Patent Publication Number: US-11653147-B2

Title: Hearing device with microphone switching and related method

Description:
RELATED APPLICATION DATA 
     This application claims priority to, and the benefit of, Danish Patent Application No. PA 2020 70531 filed on Aug. 14, 2020. The entire disclosure of the above application is expressly incorporated by reference herein. 
     FIELD 
     The present disclosure relates to a hearing device and related methods including a method of operating a hearing device. In particular, a hearing device with both an in-ear microphone and one or more behind-the-ear microphones and related method are disclosed. 
     BACKGROUND 
     For hearing device designers, spatial perception and the ability to perceive spatial cues play an important role in a hearing device user&#39;s ability to understand speech and crucial for complex listening environments. On the other hand, feedback and instabilities in hearing aids continue to represent challenges to hearing device designers and engineers in particular when microphones and receiver are placed near each other, for example in a MaRie (Microphone and Receiver in ear) hearing device. 
     SUMMARY 
     Accordingly, there is a need for hearing devices and methods with improved spatial perception and stability of the hearing device. 
     A hearing device is disclosed, the hearing device comprising a first housing configured as a behind-the-ear housing to be worn behind the pinna of a user; a second housing configured as an earpiece housing to be worn in or at the ear canal of a user; a wire connecting the first housing and the second housing; a primary set of microphones arranged in the first housing, the primary set of microphones including a primary first microphone for provision of a primary first microphone input signal, and a primary second microphone for provision of a primary second microphone input signal; a secondary microphone arranged in the second housing for provision of a secondary microphone input signal; a mixing module for provision of a mixer output based on a primary mixer input and a secondary mixer input, wherein the primary mixer input optionally is based on the primary first microphone input signal and/or the primary second microphone input signal, and the secondary mixer input is based on the secondary microphone input signal; a mixing controller configured to control the mixing module; a processor for processing the mixer output and providing an electrical output signal based on mixer output; and a receiver for converting the electrical output signal to an audio output signal. The mixing controller is optionally configured to determine presence of a switch event, such as a sound event, and in accordance with a determination that a sound event is present, control the mixing module, such as increase an amount of the secondary mixer input in the mixer output for a time period and/or reduce the amount of the secondary mixer input in the mixer output after the time period. 
     Further, a method of operating a hearing device is disclosed, the hearing device comprising a first housing configured as a behind-the-ear housing to be worn behind the pinna of a user; a second housing configured as an earpiece housing to be worn in or at the ear canal of a user; a wire connecting the first housing and the second housing; a primary set of microphones arranged in the first housing, the primary set of microphones including a primary first microphone and a primary second microphone; a secondary microphone arranged in the second housing; a mixing module; a mixing controller; a processor; and a receiver. The method comprises obtaining a primary first microphone input signal, e.g. with the primary first microphone; obtaining a primary second microphone input signal, e.g. with the primary second microphone; obtaining a secondary microphone input signal, e.g. with the secondary microphone; providing a primary mixer input based on the primary first microphone input signal and/or the primary second microphone input signal; providing a secondary mixer input based on the secondary microphone input signal; providing a mixer output based on the primary mixer input and/or the secondary mixer input; processing the mixer output for provision of an electrical output signal; and converting the electrical output signal to an audio output signal. In the method, providing a mixer output based on the primary mixer input and the secondary mixer input comprises determining presence of a switch event, such as a sound event; in accordance with determining presence of a sound event, controlling the mixing module, such as increasing an amount of the secondary mixer input in the mixer output for a time period and/or reducing the amount of the secondary mixer input in the mixer output after the time period. 
     It is an important advantage of the hearing device that improved spatial perception in a hearing device with increased stability is provided. Accordingly, the risk of feedback in the hearing device is reduced while spatial cues are preserved to a higher degree, which in turn provides an improved hearing device. 
     Further, the present disclosure provides improved user experience by improving speech intelligibility and reducing feedback or other instability in the hearing device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages will become readily apparent to those skilled in the art by the following detailed description of exemplary embodiments thereof with reference to the attached drawings, in which: 
         FIG.  1    schematically illustrates exemplary hearing devices, 
         FIG.  2    schematically illustrates an exemplary hearing device, 
         FIG.  3    schematically illustrates an exemplary hearing device, 
         FIG.  4    is a flow diagram of an exemplary method according to the disclosure, 
         FIG.  5    is a flow diagram of an exemplary method according to the disclosure, and 
         FIG.  6    illustrates sound events in speech. 
     
    
    
