Facebook Patent | Hybrid Audio System For Eyewear Devices

Patent: Hybrid Audio System For Eyewear Devices

Publication Number: 20200389716

Publication Date: 20201210

Applicants: Facebook

Abstract

An audio system for providing content to a user. The system includes a first and a second transducer assembly of a plurality of transducer assemblies, an acoustic sensor, and a controller. The first transducer assembly couples to a portion of an auricle of the user’s ear and vibrates over a first range of frequencies based on a first set of audio instructions. The vibration causes the portion of the ear to create a first range of acoustic pressure waves. The second transducer assembly is configured to vibrate over a second range of frequencies to produce a second range of acoustic pressure waves based on a second set of audio instructions. The acoustic sensor detects acoustic pressure waves at an entrance of the ear. The controller generates the audio instructions based on audio content to be provided to the user and the detected acoustic pressure waves from the acoustic sensor.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of co-pending U.S. application Ser. No. 15/967,924 filed on May 1, 2018, which is incorporated by reference in its entirety for all purposes.

BACKGROUND

[0002] This disclosure relates generally to an audio system in an eyewear device, and specifically relates to a hybrid audio system for use in eyewear devices.

[0003] Head-mounted displays in an artificial reality system often include features such as speakers or personal audio devices to provide audio content to users of the head-mounted displays. The audio devices ideally operate over the full range of human hearing while balancing being lightweight, ergonomic, low in power consumption, and minimizing crosstalk between the ears. Traditional audio devices utilize one mode of sound conduction (e.g., speakers through air conduction); however, only one mode of sound conduction may put some limits on the performance of the device, such that not all the frequency contents can be delivered using one mode of conduction. This is especially important when the user’s ears need to remain in contact with the sound conduction transducer assembly and cannot be occluded.

SUMMARY

[0004] This present disclosure describes an audio system comprising a plurality of transducer assemblies configured to provide audio content. The audio system may be a component of an eyewear device which may be a component of an artificial reality head-mounted display (HMD). Of the plurality of transducer assemblies, the audio system comprises a first transducer assembly coupled to a portion of an ear of a user of the audio system. The first transducer assembly comprises at least one transducer that is configured to vibrate the portion of the ear over a first range of frequencies to cause the portion of the ear to create a first range of acoustic pressure waves at an entrance to the user’s ear according to a first set of audio instructions. The audio system comprises a second transducer assembly including at least one transducer that vibrates over a second range of frequencies to produce a second range of acoustic pressure waves at the entrance of the user’s ear according to a second set of audio instructions. The audio system includes a controller coupled to the plurality of transducer assemblies and generates the first set and the second set of audio instructions such that the first range and the second range of acoustic pressure waves together form at least a portion of audio content to be provided to the user.

[0005] In additional embodiments, the audio system comprises an acoustic sensor configured to detect acoustic pressure waves at the entrance of the user’s ear, wherein the detected acoustic pressure waves include the first range and the second range of acoustic pressure waves. In additional embodiments, there is a third transducer assembly in the plurality of transducer assemblies that is coupled to a portion of the user’s skull bone behind the user’s ear or in front of it on a condyle and configured to vibrate the bone over a third range of frequencies according to a third set of audio instructions.

[0006] Additionally, the audio system can update audio instructions. To monitor resulting acoustic pressure waves at an entrance of the user’s ear due to the cartilage conduction transducer assembly and the air conduction transducer assembly, the audio system additionally comprises an acoustic sensor for detecting the acoustic pressure waves. As the controller receives feedback from the acoustic sensor, the controller can generate a frequency response model. The frequency response model compares the detected acoustic pressure waves to the audio content to be provided to the user. The controller can then update the audio instructions based in part on the frequency response model.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 is a perspective view of an eyewear device including an audio system, in accordance with one or more embodiments.

[0008] FIG. 2 is a profile view a portion of an audio system as a component of an eyewear device, in accordance with one or more embodiments.

[0009] FIG. 3 is a block diagram of an audio system, in accordance with one or more embodiments.

[0010] FIG. 4 is a flowchart illustrating a process of operating the audio system, in accordance with one or more embodiments.

[0011] FIG. 5 is a system environment of an eyewear device including an audio system, in accordance with one or more embodiments.

[0012] The figures depict embodiments of the present disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles, or benefits touted, of the disclosure described herein.

