Patent Publication Number: US-2021165866-A1

Title: Methods, apparatus and systems for authentication

Description:
TECHNICAL FIELD 
     Embodiments of the disclosure relate to methods, apparatus and systems for authentication of a user, and particularly to methods, apparatus and systems for authentication of a user based on ear biometric data. 
     BACKGROUND 
     It is known that the acoustic properties of a user&#39;s ear, whether the outer parts (known as the pinna or auricle), the ear canal or both, differ substantially between individuals and can therefore be used as a biometric to identify the user. One or more loudspeakers or similar transducers positioned close to or within the ear generate an acoustic stimulus, and one or more microphones similarly positioned close to or within the ear detect the acoustic response of the ear to the acoustic stimulus. One or more features may be extracted from the response signal, and used to characterize an individual. 
     For example, the ear canal is a resonant system, and therefore one feature which may be extracted from the response signal is the resonant frequency of the ear canal. If the measured resonant frequency (i.e. in the response signal) differs from a stored resonant frequency for the user, a biometric algorithm coupled to receive and analyse the response signal may return a negative result. Other features of the response signal may be similarly extracted and used to characterize the individual. For example, the features may comprise one or more mel frequency cepstrum coefficients. More generally, the transfer function between the acoustic stimulus and the measured response signal (or features of the transfer function) may be determined, and compared to a stored transfer function (or stored features of the transfer function) which is characteristic of the user. 
     SUMMARY 
     One problem faced by biometric algorithms is the need to achieve acceptable performance in two respects. First, the algorithm should provide acceptable security so that unauthorised users are not falsely recognized as authorised users. The likelihood that the algorithm will accept an access attempt by an unauthorised user is known as the false acceptance rate (FAR), and should be kept low if the algorithm is to provide reasonable security. Second, the algorithm should work reliably, so that authorised users are not falsely rejected as unauthorised. The likelihood that the algorithm will reject an access attempt by an authorised user is known as the false rejection rate (FRR), and should also be kept low if the algorithm is not to prove frustrating for authorised users seeking authentication. 
     The problem is that these two performance requirements conflict with each other. A low FRR can be achieved by relaxing the requirements for a user to achieve authentication. However, this will also have the consequence of increasing the FAR. Conversely, a low FAR can be achieved by making the requirements for a user to achieve authentication stricter. However, this will have the consequence of increasing the FRR. 
     One way to decrease both FAR and FRR is to increase the efficacy of the biometric algorithm itself. However, designing the algorithm to achieve high performance is difficult. Further, the efficacy may depend on factors which are outside the designers&#39; control. For example, the efficacy of the algorithm may depend on the quality of the biometric data. However, the user may be in a noisy environment such that poor data quality is unavoidable. 
     The efficacy of the algorithm may further depend on the discriminatory nature of the biometric itself. For example, a biometric algorithm which discriminates between users based solely on gender will only ever achieve a 50% FAR and a 50% FRR at best. 
     The efficacy of the authentication process overall may therefore be improved by combining multiple authentication processes (whether biometric or not). Each authentication process may be associated with particular FAR and FRR values; however, the FAR and FRR for the combination of multiple authentication processes may be significantly lower. 
     For example, let us assume that a first authentication process has a FAR of 10%; one in ten users will be accepted by the first authentication process (i.e. identified as an authorised user). Now let us assume that the user is required to pass a second authentication process, which also has a FAR of 10%, in addition to the first authentication process. Although one in ten users will be accepted by the second authentication process, the overall FAR (i.e. based on the combination of the first and second authentication processes) will in fact be 1%. Therefore the overall authentication process is markedly improved without having to improve either the first or second authentication process individually. 
     According to embodiments of the present disclosure, ear biometric data, which may be acquired using any of the personal audio devices described above with respect to  FIGS. 1 a    to  1   e,  is combined with authentication data acquired via one or more further mechanisms, to improve the performance of the overall authentication process. For example, in one embodiment, the ear biometric data is combined with voice biometric data. In another embodiment, the ear biometric data is combined with a security question and response. In the latter embodiment, the security question may be output to the user audibly, and the (audible) response detected with a microphone. The response may therefore additionally be used for voice biometric authentication. 
     One aspect of the disclosure provides a method in a biometric authentication system. The method comprises: obtaining ear biometric data for a user to be authenticated; identifying the user as a particular authorised user based on the ear biometric data; 
     outputting a security question message to the user, specific to the particular authorised user; and authenticating the user as the particular authorised user based on a response message from the user. 
     Another aspect of the disclosure provides a method in a biometric authentication system. The method comprises: obtaining ear biometric data for a user to be authenticated; obtaining voice biometric data from the user to be authenticated; and utilizing the ear biometric data and the voice biometric data to authenticate an identity of the user. 