     DETAILED DESCRIPTION 
     Various exemplary embodiments and details are described hereinafter, with reference to the figures when relevant. It should be noted that the figures may or may not be drawn to scale and that elements of similar structures or functions are represented by like reference numerals throughout the figures. It should also be noted that the figures are only intended to facilitate the description of the embodiments. They are not intended as an exhaustive description of the invention or as a limitation on the scope of the invention. In addition, an illustrated embodiment needs not have all the aspects or advantages shown. An aspect or an advantage described in conjunction with a particular embodiment is not necessarily limited to that embodiment and can be practiced in any other embodiments even if not so illustrated, or if not so explicitly described. 
     A hearing device is disclosed. The hearing device may be configured to be worn at an ear of a user and may be a hearable or a hearing aid, wherein the processor is configured to compensate for a hearing loss of a user. 
     The hearing device may be of the Microphone-and-Receiver-in ear (MaRIE) type. The hearing device may be a combined BTE and MaRIE type hearing device. The hearing device may be part of a binaural hearing system. Thus, the hearing device may be a binaural hearing device. 
     The hearing device comprises a first housing configured as a behind-the-ear housing to be worn behind the pinna of a user and a second housing configured as an earpiece housing to be worn in and/or at the ear canal of a user. 
     The hearing device comprises a wire connecting the first housing and the second housing. The wire comprises a plurality of conductors, e.g. three, four, five, six, or even eight or more conductors for electrically connecting electrical components of the first housing to electrical components of the second housing. 
     The hearing device comprises a primary set of microphones arranged in the first housing, the primary set of microphones including a primary first microphone for provision of a primary first microphone input signal also denoted x_1_1, and optionally a primary second microphone for provision of a primary second microphone input signal also denoted x_1_2. The primary first microphone may be denoted a front BTE (behind-the-ear) microphone and the primary second microphone may be denoted a rear BTE (behind-the-ear) microphone. The primary set of microphones may comprise a primary third microphone for provision of a primary third microphone input signal also denoted x_1_3. 
     The hearing device comprises a secondary microphone, also denoted secondary first microphone, arranged in the second housing for provision of a secondary microphone input signal also denoted x_2 or x_2_1. The secondary microphone may be denoted an in-ear microphone. The hearing device may comprise a plurality of secondary microphones arranged in the second housing. For example, a secondary second microphone, also denoted a canal microphone, may be arranged in the second housing for receiving intra-canal sounds, e.g. in order to allow for reduction in occlusion effects. 
     The hearing device comprises a mixing module for provision of a mixer output based on a primary mixer input and a secondary mixer input. The primary mixer input is optionally based on the primary first microphone input signal and/or the primary second microphone input signal, and the secondary mixer input is based on the secondary microphone input signal or a plurality of secondary microphone input signals. 
     The hearing device comprises a mixing controller configured to control the mixing module. The mixing controller is optionally configured to determine presence of a switch event, such as presence of a sound event; and in accordance with a determination that a switch event, such as a sound event, is present, control the mixing module, such as increase an amount of the secondary mixer input in the mixer output for a time period and/or reduce the amount of the secondary mixer input in the mixer output after the time period. In other words, the mixing controller optionally comprises a switch/sound event detector detecting or determining presence of a switch event/sound event. 
     In one or more exemplary hearing devices/methods, to increase an amount of the secondary mixer input in the mixer output for a time period comprises to increase a secondary coefficient or secondary coefficients applied to a component or components of the secondary mixer input. 
     The mixing controller is optionally configured to in accordance with a determination that a switch event, such as a sound event, is present, control the mixing module, such as reduce an amount of the primary mixer input in the mixer output for a time period and/or increase the amount of the primary mixer input in the mixer output after the time period. 
     The mixing controller is optionally configured to in accordance with a determination that a switch event, such as a sound event, is present, control the mixing module, such as reduce an amount of the primary mixer input in the mixer output for the time period. The mixing controller is optionally configured to increase an amount of the primary mixer input in the mixer output after the time period. The total gain applied to the primary mixer input and the secondary mixer input may be the same during and after the time period. In other words, the user does not experience an instant change in the sound volume when a sound event is detected. 
     In one or more exemplary hearing devices/methods, to reduce the amount of the secondary mixer input in the mixer output after the time period comprises to reduce a secondary coefficient or secondary coefficients applied to a component or components of the secondary mixer input. 
     The mixing controller is configured to determine the presence of a switch event, such as a sound event. A sound event may be a person starting to speak or a general increase in sound pressure or level, e.g. from a low level. 
     A switch event may be an event that is indicative of a situation where perception of directional or spatial cues is of high relevance. In one or more exemplary hearing devices/methods, a person abruptly or suddenly rotating his/her head may constitute a switch event. Thus, in one or more exemplary hearing devices, the hearing device may comprise a motion sensor, such as an accelerometer, and wherein the mixing controller is configured to determine presence of a switch event based on sensor output from the motion sensor. For example, presence of a switch event may be determined if the sensor output and/or a change in sensor output is larger than a threshold, e.g. where the sensor output is indicative of head motion. 
     The mixing controller is configured to, in accordance with a determination that a switch event, such as a sound event, is present, control the mixing module. For example, the mixing controller may be configured to increase an amount of the secondary mixer input in the mixer output for a time period and/or reduce the amount of the secondary mixer input in the mixer output after the time period in accordance with a determination that a switch event, such as a sound event, is present. 
     In one or more exemplary hearing devices/methods, the time period is in the range from 1 ms to 25 ms, such as in the range from 2 ms to 20 ms, e.g. in the range from 5 ms to 15 ms. The time period may be a fixed time period, e.g. set during fitting. The time period may be adaptive, e.g. determined during operation of the hearing device, e.g. based on one or more operating parameters of the hearing device and/or microphone input signals. The mixing controller may be configured to determine the time period, e.g. based on one or more operating parameters of the hearing device and/or microphone input signals. 
     In one or more exemplary hearing devices/methods, to determine the presence of a switch event, such as a sound event, is based on one or more of the primary first microphone input signal, the primary second microphone input signal, and the secondary microphone input signal. For example, presence of a sound event may be detected if an input signal, such as primary first microphone input signal, primary second microphone input signal, secondary microphone input signal, or a combination thereof meets a threshold value, e.g. if a level of input or an increase in a level of input is larger than a threshold. 
     In one or more exemplary hearing devices, presence of a sound event is detected if a level (e.g. amplitude or power) in microphone input signal, such as secondary microphone input signal, increases by more than a threshold, e.g. a relative threshold, such as 50% or an absolute threshold, such as 3 dB. In one or more exemplary hearing devices, presence of a sound event is detected if a level (e.g. amplitude or power) in microphone input signal, such as secondary microphone input signal, exceeds a threshold, e.g. if the level is larger than 60 dB. 
     In one or more exemplary hearing devices/methods, to determine the presence of a switch event is based on one or more operating parameters of the hearing device, such as one or more of a program identifier, gains, such as a closed-loop gain, and/or feedback parameters. 
     In one or more exemplary hearing devices/methods, a program change may constitute a switch event. In other words, to determine the presence of a switch event may comprise to determine a change in hearing device program. 
     In one or more exemplary hearing devices/methods, wherein the mixing controller is configured to determine the time period, e.g. based on one or more operating parameters of the hearing device. In other words, the time period may be adaptive. Thus, the mixing controller may adaptively determine the time period, e.g. based on one or more of a program identifier, gains, such as a closed-loop gain, and/or feedback parameters of the hearing device. Thereby, improved, e.g. longer, use of the ITE microphone is provided for by maximizing the time period where the ITE microphone is used (with improved directional perception), while at the same time reducing the risk of or substantially avoiding instability issues in the hearing device. 
     In one or more exemplary hearing devices/methods, to increase an amount of the secondary mixer input in the mixer output for a time period comprises to increase an amount of the secondary mixer input in the mixer output and optionally to determine if a switching criterion is satisfied. In one or more exemplary hearing devices/methods, to reduce the amount of the secondary mixer input in the mixer output after the time period comprises to reduce the amount of the secondary mixer input in the mixer output in accordance with a determination that the switching criterion is satisfied. The switching criterion may be based on one or more gains, such as a closed-loop gain, and/or feedback parameters of the hearing device. In other words, the hearing device may use an increased amount of secondary mixer input in the mixer output until the mixing controller determines that instability is either present or imminent. For example, the switching criterion may be satisfied if a feedback parameter indicative of the risk of feedback reaches a threshold. 
