Patent Description:
Some devices that support health monitoring, however, can be obtrusive and uncomfortable. As such, people may choose to forego health monitoring if the device negatively impacts their movement or causes inconveniences while performing daily activities. It is therefore desirable for health-monitoring devices to be reliable, portable, and affordable to encourage more users to take advantage of these features.

<CIT> discloses an apparatus for and method of applying a selected range of high frequency acoustic energy to the apical end of the eardrum of an ear canal at a predetermined, substantially absolute sound pressure level at any frequency in the range of interest. The selected range of frequencies is generated at a location remote from the ear canal and the sound pressure level of each frequency is varied at the time of generation in accordance with a calibration function to ensure the frequencies reaching the apical end of the eardrum are at the predetermined sound pressure level. The calibration function is calculated by transmitting an acoustic pulse of broad frequency spectrum into the ear canal, measuring the sound pressure of the transmitted acoustic pulse and reflection thereof adjacent the entrance of the ear canal, and removing from the spectrum of the measured sound pressure the destructive interference effects between the transmitted and reflected acoustic pulses. The resultant sound pressure level contains nonuniformities having information regarding the sound transmission characteristics of the ear canal and the acoustic pulse generation and transmission system from which the calibration function is calculated.

<NPL>), <CIT>, and <CIT> respectively disclose calibration for an earbud system.

A claimed solution is specified by a method according to claim <NUM> and by a device according to claim <NUM>. Techniques and apparatuses are described that detect heart rate variability using a hearable. Provided according to one or more preferred embodiments is a hearable, such as an earbud, that is capable of performing a novel physiological monitoring process termed herein audioplethysmography, an active acoustic method capable of sensing subtle physiologically-related changes observable at a user's outer and middle ear. Instead of relying on other auxiliary sensors, such as optical or electrical sensors, audioplethysmography involves transmitting and receiving acoustic signals that at least partially propagate within a user's ear canal. To perform audioplethysmography, the hearable forms at least a partial seal in or around the user's outer ear. This seal enables formation of an acoustic circuit, which includes the seal, the hearable, the ear canal, and an ear drum of the ear. By transmitting and receiving acoustic signals, the hearable can recognize changes in the acoustic circuit to monitor a user's biometrics, including heart rate variability and/or blood pressure. In addition to being relatively unobtrusive, some hearables can be configured to support audioplethysmography without the need for additional hardware. As such, the size, cost, and power usage of the hearable can help make health monitoring accessible to a larger group of people and improve the user experience with hearables.

Aspects described below include a method associated with determining heart rate variability using a hearable. The method includes transmitting an acoustic transmit signal that propagates within at least a portion of an ear canal of a user. The method also includes receiving an acoustic receive signal. The acoustic receive signal represents a version of the acoustic transmit signal with one or more characteristics modified due to the propagation within the ear canal. The method additionally includes determining a heart rate variability of the user based on the one or more modified characteristics of the acoustic receive signal.

Aspects described below include a another method associated with determining heart rate variability using a hearable. The method includes transmitting a first acoustic transmit signal that propagates within at least a portion of an ear canal of a user. The first acoustic transmit signal has multiple frequencies (also referred herein as "tones"). The method also includes receiving an acoustic receive signal. The acoustic receive signal represents a version of the first acoustic transmit signal with one or more characteristics modified due to the propagation within the ear canal. The method additionally includes selecting a subset of the multiple frequencies based on the one or more modified characteristics of the acoustic receive signal. The method further includes transmitting a second acoustic signal having the subset of the multiple frequencies.

Aspects described below include a system with means for determining heart rate variability using a hearable.

Apparatuses for and techniques that facilitate detecting heart rate variability using a hearable are described with reference to the following drawings. The same numbers are used throughout the drawings to reference like features and components:.

Technological advances in medicine and healthcare are making it possible for people to live longer, healthier lives. To further achieve this, individuals have become interested in tracking their personal health. Health monitoring can motivate an individual to realize a particular fitness goal by tracking incremental improvements in the performance of the body's functions. Additionally, the individual can use health monitoring to observe changes in the body caused by chronic illnesses. With active feedback through health monitoring, the individual can live an active and full life with many chronic illnesses and recognize situations in which it is necessary to quickly seek medical attention.

Some health monitoring devices, however, can be obtrusive and uncomfortable. To measure carbon dioxide levels, for example, some devices take a sample of blood from the user. Other devices may utilize auxiliary sensors, including optical or electronic sensors, that add additional weight, cost, complexity, and/or bulk. Still other devices may require constant recharging of a battery due to relatively high power usage. As such, people may choose to forego health monitoring if the health monitoring device negatively impacts their movement or causes inconveniences while performing daily activities. It is therefore desirable for health monitoring devices to be reliable, portable, efficient, and affordable to expand accessibility to more users.

Wireless technology has become prevalent in everyday life, making communication and data readily accessible to users. One type of wireless technology are wireless hearables, examples of which include wireless earbuds and wireless headphones. Wireless hearables have allowed users freedom of movement while listening to audio content from music, audio books, podcasts, and videos. With the prevalence of wireless hearables, there is a market for adding additional features to existing hearables utilizing current hardware (e.g., without introducing any new hardware).

To address this challenge and provide new features for existing hearables, techniques are described that detect heart rate variability using a hearable. Provided according to one or more preferred embodiments is a hearable, such as an earbud, that is capable of performing a novel physiological monitoring process termed herein audioplethysmography, an active acoustic method capable of sensing subtle physiologically-related changes observable at a user's outer and middle ear. Instead of relying on other auxiliary sensors, such as optical or electrical sensors, audioplethysmography involves transmitting and receiving acoustic signals that at least partially propagate within a user's ear canal. To perform audioplethysmography, the hearable forms at least a partial seal in or around the user's outer ear. This seal enables formation of an acoustic circuit, which includes the seal, the hearable, the ear canal, and an ear drum of the ear. By transmitting and receiving acoustic signals, the hearable can recognize changes in the acoustic circuit to monitor a user's biometrics, including heart rate variability and/or blood pressure. In addition to being relatively unobtrusive, some hearables can be configured to support audioplethysmography without the need for additional hardware. As such, the size, cost, and power usage of the hearable can help make health monitoring accessible to a larger group of people and improve the user experience with hearables.

It can be challenging to detect heart rate variability, which represents a shift in timing between heart beats. Cardiac activity in general may modulate approximately one percent or less of an acoustic signal. This modulation can occur within an amplitude of the acoustic signal, within a phase of the acoustic signal, or within both the amplitude and the phase of the acoustic signal. In many situations, the modulation of the acoustic signal by the cardiac activity can be obscured by noise and may not be directly measurable from a raw version of the acoustic signal. Also, various conditions can impact the signal-to-noise ratio for audioplethysmography. For example, a wear of the hearable (e.g., insertion depth and/or rotation), a physical structure of the user's ear canal, a response characteristic of the hearable (e.g., speaker, microphone, and/or housing), and a frequency of the acoustic transmit signal can impact an intensity at which cardiac activities modulate the acoustic signal.

