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Ping Pong Balls Offer Acoustic Solution

Ping Pong Balls Offer Acoustic Solution

By creating a meta surface with ping pong balls, researchers have found a new way to dampen various sound frequencies.
Researchers at the Université de Lille in France and the National Technical University of Athens in Greece have collaborated with interdisciplinary university and industry leaders to create a low-frequency sound attenuator using an everyday product: ping pong balls.

Through a trial-and-error process, the team discovered they could dampen various sound frequencies by manipulating the quantity, location, size, and arrangement of holes drilled into an array of ping pong balls—referred to as a meta surface. Interconnecting holes between the balls also had an impact on sound dampening. 

The research is outlined in “Low frequency sound isolation by a metasurface of Helmholtz ping-pong ball resonators,” recently published in the Journal of Applied Physics.

Simultaneously addressing various frequencies was possible by arranging multiple meta surfaces together, each tailored for specific frequencies. “The amplitude and the frequency can change with the size of the holes inside the ping pong ball but also on the radius and interconnection from one ping pong ball to the other. We like to understand every phenomenon by changing one parameter at a time to see the effect,” explained Yan Pennec, vice dean of research at the Université de Lille.

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A Helmholtz resonator consists of a container, typically spherical in shape, with a neck and an opening at one end. Blowing air over the top of a bottle to generate a sound is an example of a Helmholtz resonator in action. When air is blown across the neck, it enters the bottle and induces vibrations, resulting in sound production. Altering the liquid level in the bottle also changes the sound, illustrating how adjusting the cavity's volume inside the bottle (the empty space) can yield various sound frequencies.
 

Low frequency sound


The research was part of a broader project within INSPIRE—a European consortium of 14 interdisciplinary university and industry leaders collaborating to mitigate the effects of earthquakes, noise, and vibration in the built environment. Urban areas often experience low frequency sounds due to high levels of background noise from traffic, HVAC equipment, aircraft, and other sources. While some find the city's cacophony comforting, it can induce headaches, disrupt sleep and concentration, and elevate eardrum pressure in others.

This effort primarily focused on low-frequency sound, but the ping pong balls' versatility in terms of hole size, location, and quantity enabled the team to investigate a range of acoustic frequencies. To adapt the meta surface for higher frequencies, liquid can be introduced into the ping pong balls to reduce the cavity volume. “If you change the level of the liquid inside the ping pong ball, then you can change the frequency of the absorption,” explained Pennec.

Although the team initiated the project with the intention of fabricating a cavity using a 3D printer, they later discovered that ping pong balls were a cost-effective and easily accessible solution that suited their Helmholtz resonator requirements.

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“We decided to build the meta surface with a simple material and saw that the Helmholtz resonator is similar to a pierced ping pong ball,” said Robine Sabat, a Marie Sklodowska-Curie fellow, and professor doctor of acoustics at the Université de Lille. The acoustic meta surface was built by assembling ping pong balls with outward-facing pre-drilled holes.

Due to the necessity of maintaining a controlled acoustic environment for precise control and measurement of sound frequencies and dampening effects, the testing conducted thus far has been on a small scale. The team is collaborating with industry partners to establish large-scale testing facilities for further development of the ping pong ball meta surfaces.
 

Friction from sound waves


Sound waves, despite their invisibility to the naked eye, consist of small pressure fluctuations. As sound waves move through the air, they generate friction, which heats the air, causing it to become less dense and more slippery, resulting in reduced resistance to sound waves. Therefore, sound travels faster in warm air compared to cold air, though the difference is imperceptible to the human ear within our everyday temperature range. The energy dissipated as heat due to friction is referred to as thermoviscous loss. When sound waves encounter a solid object, such as a building, the boundary layers near the object's surface experience increased friction and thermoviscous losses, leading to greater sound dampening. The researchers conducted tests on their prototype across several days with varying temperatures. While they observed minor fluctuations, “We have the same results for different days with different temperatures,” said Sabat.

A ping pong ball meta surface can be applied in structures housing noisy engines, motors, or compressors to diminish the transmission of noise to other areas. The prototype can also benefit mechanical rooms supporting HVAC systems or factories with noisy equipment. Additionally, when applied to windows it can mitigate the impact of low-frequency noise on residential neighborhoods adjacent to industrial areas or highways.

The research team plans to continue refining and testing different prototype variations, with the aim of expanding to larger environments.

Nicole Imeson is an engineer and writer in Calgary, Alberta.

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