at lower frequencies. As such, small amounts of distortion may negatively impact the narrowband frequency bin while not significantly impacting the higher frequencies. Accordingly, at block  1010  the narrowband frequency bin may be rejected and the frame OED metric and frame OED confidence recomputed without the narrowband frequency bin. Then at block  1012  the recomputed frame OED metric is compared to the OED threshold. If the frame OED metric does not exceed the OED threshold, the headphones are considered off-ear and distortion is no longer an issue. As such, if the frame OED metric without the narrowband frequency bin does not exceed the OED threshold, the determination of off-ear is maintained and the method  1000  proceeds to block  1016  and ends. If the frame OED metric without the narrowband frequency bin still exceeds the OED threshold (e.g. is still considered on-ear) then the distortion may be causing an incorrect OED determination. As such, the method proceeds to block  1014 . At block  1014 , the OED confidence is set to zero, which causes the frame OED metric to be ignored. The method  1000  then proceeds to block  1016  and ends to move to the next frame of OED determination. 
     In summary, the method  1000  may allow an OED signal processor, such as OED processor  206  to determine a distortion metric based on a variance of a difference metric (e.g. frame metric) over a plurality of frequency bins, and ignore the difference metric when the distortion metric is greater than a distortion threshold. 
       FIG. 11  is an example flow diagram illustrating a method  1100  of OED, for example by employing an OED processor  206 , wideband OED circuit  304 , narrowband OED circuit  310 , network  600 , network  900 , any other processing circuitry discussed herein, and/or any combination thereof. At block  1102 , a tone generator is employed to generate an OED tone at a specified frequency bin, such as a sub-audible frequency. At block  1104 , the OED tone is injected into an audio signal forwarded to a headphone speaker. At block  1106 , a noise floor is detected from a FF microphone signal. At block  1108 , a volume of the OED tone is adjusted based on a volume of the noise floor. For example, a tone margin may be maintained between the volume of the OED tone and the volume of the noise floor. Further, a magnitude of volume adjustments to the OED tone over time are may be maintained below an OED change threshold, for example by employing equation 1 above. 
     At block  1110 , a difference metric is by comparing a FB signal from a FB microphone to the audio signal. The difference metric may be determined according to as any OED metric and/or confidence determination process discussed herein. For example, the difference metric may be generated by determining an audio frequency response of the FB signal over an OED frame as a received frequency response, determining an audio frequency response of the audio signal times an off-ear transfer function between the headphone speaker and the FB microphone as an ideal off-ear response, and generating a difference metric comparing the received frequency response to the ideal off-ear frequency response. The difference metric may be determined over a plurality of frequency bins, including the specified frequency bin (e.g. sub-audible frequency bin). Further, the difference metric may be determined by weighting the frequency bins, determining a difference metric confidence as a sum of frequency bin weights; and employing the difference metric confidence when detecting the headphone cup is disengaged from the ear. 
     Finally, at block  1112 , the difference metric is employed to detect when the headphone cup is engaged/disengaged from an ear. For example, a state change may be determined when the difference metric rises above and/or drops below an OED threshold. A confidence value may also be employed so that difference metrics with low confidence are rejected from consideration when performing OED. In another example, the as state change can be detected when a difference metric changes faster than a state change margin. As another example, a state change may be determined when a weighted average of difference metrics rises above/drops below a threshold, where weighting is based on confidence and a forgetting filter. 
     With reference to  FIG. 12 , in yet another embodiment, the injected tone may be a high-frequency tone, such as above 15 kHz. In a particular embodiment, the injected tone may be 20.5 kHz. Other embodiments may use injected tones in the range of 15 khz-192 khz. Unlike the embodiments described above that use relatively low frequencies, such as 20-30 Hz, the response of the FB microphone doesn&#39;t necessarily change with the seal of being inserted or not inserted into the ear. Rather, when using injected high-frequency tones, the response of the FB microphone/s changes primarily based on reflections from materials surrounding the earbud/headphone. Embodiments of the invention can detect the change in reflections and use that detection to determine that the earphone has been removed from the ear of a user. 
