•An engineer and his colleague have developed a “sound black hole” that absorbs and cancels out the echoing sound within pipes. The device was tested in a laboratory with a 3D-printed prototype that can be attached to existing piping systems, and it was found to be consistent with 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 enclosed spaces within pipes, such as ventilation, air conditioning, industrial exhausts, 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.”
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. The researchers 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 was largely in line with 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 directing sound in ways that were once almost impossible. “Personally, I’m most excited by the way physics and mathematics bring together seemingly unrelated fields of knowledge,” he says. “And of course, the type of wave control that is now possible is something that seemed like science fiction just a few years ago.”
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