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NTT develops active noise cancellation technology for open-ear headphones

NTT develops active noise cancellation technology for open-ear headphones. (Image source: NTT)
NTT develops active noise cancellation technology for open-ear headphones. (Image source: NTT)
Unlike closed-ear headphones with earcups that block sound leakage, open-ear headphones do not have earcups to block sound leakage. The NTT ANC technology reduces this leakage.

NTT has announced the development of active noise cancellation technology that works with open-ear headphones, such as its nwm ONE open-ear headphones. Open-ear headphones allow all noises from the surrounding environment in so the user can hear important alerts and announcements at all times.

In order to produce the additional noise cancellation sound that will reduce the overall amount of sound leaking from open-ear headphones, NTT has developed new hardware and software to achieve this goal. For hardware, reference and error feedback microphones with ultra-low latency circuitry are used with newly designed speakers. For software, ANC algorithms with low calculation delays and computational demands are used. 

Together, the result is a significant reduction in noise from the 1 kHz to 3 kHz frequency range, which is superior to current ANC technology for open-ear headphones that are limited to cancelling noise under 1 kHz. The tested average amount of noise reduction was 7.8 dB, with a maximum of 13.7 dB. NTT aims to commercialize the ANC technology after more research to expand the ANC frequency range bandwidth.

The technology will be showcased at its 2024 R&D Forum to be held on November 25-29, 2024. Readers who can't wait until NTT releases open-ear headphones with active noise cancelling can buy the current NTT nwm ONE open-ear headphones without ANC on Amazon for $299.

NTT has developed active noise reduction technology for open-ear headphones that reduces noise across 1kHz to 3kHz by an average of 7.8dB and a maximum of 13.7dB. (Image source: NTT)
NTT has developed active noise reduction technology for open-ear headphones that reduces noise across 1kHz to 3kHz by an average of 7.8dB and a maximum of 13.7dB. (Image source: NTT)

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November 14, 2024

Japan Telegraph and Telephone Corporation

The world's first wideband noise canceling technology for open-ear headphones that can reduce ambient noise without blocking your ears ~ Realizing a comfortable listening experience where you can hear clear sound without blocking your ears even in crowded streets and mobility in the city ~

Key points of the presentation:

◆Established ANC (Active Noise Control) technology, an active noise control technology that can reduce noise in a wide band of 100~3,000Hz so that the sound of open-ear headphones can be heard clearly even in noisy conditions.

◆As a method for reducing the latency of the ANC system necessary to achieve wideband noise suppression, we have developed a new design for the microphone and speaker of the headphone body to reduce acoustic delay, and a new speaker design and software process to reduce the mechanical delay for each frequency band.

◆Open-ear headphones with low ear burden provide a comfortable listening experience even in noisy environments

Japan Telegraph and Telephone Corporation (Headquarters: Chiyoda-ku, Tokyo, President & CEO: Akira Shimada, hereinafter referred to as "NTT") has established ANC technology to reduce ambient noise at the ear in open-ear headphones that do not block the ears. In this technology, as a practical noise canceling technology for open-ear headphones, we have newly developed the design of the microphone and speakers of the headphone body, as well as software processing, to achieve wideband noise suppression. With this technology, you can hear the sound reproduced from open-ear headphones clearly by reducing noise while feeling the surrounding sound naturally. In addition, it is expected that the range of use of acoustic XR technology (*1), which fuses virtual sounds heard through headphones with real sounds heard directly with the ears, will expand. Open-ear headphones using this technology will be exhibited at NTT R&D FORUM 2024 ―IOWN INTEGRAL(*2) to be held on November 25~29, 2024.

1. Background

NTT has been working on the development of open-ear headphone design technology that realizes playback that only the user can hear without blocking the ears, with the aim of establishing PSZ (Personalized Sound Zone) technology, the ultimate private sound space in which "sounds you want to hear" can be heard only by yourself without leaking to those around you, and "sounds you don't want to hear" are cut out (*3). These open-ear headphones allow you to naturally hear your surroundings and are designed to prevent sound leakage, so they can be used in public places, offices, and other common spaces. In addition, the ears do not get tired and can be worn for a long time. In addition, focusing on these features, we are advocating acoustic XR technology that fuses virtual sound heard from open-ear headphones with real sound heard directly with the ear, and is conducting research and development and demonstration experiments (*1).

Conventional open-ear headphones have only been able to reduce sound below 1,000 Hz, even if the noise canceling function is present. Therefore, the sound around 3,000 Hz, which the human ear is most sensitive to, cannot be reduced, and if the ambient noise is large, the opportunity to use it will increase the volume, so there is a risk of hearing loss. Therefore, we have established Active Noise Control (ANC) technology that can reduce sound above 1,000 Hz in open-ear headphones.

