Minxin Shares: The application of bone vibration sensor in TWS headphones

Application of bone vibration sensor in TWS headphones

Minxin shares

TWS headphones

TWS (True Wireless Stereo) True wireless Bluetooth headsets are extremely portable compared to traditional headphones. With the addition of new features such as active noise reduction and spatial audio, the functionality of TWS headphones is more abundant, the user experience has been continuously improved, and the rapid popularization of TWS headphones has been further promoted.

In daily life, the application scenarios of voice calls are more complex, such as in the subway, office, airport and other public places, there are high requirements for the effect of the call, in the uplink call, the elimination of environmental noise becomes a rigid demand. How to further improve the call noise reduction effect of TWS headphones, so as to highlight the differences in products and enhance the competitiveness of products, has become one of the key directions for many manufacturers to lay out their products.

Detection principle of bone vibration sensor

When people speak, the vibration signal generated by the vocal cords is transmitted in two ways: one is to spread outward through the air medium, and the other way is to spread outward through the skull bones and muscles of the human skull, causing the vibration of the ear pinna. Bone vibration sensors use the latter's propagation method to detect vibration signals in the auricle and pick up the wearer's voice information. Bone vibration sensor is insensitive to the sound wave signal propagated in the air, and has a natural inhibition effect on the sound signal propagated in the air, so the call noise reduction algorithm is simpler and more natural, and the noise suppression is more effective, which can provide a better upward call effect.

Comparison of noise reduction methods for TWS headphones

There are many types of TWS headphones on the market, and for ENC (Environmental Noise Cancellation) noise reduction, according to different price and performance positioning, there are single microphones, dual microphones, three microphones and bone vibration and other product forms.

The price of the single microphone solution has advantages, the main control chip requires a lower threshold, the structure is not limited, and the neural network algorithm is used to identify and filter out the noise signal. However, the noise reduction effect of the single-microphone scheme is general, and the noise recognition and suppression of complex scenes are not very obvious. Dual microphone can adapt to a variety of forms, in most high-noise scenes have a better experience, cost-effective, is the current ENC noise reduction mainstream scheme.

Dual microphone ENC noise reduction, the use of a certain pitch of two microphones, so that you can identify the sound signal in different directions, the minimum spacing of the two microphones generally requires 10mm. Algorithmically, the relatively mature beam forming algorithm is used, and the waveform is pointed at the sound source where the wearer speaks, which enhances the signal sensitivity in the direction of the sound source and suppresses the signal sensitivity in other directions, thereby achieving noise elimination.

Sanmai ENC noise reduction generally multiplex ANC active noise reduction feedback microphone (FB), FB microphone is used to detect the sound signal transmitted by the vocal cords to the headphones through the cochlea, in addition, because the FB microphone is deep inside the ear canal, most of the active noise reduction headphones are worn in the ear, and the rubber sleeve of the headphones has a better sealing effect, which can isolate the external environmental noise. Sanmai ENC noise reduction can be applied to most scenarios, and the wind noise resistance is good, but the algorithm is complex and the price is relatively high.

The microphone is combined with bone vibration for better resistance to ambient vocal interference and less vocal distortion for callers. Wind noise is a pain point for most headphones, when the wind speed is greater than 5m/s, the microphone will appear saturation distortion, at this time the microphone loses the ability to capture the voice signal, the bone vibration sensor does not respond to air fluctuations, so even in the case of large wind speeds, the voice signal can still be accurately captured. In addition, for bean headphones, due to the compact space and small size, it is difficult for the two microphones to pull apart, so the noise reduction effect of double wheat ENC is not good in this case. A call microphone plus a bone vibration sensor, can achieve a better call effect, strong anti-wind noise ability, good noise suppression effect, algorithm does not need complex beamforming, simple voice algorithm can be achieved, the overall cost performance of the program is high, for more and more headphone manufacturers to use.

Device design for bone vibration sensors

Based on MEMS (Micro-Electronic-Mechanical-System) micromachining technology, an elastic beam structure of micron size and a mass block of a certain weight are processed on a silicon substrate to form a vibration-sensitive mechanical structure. The capacitor plate on the mass block and the capacitor plate on the substrate constitute a comb-shaped movable capacitor, when there is a vibration applied to the device, the mass block produces vibration, driving the capacitor comb tooth to produce vibration, and the spacing between the capacitor plates changes, so the capacitance changes.

The internal ASIC (Application Specific Integrated Circuit) of the device extracts, converts and amplifies the weak capacitive signal, and outputs the vibration signal in the form of voltage, similar to the output signal of traditional silicon wheat, which is convenient for Bluetooth master control to perform signal processing.

In the package of Metal-Lid LGA, the overall appearance is presented as a combination of PCB substrate and metal shell. The PCB substrate is located at the bottom, with various signal traces distributed on the front and back of the substrate, and an exit electrode on the back. The substrate is surrounded by a streaked tin channel that connects the top metal shell to the substrate through solder for complete electromagnetic signal shielding. The LGA is mainly composed of MEMS chips and ASIC chips. The MEMS chip is mounted on the PCB substrate, and the ASIC chip is mounted on the MEMS chip, forming a stacked up and down form, making the structure more compact and the space utilization rate higher. Between the MEMS chip and the ASIC chip, and between the ASIC chip and the PCB substrate, the signal interconnection is realized through the Way-Bonding. 2.7x1.8x1.1 package size, small and compact, compatible with standard microphone sizes.

The sensitivity of the device is -26dBV/g @1kHz, the signal-to-noise ratio is 55dB, and the resonant frequency is 4kHz. In general, the bandwidth of the voice signal is about 1kHz, so the frequency response of the device is flat within 1.5kHz, which is easier for algorithms to process.

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