To further address these challenges, the techniques for detecting heart rate variability can include performing a calibration procedure that evaluates current conditions to select one or more tones (frequencies) that enable cardiac activity to be detected in the presence of noise. As part of the calibration procedure, the hearable transmits an acoustic signal having multiple tones. During a measurement procedure, the hearable uses audioplethysmography to transmit an acoustic signal having one or more tones that are selected based on the calibration procedure. The acoustic signal associated with the measurement procedure has fewer tones compared to the acoustic signal associated with the calibration procedure. With fewer tones, the acoustic signal associated with the measurement procedure can have a higher amplitude at the selected tones compared to the amplitude of the acoustic signal associated with the calibration procedure at the multiple tones for a given output power of the hearable. Also, the acoustic signal associated with the measurement procedure can have a longer duration at each of the selected tones compared to the duration of the acoustic signal associated with the calibration procedure at the multiple tones for a given time interval. The higher amplitude and longer duration can further improve the signal-to-noise ratio and, by extension, an accuracy of the heart rate variability measurement.

<FIG> is an illustration of an example environment <NUM> in which heart rate variability detection using a hearable can be implemented. In the example environment <NUM>, a hearable <NUM> is connected to a smart device <NUM> using a physical or wireless interface. The hearable <NUM> is a device that can play audible content provided by the smart device <NUM> and direct the audible content into a user <NUM>'s ear <NUM>. In this example, the hearable <NUM> operates together with the smart device <NUM>. In other examples, the hearable <NUM> can operate or be implemented as a stand-alone device. Although depicted as a smartphone, the smart device <NUM> can include other types of devices, including those described with respect to <FIG>.

The hearable <NUM> is capable of performing audioplethysmography <NUM>, which is an acoustic method of sensing that occurs at the ear <NUM>. The hearable <NUM> can perform this sensing without the use of other auxiliary sensors, such as an optical sensor or an electrical sensor. Through audioplethysmography <NUM>, the hearable <NUM> can perform detect (or measure) heart rate variability <NUM> and/or blood pressure <NUM>. Heart rate variability <NUM> is a shift in timing between heart beats and can reflect a physiological state and/or an emotional state of the user <NUM>. For example, heart rate variability <NUM> can indicate a heart condition or a mental health issue such as anxiety or depression. Blood pressure <NUM> represents an amount of force applied against arterial walls by blood that is pumped via the heart. Many vital organs, including the heart, kidneys, and brain, can be damaged if the blood pressure <NUM> is too high. With audioplethysmography <NUM>, the user <NUM> can actively monitor their health and take appropriate action based on any changes in their heart rate variability <NUM> and/or blood pressure <NUM> to live a longer and healthier life.

To use audioplethysmography <NUM>, the user <NUM> positions the hearable <NUM> in a manner that creates at least a partial seal <NUM> around or in the ear <NUM>. Some parts of the ear <NUM> are shown in <FIG>, including an ear canal <NUM> and an ear drum <NUM> (or tympanic membrane). Due to the seal <NUM>, the hearable <NUM>, the ear canal <NUM>, and the ear drum <NUM> couple together to form an acoustic circuit. Audioplethysmography <NUM> involves, at least in part, measuring properties associated with this acoustic circuit. The properties of the acoustic circuit can change due to a variety of different situations or actions.

For example, consider <FIG> in which a change occurs in a physical structure of the ear <NUM>. Example changes to the physical structure include a change in a geometric shape of the ear canal <NUM> and/or a change in a volume of the ear canal <NUM>. This change can be caused, at least in part, by subtle blood vessel deformations in the ear canal <NUM> caused by the user <NUM>'s heart pumping. Other changes can also can be caused by movement in the ear drum <NUM> or movement of the user <NUM>'s jaw.

At <NUM>, for instance, the tissue around the ear canal <NUM> and the ear drum <NUM> itself are slightly "squeezed" due to blood vessel deformation. This squeeze causes a volume of the ear canal <NUM> to be slightly reduced at <NUM>. At <NUM>, however, the squeezing subsides and the volume of the ear canal <NUM> is slightly increased relative to <NUM>. The physical changes within the ear <NUM> can modulate an amplitude and/or phase of an acoustic signal that propagates through the ear canal <NUM>, as further described below.

During audioplethysmography <NUM>, an acoustic signal propagates through at least a portion of the ear canal <NUM>. The hearable <NUM> can receive an acoustic signal that represents a superposition of multiple acoustic signals that propagate along different paths within the ear canal <NUM>. Each path is associated with a delay (τ) and an amplitude (a). The delay and amplitude can vary over time due to the subtle changes that occur in the volume of the ear canal <NUM>. The received acoustic signal can be represented by Equation <NUM>: <MAT> where S(t) represents the received acoustic signal, n represents noise, φini represents a relative phase between the received acoustic signal and the transmitted acoustic signal, Ωfc represents a frequency of the transmitted acoustic signal, and t represents a time vector. Cardiac activities of the user <NUM> can modulate the amplitude and/or phase of the receive acoustic signal, as further shown in Equation <NUM>: <MAT> where hamp(t) represents an amplitude modulator and hphase(t) represents a phase modulator. The interactions between the hearable <NUM> and the ear <NUM> as well as the physiological activities of the user <NUM> modulate the amplitude and phase of the received acoustic signal. The techniques for audioplethysmography <NUM> can be performed while the hearable <NUM> is playing audible content to the user <NUM>. The smart device <NUM> is further described with respect to <FIG>.

<FIG> illustrates an example smart device <NUM>. The smart device <NUM> is illustrated with various non-limiting example devices including a desktop computer <NUM>-<NUM>, a tablet <NUM>-<NUM>, a laptop <NUM>-<NUM>, a television <NUM>-<NUM>, a computing watch <NUM>-<NUM>, computing glasses <NUM>-<NUM>, a gaming system <NUM>-<NUM>, a microwave <NUM>-<NUM>, and a vehicle <NUM>-<NUM>. Other devices may also be used, such as a home service device, a smart speaker, a smart thermostat, a baby monitor, a Wi-Fi™ router, a drone, a trackpad, a drawing pad, a netbook, an e-reader, a home automation and control system, a wall display, and another home appliance. Note that the smart device <NUM> can be wearable, non-wearable but mobile, or relatively immobile (e.g., desktops and appliances).

The smart device <NUM> includes one or more computer processors <NUM> and at least one computer-readable medium <NUM>, which includes memory media and storage media. Applications and/or an operating system (not shown) embodied as computer-readable instructions on the computer-readable medium <NUM> can be executed by the computer processor <NUM> to provide some of the functionalities described herein. The computer-readable medium <NUM> also includes an audioplethysmography-based application <NUM>, which uses information provided by the hearable <NUM> to perform an action. Example actions can include displaying data associated with heart rate variability <NUM> and/or blood pressure <NUM> to the user <NUM>.

The smart device <NUM> can also include a network interface <NUM> for communicating data over wired, wireless, or optical networks. For example, the network interface <NUM> may communicate data over a local-area-network (LAN), a wireless local-area-network (WLAN), a personal-area-network (PAN), a wire-area-network (WAN), an intranet, the Internet, a peer-to-peer network, point-to-point network, a mesh network, Bluetooth®, and the like. The smart device <NUM> may also include the display <NUM>. Although not explicitly shown, the hearable <NUM> can be integrated within the smart device <NUM>, or can connect physically or wirelessly to the smart device <NUM>. The hearable <NUM> is further described with respect to <FIG>.