     A further aspect of the disclosure provides a method in a biometric authentication system. The method comprises: obtaining ear biometric data for a user to be authenticated; outputting a security question message to the user; and authenticating the user as an authorised user based on the ear biometric data and a response message from the user. 
     Another aspect provides an apparatus for biometric authentication. The apparatus comprises: an ear biometric module configured to obtain ear biometric data for a user to be authenticated; a decision module configured to identify the user as a particular authorised user based on the ear biometric data; an output, for outputting a security question message to the user, specific to the particular authorised user; and an authentication module configured to authenticate the user as the particular authorised user based on a response message from the user. 
     An aspect of the disclosure provides an electronic apparatus comprising processing circuitry and a non-transitory machine-readable medium storing instructions which, when executed by the processing circuitry, cause the electronic apparatus to: obtain ear biometric data for a user to be authenticated; identify the user as a particular authorised user based on the ear biometric data; output a security question message to the user, specific to the particular authorised user; and authenticate the user as the particular authorised user based on a response message from the user. 
     A further aspect provides a non-transitory machine-readable medium storing instructions which, when executed by processing circuitry, cause an electronic apparatus to: obtain ear biometric data for a user to be authenticated; identify the user as a particular authorised user based on the ear biometric data; output a security question message to the user, specific to the particular authorised user; and authenticate the user as the particular authorised user based on a response message from the user. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of examples of the present disclosure, and to show more clearly how the examples may be carried into effect, reference will now be made, by way of example only, to the following drawings in which: 
         FIGS. 1 a  to 1 e    show examples of personal audio devices; 
         FIG. 2  shows an arrangement according to embodiments of the disclosure; 
         FIG. 3  shows a system according to embodiments of the disclosure; 
         FIG. 4  is a flowchart of a method according to embodiments of the disclosure; and 
         FIG. 5  is a flowchart of a method according to further embodiments of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     As noted above, ear biometric data may be acquired by the generation of an acoustic stimulus, and the detection of an acoustic response of the ear to the acoustic stimulus. One or more features may be extracted from the response signal, and used to characterize the individual. 
     The acoustic stimulus may be generated and the response measured using a personal audio device. As used herein, the term “personal audio device” is any electronic device which is suitable for, or configurable to, provide audio playback substantially to only a single user. Some examples of suitable personal audio devices are shown in  FIGS. 1 a    to  1   e.    
       FIG. 1 a    shows a schematic diagram of a user&#39;s ear, comprising the (external) pinna or auricle  12   a,  and the (internal) ear canal  12   b.  A personal audio device  20  comprising a circum-aural headphone is worn by the user over the ear. The headphone comprises a shell which substantially surrounds and encloses the auricle, so as to provide a physical barrier between the user&#39;s ear and the external environment. Cushioning or padding may be provided at an edge of the shell, so as to increase the comfort of the user, and also the acoustic coupling between the headphone and the user&#39;s skin (i.e. to provide a more effective barrier between the external environment and the user&#39;s ear). 
     The headphone comprises one or more loudspeakers  22  positioned on an internal surface of the headphone, and arranged to generate acoustic signals towards the user&#39;s ear and particularly the ear canal  12   b.  The headphone further comprises one or more microphones  24 , also positioned on the internal surface of the headphone, arranged to detect acoustic signals within the internal volume defined by the headphone, the auricle  12   a  and the ear canal  12   b.    
     The headphone may be able to perform active noise cancellation, to reduce the amount of noise experienced by the user of the headphone. Active noise cancellation operates by detecting a noise (i.e. with a microphone), and generating a signal (i.e. with a loudspeaker) that has the same amplitude as the noise signal but is opposite in phase. The generated signal thus interferes destructively with the noise and so lessens the noise experienced by the user. Active noise cancellation may operate on the basis of feedback signals, feedforward signals, or a combination of both. Feedforward active noise cancellation utilizes one or more microphones on an external surface of the headphone, operative to detect the environmental noise before it reaches the user&#39;s ear. The detected noise is processed quickly, and the cancellation signal generated so as to match the incoming noise as it arrives at the user&#39;s ear. Feedback active noise cancellation utilizes one or more error microphones positioned on the internal surface of the headphone, operative to detect the combination of the noise and the audio playback signal generated by the one or more loudspeakers. This combination is used in a feedback loop, together with knowledge of the audio playback signal, to adjust the cancelling signal generated by the loudspeaker and so reduce the noise. The microphone  24  shown in  FIG. 1 a    may therefore form part of an active noise cancellation system, for example, as an error microphone. 
