Zero-Crossing-Based Parasitic Vibration Detection for Wearable Audio Devices

Abstract

A system for detecting parasitic vibrations in a wearable audio device, the system including: an electroacoustic transducer configured to generate sound for a user, a housing to hold the transducer, at least one of a feedforward microphone configured to detect sound outside the housing and output a feedforward microphone signal and a feedback microphone configured to detect sound inside the housing and output a feedback microphone signal, and an opening in the housing through which sound pressure is emitted from the transducer, the system includes a parasitic vibration detector configured to determine a fundamental frequency of at least one of the feedforward microphone signal and the feedback microphone signal and compare an amplitude of the determined fundamental frequency to a threshold level to determine the parasitic vibration.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Patent Application No. 17 / 412,062, filed August 25, 2021.

[0002] The present disclosure relates to wearable audio devices. [Background technology]

[0003] Wearable audio devices such as earphones and hearing aids can generate parasitic vibrations in the feedforward and / or feedback loops, which can result in undesirable instabilities and squealing noises. Summary of the Invention

[0004] All embodiments and features mentioned below can be combined in any technically possible manner. [Means for solving the problem]

[0005] In one aspect, a system for detecting parasitic vibrations in a wearable audio device includes an electro-acoustic transducer configured to generate sound for a user, a housing that holds the transducer, at least one of a feedforward microphone configured to detect sound outside the housing and output a feedforward microphone signal or a feedback microphone configured to detect sound inside the housing and output a feedback microphone signal, and an opening in the housing that releases sound pressure from the transducer. The system includes a parasitic vibration detector configured to determine a fundamental frequency of at least one of the feedforward microphone signal and the feedback microphone signal and compare an amplitude of the determined fundamental frequency to a threshold level to determine parasitic vibrations.

[0006] Some embodiments include one of the above and / or below features, or any combination thereof. In one example, the parasitic vibration detector is further configured to determine whether the fundamental frequency is at least at a threshold level for at least a predetermined amount of time. In one example, the wearable audio device includes an earphone configured to output sound directly to the user's ear canal. In one example, the microphone is used in an active noise reduction (ANR) system. In one example, the feedforward microphone is used in a transmission mode in which environmental sounds are reproduced by the transducer.

[0007] Some embodiments include one or any combination of the above and / or the following features: In some examples, the fundamental frequency is determined based on zero crossings of the microphone signal; In one example, the fundamental frequency is determined by measuring several samples of an operating clock between zero crossings; In one example, the fundamental frequency is determined based on monitoring zero crossings over time; In one example, the zero crossings are determined based on changes in sign of the microphone signal.

[0008] Some embodiments include one of the above and / or the following features, or any combination thereof. In some examples, the parasitic vibration detector is configured to detect parasitic vibrations within a predetermined frequency range. In one example, the frequency range is between about 300 Hz and about 1,000 Hz. In some examples, the system further includes an instability reducer configured to modify the microphone signal in response to determining the parasitic vibrations. In one example, the instability reducer is configured to mute the microphone. In one example, the microphone is muted for a predetermined amount of time. In one example, after the predetermined amount of time, the microphone is returned to an unmuted state.

[0009] In another aspect, a system for detecting parasitic vibrations in an earphone configured to output sound directly into a user's ear canal, the earphone comprising: an electro-acoustic transducer configured to generate sound for a user; a housing holding the transducer; a feedforward microphone configured to detect sound outside the housing and output a feedforward microphone signal used in a transparency mode in which environmental sound is reproduced by the transducer; a feedback microphone configured to detect sound inside the housing and output a feedback microphone signal used for active noise reduction; and an opening in the housing that releases sound pressure from the transducer that can reach the feedforward microphone, to determine the parasitic vibrations. The system includes a parasitic vibration detector configured to determine a fundamental frequency of the microphone signal based on zero-crossings of the microphone signal, compare an amplitude of the fundamental frequency of the microphone signal to a threshold level, and determine whether the fundamental frequency is at least at the threshold level for at least a predetermined amount of time.

