Sound can travel through buildings in ways that are difficult to control. In ventilation systems, air-conditioning ducts and plumbing, sound waves may bounce off enclosed surfaces, creating unwanted echoes and persistent noise.

Researchers have now developed an unusual device inspired by the physics of black holes that can absorb much of this reflected sound. Called a sonic black hole, the technology offers a promising approach to improving noise control without requiring an entirely new pipe system.

A Different Kind of Black Hole

The name may sound like something from astronomy, but a sonic black hole is not a real astronomical object. Instead, it is an engineered structure designed to influence how sound waves move through a confined space.

In astronomy, a black hole's gravitational pull creates a boundary beyond which light cannot escape. The acoustic version borrows the underlying idea of preventing something from returning, but it works through a completely different physical process.

Rather than relying on gravity, a sonic black hole gradually changes the conditions experienced by sound travelling through a pipe. Under carefully designed conditions, sound waves slow down as they move deeper into the structure, making it harder for them to return as echoes.

Why Ordinary Pipes Create Noise

Pipes and ducts are designed to transport air, water or other materials, but their enclosed shapes can also guide sound over considerable distances. When sound waves reach a closed end, an obstruction or a sudden change in the pipe's shape, some of their energy may travel back towards the source.

The resulting reflections can make machinery sound louder or create unwanted reverberation. Conventional sound-control methods may reduce the problem, but their effectiveness depends on the structure, the materials used and the frequencies involved.

Earlier sonic black hole designs used a series of rings or cavities built into a pipe's walls. Their carefully changing dimensions altered the movement of sound as it travelled along the pipe.

However, this arrangement had a practical drawback: the acoustic structure generally needed to be incorporated into the pipe during manufacturing. Adding it to an existing ventilation or industrial system could therefore be complicated and expensive.

A Removable Design

A research team led by engineer Jie Deng, working with acoustics engineer Oriol Guasch and colleagues at La Salle, Universitat Ramon Llull in Spain, developed an alternative configuration. Instead of attaching the rings to the pipe walls, the researchers mounted a sequence of rings on a central rod. Their diameters gradually decrease along the structure, creating a series of spaces that influence how sound travels.

The resulting device resembles a small, carefully arranged stack of rings. It can be manufactured separately and inserted into an existing pipe, making installation potentially more convenient than rebuilding the pipe around a permanent acoustic feature.

This compact design demonstrates how additive manufacturing can help researchers create specialised structures with carefully controlled shapes. It also offers a practical way to test different configurations without manufacturing an entire pipe system for each experiment.

How the Device Absorbs Sound

When sound enters the structure, the changing spaces between the rings influence its wavelength and movement. As the waves travel further into the device, their energy becomes concentrated in a smaller region. Some of that energy is then dissipated through friction and heat exchange involving the air and the surfaces of the structure. As a result, less sound returns towards the entrance, reducing the strength of the echo.

Potential Uses and Remaining Challenges

The most immediate application is noise management in ducts, ventilation systems and industrial silencers. A removable structure could offer engineers more flexibility when upgrading existing installations. Researchers are also examining whether similar principles might eventually be adapted for two-dimensional and three-dimensional acoustic environments, including spaces where room echoes need to be controlled.

Although the prototype requires further testing, its ability to reduce sound reflections and potentially operate across a broad frequency range makes it an interesting development in acoustic engineering. With continued refinement, this approach could help make everyday mechanical systems quieter while giving engineers a more flexible way to manage unwanted noise.