<FIG> illustrates an example hearable <NUM>. The hearable <NUM> is illustrated with various non-limiting example devices, including wireless earbuds <NUM>-<NUM>, wired earbuds <NUM>-<NUM>, and headphones <NUM>-<NUM>. The earbuds <NUM>-<NUM> and <NUM>-<NUM> are a type of in-ear device that fits into the ear canal <NUM>. Each earbud <NUM>-<NUM> or <NUM>-<NUM> can represent a hearable <NUM>. Headphones <NUM>-<NUM> can rest on top of or over the ears <NUM>. The headphones <NUM>-<NUM> can represent closed-back headphones, open-back headphones, on-ear headphones, or over-ear headphones. Each headphone <NUM>-<NUM> includes two hearables <NUM>, which are physically packaged together. In general, there is one hearable <NUM> for each ear <NUM>.

The hearable <NUM> includes a communication interface <NUM> to communicate with the smart device <NUM>, though this need not be used when the hearable <NUM> is integrated within the smart device <NUM>. The communication interface <NUM> can be a wired interface or a wireless interface, in which audio content is passed from the smart device <NUM> to the hearable <NUM>. The hearable <NUM> can also use the communication interface <NUM> to pass data measured using audioplethysmography <NUM> to the smart device <NUM>. In general, the data provided by the communication interface <NUM> is in a format usable by the audioplethysmography-based application <NUM>. The communication interface <NUM> also enables the hearable <NUM> to communicate with another hearable. During bistatic sensing, for instance, the hearable <NUM> can use the communication interface <NUM> to coordinate with the other hearable to support two-ear audioplethysmography <NUM>, as further described with respect to <FIG>. In particular, the transmitting hearable <NUM> can communicate timing and waveform information to the receiving hearable <NUM> to enable the receiving hearable <NUM> to appropriately demodulate a received acoustic signal.

The hearable <NUM> includes at least one transducer <NUM> that can convert electrical signals into sound waves. The transducer <NUM> can also detect and convert sound waves into electrical signals. These sound waves may include ultrasonic frequencies and/or audible frequencies, either of which may be used for audioplethysmography <NUM>. In particular, a frequency spectrum (e.g., range of frequencies) that the transducer <NUM> uses to generate an acoustic signal can include frequencies from a low-end of the audible range to a high-end of the ultrasonic range, e.g., between <NUM> hertz (Hz) to <NUM> megahertz (MHz). Other example frequency spectrums for audioplethysmography <NUM> can encompass frequencies between <NUM> and <NUM> kilohertz (kHz), between <NUM> and <NUM>, between <NUM> and <NUM>, or between <NUM> and <NUM>.

In an example implementation, the transducer <NUM> has a monostatic topology. With this topology, the transducer <NUM> can convert the electrical signals into sound waves and convert sound waves into electrical signals (e.g., can transmit or receive acoustic signals). Example monostatic transducers may include piezoelectric transducers, capacitive transducers, and micro-machined ultrasonic transducers (MUTs) that use microelectromechanical systems (MEMS) technology.

Alternatively, the transducer <NUM> can be implemented with a bistatic topology, which includes multiple transducers that are physically separate. In this case, a first transducer converts the electrical signal into sound waves (e.g., transmits acoustic signals), and a second transducer converts sound waves into an electrical signal (e.g., receives the acoustic signals). An example bistatic topology can be implemented using at least one speaker <NUM> and at least one microphone <NUM>. The speaker <NUM> and the microphone <NUM> can be dedicated for audioplethysmography <NUM> or can be used for both audioplethysmography <NUM> and other functions of the smart device <NUM> (e.g., presenting audible content to the user <NUM>, capturing the user <NUM>'s voice for a phone call, or for voice control).

In general, the speaker <NUM> and the microphone <NUM> are directed towards the ear canal <NUM> (e.g., oriented towards the ear canal <NUM>). Accordingly, the speaker <NUM> can direct acoustic signals towards the ear canal <NUM>, and the microphone <NUM> is responsive to receiving acoustic signals from the direction associated with the ear canal <NUM>.

The hearable <NUM> includes at least one analog circuit <NUM>, which includes circuitry and logic for conditioning electrical signals in an analog domain. The analog circuit <NUM> can include analog-to-digital converters, digital-to-analog converters, amplifiers, filters, mixers, and switches for generating and modifying electrical signals. In some implementations, the analog circuit <NUM> includes other hardware circuitry associated with the speaker <NUM> or microphone <NUM>.

The hearable <NUM> also includes at least one system processor <NUM> and at least one system medium <NUM> (e.g., one or more computer-readable storage media). In the depicted configuration, the system medium <NUM> includes an audioplethysmography measurement module <NUM> (APG measurement module <NUM>) and optionally includes an audioplethysmography calibration module <NUM> (APG calibration module <NUM>). The audioplethysmography measurement module <NUM> and the audioplethysmography calibration module <NUM> can be implemented using hardware, software, firmware, or a combination thereof. In this example, the system processor <NUM> implements the audioplethysmography measurement module <NUM> and the audioplethysmography calibration module <NUM>. In an alternative example, the computer processor <NUM> of the smart device <NUM> can implement at least a portion of the audioplethysmography measurement module <NUM> and/or the audioplethysmography calibration module <NUM>. In this case, the hearable <NUM> can communicate digital samples of the acoustic signals to the smart device <NUM> using the communication interface <NUM>.

The audioplethysmography measurement module <NUM> analyzes receive acoustic signals to measure data associated with audioplethysmography <NUM>. The audioplethysmography measurement module <NUM> can be implemented using a biometric monitor <NUM>, which can detect heart rate variability <NUM> and/or blood pressure <NUM>. The measurement procedure is further described with respect to <FIG>. An example audioplethysmography measurement module <NUM> is further described with respect to <FIG>.

The audioplethysmography calibration module <NUM> can determine appropriate characteristics (e.g., waveform or signal characteristics) of transmitted acoustic signals to improve audioplethysmography <NUM> performance. For example, the audioplethysmography calibration module <NUM> can take into account the wear of the hearable <NUM> (e.g., the position of the hearable <NUM> relative to the ear canal <NUM>) and the physical structure of the ear canal <NUM> to determine a transmission frequency that can enable the hearable <NUM> to detect the user <NUM>'s heart rate variability <NUM> and/or blood pressure <NUM> with an accuracy of <NUM>% or more. With the audioplethysmography calibration module <NUM>, the hearable <NUM> can dynamically adjust the transmission frequency each time the seal <NUM> is formed (e.g., based on the wear of the hearable <NUM>) and based on the unique physical structure of the ear <NUM>. Through this calibration procedure, the hearables <NUM> on different ears <NUM> may operate with one or more different acoustic frequencies. The calibration procedure is further described with respect to <FIG>. An example implementation of the audioplethysmography calibration module <NUM> is further described with respect to <FIG>.

Some hearables <NUM> include an active-noise-cancellation circuit <NUM>, which enables the hearables <NUM> to reduce background or environmental noise. In this case, the microphone <NUM> used for audioplethysmography <NUM> can be implemented using a feedback microphone of the active-noise-cancellation circuit <NUM>. During active noise cancellation, the feedback microphone provides feedback information regarding the performance of the active noise cancellation. During audioplethysmography <NUM>, the feedback microphone receives an acoustic signal, which is provided to the audioplethysmography measurement module <NUM> and/or the audioplethysmography calibration module <NUM>. In some situations, active noise cancellation and audioplethysmography <NUM> are performed simultaneously using the feedback microphone. In this case, the acoustic signal received by the feedback microphone can be provided to at least one of the audioplethysmography modules <NUM> or <NUM> and can be provided to the active-noise-cancellation circuit <NUM>. Different types of audioplethysmography <NUM> are further described with respect to <FIG> and <FIG>.

