An apparatus developed in the 19th century could help provide auditory relief from our noisy 21st-century world.
Using arrays of Helmholtz resonators, which were created in the mid 1800s to better understand human speech, researchers were able to dampen sound waves over a range of useful frequencies, they report in the Journal of Applied Physics.
For the resonators in his original experiment, Hermann von Helmholtz used glass spheres of various sizes with a hole on one end for air to pass across and a hollow protrusion on the other end to amplify the created sound. Each resonator has a specific resonant frequency determined by the internal volume and the size of the opening. When air outside passes across that opening, the air inside oscillates at a set frequency and produces a note. You’ve probably experienced the phenomenon when blowing across the top of a wine bottle. The same properties can also be used to absorb sound waves.
That was the subject of Robine Sabat’s PhD research, in which she worked with colleagues at the University of Lille in France and National Technical University of Athens in Greece. It began with the basic principle of destructive interference, which is used in noise-canceling headphones: When two waves of the same frequency are combined but their peaks and troughs are in opposition, the resultant wave will have a lesser amplitude than either of the original waves.
Sabat used a metasurface—an artificial material with subwavelength thickness—in between a sound source and the listener to cancel out noise. The exact frequencies that were dampened were determined by the resonant frequency of Helmholtz resonators placed on the metasurface. Most importantly, the researchers realized that a cheaper alternative to Helmholtz’s original glass bulbs were Ping-Pong balls with a hole drilled into them. Sport regulations ensure that Ping-Pong balls are manufactured as similar as possible, meaning that each one’s resonant frequency should be the same, even when purchasing them in bulk.
The dampening effect is further improved when multiple resonators are coupled. Attaching Ping-Pong balls together widens the range of frequencies because the air can move between the two balls. And the team was surprised to find that including a plexiglass support for the structure added a second frequency range of dampened sound waves, further reducing the transmitted noise.
The research is only one part of an international research effort to develop ways to reduce the noise that surrounds us in daily life. Specifically, the reduced frequencies would lessen the noise from trains and cars. In the future, such metamaterials could be integrated into the outer walls of a building to create a quieter environment. Although the researchers don’t recommend covering your apartment in Ping-Pong balls, they anticipate that their work on combining Helmholtz resonators with metamaterials could pave the way for a quieter future. (R. Sabat et al., J. Appl. Phys. 134, 144502, 2023.)