Active noise cancellation system

Abstract

An active noise cancellation system (20) comprises: a headphone (8') comprising: an electro-acoustic driver (11); and at least one sensing microphone (12, 13); tunable active noise cancellation circuitry (7) operative to receive signals from the at least one sensing microphone (12, 13), the tunable active noise cancellation circuitry (7) being preconfigured in a standard tuning for a reference ear, and comprising at least one noise control filter (14, 15); and a tuning module (24) operative to configure the headphone (8') for an individual wearer by: comparing acoustic coupling of the headphone (8') to an ear of the individual wearer with acoustic coupling to the reference ear to determine a deviation in acoustic coupling; and using the determined deviation in acoustic coupling to modify the tunable active noise cancellation circuitry (7) by a predetermined extent based on the determined deviation in acoustic coupling.

Description

TECHNICAL FIELD

[0001] The present invention relates to an active noise cancelling system, and in particular, but not exclusively, to an active noise cancelling system for headphones having a leaky coupling to the outer ear of a wearer. BACKGROUND

[0002] Headphones of the type intended to be worn substantially in the ear or in the outer ear have generally been provided with coupling means to ensure an effective seal between the acoustic output of the headphones and the ear of the wearer. The seal is provided by an elastic component of the device. The presence of the seal confers several acoustic benefits, including an increase in the passive acoustic attenuation provided by the headphones and the creation of a simple acoustic radiation load against the micro-speaker in the headphones. Nonetheless, the presence of the seal brings some negative system-level impacts to the headphones, including the accentuation of the 'occlusion effect' which some wearers find uncomfortable and other consequences of the eardrum seal.

[0003] Alternative headphone designs have recently gained popularity in the market in which there is no explicit coupling means present; the headphones are intended to operate with a more 'open' coupling to the outer ear canal. These headphones are referred to herein as 'leaky buds', to reflect the loose acoustic coupling between the source and the load.

[0004] During the development of 'leaky bud' headphones, the variation in the coupling between the headphones and the ear of each individual wearer has a huge impact on the performance of the headphones. This person-to-person variation seen in the case of leaky buds was partly mitigated in the case of traditional headphones, in which the coupling is managed by a seal, implemented by a deformable grommet.

[0005] In a subsequent attempt to increase the acoustic utility of 'leaky bud' type headphones by introducing an active noise control method, it has been observed that the additional dependence on the individual ear of the wearer, due to the absence of a seal, greatly complicates the practical deployment of the active noise control method.

[0006] Some competing attempts to address this problem have relied on significant modifications to the acoustics of the earbud, in order to intentionally introduce larger leaks than those caused by wearer-to-wearer variation (see, for example, US9473845 B2). These larger leaks have the intention of short-circuiting the wearer-dependent leaks, but have the unintended consequences of i) short-circuiting the radiation from the speaker; and ii) adding passive noise transmission paths.

[0007] An alternative approach that might address wearer-to-wearer variations is to deploy an adaptive filtering approach (see, for example, US9293128 B2), in which an adaptive filter is used to create an estimate of the secondary acoustic path. This approach has enjoyed limited commercial success for reasons including the complexity, cost and power consumption of the computational apparatus required to support the adaptive approach, and more importantly, the difficulties in providing the training signals required to guide the convergence of the adaptive filter in real-world situations. It is emphasized that this difficulty impacts not only the ability of prior art implementations to leak-compensate, but also the ability of prior art architectures to yield viable noise-cancellation systems. SUMMARY

[0008] The present applicant has identified a need for an improved active noise-cancellation system that addresses or at least mitigates the problems associated with the prior art.

[0009] According to a first aspect of the application, there is provided an active noise-cancellation system comprising: an earpiece comprising: an electro-acoustic driver; and at least one sensing microphone; tunable active noise-cancellation circuitry operative to receive a signal from the at least one sensing microphone, the tunable active noise-cancellation circuitry being preconfigured in a standard tuning for a reference ear, and comprising at least one noise-control filter; and a tuning module operative to configure the earpiece for an individual wearer by: comparing acoustic coupling of the earpiece to the individual wearer's ear with acoustic coupling to the reference ear to determine a deviation (e.g. a degree of deviation) in acoustic coupling; and using the determined deviation in acoustic coupling to modify the tunable active noise-cancellation circuitry based on the determined deviation in acoustic coupling to a predetermined extent.

[0010] In this way, there is provided an earpiece system that can be tuned (e.g. automatically tuned) to reduce deficiencies in ANC performance caused by poor acoustic coupling between the earpiece and the wearer's ear.

[0011] In one embodiment, the earpiece comprises: a body configured to be placed at an entrance to an ear canal of a wearer's ear, the body housing the electro-acoustic driver and defining a passageway from the electro-acoustic driver to an opening in an outer surface of the body so as to allow sound generated by the electro-acoustic driver to pass into the ear canal of the wearer's ear.

[0012] In one embodiment, the earpiece has a leaky coupling to the ear (e.g. the earpiece is not designed to completely seal the wearer's ear canal when worn on the wearer's ear). Typically, the earpiece is configured to engage the outer ear (e.g. the outer ear cavity) of the wearer's ear, with little (if any) penetration of the earpiece into the ear canal of the wearer's ear.

[0013] In one embodiment, the main body defines a rigid (e.g., non-compliant) outer surface that engages the ear.

[0014] In one embodiment, the rigid outer surface that engages the ear has a tapered profile with a cross-sectional area that increases as distance from the opening increases.

[0015] The active noise cancellation system can take the form of a headphone (e.g., a pair of earphones connected together by a headband) or a headband-free in-ear earphone unit configured to be placed at the entrance to the ear canal of the wearer’s ear and held in place by engagement with the wearer’s ear.

[0016] In one embodiment, the tunable active noise cancellation circuitry and / or the tuning module are provided as part of the earphone (e.g., housed in the main body of the earphone). However, these components can also be provided remote from the earphone.

[0017] In one embodiment, the at least one sensing microphone comprises a feedback microphone (e.g., for sensing pressure changes in the volume (e.g., depending on the type of earphone, a sealed volume or an unsealed volume) between the electro-acoustic driver of the earphone and the ear canal of the wearer).

[0018] In the case of a system comprising a feedback microphone, in one embodiment, the tuning module operates to: determine a voltage ratio of the voltage supplied to the electro-acoustic driver to the resulting voltage generated at the feedback microphone; and determine a degree of deviation between the determined voltage ratio and a voltage ratio expected for a reference ear; and use the degree of deviation in the ratio (e.g., a deviation of the determined voltage ratio from the expected voltage ratio) to tune the active noise cancellation circuitry (e.g., by a fixed function of the detected deviation).

[0019] In one embodiment, the tuning module operates to determine the degree of deviation between the determined voltage ratio and the voltage ratio expected for a reference ear by frequency domain analysis of the microphone voltage signal and the electro-acoustic driver voltage signal.

[0020] In one embodiment, the degree of deviation between the determined voltage ratio and the voltage ratio expected for a reference ear is estimated by a transfer function estimation method between the microphone voltage signal and the electro-acoustic driver voltage signal.

[0021] In one embodiment, the degree of deviation between the determined voltage ratio and the voltage ratio expected for a reference ear is estimated by analysing the power spectral density of the microphone voltage signal and the electro-acoustic driver voltage signal.