     In one or more exemplary hearing devices/methods, to increase an amount of the secondary mixer input in the mixer output for a time period comprises to, at least for a component of the mixer output, solely base the mixer output on the secondary mixer input. In other words, primary gain(s)/coefficient(s) applied to the primary mixer input may be set to zero during the time period. 
     The mixing controller may be configured to, in accordance with a determination that a switch event, such as a sound event, is present, reduce an amount of the primary mixer input in the mixer output for a time period and/or increase the amount of the primary mixer input in the mixer output after the time period. 
     In one or more exemplary hearing devices/methods, to reduce an amount of the primary mixer input in the mixer output for a time period comprises to reduce a primary coefficient or primary coefficients applied to a component or components of the primary mixer input. 
     In one or more exemplary hearing devices/methods, to increase the amount of the primary mixer input in the mixer output after the time period comprises to increase a primary coefficient or primary coefficients applied to a component or components of the primary mixer input. 
     In one or more exemplary hearing devices/methods, to increase an amount of the secondary mixer input in the mixer output for a time period comprises to increase a secondary coefficient or secondary coefficients applied to a component or components of the secondary mixer input. 
     In one or more exemplary hearing devices/methods, to reduce an amount of the secondary mixer input in the mixer output after the time period comprises to gradually reduce a secondary coefficient applied to a component of the secondary mixer input during a switching time period. Thereby a smooth switching between ITE microphone and BTE-microphones may be provided. 
     In one or more exemplary hearing devices/methods, to reduce the amount of the secondary mixer input in the mixer output after the time period comprises to switch to a secondary coefficient applied to a component of the secondary mixer input. 
     In one or more exemplary hearing devices/methods, to reduce an amount of the secondary mixer input in the mixer output after the time period comprises to reduce a secondary coefficient or secondary coefficients applied to a component or components of the secondary mixer input. 
     In one or more exemplary hearing devices/methods, to increase an amount of the secondary mixer input in the mixer output for a time period comprises to determine a secondary coefficient or secondary coefficients, e.g. based on one or more operating parameters of the hearing device, such as based on one or more of a program identifier, gains, such as a closed-loop gain, and/or feedback parameters of the hearing device, and apply the secondary coefficient(s) to the secondary mixer input for provision of the mixer output. In other words, the secondary coefficient(s) applied in the mixing module may be adaptive. 
     In one or more exemplary hearing devices, the hearing device comprises a primary pre-processor connected to respective primary first microphone and primary second microphone for pre-processing the primary first microphone input signal and the primary second microphone input signal for provision of the primary mixer input. The primary pre-processor may comprise a first filter and/or a second filter for filtering the primary first microphone input signal and/or the primary second microphone input signal before adding the (optionally filtered/pre-processed) primary first microphone input signal and the (optionally filtered/pre-processed) primary second microphone input signal in adder of the first pre-processor. The first filter of the primary pre-processor may be a pinna-restoration filter, i.e. a filter configured to perform pinna-restoration of the primary first microphone input signal. The second filter of the primary pre-processor may be a pinna-restoration filter, i.e. a filter configured to perform pinna-restoration of the primary second microphone input signal. 
     In one or more exemplary hearing devices, the hearing device comprises a secondary pre-processor connected to the secondary microphone for pre-processing the secondary microphone input signal for provision of the secondary mixer input. 
     The hearing device comprises a processor for processing the mixer output. The processor provides an electrical output signal based on the mixer output (fed as processor input) to the processor. The processor may be configured for hearing compensation of a user&#39;s hearing loss. The processor may be a multi-channel processor. In other words, the processor input may be a multi-channel input, where each channel or (frequency) component is processed in parallel. The processor may be configured for feedback compensation and/or feedback cancellation. 
     The hearing device comprises a receiver for converting the electrical output signal to an audio output signal. 
     The mixing module is configured to mix the primary mixer input and the secondary mixer input for provision of the mixer output. In one or more exemplary hearing devices, the mixing module is configured to mix a primary first component of the primary mixer input and a secondary first component of the secondary mixer input for provision of a first component of the mixer output. 
     In one or more exemplary hearing devices/methods, to increase an amount of the secondary mixer input in the mixer output for a time period comprises to increase one or more secondary components, such as secondary first component and/or secondary second component, of the secondary mixer input. 
     In one or more exemplary hearing devices/methods, to reduce the amount of the secondary mixer input in the mixer output after the time period comprises to reduce one or more secondary components, such as secondary first component and/or secondary second component, of the secondary mixer input. 
     The primary first component of the primary mixer input and the secondary first component of the secondary mixer input may be broadband components. 
     The primary first component of the primary mixer input and the secondary first component of the secondary mixer input may be components of a first frequency band or first frequency bin. In one or more exemplary hearing devices/methods, a center frequency of the first frequency band/frequency bin is less than 1 kHz or even less than 400 Hz. A center frequency of the first frequency band/frequency bin may be in a range from 400 Hz to 2 kHz. In one or more exemplary hearing devices/methods, a center frequency of the first frequency band/frequency bin is larger than 1 kHz, such as larger than 2 kHz or even larger than 3 kHz. 
     In the hearing device, to mix the primary mixer input and the secondary mixer input for provision of the mixer output may comprise applying primary mixing filter(s) or primary gain(s)/coefficient(s) to the primary mixer input. The primary mixing filter(s) may be a time-domain filter or a frequency-domain filter. In other words, the mixing module may comprise primary filter(s) and/or primary gain unit(s) for receiving and processing the primary mixer input. 
     In the hearing device, to mix the primary mixer input and the secondary mixer input for provision of the mixer output may comprise applying secondary mixing filter(s) or secondary gain(s)/coefficient(s) to the secondary mixer input. The secondary mixing filter(s) may be a time-domain filter or a frequency-domain filter. In other words, the mixing module may comprise secondary filter(s) and/or secondary gain unit(s) for receiving and processing the secondary mixer input. In one or more exemplary hearing devices/methods, to increase an amount of the secondary mixer input in the mixer output for a time period comprises to increase one or more secondary gain(s)/coefficient(s) applied to the secondary mixer input. In one or more exemplary hearing devices/methods, to reduce an amount of the secondary mixer input in the mixer output comprises to reduce, e.g. set to zero, one or more secondary gain(s)/coefficient(s) applied to the secondary mixer input. 
     In one or more exemplary hearing devices/methods, to reduce an amount of the primary mixer input in the mixer output for a time period comprises to reduce one or more primary gain(s)/coefficient(s) applied to the primary mixer input. In one or more exemplary hearing devices/methods, to increase an amount of the primary mixer input in the mixer output comprises to increase, e.g. set to 1 or a value larger than 0.5, one or more primary gain(s)/coefficient(s) applied to the primary mixer input. 
     In the hearing device, to mix the primary mixer input and the secondary mixer input may comprise adding the output from the primary mixing filter(s)/primary gain unit(s) and the output from the secondary mixing filter(s)/secondary gain unit(s). In other words, the mixing module may comprise an adder (single-channel or multi-channel) connected to primary mixing filter(s)/primary gain unit(s) and secondary mixing filter(s)/secondary gain unit(s) for provision of an adder output as the mixer output. 
     The mixing module may be a multi-channel mixing module configured to perform mixing of a plurality of components/channels. In one or more exemplary hearing devices, the mixing module is optionally configured to mix a primary second component of the primary mixer input and a secondary second component of the secondary mixer input for provision of a second component of the mixer output. The mixing module is optionally configured to mix a primary second component of the primary mixer input and a secondary second component of the secondary mixer input for provision of a second component of the mixer output. The primary second component of the primary mixer input and the secondary second component of the secondary mixer input may be components of a second frequency band or second frequency bin, e.g. different from the first frequency band/frequency bin. 
     In one or more exemplary hearing devices and/or methods, to mix the primary first component, also denoted x_p_1, of the primary mixer input and the secondary first component x_s_1 of the secondary mixer input comprises to apply a first linear combination to the primary first component and the secondary first component for provision of the first component of the mixer output. In other words, the first component y_1 of the mixer output may be given as:
 