DETAILED DESCRIPTION

[0013] Embodiments of the invention may include or be implemented in conjunction with an artificial reality system. Artificial reality is a form of reality that has been adjusted in some manner before presentation to a user, which may include, e.g., a virtual reality, an augmented reality, a mixed reality, a hybrid reality, or some combination and/or derivatives thereof. Artificial reality content may include completely generated content or generated content combined with captured (e.g., real-world) content. The artificial reality content may include video, audio, haptic sensation, or some combination thereof, and any of which may be presented in a single channel or in multiple channels (such as stereo video that produces a three-dimensional effect to the viewer). Additionally, in some embodiments, artificial reality may also be associated with applications, products, accessories, services, or some combination thereof, that are used to, e.g., create content in an artificial reality and/or are otherwise used in (e.g., perform activities in) an artificial reality. The artificial reality system that provides the artificial reality content may be implemented on various platforms, including an eyewear device, a head-mounted display (HMD) assembly with the eyewear device as a component, a HMD connected to a host computer system, a standalone HMD, a mobile device or computing system, or any other hardware platform capable of providing artificial reality content to one or more viewers.

System Architecture

[0014] A hybrid audio system (audio system) uses at least cartilage conduction and air conduction for providing sound to an ear of a user. The audio system comprises a plurality of transducer assemblies–one of which is configured for cartilage conduction and another of which is configured for air conduction. The audio system may additionally comprise a third transducer assembly of the plurality of transducer assemblies configured for bone conduction. Each type of transduction assembly operates differently from the others. The cartilage conduction transducer assembly vibrates a pinna of the user’s ear for creating an airborne acoustic pressure wave at an entrance of the ear that travels down an ear canal to an eardrum where it is perceived as sound by the user, wherein airborne refers to an acoustic pressure wave which travels through air in the ear canal which then vibrates the eardrum, and these vibrations are turned into signals by the cochlea (also referred to as the inner ear) which the brain perceives as sound. The air conduction transducer assembly directly creates an airborne acoustic pressure wave at the entrance of the ear which also travels to the eardrum and perceived in the same fashion as cartilage conduction. The bone conduction transducer assembly vibrates the bone to create a tissue-borne and then, bone-borne acoustic pressure wave that is conducted by the tissue/bone of the head (bypassing the eardrum) to the cochlea. The cochlea turns the bone-borne acoustic pressure wave into signals which the brain perceives as sound. A tissue-borne acoustic pressure wave refers to an acoustic pressure wave that is transmitted via tissue and is for presenting audio content to a user. Advantages of an audio system that uses a combination of these methods to provide audio content to the user allows for the audio system to designate varying methods for varying ranges of the total range of human hearing. In one embodiment, the audio system may operate a bone conduction transducer assembly over a lowest range of frequencies, a cartilage conduction transducer assembly over a medium range of frequencies, and an air conduction transducer assembly over a highest range of frequencies.

[0015] FIG. 1 is a perspective view of an eyewear device 100 including an audio system, in accordance with one or more embodiments. The eyewear device 100 presents media to a user. In one embodiment, the eyewear device 100 may be a component of or in itself a head-mounted display (HMD). Examples of media presented by the eyewear device 100 include one or more images, video, audio, or some combination thereof. The eyewear device 100 may include, among other components, a frame 105, a lens 110, a sensor device 115, a cartilage conduction transducer assembly 120, an air conduction transducer assembly 125, a bone conduction transducer assembly 130, an acoustic sensor 135, and a controller 150.

[0016] The eyewear device 100 may correct or enhance the vision of a user, protect the eye of a user, or provide images to a user. The eyewear device 100 may be eyeglasses which correct for defects in a user’s eyesight. The eyewear device 100 may be sunglasses which protect a user’s eye from the sun. The eyewear device 100 may be safety glasses which protect a user’s eye from impact. The eyewear device 100 may be a night vision device or infrared goggles to enhance a user’s vision at night. The eyewear device 100 may be a HMD that produces artificial reality content for the user. Alternatively, the eyewear device 100 may not include a lens 110 and may be a frame 105 with an audio system that provides audio (e.g., music, radio, podcasts) to a user.

[0017] The frame 105 includes a front part that holds the lens 110 and end pieces to attach to the user. The front part of the frame 105 bridges the top of a nose of the user. The end pieces (e.g., temples) are portions of the frame 105 to which the temples of a user are attached. The length of the end piece may be adjustable (e.g., adjustable temple length) to fit different users. The end piece may also include a portion that curls behind the ear of the user (e.g., temple tip, ear piece).

