Airpods Pro Acoustic Transparent Mode Principle

By cloudatlas / Apr 15, 2021

The popularity of TWS headsets, and the addition of many new features, quickly drove the rapid development of the entire Bluetooth headset market. Such as AirPods Pro, Sony.

In 2019, Apple released the third-generation true wireless headset product AirPods Pro. Its ANC active noise reduction function popularized the concept of noise-canceling headsets to ordinary consumers and once again detonated the market. Later released mid-to-high-end TWS headsets, Bluetooth headsets, and neck-mounted Bluetooth headsets all carry this feature.

airpods pro

The popularity of ANC headphones

AirPods Pro with active noise reduction is very popular, Along with the ANC active noise reduction function, the environmental sound enhancement mode-sound transmission also bring into people's field of vision. Although the names of different brands are different, the main effect is that after entering the environmental sound enhancement mode, It can clearly learn the surrounding sound without taking off the earphones. And with the continuous advancement of technology, there are still many products that support the voice enhancement mode, which can realize the function of only enhancing the voice of the surrounding environment and suppressing other environmental noises.

With the rapid popularity of noise-canceling headphones, manufacturers' propaganda and other reasons, consumers have a basic understanding of the principles of ANC. Consumers know nothing about the principles of environmental sound enhancement mode. This article will focus on sound The transparent transmission function helps consumers understand the basic principles of this function.

Transparent mode headset

Transparency/Hear-through call ambient sound mode on Sony ANC headsets, and transparent mode on Apple ANC headsets. In real life, there is no need to take off the headset and switch the ANC mode to the transparent mode to listen to the other party (although it is not polite); or when commuting, you can also pay attention to important environments including the sound of siren and driving Sound, protect personal safety.

With the Apple AirPods Pro detonating the TWS headset market, the sound transmission function is gradually recognized by consumers for its convenience and wide range of application scenarios, so that other manufacturers have to add this feature when developing ANC headsets. But compare to Apple's ANC headsets, the transparent mode of other manufacturers is always less realistic in subjective hearing. Although the industry's ANC design has become more mature, there is still room for improvement in the sound transmission design. It is the technical focus of many manufacturers that need to catch up with Apple in the next stage.

The principle of transparent transmission

The sound is transmitted through, and the ambient sound is picked up by the microphone (reusable microphone used for feedforward noise reduction) arranged outside the earphone, and it is played by the earphone speaker after filtering. This part calls electronic replay sound, which not isolate from the passive protection of the earphone shell. Ambient sound is superimposed, and finally a pseudo-ambient sound is formed at the tympanic membrane of the human ear.

The filter used for active noise control call a controller, and the filter used for equalization call an equalizer. Therefore, the filter used for transparent transmission may call a transponder.
Academically, the single wheat transparent transmission technology has long been mature. In the early stage of research, it mostly calls virtual reality audio technology. So the pass through also call virtual reality audio mixer or live equalizer.

The focus of the design of sound transmission is to solve the parameters of the transponder with excellent transmission performance. The measurement index is that the error of pseudo-ambient sound and real environmental sound is the smallest under certain criteria. Especially at high frequencies, so the fitting usually only considers the response below 2kHz.

AirPods Pro sublimation feeling

The electronic replay sound of medium and high frequency correlated compared with the passive sound signal of low frequency. When the two signals are similar, have a fixed delay, and the sound pressure level is not much different. A comb filtering effect form, which brings audible tone difference.

Therefore, the total delay of the transparent transmission path (introduced by anti-aliasing, reconstruction filtering, AD/DA conversion, and electronic transparent transmitter) must be controlled below a certain level, otherwise it will bring audible tone differences in the sense of hearing. And because the wearer judges the directionally, the ILD and ITD of low-frequency sound must be used. Therefore, if the time delay not handle properly. And the binaural sound reproduction gain is inconsistent, the original spatial cues will also destroy. This will destroy the user’s spatial awareness.

Binaural energy difference(ILD)

time delay difference(ITD)

Form 52 audio

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