 y _1= a _1 *x _ p _1+ b _1 *x _ s _1,
 
where a_1 is a primary coefficient and b_1 is a secondary coefficient. The first linear combination, also denoted LC_1, may be defined by primary coefficient a_1 and secondary coefficient b_1.
 
     The primary coefficient a_1 of the first linear combination may be larger than 0. The secondary coefficient b_1 of the first linear combination may be larger than 0. In one or more exemplary hearing devices and/or methods, the primary coefficient a_1 may be in the range between 0 and 1. In one or more exemplary hearing devices and/or methods, the secondary coefficient b_1 may be in the range between 0 and 1. In one or more exemplary hearing devices, the primary coefficient a_1 may be in the range between 0.1 and 0.9. and/or the secondary coefficient b_1 may be in the range between 0.1 and 0.9. The secondary coefficient b_1 may be less than 0.5. 
     In one or more exemplary hearing devices/methods, the sum of the primary coefficient and the secondary coefficient of a linear combination is 1. Thus, the sum of a_1 and b_1 may be 1. 
     In one or more exemplary hearing devices, the mixing module is configured to mix a primary second component, also denoted x_p_2, of the primary mixer input and a secondary second component, also denoted x_s_2, of the secondary mixer input for provision of a second component, also denoted y_2, of the mixer output. 
     The primary second component of the primary mixer input and the secondary second component of the secondary mixer input may be components of a second frequency band or second frequency bin. In one or more exemplary hearing devices/methods, a center frequency of the second frequency band/frequency bin is less than 1 kHz or even less than 400 Hz. A center frequency of the second frequency band/frequency bin may be in a range from 400 Hz to 2 kHz. In one or more exemplary hearing devices/methods, a center frequency of the second frequency band/frequency bin is larger than 1 kHz, such as larger than 2 kHz or even larger than 3 kHz. The second frequency band/frequency bin may be different from the first frequency band/frequency bin. 
     In one or more exemplary hearing devices, to mix the primary second component of the primary mixer input and the secondary second component of the secondary mixer input comprises to apply a second linear combination to the primary second component and the secondary second component for provision of the second component of the mixer output. In other words, the second component y_2 of the mixer output may be given as:
 
 y _2= a _2* x _ p _2+ b _2* x _ s _2,
 
where a_2 is a primary coefficient and b_2 is a secondary coefficient. The second linear combination, also denoted LC_2, may be defined by primary coefficient a_2 and secondary coefficient b_2. The second linear combination may be different from the first linear combination. Different linear combinations or different mixing of primary mixer input and secondary mixer for different components, e.g. in different frequency bands, may further optimize the spatial perception for the hearing device user, since feedback and instability in general depends on frequency. Thus, the present disclosure allows for increased use of the secondary microphone (high coefficient) in frequency bands where the feedback is low which may lead to improved spatial perception. Further, a low secondary coefficient may be used in frequency bands with severe feedback challenges. The sum of a_2 and b_2 may be 1.
 