[0018] The lens 110 provides or transmits light to a user wearing the eyewear device 100. The lens 110 is held by a front part of the frame 105 of the eyewear device 100. The lens 110 may be prescription lens (e.g., single vision, bifocal and trifocal, or progressive) to help correct for defects in a user’s eyesight. The prescription lens transmits ambient light to the user wearing the eyewear device 100. The transmitted ambient light may be altered by the prescription lens to correct for defects in the user’s eyesight. The lens 110 may be a polarized lens or a tinted lens to protect the user’s eyes from the sun. The lens 110 may be one or more waveguides as part of a waveguide display in which image light is coupled through an end or edge of the waveguide to the eye of the user. The lens 110 may include an electronic display for providing image light and may also include an optics block for magnifying image light from the electronic display. Additional detail regarding the lens 110 can be found in the detailed description of FIG. 5.

[0019] The sensor device 115 estimates a current position of the eyewear device 100 relative to an initial position of the eyewear device 100. The sensor device 115 may be located on a portion of the frame 105 of the eyewear device 100. The sensor device 115 includes a position sensor and an inertial measurement unit. Additional details about the sensor device 115 can be found in the detailed description of FIG. 5.

[0020] The audio system of the eyewear device 100 comprises a plurality of transducer assemblies configured to provide audio content to a user of the eyewear device 100. In the illustrated embodiment of FIG. 1, the audio system of the eyewear device 100 includes the cartilage conduction transducer assembly 120, the air conduction transducer assembly 125, the bone conduction transducer assembly 130, the acoustic sensor 135, and the controller 150. The audio system provides audio content to a user by utilizing some combination of the cartilage conduction transducer assembly 120, the air conduction transducer assembly 125, and the bone conduction transducer assembly 130. The audio system also uses feedback from the acoustic sensor 135 to create a similar audio experience across different users. The controller 150 manages operation of the transducer assemblies by generating audio instructions. The controller 150 also receives feedback as monitored by the acoustic sensor 135, e.g., for updating the audio instructions. Additional detail regarding the audio system can be found in the detailed description of FIG. 3.

[0021] The cartilage conduction transducer assembly 120 produces sound by vibrating cartilage in the ear of the user. The cartilage conduction transducer assembly 120 is coupled to an end piece of the frame 105 and is configured to be coupled to the back of an auricle of the ear of the user. The auricle is a portion of the outer ear that projects out of a head of the user. The cartilage conduction transducer assembly 120 receives audio instructions from the controller 150. Audio instructions may include a content signal, a control signal, and a gain signal. The content signal may be based on audio content for presentation to the user. The control signal may be used to enable or disable the cartilage conduction transducer assembly 120 or one or more transducers of the transducer assembly. The gain signal may be used to adjust an amplitude of the content signal. The cartilage conduction transducer assembly 120 vibrates the auricle to generate an airborne acoustic pressure wave at an entrance of the user’s ear. The cartilage conduction transducer assembly 120 may include one or more transducers to cover different parts of a frequency range. For example, a piezoelectric transducer may be used to cover a first part of a frequency range and a moving coil transducer may be used to cover a second part of a frequency range. Additional detail regarding the cartilage conduction transducer assembly 120 can be found in the detailed description of FIG. 3.

[0022] The air conduction transducer assembly 125 produces sound by generating an airborne acoustic pressure wave in the ear of the user. The air conduction transducer assembly 125 is coupled to an end piece of the frame 105 and is placed in front of an entrance to the ear of the user. The air conduction transducer assembly 125 also receives audio instructions from the controller 150. The air conduction transducer assembly 125 may include one or more transducers to cover different parts of a frequency range. For example, a piezoelectric transducer may be used to cover a first part of a frequency range and a moving coil transducer may be used to cover a second part of a frequency range. Additional detail regarding the air conduction transducer assembly 125 can be found in the detailed description of FIG. 3.

[0023] The bone conduction transducer assembly 130 produces sound by vibrating bone in the user’s head. The bone conduction transducer assembly 130 is coupled to an end piece of the frame 105 and is configured to be behind the auricle coupled to a portion of the user’s bone. The bone conduction transducer assembly 130 also receives audio instructions from the controller 150. The bone conduction transducer assembly 130 vibrates the portion of the user’s bone which generates a tissue-borne acoustic pressure wave that propagates toward the user’s cochlea, thereby bypassing the eardrum. The bone conduction transducer assembly 130 may include one or more transducers to cover different parts of a frequency range. For example, a piezoelectric transducer may be used to cover a first part of a frequency range and a moving coil transducer may be used to cover a second part of a frequency range. Additional detail regarding the air conduction transducer assembly 125 can be found in the detailed description of FIG. 3.

