•An engineer and his colleague have developed a “sound black hole” that absorbs and cancels out the echoing sound in pipes, created in a laboratory environment. 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 enclosed spaces in pipes, such as ventilation, air conditioning, industrial exhaust, or plumbing, can echo regularly when it encounters an obstruction or narrowing.
“The sound approaches the tip of the tube 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 tube, it doesn’t come back out.”
Researchers tested this with a 3D-printed prototype: The structure, consisting of 29 rings and measuring approximately 15 centimeters in length, 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 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 largely with the researchers’ computer simulations; however, it should be noted that the echoes did not completely disappear.
There are challenges to overcome before small sound-absorbing Christmas trees are deployed. Different environments pose different problems: from particles that can fill gaps to temperature, pressure and airflow effects. However, Guasch is excited by the possibilities of controlling sound in ways that were once almost impossible. “Personally, I am 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 seems like science fiction just a few years ago,” he says. The research was published in the journal Applied Acoustics.
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