     In one or more exemplary hearing devices, the primary coefficient a_2 may be in the range between 0.1 and 0.9. and/or the secondary coefficient b_2 may be in the range between 0.1 and 0.9. The secondary coefficient b_2 may be less than 0.5. The secondary coefficient b_2 may be different from the secondary coefficient b_1. 
     In or more exemplary hearing devices, the secondary coefficient b_1 is larger than 0.5, such as in the range from 0.55 to 0.95 and/or the secondary coefficient b_2 is less than 0.5, such as in the range from 0.05 to 0.45. 
     In one or more exemplary hearing devices and/or methods, to increase an amount of the secondary mixer input for a time period may comprise increasing the secondary coefficient b_1 of the first linear combination, e.g. from a value less than 0.9 to 1, and/or increasing the secondary coefficient b_2 of the second linear combination, e.g. from a value less than 0.9 to 1. To increase an amount of the secondary mixer input for a time period may comprise setting one or more secondary coefficient(s), such as b_1 and/or b_2, to a value larger than 0.5, such as 1 or in the range from 0.6 to 1. 
     In one or more exemplary hearing devices and/or methods, to reduce the amount of the secondary mixer input in the mixer output after the time period may comprise reducing the secondary coefficient b_1 of the first linear combination, e.g. from a value of 1 to less than 0.9, and/or the secondary coefficient b_2 of the second linear combination, e.g. from a value of 1 to less than 0.9. To reduce an amount of the secondary mixer input after the time period may comprise setting one or more secondary coefficient(s), such as b_1 and/or b_2, to a value less than 0.5, such as 0 or in the range from 0 to 0.4. 
     In one or more exemplary hearing devices/methods, to reduce an amount of the primary mixer input in the mixer output for a time period may comprise reducing the primary coefficient a_1 of the first linear combination and/or the primary coefficient a_2 of the second linear combination. To reduce an amount of the primary mixer input for a time period may comprise setting one or more primary coefficient(s), such as a_1 and/or a_2, to a value less than 0.5, such as 0 or in the range from 0 to 0.4. 
     In one or more exemplary hearing devices/methods, to increase the amount of the primary mixer input in the mixer output after the time period may comprise increasing the primary coefficient a_1 of the first linear combination and/or increasing the primary coefficient a_2 of the second linear combination. To increase an amount of the primary mixer input after the time period may comprise setting one or more primary coefficient(s), such as a_1 and/or a_2, to a value larger than 0.5, such as 1 or in the range from 0.6 to 1. 
     In one or more exemplary hearing devices, the mixing module is configured to mix a primary third component, also denoted x_p_3, of the primary mixer input and a secondary third component, also denoted x_s_3, of the secondary mixer input for provision of a third component, also denoted y_3, of the mixer output. 
     The primary third component of the primary mixer input and the secondary third component of the secondary mixer input may be components of a third frequency band or third frequency bin. In one or more exemplary hearing devices/methods, a center frequency of the third frequency band/frequency bin is less than 1 kHz or even less than 400 Hz. A center frequency of the third frequency band/frequency bin may be in a range from 400 Hz to 2 kHz. In one or more exemplary hearing devices/methods, a center frequency of the third frequency band/frequency bin is larger than 1 kHz, such as larger than 2 kHz or even larger than 3 kHz. The third frequency band/frequency bin may be different from the first frequency band/frequency bin and/or different from the second frequency band/frequency bin. 
     In one or more exemplary hearing devices, to mix the primary third component of the primary mixer input and the secondary third component of the secondary mixer input comprises to apply a third linear combination to the primary third component and the secondary third component for provision of the third component of the mixer output. In other words, the third component y_3 of the mixer output may be given as:
 
 y _3= a _3* x _ p _3+ b _3* x _ s _3,
 
where a_3 is a primary coefficient and b_3 is a secondary coefficient. The third linear combination, also denoted LC_3, may be defined by primary coefficient a_3 and secondary coefficient b_3. The third linear combination may be different from the first linear combination.
 
     In one or more exemplary hearing devices, the primary coefficient a_3 may be in the range between 0.1 and 0.9. and/or the secondary coefficient b_3 may be in the range between 0.1 and 0.9. The secondary coefficient b_3 may be larger than 0.5. The secondary coefficient b_3 may be different from the secondary coefficient b_1 and/or different from the secondary coefficient b_2. The sum of a_3 and b_3 may be 1. 
     In one or more exemplary hearing devices and/or methods, to increase an amount of the secondary mixer input for a time period may comprise increasing the secondary coefficient b_3 of the third linear combination, e.g. from a value less than 0.9 to 1. To increase an amount of the secondary mixer input for a time period may comprise setting secondary coefficient b_3 to a value larger than 0.5, such as 1 or in the range from 0.6 to 1. 
     In one or more exemplary hearing devices and/or methods, to reduce the amount of the secondary mixer input in the mixer output after the time period may comprise reducing the secondary coefficient b_3 of the third linear combination, e.g. from a value of 1 to less than 0.9. To reduce an amount of the secondary mixer input after the time period may comprise setting secondary coefficient b_3 to a value less than 0.5, such as 0 or in the range from 0 to 0.4. 
     In one or more exemplary hearing devices/methods, to reduce an amount of the primary mixer input in the mixer output for a time period may comprise reducing the primary coefficient a_3 of the third linear combination. To reduce an amount of the primary mixer input for a time period may comprise setting primary coefficient a_3 to a value less than 0.5, such as 0 or in the range from 0 to 0.4. 
     In one or more exemplary hearing devices/methods, to increase the amount of the primary mixer input in the mixer output after the time period may comprise increasing the primary coefficient a_3 of the third linear combination. To increase an amount of the primary mixer input after the time period may comprise setting primary coefficient a_3 to a value larger than 0.5, such as 1 or in the range from 0.6 to 1. 
     Further, it is an important advantage of the present disclosure that the linear combinations/mixing filters may be individually set during operation of the hearing device and configured to the specific hearing device user environments. Thereby, improved hearing device operation/listening experience is provided. In particular, a hearing device with improved directionality is provided. 
     In one or more exemplary hearing devices, the mixing module is configured to mix primary components x_p_i, of the primary mixer input and secondary components x_s_i of the secondary mixer input for provision of components y_i of the mixer output, e.g. by application of linear combinations LC_i=(a_i, b_i), where i is an index of the i&#39;th component of the respective mixer inputs, mixer output and linear combinations defined by primary and secondary coefficients. In other words, the mixer output components y_i, i=1 to N (N is the number of components/frequency bands) may be given as:
 