       FIG. 1 b    shows an alternative personal audio device  30 , comprising a supra-aural headphone. The supra-aural headphone does not surround or enclose the user&#39;s ear, but rather sits on the auricle  12   a.  The headphone may comprise a cushion or padding to lessen the impact of environmental noise. As with the circum-aural headphone shown in  FIG. 1   a,  the supra-aural headphone comprises one or more loudspeakers  32  and one or more microphones  34 . The loudspeaker(s)  32  and the microphone(s)  34  may form part of an active noise cancellation system, with the microphone  34  serving as an error microphone. 
       FIG. 1 c    shows a further alternative personal audio device  40 , comprising an intra-concha headphone (or earphone). In use, the intra-concha headphone sits inside the user&#39;s concha cavity. The intra-concha headphone may fit loosely within the cavity, allowing the flow of air into and out of the user&#39;s ear canal  12   b.    
     As with the devices shown in  FIGS. 1 a  and 1 b   , the intra-concha headphone comprises one or more loudspeakers  42  and one or more microphones  44 , which may form part of an active noise cancellation system. 
       FIG. 1 d    shows a further alternative personal audio device  50 , comprising an in-ear headphone (or earphone), insert headphone, or ear bud. This headphone is configured to be partially or totally inserted within the ear canal  12   b,  and may provide a relatively tight seal between the ear canal  12   b  and the external environment (i.e. it may be acoustically closed or sealed). The headphone may comprise one or more loudspeakers  52  and one or more microphones  54 , as with the others devices described above, and these components may form part of an active noise cancellation system. 
     As the in-ear headphone may provide a relatively tight acoustic seal around the ear canal  12   b,  external noise (i.e. coming from the environment outside) detected by the microphone  54  is likely to be low. 
       FIG. 1 e    shows a further alternative personal audio device  60 , which is a mobile or cellular phone or handset. The handset  60  comprises one or more loudspeakers  62  for audio playback to the user, and one or more microphones  64  which are similarly positioned. 
     In use, the handset  60  is held close to the user&#39;s ear so as to provide audio playback (e.g. during a call). While a tight acoustic seal is not achieved between the handset  60  and the user&#39;s ear, the handset  60  is typically held close enough that an acoustic stimulus applied to the ear via the one or more loudspeakers  62  generates a response from the ear which can be detected by the one or more microphones  64 . As with the other devices, the loudspeaker(s)  62  and microphone(s)  64  may form part of an active noise cancellation system. 
     All of the personal audio devices described above thus provide audio playback to substantially a single user in use. Each device comprises one or more loudspeakers and one or more microphones, which may be utilized to generate biometric data related to the frequency response of the user&#39;s ear. The loudspeaker is operable to generate an acoustic stimulus, or acoustic probing wave, towards the user&#39;s ear, and the microphone is operable to detect and measure a response of the user&#39;s ear to the acoustic stimulus, e.g. to measure acoustic waves reflected from the ear canal or the pinna. The acoustic stimulus may be sonic (for example in the audio frequency range of say 20 Hz to 20 kHz) or ultra-sonic (for example greater than 20 kHz or in the range 20 kHz to 50 kHz) or near-ultrasonic (for example in the range 15 kHz to 25 kHz) in frequency. In some examples the microphone signal may be processed to measure received signals of the same frequency as that transmitted. 
     Another biometric marker may comprise otoacoustic noises emitted by the cochlear in response to the acoustic stimulus waveform. The otoacoustic response may comprise a mix of the frequencies in the input waveform. For example if the input acoustic stimulus consists of two tones at frequencies f1 and f2, the otoacoustic emission may include a component at frequency 2*f1−f2. The relative power of frequency components of the emitted waveform has been shown to be a useful biometric indicator. In some examples therefore the acoustic stimulus may comprise tones of two or more frequencies and the amplitude of mixing products at sums or differences of integer-multiple frequencies generated by otoacoustic emissions from the cochlear may be measured. Alternatively, otoacoustic emissions may be stimulated and measured by using stimulus waveforms comprising fast transients, e.g. clicks. 
     Depending on the construction and usage of the personal audio device, the measured response may comprise user-specific components, i.e. biometric data, relating to the auricle  12   a,  the ear canal  12   b,  or a combination of both the auricle  12   a  and the ear canal  12   b.  For example, the circum-aural headphones shown in  FIG. 1 a    will generally acquire data relating to the auricle  12   a  and potentially also the ear canal  12   b . The insert headphones shown in  FIG. 1 d    will generally acquire data relating only to the ear canal  12   b.    