<FIG> illustrates example operations of two hearables <NUM>-<NUM> and <NUM>-<NUM> performing single-ear audioplethysmography <NUM>. In environment <NUM>-<NUM>, the hearables <NUM>-<NUM> and <NUM>-<NUM> independently perform audioplethysmography <NUM> on different ears <NUM> of the user <NUM>. In this case, the first hearable <NUM>-<NUM> is proximate to the user <NUM>'s right ear <NUM>, and the second hearable <NUM>-<NUM> is proximate to the user <NUM>'s left ear <NUM>. Each hearable <NUM>-<NUM> and <NUM>-<NUM> includes a speaker <NUM> and a microphone <NUM>. The hearables <NUM>-<NUM> and <NUM>-<NUM> can operate in a monostatic manner during the same time period or during different time periods. In other words, each hearable <NUM>-<NUM> and <NUM>-<NUM> can independently transmit and receive acoustic signals.

For example, the first hearable <NUM>-<NUM> uses the speaker <NUM> to transmit a first acoustic transmit <NUM>-<NUM>, which propagates within at least a portion of the user <NUM>'s right ear canal <NUM>. The first hearable <NUM>-<NUM> uses the microphone <NUM> to receive a first acoustic receive signal <NUM>-<NUM>. The first acoustic receive signal <NUM>-<NUM> represents a version of the first acoustic transmit signal <NUM>-<NUM> that is modified, at least in part, by the acoustic circuit associated with the right ear canal <NUM>. This modification can change an amplitude, phase, and/or frequency of the first acoustic receive signal <NUM>-<NUM> relative to the first acoustic transmit signal <NUM>-<NUM>.

Similarly, the second hearable <NUM>-<NUM> uses the speaker <NUM> to transmit a second acoustic transmit signal <NUM>-<NUM>, which propagates within at least a portion of the user <NUM>'s left ear canal <NUM>. The second hearable <NUM>-<NUM> uses the microphone <NUM> to receive a second acoustic receive signal <NUM>-<NUM>. The second acoustic receive signal <NUM>-<NUM> represents a version of the second acoustic transmit signal <NUM>-<NUM> that is modified by the acoustic circuit associated with the left ear canal <NUM>. This modification can change an amplitude, phase, and/or frequency of the second acoustic receive signal <NUM>-<NUM> relative to the second acoustic transmit signal <NUM>-<NUM>.

The techniques of single-ear audioplethysmography <NUM> can be particularly beneficial for biometric monitoring as it enables the smart device <NUM> to compile information from both hearables <NUM>-<NUM> and <NUM>-<NUM>, which can further improve measurement confidence. For some aspects of audioplethysmography <NUM>, it can be beneficial to analyze the acoustic channel between two ears <NUM>, as further described with respect to <FIG>.

<FIG> illustrates an example joint operation of two hearables <NUM>-<NUM> and <NUM>-<NUM> performing two-ear audioplethysmography <NUM>. In the environment <NUM>-<NUM>, the hearables <NUM>-<NUM> and <NUM>-<NUM> jointly perform audioplethysmography <NUM> across two ears <NUM> of the user <NUM>. In this case, at least one of the hearables <NUM> (e.g., the first hearable <NUM>-<NUM>) includes the speaker <NUM>, and at least one of the other hearables <NUM> (e.g., the second hearable <NUM>-<NUM>) includes the microphone <NUM>. The hearables <NUM>-<NUM> and <NUM>-<NUM> operate together in a bistatic manner during the same time period.

During operation, the first hearable <NUM>-<NUM> transmits a first acoustic transmit <NUM> using the speaker <NUM>. The acoustic transmit signal <NUM> propagates through the user <NUM>'s right ear canal <NUM>. The acoustic transmit signal <NUM> also propagates through an acoustic channel that exists between the right and left ears <NUM>. In the left ear <NUM>, the acoustic transmit signal <NUM> propagates through the user <NUM>'s left ear canal <NUM> and is represented as an acoustic receive signal <NUM>. The second hearable <NUM>-<NUM> receives the acoustic receive signal <NUM> using the microphone <NUM>. The acoustic receive signal <NUM> represents a version of the acoustic transmit signal <NUM> that is modified by the acoustic circuit associated with the right ear canal <NUM>, modified by the acoustic channel associated with the user <NUM>'s face, and modified by the acoustic circuit associated with the left ear canal <NUM>. This modification can change an amplitude, phase, and/or frequency of the acoustic receive signal <NUM> relative to the acoustic transmit signal <NUM>. In some cases, the hearable <NUM>-<NUM> measures the time-of-flight (ToF) associated with the propagation from the first hearable <NUM>-<NUM> to the second hearable <NUM>-<NUM>. Sometimes a combination of single-ear and two-ear audioplethysmography <NUM> are applied to further improve measurement confidence.

The acoustic transmit signal <NUM> of <FIG> and <FIG> can represent a variety of different types of signals. As described above with respect to <FIG>, the acoustic transmit signal <NUM> can be an ultrasonic signal and/or an audible signal. Also, the acoustic transmit signal <NUM> can be a continuous-wave signal (e.g., a sinusoidal signal) or a pulsed signal. Some acoustic transmit signals <NUM> can have a particular tone (frequency). Other acoustic transmit signals <NUM> can have multiple tones (multiple frequencies). A variety of modulations can be applied to generate the acoustic transmit signal <NUM>. Example modulations include linear frequency modulations, triangular frequency modulations, stepped frequency modulations, phase modulations, or amplitude modulations. The acoustic transmit signal <NUM> can be transmitted as part of a calibration procedure or a measurement procedure, as further described as part of <FIG>.

<FIG> illustrates an example operation of a hearable for detecting heart rate variability. At <NUM>-<NUM>, the hearable <NUM> performs a calibration procedure <NUM>. In some circumstances, the hearable <NUM> can perform on-head detection (or in-ear detection) by detecting the presence of the seal <NUM> and initiate the calibration procedure <NUM> based on a determination that on-head detection is "true. " In other circumstances, the hearable <NUM> can initiate the calibration procedure <NUM> based on a specified schedule or a timer, which can be controlled by the user <NUM> via the smart device <NUM>.

In accordance with the calibration procedure <NUM>, the hearable <NUM> transmits an acoustic transmit signal <NUM> having multiple tones <NUM> (or multiple frequencies). The multiple tones <NUM> are transmitted simultaneously (e.g., in parallel) over a given time interval. The acoustic transmit signal <NUM> can have a particular bandwidth on the order of several kilohertz. For example, the acoustic transmit signal <NUM> can have a bandwidth of approximately <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>. In example implementations, the acoustic transmit signal <NUM> is transmitted over multiple seconds, such as <NUM>, <NUM>, <NUM>, <NUM>, or more seconds. A duration of each tone <NUM> can be evenly divided over a total duration of the acoustic transmit signal <NUM>.

