When wearing acoustic devices such as earphones and headphones, closed-type designs that physically seal the ears have long been the mainstream approach to preventing sound leakage. However, while such ear-sealing devices allow users to immerse themselves in the audio being played, they also present a significant drawback: ambient sounds become difficult to hear, making it hard for users to remain aware of their surrounding environment.

To address this challenge, the award recipients devised an innovative technology that cancels sound waves over a wide area by controlling the propagation paths of the in-phase sound waves radiated from the front and the out-of-phase sound waves radiated from the rear of a single speaker unit—effectively "confining" sound to the intended listening region. This breakthrough enabled the practical realization of open-ear earphones and headphones that substantially reduce sound leakage without occluding the ear canal [1, 2, 3].

The recipients discovered that by providing multiple apertures at appropriate positions on the sides of the speaker enclosure and radiating out-of-phase sound waves to the exterior, a single speaker unit alone can dramatically reduce sound pressure levels outside the desired local listening region. They further developed a design methodology to optimally implement this new principle. This approach—achieving sound-field control solely through the structural design of the enclosure—is unprecedented in the field. Because it requires no electrical signal processing, it consumes no additional power and contributes to reduced manufacturing costs as well as device miniaturization.

Traditionally, earphone enclosures have been engineered to minimize the influence of out-of-phase sound waves radiated from the rear of the speaker unit, as these waves would otherwise cancel the reproduced sound. In contrast, the present technology is founded on the diametrically opposite concept: it actively exploits the properties of out-of-phase sound waves to suppress sound leakage to the surrounding environment. Specifically, the recipients applied the principles of Helmholtz resonance to construct an acoustic model and established a highly effective sound-confinement design methodology that accounts for the path-length difference between front-radiated and rear-radiated out-of-phase sound waves, as well as the phase inversion caused by resonance within the enclosure (Fig. 1).

Open-ear earphones and headphones offer the advantage of reduced physical burden on the ears while allowing the simultaneous perception of ambient sounds. As a result, they have found broad application, not only in digital content consumption and sports use, but also in diverse daily-life situations that have become widespread since the COVID-19 pandemic—such as remote work and web conferencing in shared office spaces. With the application of this technology, users can enjoy audio content without concern for sound leakage, further expanding the range of use cases (Fig. 2).

Products incorporating this technology—open-ear earphones and headphones that do not block the ears yet substantially prevent sound leakage—have already been commercialized and are gaining widespread adoption in the market [4]. By allowing users to enjoy audio freely without concern for disturbing others, while still naturally perceiving ambient environmental sounds, these devices offer a genuinely new acoustic experience that is transforming audio-related lifestyles and giving rise to emerging business opportunities. The achievements of the award recipients, who not only established the underlying acoustic theory but also drove the technology through to practical implementation and commercialization, thereby rapidly opening new markets, are truly outstanding and this makes them highly deserving of this Society's Achievement Award.

References

1. November 9, 2022, “Developed earphone design technology that only the user can hear without blocking the ear - NTT developed a single speaker with sound wave control that delivers sound to the user while counteracting sound leakage to the surroundings –“，Press release of Nippon Telegraph and Telephone corp.，https://group.ntt/en/newsrelease/2022/11/09/221109a.html
2. Tatsuya KAKO, “Development of open-ear earphones that minimize sound leakage by opposite-phase sound waves and realization of Acoustic XR service,” IEICE Tech. Rep., vol. 123, no. 170, EA2023-27, pp. 53-60, Aug. 2023.
3. Hironobu CHIBA, Tatsuya KAKO, Hiroaki ITOH, Kenichi NOGUCHI, Noriyoshi KAMADO, Akira NAKAYAMA, "PSZ Spot-sound-reproduction Technology: New Sound-confinement Method Using Opposite-phase Sound Waves," NTT Technical Review, Vol. 22, No. 6, pp. 23-29, June 2024．
4. January 11, 2024,，”＜CES＞ nwm, "MBH001" headphones with PSZ technology that won't cover your ears,” PHILE WEB，https://www.phileweb.com/news/d-av/202401/11/59615.html (In Japanese)