 y _ i=a _ i*x _ p _ i+b _ i*x _ i  
 
     The number i of frequency bands/components may be larger than five, such as larger than fifteen. The sum of a_i and b_i may be 1 for one or more, such as all i=1 to N. 
     In one or more exemplary hearing devices/methods, a_i and b_i are complementary, i.e. a_i+b_i=1 at least for one or a plurality, such as all, of i=1, 2, . . . , N. 
     In one or more exemplary hearing devices/methods, one or a plurality, such as all, of primary coefficients a_i, e.g. for i=1, 2 and 3, may be 0 during the time period and/or larger than 0, such as larger than 0.1, after the time period. In one or more exemplary hearing devices/methods, one or a plurality, such as all, of primary coefficients a_i, e.g. for i=1, 2 and 3, may be larger than 0, such as larger than 0.1, before detection of a switch event/sound event. 
     In one or more exemplary hearing devices/methods, one or a plurality, such as all, of secondary coefficients b_i, e.g. for i=1, 2 and 3, may be 1 at least during the time period and/or less than 1, such as less than 0.9, after the time period. In one or more exemplary hearing devices/methods, one or a plurality, such as all, of secondary coefficients b_i, e.g. for i=1, 2 and 3, may be less than 1, such as less than 0.9, before detection of a switch event/sound event. 
     In one or more exemplary hearing devices/methods, one or a plurality, such as all, of primary coefficients a_i, e.g. for i=1, 2, and 3, may be in the range from 0.1 to 0.9 or less than 0.1 during the time period and/or in the range from 0.1 to 0.9 and/or larger than 0.1 after the time period. For example, primary coefficients a_i, e.g. for i=1, 2, and 3, may be less than 0.1 during the time period and larger than 0.1 (and optionally less than 0.9) after the time period. 
     In one or more exemplary hearing devices/methods, one or a plurality, such as all, of secondary coefficients b_i, e.g. for i=1, 2, and 3, may be in the range from 0.1 to 0.9 or larger than 0.9 during the time period and/or in the range from 0.1 to 0.9 or less than 0.9 after the time period. For example, secondary coefficients b_i, e.g. for i=1, 2, and 3, may be larger than 0.9 during the time period and less than 0.9 (and optionally larger than 0.1) after the time period. 
     It is to be noted that features described in relation to the hearing device are also applicable to the method and vice versa. 
       FIG.  1    shows exemplary hearing devices. The hearing device  2 ,  2 A comprises a first housing  4  configured as a behind-the-ear housing to be worn behind the pinna of a user; a second housing  6  configured as an earpiece housing to be worn in or at the ear canal of a user; and a wire  8  connecting the first housing  4  and the second housing  6 . The hearing device  2 ,  2 A comprises a primary set of microphones  10 ,  12  arranged in the first housing  4 , the primary set of microphones including a primary first microphone  10  for provision of a primary first microphone input signal  10 A, and a primary second microphone  12  for provision of a primary second microphone input signal  12 A. The hearing device  2 ,  2 A comprises a secondary microphone  14  arranged in the second housing  6  for provision of a secondary microphone input signal  14 A. 
     The hearing device  2 ,  2 A comprises a mixing module  16  for provision of a mixer output  18  based on a primary mixer input  20  and a secondary mixer input  22 , wherein the primary mixer input  20  is optionally based on the primary first microphone input signal  10 A and/or the primary second microphone input signal  12 A, and the secondary mixer input  22  is based on the secondary microphone input signal  14 A. 
     The hearing device  2 ,  2 A comprises a processor  24  for processing the mixer output  18  and providing an electrical output signal  26  based on mixer output  18 ; and a receiver  28  for converting the electrical output signal  26  to an audio output signal. 
     In hearing device  2 , the mixing module  16  comprises a primary mixing filter  30  and a secondary mixing filter  32  for filtering broadband primary mixer input  20  and broadband secondary mixer input  22 , respectively, in the time-domain. Filtered primary mixer input  33 A and filtered secondary mixer input  33 B are fed to adder  34  of the mixing module  16  for forming the mixer output  18 . Accordingly, the mixing module  16  is configured to mix a broadband primary first component of the primary mixer input  20  and a secondary first component of the secondary mixer input  22  for provision of a broadband first component of the mixer output  18 . 
     In hearing device  2 A, the mixing module  16 A comprises a primary gain unit  30 A and a secondary gain unit  32 A for applying respective gains or coefficients of first linear combination to broadband primary mixer input  20  and broadband secondary mixer input  22 , respectively, in the time-domain. The outputs from gain units  30 A and  32 A are fed to adder  34  of the mixing module  16  for forming the mixer output  18 . Optionally, the mixing module  16 A is configured to mix a broadband primary first component of the primary mixer input  20  and a secondary first component of the secondary mixer input  22  for provision of a broadband first component of the mixer output  18  by applying a first linear combination to broadband primary first component as primary mixer input  20  and broadband secondary first component as secondary mixer input  22  for provision of broadband first component as mixer output  18 . In other words, the primary gain unit  30 A applies a primary coefficient a_1 as a primary gain to the primary mixer input  20  and the secondary gain unit  32 A applies a secondary coefficient b_1 as a secondary gain to the secondary mixer input  22 . The primary coefficient may be in the range from 0.1 to 0.9 and/or the secondary coefficient may be in the range from 0.1 to 0.9. 
     The hearing device  2 ,  2 A may comprise a primary pre-processor  36  for forming the primary mixer input  20  based on the primary first microphone input signal  10 A and/or the primary second microphone input signal  12 A. In the primary pre-processer  36 , a first filter  36 A optionally filters primary first microphone input signal  10 A and a second filter  36 B optionally filters the primary second microphone input signal  12 A. The (optionally filtered) primary first microphone input signal  10 A and the (optionally filtered) primary second microphone input signal  12 A are fed to adder  36 C for forming the primary mixer input  20  as the sum of (optionally filtered) primary first microphone input signal  10 A and (optionally filtered) primary second microphone input signal  12 A. The first filter  36 A and the second filter  36 B may be pinna-restoration filters. 
     The hearing device  2  and/or the hearing device,  2 A may comprise a secondary pre-processor  38  for forming the secondary mixer input  22  based on the secondary microphone input signal  14 A. The secondary microphone input signal  14 A may be fed to the respective mixing module  16 ,  16 A as the secondary mixer input. In the secondary pre-processer  38 , a filter and/or a delay  36 A optionally filters and/or delays secondary microphone input signal  14 A for provision of secondary mixer input  22 , e.g. for the secondary mixer input  22  to match the primary mixer input  20 . 
     The hearing device  2  and/or the hearing device  2 A comprises a mixing controller  44  configured to determine presence of a switch or sound event e.g. with switch detector  45 ; in accordance with a determination that a sound event is present, increase an amount of the secondary mixer input in the mixer output for a time period, e.g. by increasing amplitude of filter transfer function of secondary mixing filter  32  or increasing gains/coefficients of secondary gain unit  32 A; and reduce the amount of the secondary mixer input in the mixer output after the time period, e.g. by reducing amplitude of filter transfer function of secondary mixing filter  32  or reducing gains/coefficients of secondary gain unit  32 A. In other words, the mixing controller sends a control signal  52  with control parameter(s) to control the mixing module  16 ,  16 A, e.g. for controlling filter coefficients or gains in the mixing module. The time period may be adaptive and/or be in the range from 2 ms to 20 ms, such as in the range 5 ms to 15 ms. 
     The mixing controller  44  may be configured to determine the control parameter(s), e.g. based on one or more operating parameters of the hearing device, e.g. from processor  24  as indicated by dashed conductor  46  and/or one or both of electrical output signal  26  and secondary microphone input signal  14 A as indicated by respective dashed conductors  48 ,  50 . In one or more exemplary hearing devices, the primary first microphone input signal  10 A and/or primary second microphone input signal  12 A (or filtered versions thereof) is fed to the mixing controller  44  for determining presence of a switch event/sound event based on the primary first microphone input signal  10 A and/or primary second microphone input signal  12 A. Primary mixer input  20  and/or secondary mixer input  22  may be fed to the mixing controller  44  for optionally determining presence of a switch event/sound event based on one or both of these inputs  20 ,  22 . 
     Accordingly, the mixing controller  44  may be configured to determine the presence of a sound event is based on one or more of the primary first microphone input signal  10 A, the primary second microphone input signal  12 A, and the secondary microphone input signal  14 A. 
     The mixing controller  44  controls the mixing module  16 ,  16 A, e.g. by setting filter coefficient(s) or coefficients of mixing filters  30 ,  32  or gain units  30 A,  32 A, respectively with control signal  52 . 
     The mixing controller  44  may be configured to determine the time period based on one or more operating parameters, such as program identifier, gain(s), feedback parameter(s), of the hearing device optionally received from processor  24  via conductor  46 . 
     The mixing controller  44  may be configured to increase an amount of the secondary mixer input in the mixer output and determine if a switching criterion is satisfied, i.e. to increase an amount of the secondary mixer input in the mixer output for a time period optionally comprises to increase an amount of the secondary mixer input in the mixer output and determine if a switching criterion is satisfied. The switching criterion evaluated in the mixing controller  44  may be based on one or more gains, such as a closed-loop gain, and/or feedback parameters of the hearing device. In other words, the hearing device may use an increased amount of secondary mixer input in the mixer output until the mixing controller determines that instability is either present or imminent. For example, the switching criterion may be satisfied if a feedback parameter indicative of the risk of feedback reaches a threshold. The switching criterion may be satisfied if a closed-loop gain reaches a threshold. Optionally, to reduce the amount of the secondary mixer input in the mixer output after the time period comprises to reduce the amount of the secondary mixer input in the mixer output in accordance with the mixing controller  44  determining that the switching criterion is satisfied. In other words, the mixing controller  44  may be configured to reduce the amount of the secondary mixer input in the mixer output in accordance with a determination that the switching criterion is satisfied. 