In this example, the acoustic transmit signal <NUM> is shown to include tones <NUM>-<NUM>, <NUM>-<NUM>. <NUM>-M, where M represents a positive integer. The variable M can be equal to <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or more. In an example implementation, the acoustic transmit signal <NUM> has <NUM> tones <NUM> that are distributed between <NUM> and <NUM>. In some cases, the tones <NUM> are evenly distributed across an interval. For example, the tones <NUM> can be in <NUM> increments between <NUM> and <NUM> (e.g., at approximately <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>). The term "approximately" means that the tones <NUM> can be within <NUM>% of a given value or less (e.g., within <NUM>%, <NUM>%, or <NUM>% of the given value).

An amplitude of the acoustic transmit signal <NUM> can be approximately the same across the tones <NUM>-<NUM> to <NUM>-M. In this manner, power is evenly distributed across each tone <NUM>. The quantity of tones (e.g., M) can be determined based on an output power of the speaker <NUM>. Increasing the quantity of tones <NUM> can increase a likelihood that the hearable <NUM> can detect the cardiac activity across various conditions including user wear and a physical structure of the user's ear canal <NUM>. However, an amplitude of the acoustic transmit signal <NUM> can be limited across these tones <NUM> based on the output power of the speaker <NUM>. Thus, the quantity of tones <NUM> can be optimized based on an amount of output power that is available for audioplethysmography <NUM>.

The calibration procedure <NUM> selects one or more tones to be used for a measurement procedure <NUM>, which are represented by selected tones <NUM>-<NUM>. <NUM>-N, where N represents a positive integer that is less than M. In the case that M is equal to <NUM>, the variable N can be equal to <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>. The selected tones <NUM>-<NUM> to <NUM>-N represent a subset of the tones <NUM>-<NUM> to <NUM>-M (e.g., a proper subset of the tones <NUM>-<NUM> to <NUM>-M). In general, the calibration procedure <NUM> determines that the selected tones <NUM>-<NUM> to <NUM>-N improve a signal-to-noise ratio for audioplethysmography <NUM>.

At <NUM>-<NUM>, the hearable <NUM> performs the measurement procedure <NUM> to detect the heart rate variability <NUM> and/or the blood pressure <NUM>. In accordance with the measurement procedure <NUM>, the hearable <NUM> transmits another acoustic transmit signal <NUM> having the selected tones <NUM>-<NUM> to <NUM>-N. The selected tones <NUM>-<NUM> to <NUM>-N are transmitted simultaneously (e.g., in parallel) over a given time interval, which may be similar or different than the time interval of the acoustic transmit signal <NUM> at <NUM>-<NUM>.

An amplitude of the acoustic transmit signal <NUM> at <NUM>-<NUM> can be approximately the same across the selected tones <NUM>-<NUM> to <NUM>-N. In this manner, power is evenly distributed across each selected tone <NUM>. The amplitude of the acoustic transmit signal <NUM> at <NUM>-<NUM> can be higher than the amplitude of the acoustic transmit signal <NUM> at <NUM>-<NUM> because the available output power is distributed across fewer tones. Additionally or alternatively, a duration of each of the selected tones <NUM>-<NUM> to <NUM>-N of the acoustic transmit signal <NUM> at <NUM>-<NUM> can be longer than the duration of the tones <NUM>-<NUM> to <NUM>-N of the acoustic transmit signal <NUM> at <NUM>-<NUM>. The higher amplitude and/or the longer duration can further improve the signal-to-noise ratio performance of the hearable <NUM> for audioplethysmography <NUM>. By using a few selected tones <NUM>-<NUM> to <NUM>-N that were determined to improve signal-to-noise ratio performance, the measurement procedure <NUM> can achieve a higher accuracy for measuring heart rate variability <NUM> and/or blood pressure <NUM>. The audioplethysmography calibration module <NUM> is further described with respect to <FIG>.

<FIG> illustrates an example scheme implemented by the audioplethysmography calibration module <NUM>. In the depicted configuration, the audioplethysmography calibration module <NUM> includes at least one in-phase and quadrature mixer <NUM> (I/Q mixer <NUM>), at least one filter <NUM>, and at least one frequency selector <NUM>. The in-phase and quadrature mixer <NUM> performs frequency down-conversion and enables cardiac modulations that have amplitudes obscured by noise to be detected. In an example implementation, the in-phase and quadrature mixer <NUM> includes at least two mixers, at least one phase shifter, and at least one combiner (e.g., a summation circuit). The filter <NUM> attenuates intermodulation products that are generated by the in-phase and quadrature mixer <NUM>. In an example implementation, the filter <NUM> is implemented using a low-pass filter.

The frequency selector <NUM> selects one or more tones for audioplethysmography <NUM> (e.g., determines the selected tones <NUM>-<NUM> to <NUM>-N). In an example implementation, the frequency selector <NUM> includes at least one amplitude detector <NUM> (e.g., an envelope detector), at least one phase detector <NUM>, at least one peak-to-average ratio (PAR) detector <NUM> (PAR detector <NUM>), and at least one comparator <NUM>. The operation of these components are further described below.

During the calibration procedure <NUM>, the hearable <NUM> transmits the acoustic transmit signal <NUM>, as shown at <NUM>-<NUM> in <FIG>, and receives the acoustic receive signal <NUM>. The audioplethysmography calibration module <NUM> accepts the digital transmit signal <NUM>, which represents a version of the acoustic transmit signal <NUM>. Also, the audioplethysmography calibration module <NUM> accepts the digital receive signal <NUM>, which represents a digital version of the acoustic receive signal <NUM>.

The in-phase and quadrature mixer <NUM> uses the phase shifter and the two mixers to generate in-phase and quadrature components associated with the digital receive signal <NUM>. In particular, the in-phase and quadrature mixer <NUM> mixes the digital receive signal <NUM> with a first version of the digital transmit signal <NUM> that has a zero-degree phase shift to generate the in-phase component. Additionally, the in-phase and quadrature mixer <NUM> mixes the digital receive signal <NUM> with a second version of the digital transmit signal <NUM> that has a <NUM>-degree phase shift to generate the quadrature signal. This mixing operation downconverts the digital receive signal <NUM> from acoustic frequencies to baseband frequencies. Using the combiner, the in-phase and quadrature mixer <NUM> combines the in-phase and quadrature components of the digital receive signal <NUM> to generate a down-converted signal <NUM>. Use of the in-phase and quadrature mixer <NUM> can further improve the signal-to-noise ratio of the down-converted signal <NUM> compared to other mixing techniques.

In this example, the down-converted signal <NUM> represents a combination of the in-phase and quadrature components of the mixed-down digital receive signal <NUM>. In alternative implementations, the in-phase and quadrature mixer <NUM> doesn't include the combiner and passes the in-phase and quadrature components separately to the filter <NUM>. In this manner, the in-phase and quadrature components individually propagate through the filter <NUM>.

The filter <NUM> generates a filtered signal <NUM> based on the down-converted signal <NUM>. In particular, the filter <NUM> filters the down-converted signal <NUM> to attenuate spurious or undesired frequencies (e.g., intermodulation products), some of which can be associated with an operation of the in-phase and quadrature mixer <NUM>. In this example, the filtered signal <NUM> represents a combination of the in-phase and quadrature components of the down-converted signal <NUM>. Alternatively, the filtered signal <NUM> can represent separate or distinct in-phase and quadrature components, which are individually passed to the frequency selector <NUM>.