[0024] The acoustic sensor 135 detects an acoustic pressure wave at the entrance of the ear of the user. The acoustic sensor 135 is coupled to an end piece of the frame 105. The acoustic sensor 135, as shown in FIG. 1, is a microphone which may be positioned at the entrance of the user’s ear. In this embodiment, the microphone may directly measure the acoustic pressure wave at the entrance of the ear of the user.

[0025] Alternatively, the acoustic sensor 135 is a vibration sensor that is configured to be coupled to the back of the auricle of the user. The vibration sensor may indirectly measure the acoustic pressure wave at the entrance of the ear. For example, the vibration sensor may measure a vibration that is a reflection of the acoustic pressure wave at the entrance of the ear and/or measure a vibration created by the transducer assembly on the auricle of the ear of the user which may be used to estimate the acoustic pressure wave at the entrance of the ear. In one embodiment, a mapping between acoustic pressure generated at the entrance to the ear canal and a vibration level generated on the auricle is an experimentally determined quantity that is measured on a representative sample of users and stored. This stored mapping between the acoustic pressure and vibration level (e.g., frequency dependent linear mapping) of the auricle is applied to a measured vibration signal from the vibration sensor which serves as a proxy for the acoustic pressure at the entrance of the ear canal. The vibration sensor can be an accelerometer or a piezoelectric sensor. The accelerometer may be a piezoelectric accelerometer or a capacitive accelerometer. The capacitive accelerometer senses change in capacitance between structures which can be moved by an accelerative force. In some embodiments, the acoustic sensor 135 is removed from the eyewear device 100 after calibration. Additional detail regarding the acoustic sensor 135 can be found in the detailed description of FIG. 3.

[0026] The controller 150 provides audio instructions to the plurality of transducer assemblies and receives information from the acoustic sensor 135 regarding the produced sound, and updates the audio instructions based on the received information. The audio instructions may be generated by the controller 150. The controller 150 may receive audio content (e.g., music, calibration signal) from a console for presentation to a user and generate audio instructions based on the received audio content. Audio instructions instruct each transducer assembly how to produce vibrations. For example, audio instructions may include a content signal (e.g., a target waveform based on the audio content to be provided), a control signal (e.g., to enable or disable the transducer assembly), and a gain signal (e.g., to scale the content signal by increasing or decreasing an amplitude of the target waveform). The controller 150 also receives information from the acoustic sensor 135 that describes the produced sound at an ear of the user. In one embodiment, the controller 150 receives monitored vibration of an auricle by the acoustic sensor 135 and applies a previously stored frequency dependent linear mapping of pressure to vibration to determine the acoustic pressure wave at the entrance of the ear based on the monitored vibration. The controller 150 uses the received information as feedback to compare the produced sound to a target sound (e.g., audio content) and updates the audio instructions to make the produced sound closer to the target sound. For example, the controller 150 updates audio instructions for a cartilage conduction transducer assembly to adjust vibration of the auricle of the user’s ear to come closer to the target sound. The controller 150 is embedded into the frame 105 of the eyewear device 100. In other embodiments, the controller 150 may be located in a different location. For example, the controller 150 may be part of the transducer assembly or located external to the eyewear device 100. Additional detail regarding the controller 150 and the controller’s 150 operation with other components of the audio system can be found in the detailed description of FIGS. 3 & 4.

Hybrid Audio System

[0027] FIG. 2 is a profile view 200 of a portion of an audio system as a component of an eyewear device (e.g., the eyewear device 100), in accordance with one or more embodiments. A cartilage conduction transducer assembly 220, an air conduction transducer assembly 225, a bone conduction transducer assembly 230, and an acoustic sensor 235 are embodiments of the cartilage conduction transducer assembly 120, the air conduction transducer assembly 125, the bone conduction transducer assembly 130, and the acoustic sensor 135, respectively. The cartilage conduction transducer assembly 220 is coupled to a back of an auricle of an ear 210 of a user. The cartilage conduction transducer assembly 220 vibrates the back of auricle of the ear 210 of a user at a first range of frequencies to generate a first range of airborne acoustic pressure waves at an entrance of the ear 210 based on audio instructions (e.g., from the controller). The air conduction transducer assembly 220 is a speaker (e.g., a voice coil transducer) that vibrates over a second range of frequencies to generate a second range of airborne acoustic pressure waves at the entrance of the ear. The first range of airborne acoustic pressure waves and the second range of airborne acoustic pressure waves travel from the entrance of the ear 210 down an ear canal 260 where an eardrum is located. The eardrum vibrates due to fluctuations of the airborne acoustic pressure waves which are then detected as sound by a cochlea of the user (not shown in FIG. 2). The acoustic sensor 235 is a microphone positioned at the entrance of the ear 210 of the user to detect the acoustic pressure waves produced by the cartilage conduction transducer assembly 220 and the air conduction transducer assembly 225.