     One or more of the personal audio devices described above (or rather, the microphones within those devices) may be operable to detect bone-conducted voice signals from the user. That is, as the user speaks, sound is projected away from the user&#39;s mouth through the air. However, acoustic vibrations will also be carried through part of the user&#39;s skeleton or skull, such as the jaw bone. These acoustic vibrations may be coupled to the ear canal  12   b  through the jaw or some other part of the user&#39;s skeleton or skull, and detected by the microphone. Lower frequency sounds tend to experience a stronger coupling than higher frequency sounds, and voiced speech (i.e. that speech or those phonemes generated while the vocal cords are vibrating) is coupled more strongly via bone conduction than unvoiced speech (i.e. that speech or those phonemes generated while the vocal cords are not vibrating). The in-ear headphone  50  may be particularly suited to detecting bone-conducted speech owing to the tight acoustic coupling around the ear canal  12   b.    
     All of the devices shown in  FIGS. 1 a  to 1 e    and described above may be used to implement aspects of the disclosure. 
       FIG. 2  shows an arrangement  200  according to embodiments of the disclosure. The arrangement  200  comprises a personal audio device  202  and a biometric system  204 . The personal audio device  202  may be any device which is suitable for, or configurable to provide audio playback to substantially a single user. The personal audio device  202  generally comprises one or more loudspeakers, and one or more microphones which, in use, are positioned adjacent to or within a user&#39;s ear. The personal audio device may be wearable, and comprise headphones for each of the user&#39;s ears. Alternatively, the personal audio device may be operable to be carried by the user, and held adjacent to the user&#39;s ear or ears during use. The personal audio device may comprise headphones or a mobile phone handset, as described above with respect to any of  FIGS. 1 a    to  1   e.    
     The biometric system  204  is coupled to the personal audio device  202  and operative to control the personal audio device  202  to acquire biometric data which is indicative of the individual using the personal audio device. 
     The personal audio device  202  thus generates an acoustic stimulus for application to the user&#39;s ear, and detects or measures the response of the ear to the acoustic stimulus. For example, the acoustic stimulus may be in the sonic range, or ultra-sonic. In some embodiments, the acoustic stimulus may have a flat frequency spectrum over a relevant frequency range, or be preprocessed in such a way that those frequencies that allow for a good discrimination between individuals are emphasized (i.e. have a higher amplitude than other frequencies). The measured response corresponds to the reflected signal received at the one or more microphones, with certain frequencies being reflected at higher amplitudes than other frequencies owing to the particular response of the user&#39;s ear. 
     The biometric system  204  may send suitable control signals to the personal audio device  202 , so as to initiate the acquisition of biometric data, and receive data from the personal audio device  202  corresponding to the measured response. The biometric system  204  is operable to extract one or more features from the measured response and utilize those features as part of a biometric process. 
     Some examples of suitable biometric processes include biometric enrolment and biometric authentication. Enrolment comprises the acquisition and storage of biometric data which is characteristic of an individual. In the present context, such stored data may be known as an “ear print”. Authentication (alternatively referred to as verification or identification) comprises the acquisition of biometric data from an individual, and the comparison of that data to the stored ear prints of one or more enrolled or authorised users. A positive comparison (i.e. a determination that the acquired data matches or is sufficiently close to a stored ear print) results in the individual being authenticated. For example, the individual may be permitted to carry out a restricted action, or granted access to a restricted area or device. A negative comparison (i.e. a determination that the acquired data does not match or is not sufficiently close to a stored ear print) results in the individual not being authenticated. For example, the individual may not be permitted to carry out the restricted action, or granted access to the restricted area or device. 
     The biometric system  204  may, in some embodiments, form part of the personal audio device  202  itself. Alternatively, the biometric system  204  may form part of an electronic host device (e.g. an audio player) to which the personal audio device  202  is coupled, through wires or wirelessly. In yet further embodiments, operations of the biometric system  204  may be distributed between circuitry in the personal audio device  202  and the electronic host device. 
       FIG. 3  shows a system  300  according to embodiments of the disclosure. 
     The system  300  comprises processing circuitry  322 , which may comprise one or more processors, such as a central processing unit or an applications processor (AP), or a digital signal processor (DSP). The system  300  further comprises memory  324 , which is communicably coupled to the processing circuitry  322 . The memory  324  may store instructions which, when carried out by the processing circuitry  322 , cause the processing circuitry to carry out one or more methods as described below (see  FIGS. 4 and 5  for example). The one or more processors may perform methods as described herein on the basis of data and program instructions stored in memory  324 . Memory  324  may be provided as a single component or as multiple components or co-integrated with at least some of processing circuitry  322 . Specifically, the methods described herein can be performed in processing circuitry  322  by executing instructions that are stored in non-transient form in the memory  324 , with the program instructions being stored either during manufacture of the system  300  or personal audio device  202  or by upload while the system or device is in use. 
     The processing circuitry  322  comprises a stimulus generator module  303  which is coupled directly or indirectly to an amplifier  304 , which in turn is coupled to a loudspeaker  306 . 