[0022] In one embodiment, the at least one sensing microphone includes a feed-forward microphone (e.g., for sensing sound external to the earphone, e.g., for feed-forward noise reduction or binaural monitoring / talking functionality). In the case of a feed-forward system, the at least one sensing microphone can additionally include a feedback microphone as previously defined (e.g., to measure the earphone-to-ear coupling as part of a "hybrid system").

[0023] In the case of a system that includes both a feedback microphone and a feed-forward microphone (a "hybrid system"), the tuning module can operate to: determine a pressure difference (or "pressure gradient") corresponding to a difference in pressure readings between the feedback microphone and the feed-forward microphone (e.g., measure a difference in voltage generated at the feedback microphone compared to voltage generated at the feed-forward microphone); determine a degree of deviation between the determined pressure gradient and a pressure gradient expected for a reference ear; and use the degree of deviation in pressure gradient (e.g., a deviation of the detected pressure gradient relative to the expected pressure gradient) to tune the active noise cancellation circuitry (e.g., by a fixed function of the detected deviation). In one embodiment, the tuning module operates to determine the degree of deviation between the determined pressure gradient and the pressure gradient expected for a reference ear by frequency domain analysis of the feedback microphone signal and the feed-forward microphone signal.

[0024] In one embodiment, the degree of deviation between the determined pressure gradient and the pressure gradient expected for a reference ear is estimated by a transfer function estimation method between the feedback signal and the feed-forward signal.

[0025] In one embodiment, the degree of deviation between the determined pressure gradient and the pressure gradient expected for a reference ear is estimated by analyzing the power spectral density of the feedback signal and the feed-forward signal.

[0026] In one embodiment, the at least one noise control filter includes an analog filter and / or a digital (e.g., algorithm-based) filter.

[0027] In one embodiment, the at least one noise control filter defines a set of adjustable parameters (e.g., a plurality of adjustable parameters).

[0028] In one embodiment, the at least one noise control filter is a programmable filter.

[0029] In the case of a system that includes a feedback microphone, the at least one noise control filter can include a feedback control filter.

[0030] In the case of a system that includes a feed-forward microphone, the at least one noise control filter can include a feed-forward control filter.

[0031] In one embodiment, the tunable active noise cancellation circuitry includes a variable gain device (e.g., a programmable gain amplifier or a programmable attenuator) operative to apply a multiplier (e.g., > 1 or < 1) to a signal supplied to or from a noise control filter (e.g., a feedback control filter or a feedforward control filter).

[0032] In a first set of embodiments, the feedforward control filter includes an adjustable filter.

[0033] In one embodiment, the adjustable filter is configured to attenuate an upper frequency portion of a frequency range under feedforward control.

[0034] In one embodiment, the adjustable filter includes a (e.g., single) biquad filter.

[0035] In a second set of embodiments, the feedforward control filter includes a pair of filters.

[0036] In one embodiment, the pair of filters is configured to permit adjustment (e.g., independent adjustment) of gains applied to an upper frequency portion and a lower frequency portion, respectively, of a frequency range under feedforward control.

[0037] In one embodiment, one of the pair of filters is fixed, and the other is adjustable.

[0038] In one embodiment, one (e.g., the fixed filter) of the pair of filters is a high boost shelf, and the other (e.g., the adjustable filter) is a low boost shelf.

[0039] In one embodiment, the pair of filters each includes a biquad filter.

[0040] In a third set of embodiments, the feedforward control filter includes a fixed filter operative with a variable gain.

[0041] In one embodiment, the fixed filter is configured to attenuate an upper frequency portion of a frequency range under feedforward control.

[0042] In one embodiment, the fixed filter includes a (e.g., single) biquad filter.

[0043] In the case of a system that includes both a feedback microphone and a feedforward microphone (a "hybrid system"), the feedback control filter can be associated with a first variable gain device, and the feedforward control filter can be associated with a second variable gain device that operates independently of the first variable gain device.

[0044] In one embodiment, the tuning module operates to compare the acoustic coupling by comparing a low frequency acoustic coupling to the wearer's ear to a corresponding low frequency coupling to the reference ear.

[0045] For the purposes of the present disclosure, low frequency is defined as relating to frequencies below 500 Hz (e.g., below 400 Hz, such as approximately 200 Hz).

[0046] In one embodiment, the tuning module operates to compare a low frequency transfer function of the system to the wearer's ear (e.g., a low frequency open loop transfer function) to a corresponding low frequency transfer function of the system to the reference ear (e.g., a corresponding low frequency open loop transfer function).

[0047] In the case of a tunable active noise cancellation circuitry including a variable gain device, the tuning module can operate to implement the tuning by modifying the gain change of the variable gain device in proportion to the detected deviation. For example, in the case of a system including a feedback microphone, the tuning module can operate to adjust the loop gain (e.g., low frequency loop gain) of the system to correct the feedback noise cancellation performance. In the case of a system including a feedforward microphone, the tuning module can operate to adjust the path gain (e.g., low frequency path gain) of the system to correct the feedforward noise cancellation performance.

[0048] In one embodiment, the determination of the deviation (e.g., the extent of the deviation) in the acoustic coupling (e.g., the detection of the deviation between the instance of the voltage ratio and the standard voltage ratio) and the modification of the tunable active noise cancellation circuitry based on the determined deviation in the acoustic coupling is performed automatically by the system to a predetermined extent.

[0049] In one embodiment, the tuning module operates to modify an aspect of the noise control filter in proportion to the detected extent of the deviation in the acoustic coupling.

[0050] In one embodiment, the tuning module operates to modify a dominant peak segment of the feedback control filter in proportion to the detected extent of the deviation in the acoustic coupling.

[0051] In one embodiment, the active noise cancellation system further includes a memory, and the preconfigured standard tuning is stored in the memory.

[0052] In one embodiment, the system operates to save the detected deviation or a corresponding tuning value in the memory. In this way, the extent of the deviation (or the associated tuning value) can be saved by a power down, enabling the system to retrieve the deviation value upon power up. Thus, it is not necessary to take a new estimate of the acoustic coupling to the wearer's ear.

[0053] In one embodiment, the system operates to record both the audio signal applied to the electro-acoustic driver (e.g., a playback audio signal or a test signal) and the microphone signal (e.g., a feedback microphone signal). In the case of a "hybrid system," the system can operate to record both the feedback microphone signal and the feed-forward microphone signal. These signals can be recorded in synchronously sampled data frames (e.g., allowing for mobile, phase-coherent estimation of the transfer function between the electro-acoustic driver and the feedback microphone / between the feedback microphone and the feed-forward microphone).

[0054] In one embodiment, the tuning module is programmed to determine the deviation in acoustic coupling at a substantially single frequency range (e.g., a substantially single frequency).

[0055] In another embodiment, the tuning module is programmed to determine the deviation in acoustic coupling at a plurality of different frequency ranges (e.g., a plurality of different single frequencies), and to determine an average deviation in acoustic coupling.

[0056] In one embodiment, the tuning module is programmed to repeatedly (e.g., continuously) determine the deviation in acoustic coupling (e.g., for a single frequency range or a plurality of different frequency ranges).

[0057] In one embodiment, the tuning module is programmed to determine the deviation in acoustic coupling at regular intervals (e.g., every 100-1000 ms, such as every 200-500 ms). In this way, the deviation can be continuously observed with an averaging time constant suitable to track changes in the fit to an individual wearer (e.g., to account for movement of the earphone relative to the wearer's ear during use).

[0058] In one embodiment, the system includes a supervisory component.