       FIG.  2    shows a hearing device with multi-band mixing module. The hearing device  2 B comprises a primary filter-bank  40  for provision of a multi-band primary mixer input comprising N components of the primary mixer input including primary first component  20 A and primary second component  20 B of primary mixer input based on microphone input signals  10 A and  12 A. The hearing device  2 B comprises a secondary filter-bank  42  for provision of a multi-band secondary mixer input comprising N components of the secondary mixer input (N may be larger than 5 or larger than 15) including secondary first component  22 A and secondary second component  22 B of secondary mixer input based on microphone input signal  14 A. 
     The mixing module  16 B comprises a first linear combiner  17 A configured to mix primary first component  20 A and secondary first component  22 A by applying a first linear combination defined by primary coefficient a_1 and secondary coefficient b_1 to respective components  20 A,  22 A for provision of a first component  18 A of the mixer output. The mixing module  16 B comprises a second linear combiner  17 B configured to mix primary second component  20 B and secondary second component  22 B by applying a second linear combination defined by primary coefficient a_2 and secondary coefficient b_2 to respective components  20 B,  22 B for provision of a second component  18 B of the mixer output. The mixing module  16 B may comprise N linear combiners including N&#39;th linear combiner  17 C configured to mix respective N primary and secondary components for provision of N components of the mixer output. The second linear combination is different from the first linear combination. 
     The hearing device  2 B optionally comprises a mixing controller  44  as described in relation to  FIG.  1   , e.g. configured to apply a mixing scheme in the mixing module  16 B. 
     The mixing controller  44  controls the mixing module  16 B, e.g. by setting linear combinations/coefficients of linear combiners  17 A,  17 B, . . . ,  17 C with control signal  52 , e.g. according to determination of a switch event/sound event. In other words, the mixing controller  44  optionally increases and/or reduces the amount of the secondary mixer input in the mixer output by setting linear combinations/coefficients of linear combiners  17 A,  17 B, . . . ,  17 C with control signal  52 . 
       FIG.  3    shows a hearing device with multi-band primary pre-processor  37  and multi-band mixing module  16 B. The hearing device  2 C comprises a primary first filter-bank  40 A for provision of a multi-band primary first microphone input signal comprising N components that are fed to multiband pre-processor  37 . The hearing device  2 C comprises a primary second filter-bank  40 B for provision of a multi-band primary second microphone input signal comprising N components that are fed to multiband pre-processor  37 . The first filter  36 A, the second filter  36 B and adder  36 C of pre-processor  37  are multi-band implementations for provision of multi-band primary mixer input. Thereby, multi-band pinna restoration and multiband mixing is provided, which in turn may increase the user experience by improving directional or spatial cues with reduced howling and/or feedback. 
       FIG.  4    is a flow diagram of an exemplary method of operating a hearing device, the hearing device comprising a first housing configured as a behind-the-ear housing to be worn behind the pinna of a user; a second housing configured as an earpiece housing to be worn in or at the ear canal of a user; a wire connecting the first housing and the second housing; a primary set of microphones arranged in the first housing, the primary set of microphones including a primary first microphone and a primary second microphone; a secondary microphone arranged in the second housing; a mixing module; a mixing controller; a processor; and a receiver, the method  100  comprising obtaining  102  a primary first microphone input signal with the primary first microphone; obtaining  104  a primary second microphone input signal with the primary second microphone; obtaining  106  a secondary microphone input signal with the secondary microphone; providing  108  a primary mixer input based on the primary first microphone input signal and/or the primary second microphone input signal; providing  110  a secondary mixer input based on the secondary microphone input signal; providing  112  a mixer output based on the primary mixer input and the secondary mixer input; processing  114  the mixer output for provision of an electrical output signal; and converting  116  the electrical output signal to an audio output signal. In the method  100 , providing  112  a mixer output based on the primary mixer input and the secondary mixer input comprises determining  112  presence of a sound event or a switch event; in accordance with determining presence of a sound event, increasing  112 B an amount of the secondary mixer input in the mixer output for a time period; and reducing  112 C the amount of the secondary mixer input in the mixer output after the time period. Increasing  112 B an amount of the secondary mixer input in the mixer output for a time period optionally comprises reducing an amount of the primary mixer input in the mixer output for a time period. Reducing  112 C the amount of the secondary mixer input in the mixer output after the time period optionally comprises increasing the amount of the primary mixer input in the mixer output after the time period. 
       FIG.  5    is a flow diagram of an exemplary method  100 A of operating a hearing device. The method  100 A comprises providing  113  a mixer providing a mixer output based on the primary mixer input and the secondary mixer input including increasing  112 B an amount of the secondary mixer input in accordance with determining presence of a sound event. In the method  100 A, increasing  112 B an amount of the secondary mixer input in the mixer output for a time period comprises increasing  112 E an amount of the secondary mixer input in the mixer output and determine  112 D if a switching criterion is satisfied, and wherein reducing  112 C the amount of the secondary mixer input in the mixer output after the time period comprises reducing  112 C the amount of the secondary mixer input in the mixer output in accordance with a determination that the switching criterion is satisfied. 
       FIG.  6    shows an example of 4 seconds of speech (3 speakers in a reverberant room). The red lines  54  indicate instants of sound events in the speech, i.e. the mixing controller determines presence of a sound event at time instants indicated by the red lines and increases an amount of the secondary mixer input in the mixer output for a time period, e.g. by switching to only use the secondary mixer input in the mixer output or increasing one or more of the coefficients/gains applied to the secondary mixer input in the mixing module. 
     The use of the terms “first”, “second”, “third” and “fourth”, “primary”, “secondary”, “tertiary” etc. does not imply any particular order, but are included to identify individual elements. Moreover, the use of the terms “first”, “second”, “third” and “fourth”, “primary”, “secondary”, “tertiary” etc. does not denote any order or importance, but rather the terms “first”, “second”, “third” and “fourth”, “primary”, “secondary”, “tertiary” etc. are used to distinguish one element from another. Note that the words “first”, “second”, “third” and “fourth”, “primary”, “secondary”, “tertiary” etc. are used here and elsewhere for labelling purposes only and are not intended to denote any specific spatial or temporal ordering. 
     Furthermore, the labelling of a first element does not imply the presence of a second element and vice versa. 
     It may be appreciated that  FIGS.  1 - 6    comprise some modules or operations which are illustrated with a solid line and some modules or operations which are illustrated with a dashed line. The modules or operations which are comprised in a solid line are modules or operations which are comprised in the broadest example embodiment. The modules or operations which are comprised in a dashed line are example embodiments which may be comprised in, or a part of, or are further modules or operations which may be taken in addition to the modules or operations of the solid line example embodiments. It should be appreciated that these operations need not be performed in order presented. Furthermore, it should be appreciated that not all of the operations need to be performed. The exemplary operations may be performed in any order and in any combination. 
     It is to be noted that the word “comprising” does not necessarily exclude the presence of other elements or steps than those listed. 
     It is to be noted that the words “a” or “an” preceding an element do not exclude the presence of a plurality of such elements. 
     It should further be noted that any reference signs do not limit the scope of the claims, that the exemplary embodiments may be implemented at least in part by means of both hardware and software, and that several “means”, “units” or “devices” may be represented by the same item of hardware. 
     The various exemplary methods, devices, and systems described herein are described in the general context of method steps processes, which may be implemented in one aspect by a computer program product, embodied in a computer-readable medium, including computer-executable instructions, such as program code, executed by computers in networked environments. A computer-readable medium may include removable and non-removable storage devices including, but not limited to, Read Only Memory (ROM), Random Access Memory (RAM), compact discs (CDs), digital versatile discs (DVD), etc. Generally, program modules may include routines, programs, objects, components, data structures, etc. that perform specified tasks or implement specific abstract data types. Computer-executable instructions, associated data structures, and program modules represent examples of program code for executing steps of the methods disclosed herein. The particular sequence of such executable instructions or associated data structures represents examples of corresponding acts for implementing the functions described in such steps or processes. 
     Although features have been shown and described, it will be understood that they are not intended to limit the claimed invention, and it will be made obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the claimed invention. The specification and drawings are, accordingly to be regarded in an illustrative rather than restrictive sense. The claimed invention is intended to cover all alternatives, modifications, and equivalents. 
     LIST OF REFERENCES 
     
