Sound reflects off surfaces and can create unwanted echoes, especially in confined spaces such as ventilation systems, air conditioning units, industrial ducts and plumbing pipes. Scientists have developed a compact device called an “sonic black hole“ that can significantly reduce these reflections. Unlike older designs, the new design can be manufactured separately and placed inside an existing pipe.
“Imagine standing in front of an air duct that is closed at the far end and shouting into it. "After a short delay, depending on the length of the duct, you would hear an echo coming back from the end," acoustic engineer Oriol Guasch of the Universitat Ramon Llull in Spain told ScienceAlert.
"If the duct ended in a sonic black hole, you would essentially hear no echo at all. Acoustically, it would be almost as if the pipe were infinitely long and there was no end from which the sound could reflect back to you," he added.
Unlike astrophysical black holes, which trap even light due to their extremely strong gravity, the sonic version is a specially constructed structure. It exploits the property of sound waves to gradually slow down under certain conditions, until in practice they never seem to reach the end of their path.
“What happens to the wave as it approaches the edge of a sonic black hole has some similarities to what an outside observer sees near an astrophysical black hole: it seems to take an infinite amount of time for the wave to reach the boundary point“, says Guasch. “Ideally, once it enters the sonic black hole, the wave does not come back out.“
Conventional devices of this type typically use a series of rings and cavities built into the inside of the tube. They slow down sound waves, shorten their length, and concentrate their energy, but they have two major drawbacks: they are difficult to install in existing systems, and they only work effectively over a limited range of frequencies.
The team of Gouache and engineer Xie Deng took a different approach. Instead of the structure being built into the walls of the tube, it is a series of rings of gradually varying diameter mounted on a central support rod. The cavities between them affect the movement of the sound, gradually slowing it down and directing its energy towards the far end.
There, a significant part of the energy is dissipated through friction and heat exchange between the air and the surfaces of the device. This leaves less energy to return in the form of echoes.
The researchers tested the concept with a prototype printed on a 3D printer. The device is 15 centimeters long and consists of 29 rings mounted on a central rod. It was mounted in an impedance tube - a laboratory apparatus for studying the behavior of sound waves when they encounter different materials and structures.
Instead of shouting into the tube, the scientists used a loudspeaker that sent sound waves at different frequencies, and microphones measured how much sound was reflected back. The results generally matched computer simulations, although the device did not completely eliminate echoes.
The simulations also showed another advantage: the new design can operate over a significantly wider frequency range than traditional sonic black holes. According to Gouache, this is due to resonances in the cavities between the rings, which in the new geometry appear at higher frequencies and allow the effect to persist over a wider range.
The most immediate application of the technology is in noise control in air ducts and mufflers. The team is also exploring the possibility of developing similar structures in two and three dimensions, which could open the way to applications in room acoustics.
A number of practical challenges remain before the technology can be widely used. Particles can clog the cavities, and temperature, pressure and air movement can also affect the device's performance.
Nevertheless, Gouache sees significant potential in the new approach to controlling sound waves.
“Personally, what I find most stimulating is the physics and mathematics behind it all, and the way it brings together areas of knowledge that may initially seem quite unrelated,“ he tells ScienceAlert. “And of course, the type of wave control that can now be achieved would probably have seemed almost like science fiction a few years ago.“
The study was published in the scientific journal Applied Acoustics.
Source: Vesti.bg.