     The stimulus generator module  303  generates an electrical audio signal (for example, under the instruction of control module  302 ) and provides the electrical audio signal to the amplifier  304 , which amplifies it and provides the amplified signal to the loudspeaker  306 . The loudspeaker  306  generates a corresponding acoustic signal which is output to the user&#39;s ear (or ears). The audio signal may be sonic or ultra-sonic, for example. The audio signal may have a flat frequency spectrum, or be preprocessed in such a way that those frequencies that allow for a good discrimination between individuals are emphasized (i.e. have a higher amplitude than other frequencies). 
     As noted above, the audio signal may be output to all or a part of the user&#39;s ear (i.e. the auricle or the ear canal). The audio signal is reflected off the ear, and the reflected signal (or echo signal) is detected and received by a microphone  308 . The reflected signal thus comprises data which is characteristic of the individual&#39;s ear, and suitable for use as a biometric. 
     The reflected signal is passed from the microphone  308  to an analogue-to-digital converter (ADC)  310 , where it is converted from the analogue domain to the digital domain. Of course, in alternative embodiments the microphone may be a digital microphone and produce a digital data signal (which does not therefore require conversion to the digital domain). 
     The signal is detected by the microphone  308  in the time domain. However, the features extracted for the purposes of the biometric process may be in the frequency domain (in that it is the frequency response of the user&#39;s ear which is characteristic). The system  300  therefore comprises a Fourier transform module  312 , which converts the reflected signal to the frequency domain. For example, the Fourier transform module  312  may implement a fast Fourier transform (FFT). In some examples the biometric process may not be in the frequency domain, so the Fourier transform module may be omitted. 
     The transformed signal is then passed to a feature extract module  314 , which extracts one or more features of the transformed signal for use in a biometric process (e.g. biometric enrolment, biometric authentication, etc). For example, the feature extract module  314  may extract the resonant frequency of the user&#39;s ear. For example, the feature extract module  314  may extract one or more mel frequency cepstrum coefficients. Alternatively, the feature extract module may determine the frequency response of the user&#39;s ear at one or more predetermined frequencies, or across one or more ranges of frequencies. The extracted features may correspond to data for a model of the ear. 
     The extracted feature(s) are passed to a biometric module  316 , which performs a biometric process on them. For example, the biometric module  316  may perform a biometric enrolment, in which the extracted features (or parameters derived therefrom) are stored as part of biometric data  318  which is characteristic of the individual. The biometric data may be stored within the system  300  or remote from the system  300  (and accessible securely by the biometric module  316 ). Such stored data  318  may be known as an “ear print”. In another example, the biometric module  316  may perform a biometric authentication, and compare the one or more extracted features to corresponding features in the stored ear print  318  (or multiple stored ear prints). 
     The biometric module  316  may generate a biometric result (which may be the successful or unsuccessful generation of an ear print, as well as successful or unsuccessful authentication) and output the result to control module  302 . 
     In some embodiments the stimulus waveforms may be tones of predetermined frequency and amplitude. In other embodiments the stimulus generator may be configurable to apply music to the loudspeaker, e.g. normal playback operation, and the feature extract module may be configurable to extract the response or transfer function from whatever signal components the stimulus waveform contains. 
     Thus in some embodiments the feature extract module may be designed with foreknowledge of the nature of the stimulus, for example knowing the spectrum of the applied stimulus signal, so that the response or transfer function may be appropriately normalised. In other embodiments the feature extract module may comprise a second input to monitor the stimulus (e.g. playback music) and hence provide the feature extract module with information about the stimulus signal or its spectrum so that the feature extract module may calculate the transfer function from the stimulus waveform stimulus to received acoustic waveform from which it may derive the desired feature parameters. In the latter case, the stimulus signal may also pass to the feature extract module via the FFT module  312 . 
     According to embodiments of the disclosure, the system  300  further comprises an additional authentication mechanism  320 , which in the illustrated embodiment is coupled to the control module  302 . In alternative embodiments, the additional authentication mechanism  320  may be coupled to the biometric module  316 , for example. 
     The additional authentication mechanism  320  may be configured to provide one or more authentication algorithms in addition to the ear biometric algorithm described above. For example, the additional authentication mechanism  320  may comprise a voice biometric authentication module, configured to perform a biometric authentication algorithm on a voice signal, e.g. received via the microphone  308  or via another microphone such as a dedicated voice microphone (not illustrated). The voice signal may be air-conducted (i.e. travelling through the air to a microphone outside the user&#39;s ear) or bone-conducted (i.e. travelling through at least part of the user&#39;s skeleton or skull, such as the jaw bone, and detected by a suitable microphone or transducer). The bone-conducted voice signal may be detected by a microphone within the user&#39;s ear or external to the user&#39;s ear. In the former case, the microphone may be the same as that used for ear biometrics (and potentially the same as that used for active noise cancellation), or different. 
