•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 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 exhausts, 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. “In the ideal case, the wave doesn’t come back when it enters the sound tube.”
The trick to making the sound disappear lies in the design. Just as an astrophysical black hole is not 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 wavelength shortens and the energy becomes more concentrated. But there’s a problem: To build these structures into the tube, they often have to 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 radii 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 tip of the device, it slows down and its energy becomes concentrated toward the opposite end, causing the wavelength to shorten. Then the trick of disappearing the sound happens: Most of the concentrated sound energy is dissipated not by being reflected back, 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, which consists of 29 rings and is 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 corresponded largely 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 directing sound in ways that were once thought to be almost impossible. “Personally, I’m most excited by the way physics and mathematics, which initially had little to do with each other, are brought together,” 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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