In this example, the frequency selector <NUM> extracts an amplitude <NUM> of the filtered signal <NUM> using the amplitude detector <NUM> and extracts a phase <NUM> of the filtered signal <NUM> using the phase detector <NUM>. Alternatively, if the in-phase and quadrature components of the filtered signal <NUM> are received separately, the amplitude detector <NUM> and the phase detector <NUM> can respectively measure the amplitude <NUM> and phase <NUM> based on the in-phase and quadrature components.

The peak-to-average ratio detector <NUM> measures peak-to-average ratios <NUM>-<NUM> to <NUM>-<NUM> for each of the tones <NUM>-<NUM> to <NUM>-M and for each of the characteristics (e.g., amplitude <NUM> and phase <NUM>). In general, the peak-to-average ratio <NUM> represents a peak intensity within a frequency range associated with a heart beat divided by an average intensity within this frequency range. The frequency range can be, for example, between <NUM> and <NUM> to correspond with a possible range of a human heart rate, which can be between <NUM> and <NUM> beats-per-minute. A higher peak-to-average ratio <NUM> indicates a better cardiac modulation quality, or more generally, a higher signal-to-noise ratio.

In one aspect, the comparator <NUM> can evaluate the peak-to-average ratios <NUM>-<NUM> to <NUM>-M with respect to a threshold <NUM>. The threshold <NUM> can be set, for example, to a particular value, such as <NUM>. In other cases, the audioplethysmography calibration module <NUM> can dynamically determine the threshold <NUM> and update it over time based on the observed peak-to-average ratios <NUM>-<NUM> of <NUM>-<NUM>. In an example implementation, the comparator <NUM> determines the selected tones <NUM>-<NUM> to <NUM>-N based on the frequencies associated with the peak-to-average ratios <NUM>-<NUM> to <NUM>-M that are greater than or equal to the threshold <NUM>.

Additionally or alternatively, the comparator <NUM> can evaluate the peak-to-average ratios <NUM>-<NUM> to <NUM>-M with respect to each other. In an example implementation, the comparator <NUM> determines one of the selected tones <NUM> based on a frequency with the highest peak-to-average ratio <NUM> across the amplitude <NUM>. Also, the comparator <NUM> can determine one of the selected tones <NUM> based on a frequency with the highest peak-to-average ratio <NUM> across the phase <NUM>. In other implementations, the comparator <NUM> can determine a single selected tone <NUM> based on a frequency having the highest peak-to-average ratio <NUM>-<NUM> to <NUM>-<NUM> associated with either the amplitude <NUM> or the phase <NUM>.

In general, the audioplethysmography calibration module <NUM> enables the selected tones <NUM>-<NUM> to <NUM>-N to be dynamically adjusted prior to the measurement procedure <NUM> based on a current environment, which can account for a wear of the hearable <NUM> (e.g., a current insertion depth and/or rotation), a physical structure of the user <NUM>'s ear canal <NUM>, and a response characteristic of the hearable <NUM> (e.g., speaker, microphone, and/or housing). In this manner, the audioplethysmography calibration module <NUM> can improve signal-to-noise ratio performance of the hearable <NUM> for the measurement procedure <NUM>. The audioplethysmography calibration module <NUM> can also determine which tones <NUM> generate acoustic receive signals <NUM> with detectable cardiac modulations in amplitude <NUM> and/or phase <NUM>. Example amplitudes <NUM>, phases <NUM>, and peak-to-average ratios <NUM> are further described with respect to <FIG>.

<FIG> illustrates example amplitudes <NUM> and phases <NUM> of the filtered signal <NUM> across tones <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM>. At the top of <FIG>, graphs <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> depict an amplitude <NUM> of the filtered signal <NUM> across the respective tones <NUM>-<NUM> to <NUM>-<NUM>. The horizontal dimension of the graphs <NUM>-<NUM> to <NUM>-<NUM> represent time in seconds, and the vertical dimension of the graphs <NUM>-<NUM> to <NUM>-<NUM> represent a normalized amplitude. The amplitude <NUM> of the filtered signal <NUM> has peak-to-average ratios <NUM>-<NUM> to <NUM>-<NUM> respectively associated with the tones <NUM>-<NUM> to <NUM>-<NUM>. The peak-to-average ratios <NUM>-<NUM> to <NUM>-<NUM> represent a portion of the peak-to-average ratios <NUM>-<NUM> to <NUM>-<NUM>.

In this example, the peak-to-average ratio <NUM>-<NUM> is higher than the peak-to-average ratio <NUM>-<NUM>, which is higher than the peak-to-average ratio <NUM>-<NUM>. In other words, the tone <NUM>-<NUM> has the highest peak-to-average ratio <NUM>-<NUM> across the amplitude <NUM>. Also, the peak-to-average ratio <NUM>-<NUM> is greater than the threshold <NUM> while the peak-to-average ratios <NUM>-<NUM> and <NUM>-<NUM> are less than the threshold <NUM>.

At the bottom of <FIG>, graphs <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> depict a phase <NUM> of the filtered signal <NUM> across the respective tones <NUM>-<NUM> to <NUM>-<NUM>. The horizontal dimension of the graphs <NUM>-<NUM> to <NUM>-<NUM> represent time in seconds, and the vertical dimension of the graphs <NUM>-<NUM> to <NUM>-<NUM> represent a normalized phase. The phase <NUM> of the filtered signal <NUM> has peak-to-average ratios <NUM>-<NUM> to <NUM>-<NUM> respectively associated with the tones <NUM>-<NUM> to <NUM>-<NUM>. The peak-to-average ratios <NUM>-<NUM> to <NUM>-<NUM> represent a portion of the peak-to-average ratios <NUM>-<NUM> to <NUM>-<NUM>.

In this example, the peak-to-average ratio <NUM>-<NUM> is higher than the peak-to-average ratio <NUM>-<NUM>, which is higher than the peak-to-average ratio <NUM>-<NUM>. In other words, the tone <NUM>-<NUM> has the highest peak-to-average ratio <NUM>-<NUM> across the phase <NUM>. Also, the peak-to-average ratio <NUM>-<NUM> is greater than the threshold <NUM> while the peak-to-average ratios <NUM>-<NUM> and <NUM>-<NUM> are less than the threshold <NUM>.

As shown in <FIG>, cardiac activity (e.g., a heartbeat) of the user <NUM> may or may not be detectable within the amplitude <NUM> or phase <NUM>. For the tone <NUM>-<NUM>, the cardiac activity does not significantly modulate the amplitude <NUM> or phase <NUM>, which is represented by the relatively low peak-to-average ratios <NUM>-<NUM> and <NUM>-<NUM>. For the tone <NUM>-<NUM>, the cardiac activity does significantly modulate the amplitude <NUM> but not the phase <NUM>. For the tone <NUM>-<NUM>, the cardiac activity does not significantly modulate the amplitude <NUM> but does significantly modulate the phase <NUM>.

With these peak-to-average ratios <NUM>-<NUM> to <NUM>-<NUM> and <NUM>-<NUM> to <NUM>-<NUM>, the frequency selector <NUM> can select at least the tone <NUM>-<NUM> based on the peak-to-average ratio <NUM>-<NUM> and/or the tone <NUM>-<NUM> based on the peak-to-average ratio <NUM>-<NUM> for the measurement procedure <NUM>. In some cases, the frequency selector <NUM> selects only the tone <NUM>-<NUM>, only the tone <NUM>-<NUM>, or both the tones <NUM>-<NUM> and <NUM>-<NUM>. In situations in which other tones <NUM> (not shown) have peak-to-average ratios <NUM> that are greater than the threshold <NUM>, these tones <NUM> may also be optionally selected by the frequency selector <NUM>.