2. Technical Challenges

There are two ways to reduce noise used in headphones: active noise control (ANC) and passive noise control (PNC). ANC uses a reference microphone built into the headphones to pick up the noise, and then uses an error microphone to determine if the noise is off in the ear to generate an out-of-phase signal of the noise, which is then played back through the headphones to offset the noise. In particular, it can be reduced mainly for low frequencies below 1,000 Hz. PNC, on the other hand, blocks the noise by plugging the ears with the physical structure of the headphones themselves. In particular, it can be reduced mainly for high frequencies above 1,000 Hz. Sealed headphones with ANC, which are already widely available in the general public, combine ANC and PNC to achieve effective noise reduction.

However, open-ear headphones do not have a structure that blocks the ears, so high-frequency noise reaches the ears as they are. In addition, since human hearing is sensitive to the area around 3,000 Hz, ANC requires a high frequency of 1,000 Hz or more to be silenced in order to achieve effective noise reduction in the sense of hearing with open-ear headphones.

Figure 1. ANC in Closed and Open-Ear Headphones

3. Technology Points

In order to reduce noise above 1,000 Hz using ANC technology, the ANC system must perform an ultra-low latency (in submilliseconds) from the noise picked up by the reference microphone to the out-of-phase signal of the noise until it reaches the error microphone position. Especially in the case of headphones, the distance between them is only a few centimeters because the reference microphone, error microphone, and speaker that reproduces the canceled sound are built into the headphone body. Therefore, ANC systems need to be very fast, and the latency of ANC systems must be as low as possible. Therefore, in the ANC system,

(1) Acoustic delay

(2) Mechanical delay

We have developed a technology that reduces the two delay factors. (1) Acoustic delay is the time difference between the sound from the source to the target position. Sound travels through the air at a speed of about 340 meters per second, which is very slow compared to light and electrical signals. Therefore, we have established a new design in which the microphone is positioned so that the noise source and the reference microphone are close to the error microphone and the speaker unit built into the headphones to reduce acoustic delay. (2) Mechanical delay is a delay that occurs when the mechanical components inside the equipment or headphones are operating. For example, the diaphragm of the speaker unit built into the headphones takes a physical time between receiving an electrical signal and actually producing sound. And this delay varies from frequency band to frequency depending on the design of the speaker unit. Therefore, in this technology, a new hardware design and software processing have been used to reduce the mechanical delay for each frequency band. In the hardware design, we designed the position and characteristics of the speakers anew, and in the software processing, we designed a filter with low latency and low computational content. In addition, this technology is combined with the design technology of open-ear headphones (*3) that prevents sound leakage to prevent the sound from leaking to the surroundings and preventing an increase in noise around the user.

This technology, which reduces (1) acoustic delay and (2) mechanical delay, has made it possible to widen the ANC bandwidth of open-ear headphones for the first time in the world. When this technology was evaluated using the noise in the airplane, it was confirmed that it could be reduced to around 3,000 Hz. Specifically, it achieves noise suppression of up to 13.7 dB and an average of 7.8 dB at 1,000 ~ 3,000 Hz.

Figure 2. Acoustic and mechanical delays in ANC

Figure 3. Prototype open-ear headphones designed using this technology

Figure 4. Evaluation of noise suppression performance in ANC using this technology

4. Future Developments

In the future, we will aim to commercialize open-ear headphones equipped with this technology, and we will further expand the bandwidth to reduce noise and verify it in various real-world environments. In addition, NTT Group companies will develop services such as immersive entertainment and audio guides that integrate this technology with acoustic XR technology, which fuses virtual sound heard through headphones with real sound heard directly with the ear.

[Glossary]

※1

Taking on the challenge of creating a new spatial sound production for Super Kabuki with "Super Kabuki Powered by NTT" ~Realizing a spatial sound production that crosses over the realistic sound of the stage and the sound effect in the ears~

URL:https://group.ntt/jp/newsrelease/2023/04/29/230429b.html

※2

"NTT R&D FORUM 2024 -IOWN INTEGRAL" official website www.rd.ntt/forum/2024/This page will open in a new window

NTT R&D FORUM 2024 - Image of IOWN INTEGRAL

※3

Development of design technology for earphones that can only be heard by the user without blocking the ears

~Realization of sound wave control that cancels sound leakage to the surroundings while delivering sound to the user with a single speaker~

Inquiries from the press regarding this matter

Japan Telegraph and Telephone Corporation,

Service Innovation Research Institute,

Planning Department, Public Relations Officer

tools.group.ntt/jp/rd/contact/index.php page will open in a new window

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> Expert Reviews and News on Laptops, Smartphones and Tech Innovations > News > News Archive > Newsarchive 2024 11 > NTT develops active noise cancellation technology for open-ear headphones
David Chien, 2024-11-15 (Update: 2024-11-15)