[0028] The bone conduction transducer assembly 230 is coupled to a portion of the user’s bone behind the user’s ear 210. The bone conduction transducer assembly 230 vibrates over a third range of frequencies. The bone conduction transducer assembly 230 vibrates the portion of the bone to which it is coupled. The portion of the bone conducts the vibrations to create a third range of tissue-borne acoustic pressure waves at the cochlea which is then perceived by the user as sound. Although the portion of the audio system, as shown in FIG. 2, illustrates one cartilage conduction transducer assembly 120, one air conduction transducer assembly 125, one bone conduction transducer assembly 130, and one acoustic sensor 135 configured to produce audio content for one ear 210 of the user, other embodiments include an identical setup to produce audio content for the other ear of the user. Other embodiments of the audio system comprise any combination of one or more cartilage conduction transducer assemblies, one or more air conduction transducer assemblies, and one or more bone conduction transducer assemblies. Examples of the audio system include a combination of cartilage conduction and bone conduction, another combination of air conduction and bone conduction, another combination of air conduction and cartilage conduction, etc.

[0029] FIG. 3 is a block diagram of an audio system, in accordance with one or more embodiments. The audio system in FIG. 1 is an embodiment of the audio system 300. The audio system 300 includes a plurality of transducer assemblies 310, an acoustic assembly 320, and a controller 340. In one embodiment, the audio system 300 further comprises an input interface 330. In other embodiments, the audio system 300 can have any combination of the components listed with any additional components.

[0030] The plurality of transducer assemblies 310 comprises any combination of one or more cartilage conduction transducer assemblies, one or more air conduction transducer assemblies, and one or more bone conduction transducer assemblies, in accordance with one or more embodiments. The plurality of transducer assemblies 310 provide sound to a user over a total range of frequencies. For example, the total range of frequencies is 20 Hz-20 kHz, generally around the average range of human hearing. Each transducer assembly of the plurality of transducer assemblies 310 comprises one or more transducers configured to vibrate over various ranges of frequencies. In one embodiment, each transducer assembly of the plurality of transducer assemblies 310 operates over the total range of frequencies. In other embodiments, each transducer assembly operates over a subrange of the total range of frequencies. In one embodiment, one or more transducer assemblies operate over a first subrange and one or more transducer assemblies operate over a second subrange. For example, a first transducer assembly is configured to operate over a low subrange (e.g., 20 Hz-500 Hz) while a second transducer assembly is configured to operate over a medium subrange (e.g., 500 Hz-8 kHz) and a third transducer assembly is configured to operate over a high subrange (e.g., 8 kHz-20 kHz). In another embodiment, subranges for the transducer assemblies 310 partially overlap with one or more other subranges.

[0031] In some embodiments, the transducer assemblies 310 includes a cartilage conduction transducer assembly. A cartilage conduction transducer assembly is configured to vibrate a cartilage of a user’s ear in accordance with audio instructions (e.g., received from the controller 340). The cartilage conduction transducer assembly is coupled to a portion of a back of an auricle of an ear of a user. The cartilage conduction transducer assembly includes at least one transducer to vibrate the auricle over a first frequency range to cause the auricle to create an acoustic pressure wave in accordance with the audio instructions. Over the first frequency range, the cartilage conduction transducer assembly can vary amplitude of vibration to affect amplitude of acoustic pressure waves produced. For example, the cartilage conduction transducer assembly is configured to vibrate the auricle over a first frequency subrange of 500 Hz-8 kHz. In one embodiment, the cartilage conduction transducer assembly maintains good surface contact with the back of the user’s ear and maintains a steady amount of application force (e.g., 1 Newton) to the user’s ear. Good surface contact provides maximal translation of vibrations from the transducers to the user’s cartilage.