         
           2 ,  2 A,  2 B,  2 C hearing device 
           4  first housing 
           6  second housing 
           8  wire 
           10  primary first microphone 
           10 A primary first microphone input signal 
           12  primary second microphone 
           12 A primary second microphone input signal 
           14  secondary microphone 
           14 A secondary microphone input signal 
           16 ,  16 A,  16 B mixing module 
           17 A first linear combiner 
           17 B second linear combiner 
           17 C N&#39;th linear combiner 
           18  mixer output 
           18 A first component of mixer output 
           18 B second component of mixer output 
           20  primary mixer input 
           20 A primary first component of primary mixer input 
           20 B primary second component of primary mixer input 
           22  secondary mixer input 
           22 A secondary first component of secondary mixer input 
           22 B secondary second component of secondary mixer input 
           24  processor 
           26  electrical output signal 
           28  receiver 
           30  primary mixing filter 
           30 A primary gain unit 
           32  secondary mixing filter 
           32 A secondary gain unit 
           33 A filtered primary mixer input 
           33 B filtered secondary mixer input 
           34  adder 
           36 ,  37  primary pre-processor 
           36 A first filter 
           36 B second filter 
           36 C adder 
           38  secondary pre-processor 
           40  primary filter-bank 
           40 A primary first filter-bank 
           40 B primary second filter-bank 
           42  secondary filter-bank 
           44  mixing controller 
           46  conductor 
           48  conductor 
           50  conductor 
           52  control signal 
           54  sound event 
           100 ,  100 A method of operating a hearing device 
           102  obtaining a primary first microphone input signal 
           104  obtaining a primary second microphone input signal 
           106  obtaining a secondary microphone input signal 
           108  providing a primary mixer input 
           110  providing a secondary mixer input 
           112 ,  113  providing a mixer output 
           112 A determining presence of a sound event 
           112 B increasing an amount of the secondary mixer input in accordance with determining presence of a sound event 
           112 C reducing the amount of the secondary mixer input in the mixer output after the time period 
           112 D determining if a switching criterion is satisfied 
           112 E increasing an amount of the secondary mixer input 
           114  processing the mixer output for provision of an electrical output signal; and 
           116  converting the electrical output signal to an audio output signal,