     For example, the additional authentication mechanism  320  may comprise an input-output mechanism for accepting and authorising the user based on a passphrase, password, or pin number entered by the user and associated with the authorised user. The input-output mechanism may pose a question to the user based on the passphrase, password or pin number, the answer to which does not reveal the entire passphrase, password or pin number. For example, the question may relate to a particular character or digit of the passphrase, password or pin number (e.g., “what is the third character of the password?”). Thus only part of the user&#39;s passphrase, password or pin number is input by the user in response to the question. The question may require the performance of a mathematical or logical operation on the pin number or part thereof (e.g., “what is the first digit of the pin number plus three?”). The input-output mechanism may output the question audibly (e.g. through playback over the loudspeaker  306 ), so that only the user can hear the question. Further, the input-output mechanism may provide for input of the answer audibly (e.g. through the microphone  308  or some other microphone such as a voice mic), or via some other input mechanism, such as a touchscreen, keypad, keyboard, or similar. 
     As the question is provided only to the user (e.g. via the personal audio device), third parties in the vicinity of the user may be unable to hear it. Thus, although the third parties may overhear the spoken answer, they are unable to determine the question which was asked and therefore acquire no useful knowledge as to the user&#39;s password, passphrase or pin number. 
       FIG. 4  is a flowchart of a method according to embodiments of the disclosure. 
     In step  400 , ear biometric data is acquired from a user seeking authentication. For example, the biometric system may acquire ear model data from a personal audio device, which generates an acoustic stimulus for application to the user&#39;s ear, and extract one or more features from the measured response to that acoustic stimulus (e.g. as detected with a microphone in the personal audio device). 
     In step  402 , additional authentication data is obtained from the user. For example, the additional authentication data may comprise voice biometric data. e.g. received via the microphone  308  or via another microphone such as a dedicated voice microphone (not illustrated). 
     For example, the additional authentication data may comprise a response to a security question output to the user. The question may relate to a passphrase, password or pin number. In some embodiments, the question may be configured such that the correct answer does not reveal the entire passphrase, password or pin number. For example, the question may relate to a particular character or digit of the passphrase, password or pin number (e.g., “what is the third character of the password?”). Thus only part of the user&#39;s passphrase, password or pin number is input by the user in response to the question. The question may require the performance of a mathematical or logical operation on the pin number or part thereof (e.g., “what is the first digit of the pin number plus three?”). The question may be output audibly (e.g. through playback over the loudspeaker  306 ), so that only the user can hear the question. Further, the input-output mechanism may provide for input of the answer audibly (e.g. through the microphone  308  or some other microphone such as a voice mic), or via some other input mechanism, such as a touchscreen, keypad, keyboard, or similar. In some embodiments, the audible answer may be used for voice biometric authentication as well as a response to the security question. 
     In step  404 , the user is authenticated based on the ear biometric data and the additional data obtained in step  402 . 
     The authentication may be carried out on the combination of data in multiple different ways. For example, in one embodiment separate authentication algorithms may be carried out on each of the sets of data acquired in steps  400  and  402 , and separate authentication scores acquired from each of the sets of data. The scores may then be combined to generate an overall score, indicating the overall likelihood that the user is an authorised user, with the authentication decision being taken on this score (e.g. by comparing the score to a threshold). In an alternative embodiment, the individual biometric scores may be handled separately (e.g., compared to separate thresholds) and individual authentication decisions being taken on each score. Overall authentication is then based on a combination of the decisions. For example, failure at any one of the authentication algorithms may result in failure of the authentication overall. Thus, if the ear biometric algorithm results in an authentication, but one or more of the other mechanisms results in a rejection (i.e. because the voice does not match a stored voice print for the user, or the response to the security question was wrong), the user may be rejected overall. 
       FIG. 5  is a flowchart of a method according to further embodiments of the disclosure. The method may be carried out in the context of a user seeking authentication with a system (such as the system  300  described above), and utilizing a personal audio device to communicate with the system (i.e. providing speech input to the system and/or receiving audio output from the system). 
     In step  500 , ear biometric data is obtained from the user via the personal audio device. One or more loudspeakers or similar transducers positioned close to or within the ear generate an acoustic stimulus, and one or more microphones similarly positioned close to or within the ear detect the acoustic response of the ear to the acoustic stimulus. One or more features may be extracted from the response signal, and used to characterize the individual. The acoustic stimulus may comprise a flat-spectrum signal, a signal in which frequencies found to be discriminative of individuals have greater amplitude than other frequencies, or may even be a normal playback signal (e.g. music). 