[0010] Some embodiments include one or any combination of the above and / or below features. In one example, the fundamental frequency is determined by measuring several samples of the operating clock between zero crossings. In one example, the fundamental frequency is determined based on monitoring the zero crossings over time. In one example, the zero crossings are determined based on a change in sign of the microphone signal. In one example, the parasitic vibration detector is configured to detect parasitic vibrations in a frequency range of about 300 Hz to about 1,000 Hz. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a perspective view of a wearable audio device. [Figure 2] FIG. 2 is a partial cross-sectional view of a wearable audio device. [Figure 3] FIG. 1 is a block diagram of an embodiment of a wearable audio device. [Figure 4] Shows frequency as a function of zero crossings. [Figure 5] 1 is a plot of microphone signal amplitude showing microphone saturation caused by undesired parasitic vibrations. [Figure 6] 1 is a plot of microphone signal zero crossings showing detection of the fundamental frequency of a low frequency vibration. [Figure 7] 1 is a flowchart of the operation of a parasitic vibration detection and mitigation method. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present disclosure relates to wearable audio devices. Some non-limiting examples of the present disclosure describe a type of wearable audio device known as an earphone. An earphone generally includes an electro-acoustic transducer for generating sound and is configured to deliver sound directly to a user's ear canal. The earphone can be wireless or wired. In non-limiting examples described herein, the earphone includes one or more feedforward (external) microphones that sense external sound outside the housing. In non-limiting examples described herein, the earphone includes one or more feedback (internal) microphones that sense internal sound inside the housing. The feedforward microphones and feedback microphones can be used for functions such as active noise reduction (ANR). The feedforward microphone can also be used in a transparent mode of operation, where external sound is reproduced for the user by the electro-acoustic transducer. Other aspects of the earphone not included in this disclosure are not shown or described.

[0013] Some embodiments of the present disclosure also describe a type of wearable audio device known as an open-type audio device. Open-type audio devices have one or more electro-acoustic transducers (i.e., audio drivers) located away from the ear canal opening. In some embodiments, the open-type audio device also includes one or more microphones, which can be used to pick up the user's voice, for ANR, and / or for operation in a transparent mode. Open-type audio devices are further described in U.S. Pat. No. 10,397,681, the entire disclosure of which is incorporated herein by reference for all purposes.

[0014] Open-type audio devices include, but are not limited to, off-ear headphones, i.e., devices having one or more electro-acoustic transducers that are coupled to the head or ear (typically by a support structure) but do not occlude the ear canal opening. In some examples, open-type audio devices are off-ear headphones with audio glasses, although this is not intended to limit the present disclosure, as in open-type audio devices, the device is configured to deliver sound to one or both ears of the wearer, typically without earcups or earbuds. Wearable audio devices contemplated herein may include a variety of devices, including over-ear hooks, such as wireless headsets, hearing aids, eyeglasses, protective helmets, and other open-ear audio devices.

[0015] Some examples of the present disclosure describe headphones. Headphones typically refer to devices worn around, on, or in the ear and radiate acoustic energy directly or indirectly into the ear canal. Headphones are sometimes referred to as earbuds, earphones, headsets, mini-ears, or sports headphones and can be wired or wireless. Headphones include drivers that convert electronic audio signals into acoustic energy. The drivers may or may not be housed in earcups or housings configured to rest on the head or ear or to be inserted directly into the user's ear canal. A headphone may be a single, standalone unit, one for each ear, or one of a pair of headphones (each including at least one acoustic driver). One headphone may be mechanically connected to the other headphone, for example, by a headband and / or by leads that carry audio signals to the acoustic drivers in the headphones. The headphones may include components that wirelessly receive audio signals. The headphones may include components of an ANR system, which may include an internal microphone within the headphone housing and one or more external microphones that sense sound outside the housing. The headphones may also include other features such as additional microphones for ANR systems, awareness mode systems, and one or more microphones used to pick up the user's voice.

[0016] One or more of the devices, systems, and methods described herein may be used in various embodiments and combinations in a wide variety of wearable audio devices or systems, including wearable audio devices of various form factors. One such form factor is earbuds. Another is headphones. Unless otherwise specified, a wearable audio device or system includes headphones and various other types of wearable audio devices, such as head-, shoulder-, or body-worn acoustic devices (e.g., audio glasses or other ear- or head-worn audio devices) that include one or more acoustic transducers for receiving and / or generating sound with or without contact with a user's ear.