[0059] In one embodiment, the supervisory component operates to monitor for the presence of an audio signal (e.g., an audio playback signal or a test signal (e.g., applied to the electro-acoustic driver)), and the tuning module operates to observe (e.g., once or repeatedly) the deviation in acoustic coupling while the audio signal is observed by the supervisory component.

[0060] In one embodiment, the supervision component operates to request an audio signal (e.g., an audio playback signal from a playback system or a test signal (e.g., to be applied to an electro-acoustic driver) from a test signal resource), and the tuning module operates to observe (e.g., once or repeatedly) a deviation in acoustic coupling while the audio signal is observed by the supervision component. In one embodiment, the supervision component operates in this way during a calibration mode selected by a wearer. In another embodiment, the supervision component operates in this way under power up, and / or at regular intervals during periods of time in which the supervision component does not observe audio playback.

[0061] In the case of a system including a feed-forward microphone, in one embodiment, the supervision component monitors an external ambient pressure sensed by the feed-forward microphone, and compares the external ambient pressure to the audio playback level.

[0062] In one embodiment, if the determined ratio of the audio playback level to the external ambient pressure is below a threshold, the supervision component operates to prevent operation of the tuning module (e.g., to determine a deviation in acoustic coupling and / or tuning of the tunable active noise cancellation circuitry). In this way, the system can be configured to avoid tuning of the tunable active noise cancellation circuitry under high ambient noise conditions, and thereby reduce the likelihood of mis-tuning.

[0063] In the case of a hybrid system, the supervision component can be configured to monitor for the presence of an external ambient pressure sensed by the feed-forward microphone (e.g., the presence of an external ambient pressure above a predetermined threshold), and to prevent operation of the tuning module (e.g., to determine a deviation in acoustic coupling and / or tuning of the tunable active noise cancellation circuitry) when the external ambient pressure sensed by the feed-forward microphone is determined to be absent (e.g., below a predetermined threshold).

[0064] In one embodiment, the supervision component is configured to monitor pressure gradient estimates produced by the tuning module. In one embodiment, the supervision component operates to classify the pressure gradient estimates into groups associated with: i) an external sound source; ii) a near-end speech; iii) a sound originating from the electro-acoustic driver; and iv) a mixture of groups i) - iii). In one embodiment, the classification is made based on some simple heuristic rules: group i) is associated with strong negative pressure gradients; group ii) has pressure gradients close to zero in an octave band from 125 Hz to 1 kHz; group iii) has strong positive pressure gradients; and group iv) is exceptional. In one embodiment, only estimates associated with group i) are used to determine the degree of acoustic coupling using the pressure gradient method, and thus, the tuning module is prevented from operating when group i) pressure is not identified by the supervision component as being present.

[0065] In one embodiment, the tuning module operates to perform the acoustic coupling comparison step by evaluating acoustic coupling within a first frequency range (e.g., a substantially single first frequency); and adjust the performance of the tunable active noise cancellation circuitry within a second frequency range (e.g., a substantially single second frequency) different from the first frequency range.

[0066] In one embodiment, the first frequency range ("higher frequency range") encompasses higher frequencies than the second frequency range ("lower frequency range").

[0067] In this way, comparing the acoustic coupling of the earphone to the ear of the individual wearer to the acoustic coupling to the reference ear to determine a deviation in acoustic coupling is performed at one higher frequency in order to extrapolate behavior at a second lower frequency at which adjustment is made to modify the tunable active noise cancellation circuitry to a predetermined degree based on the determined deviation in acoustic coupling at the first higher frequency.

[0068] In one embodiment, the first frequency range is centered at a relatively high frequency, and the second frequency range is centered at a relatively low frequency.

[0069] In one embodiment, the first frequency range and the second frequency range do not overlap.

[0070] For example, in the case of a hybrid system operating to determine a pressure gradient between a feedback microphone signal and a feedforward microphone signal, the determined pressure gradient can be compared to a pressure gradient expected on the reference ear within any frequency or within a band (such as an octave band) of frequencies significantly different from those low frequencies at which leakage compensation of active noise compensation can be expected to operate. For example, an observation of a deviation in pressure gradient detected at 805 Hz (or within a frequency band centered at 805 Hz) can provide sufficient information to adjust the tuning of the tunable active noise cancellation circuitry operating with a peak active attenuation at 120 Hz.

[0071] According to a second aspect of the application, there is provided a method of operating an active noise cancelling system, the system comprising: a headphone comprising: an electro-acoustic driver; and at least one sensing microphone; tunable active noise cancelling circuitry operative to receive a signal from the at least one sensing microphone, the tunable active noise cancelling circuitry being pre-configured in a standard tuning for a reference ear, and comprising at least one noise control filter; and a tuning module; wherein the method comprises: the tuning module configuring the headphone for an individual wearer by: comparing an acoustic coupling of the headphone to an ear of the individual wearer with an acoustic coupling to the reference ear (e.g. when the headphone is in use positioned in or on an ear of a user) to determine a deviation in acoustic coupling; and using the determined deviation in acoustic coupling to modify the tunable active noise cancelling circuitry by a predetermined extent based on the determined deviation in acoustic coupling.

[0072] In one embodiment, the at least one sensing microphone comprises a feedback microphone, and the at least one noise control filter comprises a feedback control filter.

[0073] In one embodiment, the step of comparing the acoustic coupling of the headphone comprises: determining a voltage ratio of a voltage supplied to the electro-acoustic driver to a resulting voltage generated at the feedback microphone; determining a degree of deviation between the determined voltage ratio and a voltage ratio expected for the reference ear; and using the degree of deviation in ratio to tune the tunable active noise cancelling circuitry.

[0074] In one embodiment, the at least one sensing microphone comprises a feedforward microphone, and the at least one noise control filter comprises a feedforward control filter.

[0075] In one embodiment, the step of comparing the acoustic coupling of the headphone comprises: determining a pressure gradient corresponding to a difference in pressure reading between the feedback microphone and the feedforward microphone; determining a degree of deviation between the determined pressure gradient and a pressure gradient expected for the reference ear; and using the degree of deviation in pressure gradient to tune the active noise cancelling circuitry.

[0076] In one embodiment, the step of determining the deviation in acoustic coupling is performed automatically and continuously when the active noise cancelling system is in use (i.e. where the headphone is positioned in or on an ear of a user).

[0077] In one embodiment, the system further comprises a supervisory component.

[0078] In one embodiment, the supervisory component monitors for the presence of an audio signal (e.g. an audio signal requested by the supervisory component); and the step of comparing the acoustic coupling of the earphone comprises observing a deviation in acoustic coupling only while the audio signal is being observed by the supervisory component.

[0079] In one embodiment, the supervisory component performs the steps of monitoring an external ambient pressure sensed by the feed-forward microphone, and comparing the external ambient pressure to the audio playback level; and preventing operation of the tuning module if the determined ratio of the audio playback level to the external ambient pressure is below a threshold value.

[0080] In one embodiment, the supervisory component performs the steps of monitoring for the presence of an external ambient pressure sensed by the feed-forward microphone; and preventing operation of the tuning module when the external ambient pressure sensed by the feed-forward microphone is determined to be absent.

[0081] In one embodiment, the step of comparing the acoustic coupling of the earphone is performed within a first frequency range (e.g. a substantially single first frequency); and the step of modifying the tunable active noise cancellation circuitry by a predetermined amount is performed within a second frequency range (e.g. a substantially single second frequency) different from the first frequency range.

[0082] In one embodiment, the first frequency range ("higher frequency range") encompasses higher frequencies than the second frequency range ("lower frequency range").

