•An engineer and his colleague have developed a “sound black hole” that absorbs and eliminates the echoing sound within pipes, in a laboratory setting. The device was tested with a 3D-printed prototype that can be attached to existing piping systems, and it was found to match computer simulations of the amount of echo returned. Sound bounces and echoes from the many sources around us and reflects off surfaces, making it extremely useful or annoying, depending on the circumstances. Sound passing through closed spaces within pipes, such as ventilation, air conditioning, industrial exhaust, or plumbing, can echo regularly when it encounters an obstruction or narrowing.
“The sound approaches the edge of the resonator, and something happens to the wave; it looks like what an outside observer would see near an astrophysical black hole: The wave takes an infinite amount of time to reach the boundary,” says Guasch. “Ideally, when the wave enters the resonator, it doesn’t come back out.”
The trick to making the sound disappear lies in the design. Just as an astrophysical black hole isn’t a real hole, neither is the sound version; it’s an engineered structure that alters how sound propagates through the tube.
In conventional designs, the sound waves travel through the tube, passing through rings and gaps that gradually slow down as the wavelengths shrink and the energy becomes more concentrated. But there’s a problem: To build these structures into the tube, they must often be produced together, making it difficult to fit them into existing systems, and the black hole effect only works in a limited range of sound frequencies.
Deng, Guasch, and their colleagues thought they could solve both problems by turning everything on its head: Instead of a tube, they used a “sound Christmas tree.”
The concept is surprisingly simple, and it works very well.
Oriol Guasch
Practically, the sound black hole can be produced independently of the channel and then simply inserted into it.
The device consists of rings mounted on a rod in the center; their diameters vary gradually along the length. The gaps between the rings create the spaces that affect how the sound propagates through the tube.
As the sound approaches the end of the device, it slows down and its energy becomes concentrated toward the opposite end, causing the wavelength to shrink. Then the trick to making the sound disappear happens: Most of the concentrated sound energy is dissipated not by bouncing back out, but by friction and heat exchange between the air and the surfaces of the device; very little escapes as an echo.
Researchers tested this with a 3D-printed prototype: The structure, consisting of 29 rings and about 15 centimeters long, was placed on a central rod and then placed in an impedance tube, which is used to measure how sound waves behave when they encounter a material or structure. They did not shout into the tube; instead, a speaker sent out sound waves at different frequencies, and microphones were used to measure how much of the sound returned. The results were encouraging: The amount of sound that returned from the black hole corresponded closely to the researchers’ computer simulations; however, it should be noted that the echoes did not completely disappear.
There are challenges to overcome before the small sound-sucking Christmas trees are deployed. Different environments bring different problems: from particles that can fill the gaps to temperature, pressure and airflow effects. However, Guasch is excited by the possibilities of controlling sound in ways that were once thought to be almost impossible. “Personally, I’m most excited by the way physics and mathematics bring together seemingly unrelated fields of knowledge. And of course, the type of wave control that is now possible is something that seemed like science fiction just a few years ago,” he says. The research was published in the journal Applied Acoustics.
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