World's first wideband noise-canceling technology for open-ear headphones that reduces ambient noise without blocking your ears
A comfortable and clear listening experience without blocking your ears, even in a crowded city and in mobility

News Highlights:

TOKYO - November 14, 2024 - NTT Corporation (Headquarters: Chiyoda Ward, Tokyo; Representative Member of the Board and President: Akira Shimada; hereinafter "NTT") has developed ANC technology for open-ear headphones that do not cover the ears, reducing ambient noise near the ears. As a practical noise-canceling technology for open-ear headphones, we have newly developed the microphone and speaker design of the headphone body and software processing to achieve wideband noise suppression. With this technology, you can hear the playback sound from open-ear headphones clearly by reducing noise while feeling the surrounding sound naturally. It is also expected that the use case of acoustic XR technology¹, which combines virtual sound heard through headphones with real sound heard directly by the ear, will expand. Open-ear headphones utilizing this technology will be exhibited at NTT R&D FORUM 2024 IOWN INTEGRAL² to be held in Tokyo from November 25-29, 2024.

Background

NTT has established PSZ (Personalized Sound Zone) technology, which is the ultimate private sound space in which "desired sounds" can be heard only by oneself without leaking to others, and "undesired sounds" can be reduced. NTT has been working on the development of design technology for open-ear headphones that realize playback that can only be heard by the user without blocking the ears³. These open-ear headphones allow users to listen to their surroundings naturally and are designed to prevent sound leakage, so they can be used in public places, offices and other shared spaces. In addition, it can be worn for a long time without tiring the ears. Focusing on these features, we have proposed an acoustic XR technology that integrates the virtual sound heard from open-ear headphones with the real sound heard directly by the ear, and are conducting R&D and demonstration tests¹.
　Conventional open-ear headphones can only reduce sound below 1,000 Hz, even if they have a noise-canceling feature. For this reason, it was not possible to reduce the sound around 3,000 Hz, which the human ear is most sensitive to, and there was a risk of hearing loss due to having to increase volume when surrounding noise was high. To address this issue, we have developed an active noise control ANC technology that can reduce noise above 1,000 Hz in open-ear headphones.

Technical challenge

There are two methods of noise reduction used in headphones: active noise control (ANC) and passive noise control (PNC). The ANC collects the noise with a reference microphone built into the headphones, and uses an error microphone to determine whether the noise is eliminated in the ear. The ANC generates an anti-phase signal of the noise and plays it back from the headphones to compensate for the noise. It can reduce low frequencies, especially those below 1,000 Hz. PNC, on the other hand, blocks noise by blocking the ear with the physical structure of the headphone itself. It can reduce noise, especially at high frequencies above 1,000 Hz. Closed-back headphones equipped with ANC, which are already widely used, achieve effective noise reduction by combining ANC and PNC.
　Open-ear headphones, however, have no structure to block your ears, so high-frequency noise reaches your ears as it is. In addition, since human hearing characteristics are sensitive at around 3,000 Hz, to achieve effective audible noise reduction in open-ear headphones, it is necessary to use ANC technology to silence high frequencies above 1,000 Hz.

Figure 1 Comparison of ANC Between Closed-Back Headphones and Open-Ear Headphones

Technical point

To reduce noise above 1,000 Hz using ANC technology, the ANC system must perform the processing from the noise captured by the reference microphone until the anti-phase of the noise reaches the position of the error microphone with very low delay (in sub-milliseconds). In the case of headphones, since the reference microphone, error microphone, and speakers for playing back the cancellation sound are built into the headphones themselves, the distance between them is approximately a few centimeters. Therefore, the ANC system must be very fast, so the delay of the ANC system must be as low as possible. Therefore, we have developed a technology to reduce the two delay factors:
(1) Acoustic delay
(2) Mechanical delay
(1) The acoustic delay in is the time difference between the sound source and the target location. Sound travels through air at a speed of about 340 meters per second, which is much slower than light or electrical signals. Therefore, we established a new design in which the microphones are positioned closer to the source of the noise and the reference microphones, as well as to the headphone speaker unit and the error microphones, to reduce acoustic delay. (2) The mechanical delay refers to the delay that occurs when the mechanical components inside the device or headphones move. For example, the diaphragm of the speaker unit in headphones takes a physical time between receiving an electrical signal and producing sound. The delay varies for each frequency band, depending on the design of the speaker unit. In this technology, the mechanical delay of each frequency band was reduced by new hardware design and software processing. In the hardware design, the position and characteristics of the loudspeaker were newly designed, and in the software processing, the filter design with low delay and low computation was performed. By combining this technology with the design technology for open-ear headphones³, which prevents sound leakage, the cancellation sound is not leaked to the surroundings and noise increases around the user.
　This technology, which reduces acoustic and mechanical delays, enables the world's first wideband ANC in open-ear headphones. We evaluated this technology using aircraft noise and found that it reduced noise to around 3,000 Hz. Specifically, noise reduction of up to 13.7 dB and an average of 7.8dB is achieved at 1 - 3 kHz.

Figure 2 Acoustic and Mechanical Delay in ANC

Figure 3 Prototype Open-Ear Headphones Designed Using This Technology

Figure 4 Evaluation of Noise Suppression Performance in ANC Using This Technology

Outlook

With the aim of commercializing open-ear headphones incorporating this technology in the future, we will study applications in real environments such as crowded streets and mobility. In addition, we will create new usage scenarios by integrating this technology with the Acoustic XR Technology, which combines virtual sound heard through headphones with real sound heard directly by the ear, and develop new audio experiences such as immersive entertainment and audio guides through NTT Group.

[Glossary]

¹"Cho-Kabuki Powered by NTT" Challenges New Spatial Sound Production
https://group.ntt/jp/newsrelease/2023/04/29/230429b.html

²NTT R&D FORUM 2024 -IOWN INTEGRAL Official Website https://www.rd.ntt/e/forum/2024/

³Developed earphone design technology that only the user can hear without blocking the ear
https://group.ntt/en/newsrelease/2022/11/09/221109a.html

About NTT

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