     In step  502 , a biometric algorithm is performed on the acquired data to determine whether the user is an authorised user. For example, the one or more extracted features may be compared to one or more stored ear models (i.e. ear prints), and a biometric score generated, indicating the level of similarity or closeness between the acquired data and the stored ear models. If the acquired data does not match any of the stored ear models to a sufficient degree (e.g. the biometric scores are less than a threshold value for each of the stored ear models), the method proceeds to step  504  in which the user is rejected by the authentication system. For example, the user may be prevented from performing a restricted operation or accessing a restricted application or area. Alternatively, the method may be restarted and authentication re-attempted. 
     If the acquired data matches one of the stored ear models to a sufficient degree (e.g. the biometric score is greater than or equal to a threshold value), the user may be identified as the authorised user associated with that stored ear model and the method proceeds to step  506 . 
     In step  506 , a security question is output to the user. The security question may be specific to the user identified in step  502 , or the answer to the security question may be specific to the user identified in step  502 . The security question may be played back to the user through the speaker used to obtain the ear biometric data. 
     The question may relate to a passphrase, password or pin number associated with the authorised user. In some embodiments, the question may be configured such that the correct answer does not reveal the entire passphrase, password or pin number. For example, the question may relate to a particular character or digit of the passphrase, password or pin number (e.g., “what is the third character of the password?”). Thus only part of the user&#39;s passphrase, password or pin number is input by the user in response to the question. The question may require the performance of a mathematical or logical operation on the pin number or part thereof (e.g., “what is the first digit of the pin number plus three?”). 
     The user thus speaks the answer to the security question and a corresponding voice signal is received by the system. The speech may be detected by the microphone used to acquire the ear biometric data and/or another microphone (such as a dedicated voice microphone). 
     The system may comprise a speech recognition module, or an interface with an external speech recognition module over which the voice signal can be sent for analysis. In either case, the voice signal from the user is analysed in step  508  to determine if the answer uttered by the user is correct. If the answer is incorrect, the method may proceed to step  504 , and rejection of the user. 
     If the answer is correct, the method proceeds to step  510 , in which a voice biometric algorithm is performed on the voice signal (i.e. the response to the question posed in step  506 ). (In alternative embodiments, steps  508  and  510  may be carried out simultaneously with each other.) The voice biometric algorithm may comprise a comparison of one or more features extracted from the voice signal to a stored voice model (i.e. a voice print) of the authorised user identified in step  502 . A voice biometric score may be generated, indicating the level of similarity between the voice signal and the stored voice model. 
     In one embodiment, the voice biometric score is compared to a respective threshold value and a decision taken on whether the voice signal is a match to the stored voice model. If the decision is positive (i.e. if the voice biometric score equals or exceeds the threshold value), the method proceeds to step  512  and the user can be authenticated as the authorised user. If the decision is negative (i.e. the voice biometric score is less than the threshold), the method proceeds to step  504  and the user is rejected. 
     In an alternative embodiment, the voice biometric score is combined with the ear biometric score (i.e. obtained in step  502 ) to generate an overall score, and the overall score compared to an overall threshold value. The biometric scores may be combined by simply summing. 
     Alternatively, the biometric scores may subject to a weighted summation, with each biometric score weighted by a respective coefficient. For example, one method of achieving such weighting is as follows: 
     
       
      
       S 
       total 
       =p 
       1 
       s 
       1 
       +p 
       2 
       s 
       2  
      
     
     where S total  is the total (i.e. combined, overall) biometric score, s 1  and s 2  are the biometric scores obtained via the separate authentication algorithms (e.g. ear and voice, respectively), p 1  and p 2  are weighting coefficients, and p 1 +p 2 =1. The biometric scores s 1  and s 2  may also take values between 0 and 1. 
     The weighting coefficients may be fixed, or dynamically adjustable. For example, in one embodiment the weighting coefficients may be determined based on one or more quality metrics related to the biometric data (e.g. the ear and voice biometric data). The one or more quality metrics may comprise one or more of: a signal to noise ratio; the presence of clipping in the signal; one or more spectral parameters (such as spectral peaking, spectral flatness, spectral tilt, etc); energy per frequency bin, etc. Thus if the quality of one of the sets of biometric data is low (or relatively low compared to the other biometric data), the weighting coefficients may be adjusted to emphasize the biometric score for the higher-quality biometric algorithm. 
     Methods described herein may be utilized to achieve power savings in the authentication system. For example, one or more parts of the system  300  may be kept in a low-power state until the user has passed at least a first authentication algorithm (e.g. the ear biometric algorithm). Upon passing the first authentication algorithm, further parts of the system may be powered on to carry out one or more second authentication algorithms (e.g. voice biometric, or security question algorithms). For example, the authentication module  320  may be kept in a low-power state until the biometric module  316  generates a positive authentication result based on the ear biometric algorithm (e.g. in step  502 ). 
