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Ping-Pong Ball Insulators Can Block Noise

Researchers harness acoustics principles to seal out noise pollution

Illustration of a person with a paddle bouncing a ball, with the same balls sitting in their ears.

Constant city noise has been linked to long-term health problems, but it’s hard to keep out of homes and businesses; low-frequency sounds such as traffic and construction propagate easily through walls and other solid materials. Expensive, specialized paneling can help, but a new study in the Journal of Applied Physics shows how everyday materials and clever physics can also do the trick, creating a kind of sound insulator from strategically pin-pricked ping-pong balls.

Robine Sabat, an acoustics researcher at the University of Lille in France, has been trying to improve noise insulation by studying how sound waves bounce around in hollow cavities. When a sound wave passes over an opening in such a space, the wave squeezes and releases the air inside. This makes the air vibrate at a particular frequency depending on the cavity’s size, shape and any holes it might have (just as blowing across a bottle’s lip causes a hum, with the pitch depending on bottle size). And if cavities are constructed in just the right way, the bouncing sound waves inside will cancel one another out, dampening the noise.

Sabat chose ping-pong balls as a low-cost option with geometric properties that create resonance in the right low-frequency range. By drilling five holes in each ball, her team turned them into resonant cavities that each filter one frequency band out of the surrounding noise.

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But combining the resonating balls to dampen large ranges of sound is tricky because the sound waves interact and affect which frequencies get dampened. To find the right arrangement, the researchers placed microphones inside two balls and adjusted the holes’ positions and sizes until the combination captured multiple frequency bands. They added and adjusted more balls until the structure absorbed a wide range of frequencies.

“Getting the holes aligned perfectly took some practice,” Sabat says. Her team’s arrangement of 90 balls, fixed to a sheet of plexiglass, reduced low-frequency sounds heard on the other side by up to 50 percent compared with the plexiglass surface alone.

“The design gives excellent sound attenuation, even below 500 hertz” (the range most associated with long-term health effects), says Olga Umnova, an acoustics researcher at the University of Salford in England, who was not involved in the new study. She adds that a systematic, real-world comparison with simpler options, such as plexiglass sheets separated by an air gap, would be an important next step. Computer simulations have estimated that the ping-pong paneling improves sound reduction by 30 percent compared with an air gap alone.

Sabat’s team hopes low-tech adjustments to the new technique could also help with other acoustic aims, such as focusing sound waves to improve sound quality in concert halls.

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