[0017] While specific implementations of wearable audio devices primarily serving the purpose of acoustically outputting audio have been presented in some detail, it should be noted that the presentation of such specific implementations is intended to facilitate understanding through the provision of examples and should not be construed as limiting either the scope of the disclosure or the scope covered by the claims.

[0018] In some examples, the wearable audio device includes an electro-acoustic transducer configured to generate sound for a user, a housing that holds the transducer, a feedforward microphone configured to detect sound outside the housing and output a feedforward microphone signal, a feedback microphone configured to sense sound inside the housing and output a feedback microphone signal, and at least one opening in the housing that emits sound pressure from the transducer that can reach the feedforward microphone. The processor system is programmed to perform a parasitic vibration detector functionality, configured to determine a fundamental frequency of one or more of the microphone signals by monitoring zero-crossings of the microphone signals, and then determine whether the fundamental frequency remains above a threshold level for at least a minimum period of time. If these conditions are met, the system is vibrating. In some examples, vibration mitigation activities are then performed.

[0019] FIG. 1 is a perspective view of a wireless in-ear earphone 10. An earphone is a non-limiting example of a wearable audio device. Another example of a wearable audio device is a headphone, e.g., an over-ear headphone. Earphone 10 includes a body or housing 12 that houses the active components of the earphone. Portion 14 is coupled to body 12 and is flexible so that it can be inserted into the entrance of the ear canal. Sound is delivered through opening 15. Retention loop 16 is configured and arranged to be positioned within the outer ear, e.g., within the antihelix, to help retain the earphone within the ear. Earphones are known in the art (e.g., as disclosed in U.S. Pat. No. 10,993,009, the disclosure of which is incorporated herein by reference in its entirety for all purposes), and therefore specific details of earphones will not be further described herein.

[0020] FIG. 2 is a partial cross-sectional view of only certain elements of earphone 20 that is useful for better understanding the present disclosure. Earphone 20 includes housing 21 that encloses electro-acoustic transducer (audio driver) 30. Housing 21 includes front housing portion 50 and rear housing portions 60 and 62 that define rear housing interior 66. Transducer 30 has a diaphragm 32 that is driven to create sound pressure within front cavity 52. ​​Sound is also generated within rear cavity 53. The sound pressure is directed outward from front housing portion 50 through sound outlet 54. An internal microphone 80 is located within housing 21. In one example, microphone 80 is located within sound outlet 54 and configured to sense sound within the cavity formed by front cavity 52 and the user's ear canal (not shown), as shown in FIG. 2. An external microphone 81 is configured to sense sound outside of housing 21. An exemplary external microphone 81 is disposed inside the housing and acoustically coupled to the external environment via a housing opening 82 that allows ambient sounds to reach the microphone 81. In one example, the internal microphone 80 senses sounds inside the housing (e.g., within the front cavity 52) and is used as a feedback microphone for active noise reduction. In an illustrative example, the external microphone 81 is used as a feedforward microphone for active noise reduction and / or for transparency mode operation in which ambient sounds are played back to the user to make them more aware of their environment, hear other speech, etc. Earphones, such as those shown by earphone 10 of FIG. 1, typically include a flexible tip (not shown) that engages with the neck 51 of housing portion 50 to help direct sound into the ear canal (not shown). The earphone housing 21 further includes a rear enclosure made from rear housing portions 60 and 62 and a grille 64. It should be noted that the details of earphone 20 are illustrative of aspects of an earphone and are not intended to limit the scope of the present disclosure, as the present parasitic vibration detection can be used in earphones, headphones, and other types of wearable audio devices of various types and designs.

[0021] Transducer 30 further comprises a magnetic structure 34. Magnetic structure 34 includes a transducer magnet 38 and magnetic material that functions to contain and guide the magnetic field from magnet 38 so that the magnetic field appropriately interacts with coil 33 to drive diaphragm 32, as is well known in the art of electro-acoustic transducers. The magnetic material includes cup 36 and front plate 35, both of which are preferably made from a material with relatively high magnetic susceptibility, also as is known in the art. Transducer printed circuit board (PCB) 40 carries the electrical and electronic components (not shown) responsible for driving the transducer. Pads 41 and 42 are locations where wires (not shown) can be coupled to PCB 40.

