Researchers in Spain have developed a device that causes sound waves to enter a structure and lose their energy there, with almost no reflection back. Unlike a real black hole, it is not related to gravity: it is a compact acoustic absorber that could find applications in ventilation systems, mufflers, and sound insulation. The results of the work were published in the journal Applied Acoustics.

The key feature of the development is a design that can be manufactured on a 3D printer and installed inside an existing pipe. This makes it easier to create noise-absorbing devices without having to modify the air ducts themselves.

How the sound “black hole” works

In a real black hole, gravity does not allow light to escape once it has crossed the event horizon. The acoustic counterpart works differently, but uses a similar idea: a sound wave enters a special structure and gradually loses the ability to propagate back effectively.

As it travels inside the device, the speed of sound and the wavelength change, while the energy becomes concentrated closer to the far end of the structure. There it is dissipated through friction and heat exchange, so much less sound returns outside.

“In the ideal case, once a wave enters the sound black hole, it no longer comes back out,” explained one of the authors of the study, Oriol Guasch.

In practice, absolute absorption could not be achieved, but tests showed that the device reflects sound less than a conventional design across a wide range of frequencies. According to the researchers, the new scheme also makes it possible to broaden the range of effective absorption compared with traditional acoustic black hole analogs.

Why the rings were moved inside the pipe

In conventional designs, the rings that form the acoustic structure are attached directly to the inner walls of the pipe. The researchers proposed a different solution: placing the rings on a central rod.

The 15-centimeter prototype consisted of a rod with 29 rings. It was manufactured separately and then installed in a test pipe. The experiment was carried out using a speaker and microphones that measured the propagation and reflection of sound waves.

The measurements generally matched the calculations. The device did not eliminate echo completely, but it reduced sound reflection. At the same time, the modular design offers a practical advantage: the absorber can be manufactured separately and inserted into an existing air duct without altering its walls.

Where the technology could be useful

One of the main potential applications is noise reduction in ventilation ducts and mufflers. Such devices must absorb sound without creating excessively large obstacles to airflow and without taking up much space.

The researchers are also studying the possibility of creating two-dimensional and three-dimensional versions for controlling room acoustics. In the future, such structures could become the basis for compact noise absorbers for engineering equipment and buildings.

But before practical use, several issues still need to be resolved. Cavities inside the device may become clogged with particles, and temperature, pressure, and air movement in real systems could affect its performance. For now, the development remains a laboratory prototype.

Nevertheless, the idea of placing the acoustic structure on a separate rod could simplify the production and installation of absorbers. If further tests confirm the design’s effectiveness under real-world conditions, it could make it possible to reduce noise in pipes and air ducts without large-scale reconstruction of existing systems.