     In yet other embodiments, both high-frequency and low-frequency tones are generated and inserted into the audio stream simultaneously, and one or more reflections-based metrics from the high-frequency tone and the seal-based metrics from the low-frequency tone may be monitored simultaneously for changes. In such embodiments, both metrics are used jointly to determine if the earphones are in-ear or off-ear. 
     For optimal performance, the joint use of the two off-ear metrics (low-frequency and high-frequency) takes into account their respective reliability. For example, the high-frequency metric is much less susceptible to wind noise than the low-frequency metric, so it is more reliable in the presence of wind or other low-frequency disturbances. On the other hand, the high-frequency metric is more susceptible to be affected by reflective surfaces very close to the ear-tip. The joint algorithm may also account for the fact that the low-frequency metric changes more slowly than the high-frequency metric in response to changes in the headphone position. In particular, the low-frequency metric may take much longer than the high-frequency metric to detect the on-ear to off-ear transition. The joint detection logic may be designed to rely more on the high-frequency metric for faster on-ear to off-ear transition than possible with just the low-frequency metric.  FIG. 12  is a state transition diagram that illustrates one such example of the joint detection logic. In yet other embodiment, the two metrics can be summed together with different weights according to respective reliability to result in an aggregate metric which may then be used for determining if the headphones are off-ear. 
     Examples of the disclosure may operate on a particularly created hardware, on firmware, digital signal processors, or on a specially programmed general purpose computer including a processor operating according to programmed instructions. The terms “controller” or “processor” as used herein are intended to include microprocessors, microcomputers, Application Specific Integrated Circuits (ASICs), and dedicated hardware controllers. One or more aspects of the disclosure may be embodied in computer-usable data and computer-executable instructions (e.g. computer program products), such as in one or more program modules, executed by one or more processors (including monitoring modules), or other devices. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types when executed by a processor in a computer or other device. The computer executable instructions may be stored on a non-transitory computer readable medium such as Random Access Memory (RAM), Read Only Memory (ROM), cache, Electrically Erasable Programmable Read-Only Memory (EEPROM), flash memory or other memory technology, and any other volatile or nonvolatile, removable or non-removable media implemented in any technology. Computer readable media excludes signals per se and transitory forms of signal transmission. In addition, the functionality may be embodied in whole or in part in firmware or hardware equivalents such as integrated circuits, field programmable gate arrays (FPGA), and the like. Particular data structures may be used to more effectively implement one or more aspects of the disclosure, and such data structures are contemplated within the scope of computer executable instructions and computer-usable data described herein. 
     Aspects of the present disclosure operate with various modifications and in alternative forms. Specific aspects have been shown by way of example in the drawings and are described in detail herein below. However, it should be noted that the examples disclosed herein are presented for the purposes of clarity of discussion and are not intended to limit the scope of the general concepts disclosed to the specific examples described herein unless expressly limited. As such, the present disclosure is intended to cover all modifications, equivalents, and alternatives of the described aspects in light of the attached drawings and claims. 
     References in the specification to embodiment, aspect, example, etc., indicate that the described item may include a particular feature, structure, or characteristic. However, every disclosed aspect may or may not necessarily include that particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same aspect unless specifically noted. Further, when a particular feature, structure, or characteristic is described in connection with a particular aspect, such feature, structure, or characteristic can be employed in connection with another disclosed aspect whether or not such feature is explicitly described in conjunction with such other disclosed aspect. 
     The previously described examples of the disclosed subject matter have many advantages that were either described or would be apparent to a person of ordinary skill. Even so, all of these advantages or features are not required in all versions of the disclosed apparatus, systems, or methods. 
     Additionally, this written description makes reference to particular features. It is to be understood that the disclosure in this specification includes all possible combinations of those particular features. Where a particular feature is disclosed in the context of a particular aspect or example, that feature can also be used, to the extent possible, in the context of other aspects and examples. 
     Also, when reference is made in this application to a method having two or more defined steps or operations, the defined steps or operations can be carried out in any order or simultaneously, unless the context excludes those possibilities. 
     Although specific examples of the disclosure have been illustrated and described for purposes of illustration, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, the disclosure should not be limited except as by the appended claims.