[0083] In one embodiment, the first frequency range is centred on a relatively high frequency, and the second frequency range is centred on a relatively low frequency.

[0084] In one embodiment, the first frequency range and the second frequency range do not overlap.

[0085] In one embodiment, the active noise cancellation system is a system according to any embodiment of the first aspect of the application. BRIEF DESCRIPTION OF DRAWINGS

[0086] Embodiments of the application will now be described, by way of example only, with reference to the accompanying drawings in which:

[0087] Figure 1 is a schematic illustration of an earphone of conventional construction showing how it achieves a sealed coupling to the ear;

[0088] Figure 2 is a schematic illustration of a prior art earphone intended to achieve a 'leaky' coupling to the ear of the wearer;

[0089] Figure 3 is a schematic illustration of a 'leaky plug' earphone;

[0090] Figure 4 is based on Figure 3 a schematic illustration of an earphone for use in the active noise cancellation system of the invention;

[0091] Figure 5 a series of measurements of the ratio of the voltage applied at the terminals of an earphone with leakage coupling to the ear to the resulting voltage at the terminals of a pressure sensitive microphone inside the 'nozzle' of the earphone, thereby representing the receive response for seven subjects;

[0092] Figure 6 performance of an earphone with leakage coupling to the ear operating in an active noise cancellation system with fixed tuning;

[0093] Figure 7 a series of measurements of the ratio of the pressure at the external microphone and the internal microphone of an earphone with leakage coupling to the ear;

[0094] Figure 8 a parameter of the measured receive response (G) Figure 5 and the feedforward path gain for optimal active noise cancellation;

[0095] Figure 9 performance of an earphone with leakage coupling to the ear operating in an active noise cancellation system with fixed tuning, but with controller gains adjusted according to the rules identified in Figure 8

[0096] Figure 10 is a schematic illustration of an active noise cancellation system according to a first embodiment of the invention;

[0097] Figure 11 is a schematic illustration of an active noise cancellation system according to a second embodiment of the invention in a first mode of operation;

[0098] Figure 12 is a schematic illustration of the system of Figure 11 in a second mode of operation;

[0099] Figure 13 is a schematic illustration of the system of Figure 11 in a third mode of operation;

[0100] Figure 14 is a schematic illustration of an active noise cancellation system according to a third embodiment of the invention;

[0101] Figure 15 shows a schematic illustration of an active noise cancellation system according to a fourth embodiment of the invention; Figure 14 ​The overall amplitude-frequency response of a pair of biquadratic compensation filters used in an active noise cancellation system of the type shown in

[0102] Figure 16 The overall amplitude-frequency response of a pair of biquadratic compensation filters used in an active noise cancellation system of the type shown in Figure 14 The overall amplitude-frequency response of a pair of biquadratic compensation filters used in an active noise cancellation system of the type shown in

[0103] Figure 17 The overall amplitude-frequency response of a pair of biquadratic compensation filters used in an active noise cancellation system of the type shown in Figure 14 The overall amplitude-frequency response of a pair of biquadratic compensation filters used in an active noise cancellation system of the type shown in

[0104] Figure 18 The overall amplitude-frequency response of a pair of biquadratic compensation filters used in an active noise cancellation system of the type shown in DETAILED DESCRIPTION

[0105] Figure 1 A conventional prior art earphone 1 is shown, having a main body 2 and a flexible tip or 'collar' 3 designed to provide both a mechanical and acoustic seal to the ear. When the earphone 1 is placed in the ear 4, the earphone 1 occupies the concha 5 and the tip 3 engages the interface between the concha and the distal end of the external auditory canal or 'ear canal' 6, with the elastic deformation of the collar 3 achieving the seal.

[0106] Figure 2 A 'leaky plug' type of prior art earphone 8 is shown, in which a wide conical nozzle 9 is intended to produce a leaky acoustic interface to the wearer's ear. When the earphone 8 is placed in the ear 4, the earphone 8 occupies the concha 5 and the nozzle 9 engages the transition between the concha and the distal end of the external auditory canal or 'ear canal' 6.

[0107] Depending on the relative size and shape of the earphone 8 (and in particular the nozzle 8) and the user's ear 4 (and in particular the details of the transition between the concha 5 and the distal end of the ear canal 6), a partial seal is made between the earphone 8 and the ear 4. In some wearers, this seal is very effective, even though the body of the nozzle is rigid and impermeable. In other wearers, there is no seal. Most wearers will experience a significant change in the degree of sealing when the earphone is moved slightly within the ear; this is a manifestation of the performance changing with individual 'fit', as opposed to performance variation from wearer to wearer. The methods taught in this invention can be used to compensate for the effects of fit.

[0108] Figure 3A 'leaky-bung' type earphone 8' is shown, equipped with transducers sufficient to support hybrid active noise control. The earphone comprises a body 10 which houses a conventional electro-acoustic driver (micro-speaker or'receiver') 11, the body 10 defining a passageway 10A from the electro-acoustic driver 11 to an opening 10B in the body. The body 10 has a frusto-conical nozzle portion 9' which defines a tapering ear engaging surface 9A configured to engage the outer ear of a wearer's ear. The body 10 also includes a microphone 12 which is pressure sensitive to the inside of the nozzle 9, the pressure will consist of front radiation from the receiver 11 and loosely coupled sound from the ear and environment. This microphone 12 is used to provide the control signal on which the 'feedback' noise canceller is based, and is therefore referred to as the 'feedback' microphone. The earphone 8' further incorporates a second microphone 13 which is positioned so as to be sensitive to the pressure outside the ear. This microphone is intended to provide a reference pressure for use in a 'feed-forward' noise cancellation architecture, and is therefore referred to as the 'feed-forward' microphone. The feed-forward microphone is positioned (and both acoustically and mechanically isolated) so as to minimise its sensitivity to radiation from the receiver 11.

[0109] Figure 4 The earphone 8' is shown from Figure 3 within a tunable 'hybrid' noise cancellation topology circuitry 7, which includes filters 14, 15 along with a summing node 16 and an amplifier 17, in which both feedback control and feed-forward control are provided. The signal from the feedback microphone 12 is passed through a filter 14, which can implement a reference tuning and is adjusted to express any individual wearer-to-reference bias. This bias can be expressed, for example, by scalar multiplication (i.e. a gain change), by modification of the parameters of a peak section of the filter 14 or by more significant modification / reconfiguration of the filter 14 (which can be an analogue or digital filter defining a set of parameters).

[0110] The signal from the feed-forward microphone 13 is passed through a filter 15, which can implement a reference tuning and is adjusted to express any individual wearer-to-reference bias. This bias can be expressed, for example, by scalar multiplication (i.e. a gain change) or by more significant modification / reconfiguration of the filter 15.

[0111] The filtered microphone signals are combined at a summing node 16 and passed to an amplifier 17, which drives the receiver 11. Other signals such as audio programmes for entertainment or communication or test signals required for measurement system low frequency receive sensitivity are applied to the summing node at a signal input 18.

[0112] Figure 5 and Figure 6Validation of the central proposition of the present invention; specifically, the performance of noise-cancelling earplugs with leaky coupling to the ear and fixed tuning is not useful in a range of wearers, but can be restored by simply applying a gain correction that is functionally derived from the observed deviation in low-frequency receive sensitivity. For simplicity, the functional relationship is a linear one in the cases taught herein, but more complex relationships can be implemented.

