Ear-wearable device with active noise cancellation system that uses internal and external microphones
An ear-wearable device is operable to receive a reference signal from outside an ear canal of a user and an error signal from inside of the ear canal. A physical propagation path between the outside and inside of the ear canal defines a primary path, and amplified sound produced inside of the ear canal propagates over a secondary path to combine with direct noise at the ear canal. A noise signal inside the ear canal is estimated from the reference signal based on estimate of the primary and secondary paths. The estimated noise signal and the error signal are used to produce coefficients of an adaptive filter. The adaptive filter is used to produce an anti-noise signal, which is used actively cancel noise in the ear canal.
1 . An ear-wearable device, comprising:
a reference microphone producing a reference signal in response to external sound outside an ear canal of a user;
an error microphone locatable within the ear canal and producing an error signal in response to sound inside of the ear canal, wherein a physical propagation path between the reference microphone and the error microphone defines a primary path;
a speaker locatable within the ear canal that produces amplified sound inside of the ear canal, wherein the amplified sound propagates over a secondary path to combine with direct noise at the ear canal, the combination of which is sensed by the error microphone to produce the error signal;
a processor coupled to the reference microphone, the error microphone, and the speaker; the processor operable via instructions to:
estimate a noise signal from inside the ear canal from the reference signal by filtering with an equalization filter that is based on an estimate of the secondary path and by filtering with a spectrum shaping filter that is based on an estimate of the primary path, wherein the spectrum shaping filter reduces effects of high-frequency resonances in the primary path;
input the estimated noise signal from inside the ear canal and the error signal into a least mean square (LMS) algorithm, the LMS algorithm producing coefficients of an adaptive filter; and
apply the adaptive filter to the reference signal to produce an anti-noise signal, the anti-noise signal being reproduced by the speaker to actively cancel noise in the ear canal.
2 . The ear-wearable device of claim 1 , wherein the LMS algorithm comprises a normalized least mean square (NLMS) algorithm.
3 . The ear-wearable device of claim 2 , wherein the NLMS algorithm comprises a filtered-x NLMS algorithm.
4 . The ear-wearable device of claim 1 , wherein the reference signal is downsampled, the estimated noise signal from inside the ear canal being estimated based on the downsampled reference signal.
5 . The ear-wearable device of claim 1 , wherein the error signal is downsampled, the estimated residual noise signal being estimated based on the downsampled error signal.
6 . The ear-wearable device of claim 1 , wherein the adaptive filter comprises a finite-impulse response filter with 40 or fewer taps.
7 . The ear-wearable device of claim 1 , reducing the effects of the high-frequency resonances in the primary path comprises low-pass filtering with a cutoff frequency of about 2-2.5 kHz.
8 . The ear-wearable device of claim 7 , wherein the spectrum shaping filter further deemphasizes low frequencies where the response of the speaker is low.
9 . The ear-wearable device of claim 7 , wherein the spectrum shaping filter comprises a cascaded biquad filter.
10 . The ear-wearable device of claim 1 , wherein the equalization filter inverses a minimum phase part of the estimated noise signal from inside the ear canal and applies low-pass and high-pass filters, wherein cutoff frequencies of the low-pass and high-pass filters are determined by characteristics of the secondary path.
11 . The ear-wearable device of claim 1 , wherein the processor is further configured to estimate the secondary path via a calibration process comprising:
sending a stimulus signal to the speaker, the stimulus signal comprising a combination of tones at a selected set of frequencies;
measuring, via the error microphone, an error microphone signal that is produced in response to the stimulus signal; and
determining a transfer function between the stimulus signal and the error microphone signal, the transfer function being stored in a memory of the ear-wearable device and used as the estimate of the secondary path.
12 . The ear-wearable device of claim 11 , wherein the error signal is averaged in a time domain before determining the transfer function.
13 . The ear-wearable device of claim 11 , wherein the tones have differing magnitudes that emphasize low frequencies.
14 . The ear-wearable device of claim 1 , wherein the processor is further configured to estimate the primary path via a calibration process comprising:
receiving a stimulus signal via the external reference microphone, the stimulus signal generated in response to a combination of tones at a selected set of frequencies rendered via a headset worn over the ear-wearable device;
determining a response to the stimulus signal at the error microphone; and
determining a transfer function between the external microphone and the error microphone, the transfer function being stored in a memory of the ear-wearable device and used as the estimate of the primary path.
15 . The ear-wearable device of claim 1 , wherein the processor is further configured to:
modify the reference signal to produce an enhanced hearing signal that compensates for hearing loss; and
combine the enhanced hearing signal with the anti-noise at the speaker.
16 . A method of active noise cancellation via an ear-wearable device, comprising:
receiving a reference signal from a reference microphone in response to external sound outside an ear canal of a user;
receiving an error signal from an error microphone in response to sound inside of the ear canal, wherein the error microphone is located within the ear canal and a physical propagation path between the reference microphone and the error microphone defines a primary path, and wherein amplified sound produced inside of the ear canal by a speaker located within the ear canal propagates over a secondary path to combine with direct noise at the ear canal, the combination of which is sensed by the error microphone to produce the error signal;
estimating a noise signal inside the ear canal from the reference signal by filtering with an equalization filter that is based on an estimate of the secondary path and by filtering with a spectrum shaping filter that is based on an estimate of the primary path, wherein the spectrum shaping filter reduces effects of high-frequency resonances in the primary path and deemphasizes low frequencies where the response of the speaker is low;
inputting the estimated noise signal from inside the ear canal and the error signal into a least mean square (LMS) algorithm, the LMS algorithm producing coefficients of an adaptive filter;
applying the adaptive filter to the reference signal to produce an anti-noise signal; and
reproducing the anti-noise signal in the ear canal by the speaker to actively cancel noise.
17 . The method of claim 16 , further comprising equalizing the estimated noise signal from inside the ear canal via an equalization filter that inverses a minimum phase part of the estimated noise signal from inside the ear canal and applies low-pass and high-pass filters, wherein cutoff frequencies of the low-pass and high-pass filters are determined by characteristics of the secondary path.
18 . The method of claim 16 , further comprising:
sending a stimulus signal to the speaker, the stimulus signal comprising a combination of tones at a selected set of frequencies;
measuring, via the error microphone, an error microphone signal that is produced in response to the stimulus signal; and
determining a transfer function between the stimulus signal and the error microphone signal, the transfer function being stored in a memory of the method and used as the estimate of the secondary path.
19 . The method of claim 16 , further comprising:
receiving a stimulus signal via the external reference microphone, the stimulus signal generated in response to a combination of tones at a selected set of frequencies rendered via a headset worn over the ear-wearable device;
determining a response to the stimulus signal at the error microphone; and
determining a transfer function between the external microphone and the error microphone, the transfer function being stored in a memory of the ear-wearable device and used as the estimate of the primary path.
20 . The method of claim 16 , further comprising:
modifying the reference signal to produce an enhanced hearing signal that compensates for hearing loss; and
combining the enhanced hearing signal with the anti-noise at the speaker.