     Embodiments of the disclosure thus provide methods, apparatus and systems for authenticating a user. 
     Embodiments described above have focussed on an implementation in which ear biometrics and/or voice biometrics are performed on signals detected in a single ear. It will be appreciated by those skilled in the art that the embodiments may straightforwardly be adapted to take into consideration biometric data obtained from both ears of a user. Thus, where the description above discloses acquiring data from an ear (e.g. through application of an acoustic stimulus and detection of the response, or acquisition of a bone-conducted voice signal in the ear), data may similarly be acquired from two ears. For example, the system  300  described above may comprise respective signal processing chains for data from each ear (e.g. respective ADCs, Fourier transform modules, and/or feature extract modules), or a single signal processing chain which is multiplexed between data streams generated within each ear. Biometric algorithms may similarly be performed on data from both ears, and this may be combined as described above, i.e. separate biometric authentication scores combined to form a combined score on which an overall decision is determined, or separate biometric authentication decisions which are then combined to determine an overall decision. 
     Embodiments may be implemented in an electronic, portable and/or battery powered host device such as a smartphone, an audio player, a mobile or cellular phone, or a handset. Embodiments may be implemented on one or more integrated circuits provided within such a host device. Embodiments may be implemented in a personal audio device configurable to provide audio playback to a single person, such as a smartphone, a mobile or cellular phone, headphones, earphones, etc., see  FIGS. 1 a    to  1   e.  Again, embodiments may be implemented on one or more integrated circuits provided within such a personal audio device. In yet further alternatives, embodiments may be implemented in a combination of a host device and a personal audio device. For example, embodiments may be implemented in one or more integrated circuits provided within the personal audio device, and one or more integrated circuits provided within the host device. 
     It should be understood—especially by those having ordinary skill in the art with the benefit of this disclosure—that the various operations described herein, particularly in connection with the figures, may be implemented by other circuitry or other hardware components. The order in which each operation of a given method is performed may be changed, and various elements of the systems illustrated herein may be added, reordered, combined, omitted, modified, etc. It is intended that this disclosure embrace all such modifications and changes and, accordingly, the above description should be regarded in an illustrative rather than a restrictive sense. 
     Similarly, although this disclosure makes reference to specific embodiments, certain modifications and changes can be made to those embodiments without departing from the scope and coverage of this disclosure. Moreover, any benefits, advantages, or solutions to problems that are described herein with regard to specific embodiments are not intended to be construed as a critical, required, or essential feature or element. 
     Further embodiments likewise, with the benefit of this disclosure, will be apparent to those having ordinary skill in the art, and such embodiments should be deemed as being encompassed herein. 
     The skilled person will recognise that some aspects of the above-described apparatus and methods, for example the discovery and configuration methods may be embodied as processor control code, for example on a non-volatile carrier medium such as a disk, CD- or DVD-ROM, programmed memory such as read only memory (Firmware), or on a data carrier such as an optical or electrical signal carrier. For many applications embodiments of the invention will be implemented on a DSP (Digital Signal Processor), ASIC (Application Specific Integrated Circuit) or FPGA (Field Programmable Gate Array). Thus the code may comprise conventional program code or microcode or, for example code for setting up or controlling an ASIC or FPGA. The code may also comprise code for dynamically configuring re-configurable apparatus such as re-programmable logic gate arrays. Similarly the code may comprise code for a hardware description language such as Verilog™ or VHDL (Very high speed integrated circuit Hardware Description Language). As the skilled person will appreciate, the code may be distributed between a plurality of coupled components in communication with one another. Where appropriate, the embodiments may also be implemented using code running on a field-(re)programmable analogue array or similar device in order to configure analogue hardware. 
     Note that as used herein the term module shall be used to refer to a functional unit or block which may be implemented at least partly by dedicated hardware components such as custom defined circuitry and/or at least partly be implemented by one or more software processors or appropriate code running on a suitable general purpose processor or the like. A module may itself comprise other modules or functional units. A module may be provided by multiple components or sub-modules which need not be co-located and could be provided on different integrated circuits and/or running on different processors. 
     It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims or embodiments. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim or embodiment, “a” or “an” does not exclude a plurality, and a single feature or other unit may fulfil the functions of several units recited in the claims or embodiments. Any reference numerals or labels in the claims or embodiments shall not be construed so as to limit their scope. 
     Although the present disclosure and certain representative advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims or embodiments. Moreover, the scope of the present disclosure is not intended to be limited to the particular embodiments of the process, machine, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments herein may be utilized. Accordingly, the appended claims or embodiments are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.