[0022] The earphones 20 also include a processor 74 located on the PCB 70. In some examples, the processor 74 is configured to process the outputs of the microphones 80 and 81. As will be apparent to those skilled in the art, the processor is typically involved in other processing necessary for earphone function, such as processing digital sound files played by the earphones. In one example, the processor is configured to detect parasitic vibrations. In some examples, the processor is also configured to mitigate parasitic vibrations or instabilities. In one example, parasitic vibrations can be caused when a feedforward microphone (used to sense environmental sounds outside the earphones) picks up sound from the earphones' audio driver. This can occur, for example, when sound pressure exiting the housing through a resistive port 84 in the rear cavity 53 is sensed by the microphone 81. In some examples, the port 84 is covered by a resistive fabric 85. Direct coupling through other ports or even leakage within the acoustic cavity can also result in parasitic vibrations. In one example, parasitic vibrations are induced in a feedback system when the pressure sensed at the internal microphone 80 as a function of driver voltage varies enough to drive the control loop unstably. The resulting parasitic vibrations can cause undesirable audio vibrations or squealing noises. Squealing noises can occur even when the earphones are properly positioned in the user's ears. Squealing noises can also occur when the earphones are placed in their case and not powered down. This can occur when communication between the earphones and the case is improper, for example, when the case's battery is depleted.

[0023] FIG. 3 is a block diagram of an embodiment of a wearable audio device 100. The exemplary device 100 is an earphone or headphone, although this is not a limitation of the present disclosure. The wearable audio device 100 includes a processor 102 that receives audio data from an external source via a wireless transceiver 104. The processor 102 also receives the output of feedback microphone(s) 108 and feedforward microphone(s) 110. The processor 102 outputs the audio data, which is converted to an analog signal that is supplied to an audio driver 106. The exemplary device 100 includes a memory containing instructions that, when executed by the processor, accomplish the processes described herein configured to detect parasitic vibrations. In some examples, detected instabilities are also mitigated via a suitably programmed processor. In some examples, the device 100 is configured to store a computer program product using a non-transitory computer-readable medium, the medium including computer program logic encoded thereon that, when executed on the wearable audio device (e.g., by a processor), causes the device to filter and otherwise process signals as described herein. It should be noted that the details of wearable audio device 100 are examples of earphone and headphone aspects and are not intended to limit the scope of the present disclosure, as this parasitic vibration detection can be used in various types and designs of earphones, headphones, and other wearable audio devices. Also, it should be noted that aspects of wearable audio device 100 that are not involved in parasitic vibration detection and mitigation are not shown in FIG. 3 for simplicity.

[0024] For low-frequency parasitic vibrations (e.g., those in the range of about 300 Hz to about 1,000 Hz), the onset of feedforward or feedback-based vibrations can be so fast that the system transitions from no vibration to microphone saturation in a few milliseconds. In one example, vibrations can saturate a microphone within about five cycles after vibration onset. As a result, existing vibration detection algorithms that look for energy in narrow bands may not react quickly enough to detect and suppress vibrations when the microphone response is saturated. Harmonic distortion sufficient to cause detector failure may occur.

[0025] In some examples of the present disclosure, the processor is programmed to detect parasitic vibrations by determining a fundamental frequency of at least one of the feedforward microphone signal and the feedback microphone signal and comparing the amplitude of the determined fundamental frequency to a threshold level to determine the parasitic vibrations. In some examples, the fundamental frequency is determined based on zero crossings of the microphone signal. In one example, the fundamental frequency is determined by measuring several samples of an operating clock between zero crossings. In one example, the fundamental frequency is determined based on monitoring zero crossings over time. In one example, the zero crossings are determined based on changes in sign of the microphone signal.

[0026] In a more specific example, the parasitic vibration detector is also configured to determine whether the fundamental frequency is at least at a threshold level (amplitude) for at least a predetermined amount of time. In one example, the parasitic vibration detector is configured to detect parasitic vibrations in a predetermined frequency range. This frequency range can be from about 300 Hz to about 1,000 Hz.