[0032] In one embodiment, a transducer is a single piezoelectric transducer. A piezoelectric transducer can generate frequencies up to 20 kHz using a range of voltages around +/-100V. The range of voltages may include lower voltages as well (e.g., +/-10V). The piezoelectric transducer may be a stacked piezoelectric actuator. The stacked piezoelectric actuator includes multiple piezoelectric elements that are stacked (e.g. mechanically connected in series). The stacked piezoelectric actuator may have a lower range of voltages because the movement of a stacked piezoelectric actuator can be a product of the movement of a single piezoelectric element with the number of elements in the stack. A piezoelectric transducer is made of a piezoelectric material that can generate a strain (e.g., deformation in the material) in the presence of an electric field. The piezoelectric material may be a polymer (e.g., polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF)), a polymer-based composite, ceramic, or crystal (e.g., quartz (silicon dioxide or SiO.sub.2), lead zirconate-titanate (PZT)). By applying an electric field or a voltage across a polymer which is a polarized material, the polymer changes in polarization and may compress or expand depending on the polarity and magnitude of the applied electric field. The piezoelectric transducer may be coupled to a material (e.g., silicone) that attaches well to an ear of a user.

[0033] In another embodiment, a transducer is a moving coil transducer. A typical moving coil transducer includes a coil of wire and a permanent magnet to produce a permanent magnetic field. Applying a current to the wire while it is placed in the permanent magnetic field produces a force on the coil based on the amplitude and the polarity of the current that can move the coil towards or away from the permanent magnet. The moving coil transducer may be made of a more rigid material. The moving coil transducer may also be coupled to a material (e.g., silicone) that attaches well to an ear of a user.

[0034] In some embodiments, the transducer assemblies 310 includes an air transducer assembly. An air conduction transducer assembly is configured to vibrate to generate acoustic pressure waves at an entrance of the user’s ear in accordance with audio instructions (e.g., received from the controller 340). The air conduction transducer assembly is in front of an entrance of the user’s ear. Optimally, the air conduction transducer assembly is unobstructed being able to generate acoustic pressure waves directly at the entrance of the ear. The air conduction transducer assembly includes at least one transducer (substantially similar to the transducer described in conjunction with the cartilage conduction transducer assembly) to vibrate over a second frequency range to create an acoustic pressure wave in accordance with the audio instructions. Over the second frequency range, the air conduction transducer assembly can vary amplitude of vibration to affect amplitude of acoustic pressure waves produced. For example, the air conduction transducer assembly is configured to vibrate over a second frequency subrange of 8 kHz-20 kHz (or a higher frequency that is hearable by humans).

[0035] In some embodiments, the transducer assemblies 310 includes a bone conduction transducer assembly. A bone conduction transducer assembly is configured to vibrate the user’s bone to be detected directly by the cochlea in accordance with audio instructions (e.g., received from the controller 340). The bone conduction transducer assembly may be coupled to a portion of the user’s bone. In one implementation, the bone conduction transducer assembly is coupled to the user’s skull behind the user’s ear. In another implementation, the bone conduction transducer assembly is coupled to the user’s jaw. The bone conduction transducer assembly includes at least one transducer (substantially similar to the transducer described in conjunction with the cartilage conduction transducer assembly) to vibrate over a third frequency range in accordance with the audio instructions. Over the third frequency range, the bone conduction transducer assembly can vary amplitude of vibration. For example, the bone conduction transducer assembly is configured to vibrate over a third frequency subrange of 100 Hz (or a lower frequency that is hearable by humans)-500 Hz.

[0036] The acoustic assembly 320 detects acoustic pressure waves at the entrance of the user’s ear. The acoustic assembly 320 comprises one or more acoustic sensors. One or more acoustic sensors may be positioned at an entrance of each ear of a user. The one or more acoustic sensors are configured to detect the airborne acoustic pressure waves formed at an entrance of the user’s ears. In one embodiment, the acoustic assembly 320 provides information regarding the produced sound to the controller 340. The acoustic assembly 320 transmits feedback information of the detected acoustic pressure waves to the controller 340.

[0037] In one embodiment, the acoustic sensor is a microphone positioned at an entrance of an ear of a user. A microphone is a transducer that converts pressure into an electrical signal. The frequency response of the microphone may be relatively flat in some portions of a frequency range and may be linear in other portions of a frequency range. The microphone may be configured to receive a signal from the controller to scale a detected signal from the microphone based on the audio instructions provided to the transducer assembly 310. For example, the signal may be adjusted based on the audio instructions to avoid clipping of the detected signal or for improving a signal to noise ratio in the detected signal.

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