Ping-Pong Ball Insulators Can Block Noise

By Rachel Berkowitz

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…