[0027] In some examples, the processor is further configured to mitigate the detected parasitic vibration or instability. In one example, the instability mitigator is configured to modify the microphone signal in response to determining the parasitic vibration. In an example, the instability mitigator is configured to mute the microphone, and the microphone may be muted for a predetermined amount of time. After the predetermined amount of time, the microphone may be returned to an unmuted state. Other aspects of mitigation are described elsewhere herein.

[0028] The processor can be configured to determine the fundamental frequency by detecting zero-crossings of the microphone signal. A tonal signal has a fundamental frequency, and the distance between zero-crossings determines what that fundamental frequency is. In one example, the processor maps the distance between zero-crossings in integer samples of a clock to a given frequency range. In a specific, non-limiting example, the processor runs a 48 kHz clock. FIG. 4 shows a plot of the determined frequency as a function of the "distance" (measured as the number of clock cycles) between zero-crossings. In one example, the processor is configured to detect fundamental frequencies ranging from 300 Hz (equivalent to 80 clock cycles between zero-crossings) to 1 kHz (equivalent to 24 clock cycles between zero-crossings). Thus, the processor is configured to monitor zero-crossings as a function of time. In one example, zero-crossings are detected by detecting a change in sign of the microphone signal, i.e., from positive to negative and vice versa.

[0029] 5 is a plot 120 of a feedforward or feedback microphone signal at the onset of parasitic vibration, where the signal reaches saturation in only about 5 cycles, or about 5 milliseconds. Microphone saturation changes the shape of the output waveform from a pure tone (a sine wave, as shown in region 121) to more of a square wave with all the harmonics, as shown in region 122, beginning at about 515 ms. Such low-frequency vibrations cannot be detected because the saturated output contains a lot of energy at harmonic frequencies that cannot be monitored by vibration detection algorithms that look for energy in narrow bands.

[0030] 6 is a plot 150 of zero-crossings over time corresponding to the microphone signal shown in FIG. 5. The determined frequency is a function of the "distance" (measured as the number of clock cycles) between zero-crossings. In one example, the processor is configured to detect fundamental frequencies ranging from 300 Hz (equivalent to 80 clock cycles between zero-crossings) to 1 kHz (equivalent to 24 clock cycles between zero-crossings). Thus, the processor is configured to monitor zero-crossings as a function of time. In one example, the zero-crossings are detected by detecting a change in sign of the microphone signal, i.e., from positive to negative and vice versa.

[0031] Curve 152 is a plot of zero-crossing distance (i.e., samples at the sampling clock rate). Starting near 515 ms and progressing through 540 ms (region 154), the distance between zero-crossings is consistently within a range of about 30 samples, indicating a fairly stable fundamental frequency. Curve 156 is a plot of low-pass filtered zero-crossing distance. The 300 Hz to 1,000 Hz boundary is also shown. When curves 152 and 156 correspond or overlap (as in plot region 158), the system can be more confident that a fundamental frequency has been detected (compared to detecting a short-term stimulus, where the low-pass filtered plot would not overlap with the zero-crossing distance plot). The idea is that when the absolute value of the difference between curves 156 and 152 is below a threshold, there is more confidence that there is relative consistency in the zero-crossing frequency. More generally, some type of averaging of zero-crossing measurements can be compared to instantaneous values ​​to establish greater confidence that a fundamental frequency has been detected.

[0032] Another approach to determining confidence in zero-crossing-based fundamental frequency detection is to compare multiple zero-crossing measurements for consistency. For example, the last N zero-crossings (stored in memory) can be examined to determine whether they are all within a predetermined range or whether their range of values ​​(max-min) is small. If so, there is greater confidence that a fundamental frequency has been detected. In one example, this consistency-based determination is used in addition to another zero-crossing-based fundamental frequency measurement described herein as a check or to build further confidence that a parasitic oscillation (typically dominated by a single frequency) has been detected.