[0113] Figure 5 The ratio of the voltage applied to the receiver 11 of the leaky earplug to the voltage generated at the feedback microphone 12 is shown for seven subjects measured in a pseudo-diffuse noise field. This ratio is the 'feedback-device' response. Figure 5 The bolded traces are for the subject for which the reference tuning was developed. Figure 5 The dashed traces are additional subjects that will be used in the validation of the process (to be reported in Figure 9 Note the range of receive sensitivity exhibited by these 'device responses' at low and mid-frequencies (below 1 kHz). This is evidence of varying degrees of leakage caused by the coupling between the earplug nozzle 9 and the individual wearer's ear.

[0114] Figure 6 Active Noise Reduction achieved when the noise control system with fixed tuning designed for the reference subject is used with the same six subjects reported by the continuous traces in Figure 5 The bolded trace is the reference subject who (naturally) experiences the best noise cancellation. The other wearers experience poor performance, with several experiencing noise enhancement, particularly at low frequencies (<100 Hz) or in the psycho-acoustically important range from 500-2 kHz.

[0115] Figure 7 The ratio of the voltage outputs of the feedback and feedforward microphones of the leaky earplug worn by the same subjects as those reported in Figure 5 Unlike the previous case, this 'feedforward-device' response exhibits less wearer dependence. More precisely, it is surprisingly constant.

[0116] Figure 5 The six subjects whose 'feedback-device' responses were reported by the continuous lines in participated in the experiment. All six subjects used a single reference tuning (for the earplug with nozzle 9 and the earplug with nozzle 10) and a single gain correction (for the feedback microphone 12). Figure 5feedback device response of the subject) and their feedforward controller gains were artificially adjusted until both i) the noise cancellation performance measured at the feedback microphone was observed to reach an optimal level when monitored on an audio analyser and ii) the reported subjective level of noise cancellation reached an optimum. The feedforward gain adjustment that gave this optimal noise cancellation was recorded for each wearer.

[0117] Figure 8 The gain adjustments that were found to result in optimal active noise cancellation are plotted against a simple scalar measure of the deviation between the individual's feedback device response and the feedback device response of the reference user. It is seen that these follow a functional relationship as expected. In this case, the'simple scalar measure of the deviation between the individual's feedback device response and the feedback device response of the reference user' was simply the difference in magnitude at 200Hz, and, Figure 8 The functional relationship is revealed to be linear.

[0118] In practice, the feedback device response can be interrogated at more than one frequency and a weighted average of the differences at these frequencies computed to compare the feedback device response of the wearer to that of the reference. This will produce a more robust estimate of the deviation between the individual wearer's leakage condition and that experienced by the reference wearer but it comes at the expense of additional computational load. In practice, if more than ~7 frequencies are added, the additional computational load is likely to be trivial.

[0119] Figure 9 The active noise reduction achieved when the same noise control system with fixed tuning designed for the reference subject is used with the seven subjects reported in Figure 5 is shown. Note that all wearers experience good noise cancellation performance without any experiencing enhancement. The performance curves described by the continuous traces all have their feedforward controller gains adjusted according to the findings of the experiments described in Figure 8 and specified in detail in Figure 5 The performance described by the dashed trace is associated with a subject that did not participate in the experiments. This performance is achieved by identifying the deviation of the feedback device of the subject from Figure 8 the dashed trace in Figure 9 and passing this deviation into the function described in The resulting gain correction immediately produces the 'leakage compensated' performance described in

[0120] for this subject. This has been repeated for other subjects to validate the process.The present invention is independent of the means by which the response of the feedback device is measured, but in the absence of means to estimate this feature of the headphone behaviour in vivo, practical exploitation of the present invention is impossible.

[0121] Figure 10 An active noise cancellation system 20 is shown. Figure 4 'Leaky plug' earphones 8' are incorporated, comprising the tunable 'hybrid' noise cancellation topology circuitry 7 as previously described. System 20 includes a processing element 24 operative to perform supervisory functions and capable of observing the audio playback signal 21 and the output 22a of the feedback microphone 12 via a data converter 23 and performing tuning module functions. Note that the presence of data converter 23 does not imply the presence of an analog feedback signal; any of signals 21, 22a, and 22b (the output of the feedforward microphone 13) can be represented as analog or digital signals without bias or limitation. These observations are passed from data converter 23 to processing element 24, which is capable of operating on the signal observations. Processing element 24 is typically implemented as a microcontroller or similar programmable device.

[0122] The typical observations required to maintain an estimate of the low-frequency feedback device estimate involve the compilation of time-aligned frames of the two signals, (optional) imposition of a time-domain window, Fourier transform calculation, and maintenance of automatic and cross-spectral estimates as described above. The required deviation from the reference feedback device amplitude can be calculated from these estimates.

[0123] Maintenance of the automatic and cross-spectral estimates involves an explicit averaging process, advantageously implemented using a simple first-order filter with a long time constant on the order of one second. Such averaging is useful in establishing noise immunity, allowing the transfer function estimate to suppress the destructive effects of noise sources such as ambient sound, which would otherwise corrupt the correct estimate of the receive response. The averaging time constant should not be too long, as it is useful to enable the system to track changes in the low-frequency coupling between the earphone and the ear for a particular wearer. Such changes inevitably occur when the earphones move slightly with use; this is known as 'fit'.

[0124] As long as continuous observation of low-frequency coupling is performed using an automated system such as that described above, the teachings of the present invention can be used to account for changes in noise cancellation performance over time for a wearer that are associated with fit. This requires careful selection of the averaging time constant; long enough to ensure good noise suppression, yet short enough to allow good tracking of changes due to fit.

[0125] The active noise cancellation system 20 is equipped with an interface 25 suitable for transmitting control outputs 26 and 27 respectively to modify the feedback filter 14 and the feedforward filter 15 in order to implement leakage compensation according to the observed feedback device bias.

[0126] The active noise cancellation system 20 is further equipped with an interface 28 internal or external to the memory element(s) 29. These memory elements 29 allow the system to store recent values of the observed feedback device bias so that the system is powered in a state suitable for its owner without having to wait for the convergence of a new estimate of the feedback device bias.

[0127] In addition, the active noise cancellation system 20 has a support interface 30 (via an interface component 30a) for a user interface or additional capabilities controlled by a host device so that its operation can be modified or suspended as appropriate.

[0128] In particular, the design of the earbud earphone 8' (and of its integral transducers 11, 12, 13) anticipates applications such as Figure 10 The teachings of the present application are facilitated within the system shown in Fig. 3, where an active measurement is applied to compensate for leakage. The design for active leakage compensation can include: i) placing the error microphone 12 closer to the outer (ear) end of the nozzle 9' of the earbud earphone 8'; and ii) one should ensure an earbud design with low nozzle impedance.

[0129] Figure 11 Fig. 4 shows the active noise cancellation system 20 based on the Figure 10 of Fig. 1 and incorporating the 'leaky plug' earphone 8' of Fig. 2 (the corresponding features being marked accordingly) and Fig. 5 shows the active noise cancellation system 20' of Fig. 4 in a first operating mode. Figure 4 Fig. 6 shows the active noise cancellation system 20' of Fig. 4 in a second operating mode. Figure 11 Fig. 5 shows the active noise cancellation system 20' in a first operating mode, in which the required modifications of the feedback filter 14 and of the feedforward filter 15 are limited to simple scalar (gain) modifications, and in which the control outputs 31' and 32' are directed to multipliers 33 and 34 (e.g. gain amplifiers or attenuators provided as part of the feedback and feedforward circuitry) in the feedback and feedforward paths respectively.