<FIG> illustrates an example scheme implemented by the audioplethysmography measurement module <NUM>. In the depicted configuration, the audioplethysmography measurement module <NUM> includes at least one in-phase and quadrature mixer <NUM>, at least one filter <NUM>, and at least one biometric monitor <NUM>. The in-phase and quadrature mixer <NUM> and the filter <NUM> of the audioplethysmography measurement module <NUM> can be similar to the in-phase and quadrature mixer <NUM> and the filter <NUM> of the audioplethysmography calibration module <NUM>.

The biometric monitor <NUM> determines one or more physiological metrics of the user <NUM> for monitoring heart rate variability <NUM> and/or blood pressure <NUM>. In this example, the biometric monitor <NUM> includes a heart-rate-variability detector <NUM> and/or a blood-pressure detector <NUM>. The heart-rate-variability detector <NUM> measures the heart rate variability <NUM> of the user <NUM>. The blood pressure detector <NUM> measures the blood pressure <NUM> of the user <NUM>. The biometric monitor <NUM> can pass the measured heart rate variability <NUM> and/or the measured blood pressure <NUM> to the audioplethysmography-based application <NUM> for further processing or for displaying to the user <NUM>.

During the measurement procedure <NUM>, the hearable <NUM> transmits the audio transmit signal <NUM>, as shown at <NUM>-<NUM> in <FIG>, and receives the acoustic receive signal <NUM>. The audioplethysmography measurement module <NUM> accepts the digital transmit signal <NUM>, which represents a version of the acoustic transmit signal <NUM>, which represents a version of the acoustic transmit signal <NUM>. Also, the audioplethysmography measurement module <NUM> accepts the digital receive signal <NUM>, which represents a digital version of the acoustic receive signal <NUM>.

The audioplethysmography measurement module <NUM> performs similar operations as the audioplethysmography calibration module <NUM> to generate the filtered signal <NUM>. The heart-rate-variability detector <NUM> uses peak finding estimation to localize the peak of each heartbeat within the filtered signal <NUM>. This estimation can be performed across the amplitude and/or phase of the filtered signal <NUM>. Example peak finding estimation techniques include Z-score, local maxima, and divide and conquer. The heart-rate-variability detector <NUM> can measure the heart rate variability <NUM> by calculating a root mean square of successive differences (RMSSD) between each peak (e.g., between each heartbeat).

The blood-pressure detector <NUM> can detect occurrences of a dicrotic notch within the filtered signal <NUM>. The blood-pressure detector <NUM> can determine the blood pressure <NUM> of the user based on the occurrences of the dicrotic notch.

In <FIG>, <FIG>, and <FIG>, the calibration procedure <NUM> and the measurement procedure <NUM> are described as individual procedures that occur at different time intervals. In particular, the calibration procedure <NUM> occurs before the measurement procedure <NUM>. This enable the acoustic transmit signal <NUM> for the measurement procedure <NUM> to be transmitted with fewer tones than the acoustic transmit signal <NUM> for the calibration procedure <NUM>, which can further enable cardiac activity to be detected in the presence of noise by increasing the signal-to-noise ratio. In some implementations, however, the hearable <NUM> can have sufficient output power to perform the measurement procedure <NUM> with the multiple tones <NUM>-<NUM> to <NUM>-M using a single acoustic transmit signal <NUM>. In this case, aspects of the frequency selector <NUM> are integrated within the audioplethysmography measurement module <NUM> and effectively pass selected tones <NUM>-<NUM> to <NUM>-N of the filtered signal <NUM> to the biometric monitor <NUM>.

<FIG> illustrates an example graph <NUM> of an amplitude <NUM> of the filtered signal <NUM> for detecting heart rate variability <NUM> or blood pressure <NUM> using the hearable <NUM>. A horizontal dimension of the graph <NUM> represents time in seconds, and a vertical dimension of the graph <NUM> represents a normalized amplitude. The filtered signal <NUM> has peaks <NUM>, which are identified using triangles. The filtered signal <NUM> also has dicrotic notches <NUM>, an example of which is circled in <FIG>. Each peak <NUM> is associated with a heartbeat of the user, and can be identified by the heart-rate-variability detector <NUM> to measure the heart rate variability <NUM>. Each dicrotic notch <NUM> can be identified by the blood-pressure detector <NUM> to measure the blood pressure <NUM>.

<FIG> and <FIG> depict example methods <NUM> and <NUM> for implementing aspects of heart rate variability detection using a hearable. Methods <NUM> and <NUM> are shown as sets of operations (or acts) performed but not necessarily limited to the order or combinations in which the operations are shown herein. Further, any of one or more of the operations may be repeated, combined, reorganized, or linked to provide a wide array of additional and/or alternate methods. In portions of the following discussion, reference may be made to the environment <NUM> of <FIG>, and entities detailed in <FIG> and <FIG>, reference to which is made for example only. The techniques are not limited to performance by one entity or multiple entities operating on one device.

At <NUM> in <FIG>, a first acoustic transmit signal that propagates within at least a portion of an ear canal of a user is transmitted. The first acoustic transmit signal has multiple tones. For example, as part of a calibration procedure <NUM>, the hearable <NUM> transmits the acoustic transmit signal <NUM> shown at <NUM>-<NUM> in <FIG>. The acoustic transmit signal <NUM> propagates within at least a portion of the ear canal <NUM> of the user <NUM>, as described with respect to <FIG> and <FIG>. The acoustic transmit signal <NUM> has multiple tones, such as tones <NUM>-<NUM> to <NUM>-M shown in <FIG>. The calibration procedure <NUM> can be initiated based on the hearable <NUM> determining that on-head detection is "true.

At <NUM>, an acoustic receive signal is received. The acoustic receive signal represents a version of the first acoustic transmit signal with one or more characteristics modified due to the propagation within the ear canal. For example, as part of the calibration procedure <NUM>, the hearable <NUM> receives the acoustic receive signal <NUM> shown in <FIG> and <FIG>. The acoustic receive signal <NUM> represents a version of the acoustic transmit signal <NUM> with one or more characteristics (e.g., signal characteristics or waveform characteristics) modified due to the propagation within the ear canal <NUM>. Example characteristics can include amplitude <NUM> and/or phase <NUM>.

At <NUM>, a subset of the multiple tones is selected based on the one or more modified characteristics of the acoustic receive signal. For example, the audioplethysmography calibration module <NUM> selects the tones <NUM>-<NUM> to <NUM>-N, which represent a subset (e.g., a proper subset) of the multiple tones <NUM>-<NUM> to <NUM>-M. In some implementations, there are fewer selected tones <NUM> compared to the tones <NUM> (e.g., N is less than M). The selected tones <NUM> can have better signal-to-noise ratio performance relative to the non-selected tones based on current environmental conditions (e.g., the wear of the hearable <NUM> and/or the physical structure of the user <NUM>'s ear canal <NUM>).