[0033] In some examples, zero-crossing detection is paired with processor logic around the absolute value. In one example, if the microphone signal has an amplitude greater than a certain magnitude at the moment a very fast, high-onset vibration is detected, there is a timer that must be true for a certain duration before mitigation action is initiated. In one example, the mitigation action may be to mute the awareness output (i.e., the output from the feedforward microphone) for a certain duration or until the vibration is no longer present. In another example, the processor is configured to determine the consistency of the detected tone over time. For example, the processor may apply a smoothing function (e.g., an exponential smoothing function) to the detected vibration. The processor may then compare such averaged magnitude to the instantaneous magnitude. Such a technique may help ensure that the fundamental vibration frequency is being detected compared to a fluctuating input (such as a sound desired to be sensed). This may help avoid muting environmental sounds and other desirable sounds that should not be canceled.

[0034] FIG. 7 is a flowchart of an exemplary operation of earphone parasitic vibration detection and mitigation method 180. In one example, all steps are performed by a processor. Thus, the operation can be modified as needed by appropriately programming the processor. The input signal is the output of a feedforward or feedback microphone. In step 182, the distance between zero-crossings is mapped at the processor sampling rate, as shown in FIG. 4. In step 184, the detected frequency is compared to a predetermined frequency or range of vibration frequencies to be detected and resolved. If the frequency is within the range, in step 186, the magnitude of the microphone signal is compared to a predetermined threshold. If the signal is above the threshold, in step 188, the processor determines whether the signal remains above the threshold for a predetermined duration. If so, in some examples, the vibration is mitigated (step 190). If any of steps 184, 186, and 188 are not met, operation returns to step 182, and no mitigation action is taken.

[0035] In optional step 190, if an undesired parasitic vibration is detected, the vibration is mitigated. One goal is to quickly eliminate the vibration without reducing or eliminating the desired sound, even if the mitigation algorithm is triggered during a false positive event (e.g., an external sound). In one example of step 190, the mitigation action is to mute the microphone for a predetermined time calculated to avoid the vibration recurring, or until the vibration stops. In another example, the mitigation includes adjusting the gain applied to the signal from the associated feedforward or feedback microphone before the signal is provided to the driver. In extreme cases, the entire gain applied to the microphone is reduced; however, this may be audible to the user. In some examples, the gain is reduced in a more controlled manner to reduce and eliminate the vibration. In some examples, the gain is gradually reduced (e.g., to zero) over a predetermined period of time, held at the reduced level for a predetermined amount of time, and then increased back to its original value. The increase may be instantaneous or over a predetermined period of time, and may be gradual over that time. In some examples, the gain adjustment is frequency-dependent. In one example, the gain is gradually reduced by about 20 dB over a period of about 0.5 seconds. In one example, the gain is then gradually restored to its original value over a period of about 0.5 seconds. This restoration can be done in several steps to reduce the likelihood that a user will detect the anomaly. In other examples, mitigation involves allowing the gain of the microphone to be altered in an alternative way, shaping the frequency response of the microphone to reduce the gain, or changing the phase of the microphone in certain areas. Alternatively, mitigation involves enabling an echo canceller.

[0036] When a process is depicted or suggested by a block diagram, the steps may be performed by one element or by multiple elements. These steps may be performed together or at different times. Elements performing activities may be physically the same, or may be in close proximity to each other, or may be physically separate. One element may perform more activities than one block. Audio signals may be coded or uncoded and may be transmitted in either digital or analog form. Conventional audio signal processing equipment and operations may be omitted from the drawings.

[0037] The example systems and methods described herein include computer components and computer-implemented steps that will be apparent to those skilled in the art. For example, it should be understood by those skilled in the art that the computer-implemented steps may be stored as computer-executable instructions on a computer-readable medium, such as, for example, a hard disk, an optical disk, a flash ROM, a non-volatile ROM, and a RAM. Furthermore, it should be understood by those skilled in the art that the computer-executable instructions may be executed on a variety of processors, such as, for example, a microprocessor, a digital signal processor, a gate array, and the like. For ease of description, not all steps or elements of the systems and methods are described herein as part of a computer system, but those skilled in the art will recognize that each step or element may have a corresponding computer system or software component. Accordingly, such computer systems and / or software components are enabled by the description of their corresponding steps or elements (i.e., their functionality) and are within the scope of the present disclosure.

[0038] Although multiple implementations have been described, it is nevertheless understood that additional modifications can be made without departing from the scope of the inventive concepts described herein, and accordingly, other examples are within the scope of the following claims.