[0130] Figure 12 Fig. 6 shows the active noise cancellation system 20' in a second operating mode, in which the required modifications of the feedback filter 14 and of the feedforward filter 15 are again limited to simple scalar (gain) modifications, but this time applied as a common factor to both the feedback and feedforward paths. In this simple and preferred embodiment, a single control output 35 from the supervisory layer is sufficient to control both multipliers 33 and 34.

[0131] Figure 13 Active noise cancellation system 20' is shown in a third operating mode, in which there is an additional capability 36 to observe external pressure at the 'feedforward' microphone via data converter 23. This observation provides a means to assess the signal-to-noise ratio of the feedback device estimation performed by the supervisory system. External noise is one mechanism that can corrupt the transfer function estimates used in performing feedback device estimation. While averaging and phase coherence methods are not good for this noise source, it is desirable to monitor external noise conditions and gate out measurements made under high ambient noise conditions.

[0132] By being closely connected to the user and / or host interface 30 and explicitly observing the audio playback signal 21, the supervisory system can operate only in the presence of a playback signal that has been intentionally selected by the user (such as when the user has turned on music playback or enabled a calibration mode for the headphones). Thus, the supervisory system will only update the control filters 14, 15 when there is 'valid' information, thereby avoiding 'hunting'. Alternatively, the supervisory system can request playback of a test signal through the host interface 30 and thus be able to initiate coupling measurements to the ear (e.g., at power-up, or at regular intervals if there is no audio playback).

[0133] In summary, the present system 20 and system 20' provide a practical strategy suitable for high-volume deployment in consumer applications to correct the performance of active noise control systems in 'leaky' headphones. This correction allows a single 'tuning' of the system to be adapted to any ear with simple modifications. The required modifications are simple enough that they can be performed automatically in situ by the headphones themselves.

[0134] The teachings of the present invention can be utilized to allow the method to compensate for changes in the performance of any earphone that naturally occur as the earphone moves within the ear of an individual wearer.

[0135] The present invention uses a simple fixed filter solution designed for one 'reference' user during product development and simple rules that express how the reference solution should be modified for different wearers in application. The changes (of the reference control solution) required to maintain good noise cancellation performance across a group of wearers can be as simple as a scalar adjustment; no complete filter redesign, adaptive filtering, or computationally expensive processing is required.

[0136] It has been observed that the most important change (relative to the reference) observed in the active noise control in the band of interest when the 'leaky' earphone is placed in the ear of a number of wearers is a change in the low frequency receive response of the system (which shall later in this specification be denoted the 'feed-forward device' response). This is the factor by which the user hears the reproduced sound and the dominant component of the open loop response of the 'feed-back' active noise reduction system.

[0137] It has further been observed that the ratio of the pressure outside the earphone to the pressure inside the earphone in the same band of interest remains constant when measured over a number of ears, this ratio remains fairly constant despite variations in the coupling to the individual's ear. This ratio can usefully be referred to as the 'feed-forward device'.

[0138] By changing the low frequency feedback loop gain by an amount equal to the observed change in the low frequency receive sensitivity of the system, the performance of the feedback control system on the new wearer can be restored to the performance experienced by the reference user. In many practical cases where the feedback control system has been designed with sufficient stability margin, this can simply be achieved by scaling the feedback loop gain by an amount equal to the observed deviation in the other components of the open loop response.

[0139] In other cases, the low frequency feedback active control is dominated by the action of a multiplication factor of the feedback control filter 14 having a 'peaking' magnitude response, as in the case of a canonical "peaking parametric EQ" filter having a Laplace domain response:

[0140]

[0141] where ω ０ is the peaking frequency, Q p is the quality factor of the peak, and the height of the peak is log 10 (A0) dB.

[0142] In such cases, by reducing the magnitude of the peaking segment response, the performance of the feedback control system on the 'new' wearer can be restored to a level close to the level of performance experienced by the reference user, facilitated by reducing A0 by an amount proportional to the observed change in the low frequency receive sensitivity of the system (some concomitant change in Q p may also be preferred).

[0143] Similarly, since the target feedforward filter 15 effectively involves the ratio of two elements (feedback device and feedforward device), one of which is not subject to wearer-to-wearer variations, the performance of the feedforward control system on a new wearer can be restored to that experienced by the reference user by altering the low frequency feedforward path gain by an amount equal to the observed deviation in the low frequency receive sensitivity of the system from that observed on the'reference' wearer. In many practical cases where the feedforward control system has been designed with due regard to the high frequency behaviour of the feedforward filter 15, this can be achieved simply by scaling the feedforward path gain.

[0144] In one embodiment, the reference tuning of the control filters 14, 15 of the active noise cancellation system 20, 20' is provided for a median (or other representative) wearer, and observed during a wear period of the low frequency acoustic coupling to the wearer's ear, and compared to the coupling to the reference ear. This comparison is expressed as a "deviation". The deviation D (dB) between the (magnitude) open loop transfer function measured with the wearer and that predicted on the reference wearer is estimated. The instantaneous low frequency loop gain of the system is adjusted by -D (dB) to correct the feedback noise cancellation performance.

[0145] In one embodiment, the reference tuning of the control filters 14, 15 of the active noise cancellation system 20, 20' is provided for a median (or other representative) wearer, and observed during a wear period of the low frequency open loop transfer function of the system, by injection of an audio signal (for audio playback or explicit test purposes) and monitoring of the resulting response at the 'error microphone' of the system. The deviation D (dB) between the (magnitude) open loop transfer function measured with the wearer and that predicted on the reference wearer is estimated. The instantaneous low frequency loop gain of the system is adjusted by -D (dB) to correct the feedback noise cancellation performance.

[0146] In another embodiment, the reference tuning of the control filters 14, 15 of the active noise cancellation system 20, 20' is provided for a median (or other representative) wearer, and observed during a wear period of the low frequency open loop transfer function of the system, by injection of an audio signal (for audio playback or explicit test purposes) and monitoring of the resulting response at the 'error microphone' of the system. The deviation D (dB) between the (magnitude) open loop transfer function measured with the wearer and that predicted on the reference wearer is estimated. The reference feedback control law H FB is modified so that its peak factor is attenuated by D (dB) to correct the feedback noise cancellation performance.

[0147] In one embodiment, the reference tuning of the control filters 14, 15 of the active noise cancellation system 20, 20' is provided for a median (or other representative) wearer, and, during the wearing of the low frequency open loop transfer function of the system, observations are made by injection of an audio signal (for audio playback or explicit test purposes) and monitoring of the resulting response at the 'error microphone' of the system, or by other means. The deviation D (dB) between the (amplitude) open loop transfer function measured with the wearer and the (amplitude) open loop transfer function expected on the reference wearer is estimated. The instantaneous loop gain of the system is adjusted by -D (dB) to correct the feedback noise cancellation performance, as the open loop response of the system is such that a sufficient stability margin is maintained.

[0148] In one embodiment, the reference tuning of the control filters 14, 15 of the active noise cancellation system 20, 20' is provided for a median (or other representative) wearer, and, during the wearing of the low frequency open loop transfer function of the system, observations are made by injection of an audio signal (for audio playback or explicit test purposes) and monitoring of the resulting response at the 'error microphone' of the system, or by other means. The deviation D (dB) between the (amplitude) open loop transfer function measured with the wearer and the (amplitude) open loop transfer function expected on the reference wearer is estimated. The instantaneous loop gain of the feedback control system is adjusted by -D (dB) to correct the feedback noise cancellation performance. The instantaneous path gain of the feedforward control system is adjusted by -D (dB) to correct the feedforward noise cancellation performance.