At <NUM>, a second acoustic transmit signal having the subset of the multiple tones is transmitted. For example, as part of a measurement procedure <NUM>, the hearable <NUM> transmits the acoustic transmit signal <NUM> shown at <NUM>-<NUM> in <FIG>. This acoustic transmit signal <NUM> can have a higher amplitude and/or a longer duration at each of the selected tones <NUM>-<NUM> to <NUM>-N compared to the previous acoustic transmit signal <NUM> associated with the calibration procedure <NUM>. By receiving a reflection of the second acoustic transmit signal <NUM>, the hearable <NUM> can detect the heart rate variability <NUM> and/or the blood pressure <NUM> of the user <NUM>, as further described with respect to <FIG>.

At <NUM> of <FIG>, an acoustic transmit signal that propagates within at least a portion of an ear canal of a user is transmitted. For example, the transducer <NUM> (or speaker <NUM>) of the hearable <NUM> transmits the acoustic transmit signal <NUM>. The acoustic transmit signal <NUM> propagates within at least a portion of the ear canal <NUM> of the user <NUM>, as described with respect to <FIG> and <FIG>. The acoustic transmit signal <NUM> can have one or more tones <NUM> (e.g., frequencies), which may have been determined by a calibration process <NUM> to improve signal-to-noise ratio performance based on current environmental conditions. Example tones can include those within the audible range (e.g., between <NUM> and <NUM>) and/or those within the ultrasonic range (e.g., between <NUM> and <NUM>). The acoustic transmit signal <NUM> can be a sinusoidal signal or a pulsed signal.

At <NUM>, an acoustic receive signal is received. The acoustic receive signal represents a version of the acoustic transmit signal with one or more characteristics modified due to the propagation within the ear canal. For example, the transducer <NUM> (or the microphone <NUM>) of the hearable <NUM> receives the acoustic receive signal <NUM>. The acoustic receive signal <NUM> represents a version of the acoustic transmit signal <NUM> with one or more characteristics modified due to the propagation within the ear canal <NUM>. The characteristics can also be modified, at least in part, by the user <NUM>'s biometrics. Example characteristics include amplitude <NUM> and/or phase <NUM>. In some implementations, a feedback microphone of an active-noise-cancellation circuit <NUM> can receive the acoustic receive signal <NUM>.

At <NUM>, a heart rate variability of the user is determined based on the one or more modified characteristics of the acoustic receive signal. For example, the hearable <NUM> determines (e.g., measures) the heart rate variability <NUM> of the user <NUM> using the audioplethysmography measurement module <NUM>. In particular, the hearable <NUM> can use peak finding estimation to localize peaks associated with each heartbeat and calculate a root mean square of successive differences across these peaks to measure the heart rate variability <NUM>. Additionally or alternatively, the hearable <NUM> can determine the blood pressure <NUM> of the user <NUM> using the audioplethysmography measurement module <NUM>.

In some situations, the methods <NUM> and/or <NUM> are performed using one hearable <NUM> for single-ear audioplethysmography <NUM>, as described with respect to <FIG>. In other situations, the methods <NUM> and/or <NUM> are performed using two hearables <NUM> for two-ear audioplethysmography <NUM>, as described with respect to <FIG>.

<FIG> illustrates various components of an example computing system <NUM> that can be implemented as any type of client, server, and/or computing device as described with reference to the previous <FIG> and <FIG> to implement aspects of active acoustic sensing using a hearable.

The computing system <NUM> includes communication devices <NUM> that enable wired and/or wireless communication of device data <NUM> (e.g., received data, data that is being received, data scheduled for broadcast, or data packets of the data). The communication devices <NUM> or the computing system <NUM> can include one or more hearables <NUM>. The device data <NUM> or other device content can include configuration settings of the device, media content stored on the device, and/or information associated with a user of the device. Media content stored on the computing system <NUM> can include any type of audio, video, and/or image data. The computing system <NUM> includes one or more data inputs <NUM> via which any type of data, media content, and/or inputs can be received, such as human utterances, user-selectable inputs (explicit or implicit), messages, music, television media content, recorded video content, and any other type of audio, video, and/or image data received from any content and/or data source.

The computing system <NUM> also includes communication interfaces <NUM>, which can be implemented as any one or more of a serial and/or parallel interface, a wireless interface, any type of network interface, a modem, and as any other type of communication interface. The communication interfaces <NUM> provide a connection and/or communication links between the computing system <NUM> and a communication network by which other electronic, computing, and communication devices communicate data with the computing system <NUM>.

The computing system <NUM> includes one or more processors <NUM> (e.g., any of microprocessors, controllers, and the like), which process various computer-executable instructions to control the operation of the computing system <NUM>. Alternatively or in addition, the computing system <NUM> can be implemented with any one or combination of hardware, firmware, or fixed logic circuitry that is implemented in connection with processing and control circuits which are generally identified at <NUM>. Although not shown, the computing system <NUM> can include a system bus or data transfer system that couples the various components within the device. A system bus can include any one or combination of different bus structures, such as a memory bus or memory controller, a peripheral bus, a universal serial bus, and/or a processor or local bus that utilizes any of a variety of bus architectures.

The computing system <NUM> also includes a computer-readable medium <NUM>, such as one or more memory devices that enable persistent and/or non-transitory data storage (i.e., in contrast to mere signal transmission), examples of which include random access memory (RAM), non-volatile memory (e.g., any one or more of a read-only memory (ROM), flash memory, EPROM, EEPROM, etc.), and a disk storage device. The disk storage device may be implemented as any type of magnetic or optical storage device, such as a hard disk drive, a recordable and/or rewriteable compact disc (CD), any type of a digital versatile disc (DVD), and the like. The computing system <NUM> can also include a mass storage medium device (storage medium) <NUM>.

The computer-readable medium <NUM> provides data storage mechanisms to store the device data <NUM>, as well as various device applications <NUM> and any other types of information and/or data related to operational aspects of the computing system <NUM>. For example, an operating system <NUM> can be maintained as a computer application with the computer-readable medium <NUM> and executed on the processors <NUM>. The device applications <NUM> may include a device manager, such as any form of a control application, software application, signal-processing and control module, code that is native to a particular device, a hardware abstraction layer for a particular device, and so on.

The device applications <NUM> also include any system components, engines, or managers to implement audioplethysmography <NUM> for detecting heart rate variability <NUM> and/or blood pressure <NUM>. In this example, the device applications <NUM> include the audioplethysmography-based application <NUM> (APG-based application <NUM>) of <FIG>, the audioplethysmography measurement module <NUM> of <FIG>, and optionally the audioplethysmography calibration module <NUM> of <FIG>.

Claim 1:
A method comprising:
transmitting, with a hearable (<NUM>) that forms at least a partial seal in or around a user's outer ear, a first acoustic transmit signal (<NUM>) that propagates within at least a portion of an ear canal (<NUM>) of the user (<NUM>), the first acoustic transmit signal (<NUM>) having multiple frequencies (<NUM>);
receiving, with the hearable, an acoustic receive signal (<NUM>), the acoustic receive signal (<NUM>) representing a version of the first acoustic transmit signal (<NUM>) with one or more characteristics modified due to the propagation within the ear canal (<NUM>);
selecting a subset of the multiple frequencies (<NUM>) based on the one or more modified characteristics of the acoustic receive signal (<NUM>); and
transmitting a second acoustic transmit signal (<NUM>) that propagates within at least a portion of the ear canal of the user having the subset of the multiple frequencies (<NUM>).