Claims

1. 1. A system for detecting parasitic vibrations in a wearable audio device, the system comprising: an electro-acoustic transducer configured to generate sound for a user; a housing that holds the transducer; at least one of a feedforward microphone configured to detect sound outside the housing and output a feedforward microphone signal; or a feedback microphone configured to detect sound inside the housing and output a feedback microphone signal; and an opening in the housing that emits sound pressure from the transducer, the system comprising:

1. A parasitic vibration detector, comprising: determining a fundamental frequency of at least one of the feedforward microphone signal and the feedback microphone signal based on zero crossings of at least one of the feedforward microphone signal and the feedback microphone signal, wherein an absolute value of a difference between a plot of zero crossing distances and a plot of low-pass filtered zero crossing distances is less than a threshold; A system comprising: a parasitic vibration detector configured to compare an amplitude of the determined fundamental frequency with a threshold level to determine a parasitic vibration.

2. 10. The system of claim 1, wherein the wearable audio device comprises an earphone configured to output sound directly to the user's ear canal.

3. The system of claim 1 , wherein the microphone is used in an active noise reduction (ANR) system.

4. The system of claim 1 , wherein the feedforward microphone is used in a transparent mode in which ambient sounds are reproduced by the transducer.

5. 2. The system of claim 1, wherein the fundamental frequency is determined by measuring several samples of the operating clock between zero crossings.

6. The system of claim 1 , wherein the fundamental frequency is determined based on monitoring zero crossings over time.

7. The system of claim 1 , wherein a zero-crossing is determined based on a change in sign of at least one of the feedforward microphone signal and the feedback microphone signal.

8. The system of claim 1 , wherein the parasitic vibration detector is further configured to determine whether the fundamental frequency is at least at the threshold level for at least a predetermined amount of time.

9. The system of claim 1 , wherein the parasitic vibration detector is configured to detect parasitic vibrations in a predetermined frequency range.

10. 10. The system of claim 9, wherein the frequency range is from about 300 Hz to about 1,000 Hz.

11. The system of claim 1 , further comprising an instability reducer configured to modify the microphone signal in response to determining the parasitic vibration.

12. The system of claim 11 , wherein the instability reducer is configured to mute at least one of the feedforward microphone and the feedback microphone.

13. The system of claim 12 , wherein at least one of the feedforward microphone and the feedback microphone is muted for a predetermined amount of time.

14. 14. The system of claim 13, wherein after the predetermined amount of time, at least one of the feedforward microphone and the feedback microphone is returned to an unmuted state.

15. 1. A system for detecting parasitic vibrations in an earphone configured to output sound directly into a user's ear canal, the earphone comprising: an electro-acoustic transducer configured to generate sound for a user; a housing holding the transducer; a feedforward microphone configured to detect sound outside the housing and output a feedforward microphone signal to be used in a transparency mode in which ambient sound is reproduced by the transducer; a feedback microphone configured to detect sound inside the housing and output a feedback microphone signal to be used for active noise reduction; and an opening in the housing to release sound pressure from the transducer that can reach the feedforward microphone, the system comprising:

1. A parasitic vibration detector, comprising: determining a fundamental frequency of at least one of the feedforward microphone signal and the feedback microphone signal based on zero crossings of at least one of the feedforward microphone signal and the feedback microphone signal, wherein an absolute value of a difference between a plot of zero crossing distances and a plot of low-pass filtered zero crossing distances is less than a threshold; comparing the amplitude of the fundamental frequency of at least one of the feedforward microphone signal and the feedback microphone signal to a threshold level; A system comprising: a parasitic vibration detector configured to determine whether the fundamental frequency is at least at the threshold level for at least a predetermined amount of time to determine parasitic vibration.

16. 16. The system of claim 15, wherein the fundamental frequency is determined by measuring several samples of the operating clock between zero crossings.

17. The system of claim 15 , wherein the fundamental frequency is determined based on monitoring zero crossings over time.

18. The system of claim 15 , wherein a zero-crossing is determined based on a change in sign of at least one of the feedforward microphone signal and the feedback microphone signal.

19. The system of claim 15 , wherein the parasitic vibration detector is configured to detect parasitic vibrations in a frequency range of about 300 Hz to about 1,000 Hz.

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