[0149] In one embodiment of the application, the observations during the wearing of the low frequency open loop transfer function use i) recordings of the playback audio signal applied to the loudspeaker and ii) recordings of the feedback microphone signal. These signal recordings are in synchronously sampled 'frames' of data, allowing for a moving, phase-coherent estimation of the transfer function between these two points. Such transfer function estimation is made at regular intervals (200-500 ms) and at one or more frequencies chosen to most clearly reveal wearer dependence. The deviation from the reference performance can be estimated at one frequency or averaged over several frequencies or a range of frequencies to improve the quality of the estimate and its robustness to noise.

[0150] Figure 14 A further additional example of an active noise cancellation system 20" incorporating a modified 'leaky bung' earpiece 8" (corresponding features being labelled accordingly) is shown based on the active noise cancellation system 20 of Figure 10 and based on the 'leaky bung' earpiece of Figure 4 .

[0151] As shown, the earphone 8" includes a modified feed-forward filter 15' that includes an adjustable filter section 37 cascaded with a fixed filter 38 stage operating under the control of a processing element 24" that operates to perform both a supervisory function and a tuning module function.

[0152] The noise cancellation system 20" is configured to estimate a pressure gradient across the earphone that simultaneously observes both the external pressure at the feed-forward microphone 13 via signal 22b" and the internal pressure at the feedback microphone 12 via signal 22a". This pressure gradient is compared to a reference value that represents the expected value under normal fitting conditions, and the deviation (degree of deviation) will be able to be used as a leakage measure. This leakage measure can be used to adjust (via control outputs 26" and 27") the tuning of the active noise cancellation circuitry 14', 15' in order to compensate for the effects of leakage.

[0153] In this embodiment, the pressure gradient can be estimated by a transfer function estimation method between the feedback microphone signal and the feed-forward microphone signal. Alternatively, the pressure gradient can be estimated by the difference in power spectral density of the feedback microphone signal and the feed-forward microphone signal. In either of the previously mentioned cases, it is understood that the pressure gradient estimation is made in the frequency domain.

[0154] In one embodiment, the adjustable filter 37 is conveniently implemented as a pair of biquad filters configured to allow independent adjustment of the gain of the upper and lower portions of the frequency range of the feed-forward controller 15'. In one embodiment, the adjustable filter 37 includes a low boost shelf and a high boost shelf, and the crossover between the two is a fixed property of the design. The boost is a function of the determined deviation. In a practical implementation, the boost can be a linear function of the determined deviation. Scaling on the boost function for each of the two filters can introduce a differential compensation of the upper and lower frequency ranges of the feed-forward controller 15'.

[0155] In practice, the adjustable filter 37 can vary in complexity from a scalar multiplier (as previously taught in this specification) to a single biquad section implementing a shelving filter response or to a pair of biquad filters. Higher order filters can be utilized in 37, but this has been found to offer little practical advantage.

[0156] In the absence of leakage, the adjustable filter 37 presents a unity transfer function, leaving the non-trivial characteristics of the feed-forward filter defined by the fixed element 38 alone.

[0157] Figure 15A plot showing the magnitude frequency response of the adjustable filter 37 in four different leakage configurations. The adjustable filter 37 in this example is implemented by a cascade of two biquad filters. The first filter response group (left-hand set of four lines in the plot) is intended to compensate for the interruption of the earphone's receive response caused by leakage; it is a low-frequency effect. The second filter response group (right-hand set of four lines in the plot) is intended to compensate for the interruption of the earphone's feedback device response caused by leakage.

[0158] Figure 16 A plot showing the magnitude frequency response of the adjustable filter 37 in four different leakage configurations. The adjustable filter 37 in this example is implemented by a single biquad filter, providing a controllable boost in the shelf. The 'S-shaped' shape of the response is formed from the product of two biquad filter responses of the form seen in Figure 15 and imparts attenuation to the upper frequency portion of the frequency range of the feedforward controller, which helps to prevent unwanted noise enhancement during leakage compensation; thus, the system provides the same degree of compensation. The response associated with the product of two biquad filters is considered to be a continuous trace, and a fully acceptable single filter implementation is shown in a dotted trace. The single filter solution is actually more efficient to implement. The filter is a low-boost shelf design, and the shelf corner frequency is a fixed property of the design. The 'boost' and 'gain' are functions of the determined deviation. In a practical implementation, the boost and gain can be linear functions of the determined deviation.

[0159] Figure 17 A plot showing the magnitude frequency response of the adjustable filter 37 in four different leakage configurations. The adjustable filter 37 in this example is implemented by a single biquad filter, providing a fixed boost in the shelf; only the gain is a function of the determined deviation. Again, the filter allows adjustment of the feedforward controller, and its S-shaped form imparts a fixed attenuation to the upper portion of the frequency range of the feedforward controller, which helps to prevent unwanted noise enhancement during leakage compensation. The filter is a low-boost shelf design, and the shelf corner frequency is a fixed property of the design. The gain is a function of the determined deviation. In a practical implementation, the gain can be a linear function of the determined deviation.

[0160] In all of the embodiments described above, the adjustment of the tuning of the active noise cancellation circuitry to compensate for the effects of leakage is understood to operate at generally low frequencies (e.g. frequencies below 800Hz), with the maximum effect being at frequencies in the order of 100 to 200Hz. This is also the frequency range in which the feedback device response has been observed to be weakly related to, or completely independent of, leakage in several headphone types. However, the adjustment can be implemented based on an observation of the degree of deviation between a measurement of the acoustic coupling between the headphone and the wearer's ear and a reference, expected value of this acoustic coupling, made at a frequency substantially different from the lower frequencies at which the leakage compensation operation of the active noise compensation can be expected to operate. Importantly, this different frequency can be significantly higher than 100 to 200Hz, typically 800Hz or 2.1 kHz. This is true in both of the following cases: when the measurement of the acoustic coupling is derived from the voltage ratio between the voltage input to the electro-acoustic driver 11 and the voltage output from the feedback microphone 12; when the measurement of the acoustic coupling is derived from the pressure ratio between the feedback microphone pressure and the feed-forward microphone pressure. These higher observation frequencies are the conditions in which both the feed-forward device response and the receive response are functionally related to leakage. However, changes in the low frequency receive response that are difficult to measure directly can be made by observing the high frequency feed-forward device response and utilising the correlation between these two functions.

[0161] To illustrate this point, Figure 18 Measurements of the amplitude feed-forward device response at 805Hz and the (simultaneous) receive frequency response at 150Hz for headphones in a nominal fit and under four conditions of leakage are shown. The tests were conducted on eight human ears. It is seen that there is a clear correlation between the two parameters; the dotted line is a least squares linear fit to the data. This indicates that observation of the behaviour at 805Hz (in this case, pressure gradient) can be used to provide an indirect observation of the receive frequency response at 150Hz. Using this indirect derived information, the tuning of the active noise cancellation circuitry to compensate for the effects of leakage can continue, including by adjusting the filter as previously taught.

Claims

1. An active noise cancellation system, comprising: Headphones, comprising: electroacoustic drivers; and at least one sensing microphone; tunable active noise cancellation circuitry operative to receive signals from the at least one sensing microphone, the tunable active noise cancellation circuitry being preconfigured in a standard tuning for a reference ear and including at least one noise control filter; and A tuning module that operates to configure the earphones for an individual wearer by: comparing the acoustic coupling of the earphone to the individual wearer's ear with the acoustic coupling to the reference ear to determine a deviation in acoustic coupling based at least in part on the expected voltage ratio associated with the reference ear; and The determined deviation in acoustic coupling is used to modify the tunable active noise cancellation circuitry by a predetermined degree based on the determined deviation in acoustic coupling.

2. The system according to claim 1, wherein: The at least one sensing microphone comprises a feedback microphone, and the at least one noise control filter comprises a feedback control filter.

3. The system according to claim 2, wherein: The tuning module operates to: determining a voltage ratio of a voltage supplied to the electroacoustic driver and a resulting voltage generated at the feedback microphone; determining a degree of deviation between the determined voltage ratio and an expected voltage ratio for the reference ear; and The degree of deviation in the ratio is used to tune the active noise cancellation circuitry.

4. The system according to claim 2 or 3, wherein: The at least one sensing microphone comprises a feedforward microphone, and the at least one noise control filter comprises a feedforward control filter.

5. The system according to claim 4, wherein: The tuning module operates to: determining a pressure gradient corresponding to a difference in pressure readings between the feedback microphone and the feedforward microphone; determining a degree of deviation between the determined pressure gradient and a pressure gradient expected for the reference ear; and The degree of deviation in the pressure gradient is used to tune the active noise cancellation circuitry.

6. The system according to claim 1, wherein: The tunable active noise cancellation circuitry includes a variable gain device operative to apply a multiplier to a signal supplied to or from the noise control filter.

7. The system according to claim 6, wherein: The tuning module operates to compare acoustic coupling by comparing low frequency acoustic coupling to the wearer's ear with corresponding low frequency coupling to the reference ear.

8. The system according to claim 7, wherein: The tuning module operates to compare a low frequency transfer function of the system to the wearer's ear with a corresponding low frequency transfer function of the system to the reference ear.

9. The system according to claim 6, wherein: The tuning module operates to achieve tuning by modifying a gain change of the variable gain device in proportion to the deviation.

10. The system according to claim 6, wherein: The determination of the deviation in acoustic coupling and the modification of the tunable active noise cancellation circuitry to a predetermined degree based on the determined deviation in acoustic coupling are performed automatically by the system.

11. The system according to claim 6, wherein: The tuning module operates to modify aspects of the noise control filter in proportion to the degree of deviation in acoustic coupling.

12. The system according to claim 2, wherein: The tuning module operates to modify a dominant peak section of the feedback control filter in proportion to the degree of deviation in acoustic coupling.

13. The system according to claim 6, wherein: The active noise cancellation system further includes a memory, and the preconfigured standard tuning is stored in the memory.

14. The system according to claim 13, wherein: The system operates to save the deviation or corresponding tuning value in the memory.

15. The system according to claim 6, wherein: The tuning module is programmed to determine the deviation in acoustic coupling at a plurality of different frequency ranges and to determine an average deviation in acoustic coupling.

16. The system according to claim 6, wherein: The tuning module is programmed to repeatedly determine the deviation in acoustic coupling.

17. The system according to claim 16, wherein: The tuning module is programmed to determine the deviation in acoustic coupling at regular intervals.

18. The system of claim 4, wherein: The system includes a supervisory component.

19. The system according to claim 18, wherein: The supervisory component operates to monitor for the presence of an audio signal, and the tuning module operates to observe the deviation in acoustic coupling while the audio signal is observed by the supervisory component.

20. The system of claim 18, wherein: The supervisory component operates to request an audio signal, and the tuning module operates to observe the deviation in acoustic coupling while the audio signal is observed by the supervisory component.

21. The system of claim 18, wherein: The supervisory component: monitoring external ambient pressure sensed by the feed-forward microphone and comparing the external ambient pressure to an audio playback level; and If the determined ratio of the audio playback level to the external ambient pressure is below a threshold, operation of the tuning module is prevented.

22. The system of claim 18, wherein: The supervisory component: monitoring for the presence of external ambient pressure sensed by the feedforward microphone; as well as When external ambient pressure sensed by the feedforward microphone is determined to be absent, operation of the tuning module is prevented.

23. The system of claim 6, wherein: The tuning module operates to: performing said acoustic coupling comparison step by evaluating acoustic coupling within a first frequency range; as well as adjusting the performance of the tunable active noise cancellation circuitry within a second frequency range different from the first frequency range; The first frequency range covers higher frequencies than the second frequency range.

24. A method of operating an active noise cancellation system, the system comprising: Headphones, comprising: electroacoustic drivers; and at least one sensing microphone; tunable active noise cancellation circuitry operative to receive signals from the at least one sensing microphone, the tunable active noise cancellation circuitry being preconfigured in a standard tuning for a reference ear and including at least one noise control filter; and Tuning module; The method comprises: The tuning module configures the earphones for an individual wearer by: comparing the acoustic coupling of the earphone to the individual wearer's ear with the acoustic coupling to the reference ear to determine a deviation in acoustic coupling based at least in part on the expected voltage ratio associated with the reference ear; and The determined deviation in acoustic coupling is used to modify the tunable active noise cancellation circuitry by a predetermined degree based on the determined deviation in acoustic coupling.

25. The method of claim 24, wherein: the at least one sensing microphone comprising a feedback microphone, and the at least one noise control filter comprising a feedback control filter; as well as The step of comparing the acoustic coupling of the earphones comprises: determining a voltage ratio of a voltage supplied to the electroacoustic driver and a resulting voltage generated at the feedback microphone; determining a degree of deviation between the determined voltage ratio and an expected voltage ratio for the reference ear; and The degree of deviation in the ratio is used to tune the tunable active noise cancellation circuitry.

26. The method according to claim 25, wherein The at least one sensing microphone comprises a feedforward microphone, and the at least one noise control filter comprises a feedforward control filter.

27. The method according to claim 26, wherein The step of comparing the acoustic coupling of the earphones comprises: determining a pressure gradient corresponding to a difference in pressure readings between the feedback microphone and the feedforward microphone; determining a degree of deviation between the determined pressure gradient and a pressure gradient expected for the reference ear; and The degree of deviation in the pressure gradient is used to tune the active noise cancellation circuitry.

28. The method according to claim 24, wherein The step of determining the deviation in acoustic coupling is performed automatically and continuously while the active noise cancellation system is in use.

29. The method according to claim 27, wherein The system further includes a supervisory component.

30. The method according to claim 29, wherein The method further comprises: The supervisory component monitors for the presence of an audio signal requested by the supervisory component; and The step of comparing the acoustic coupling of the earphones comprises: The deviation in acoustic coupling is observed only while the audio signal is being observed by the monitoring component.

31. The method of claim 29, wherein: The supervisory component performs the following steps: monitoring external ambient pressure sensed by the feed-forward microphone and comparing the external ambient pressure to an audio playback level; and If the determined ratio of the audio playback level to the external ambient pressure is below a threshold, operation of the tuning module is prevented.

32. The method of claim 29, wherein: The supervisory component performs the following steps: monitoring for the presence of external ambient pressure sensed by the feedforward microphone; as well as When external ambient pressure sensed by the feedforward microphone is determined to be absent, operation of the tuning module is prevented.

33. The method of claim 24, wherein: The step of comparing the acoustic coupling of the earphones is performed within a first frequency range; and The step of modifying the tunable active noise cancellation circuitry to a predetermined degree is performed within a second frequency range different from the first frequency range; The first frequency range covers higher frequencies than the second frequency range.

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