How do you block sound without cutting off airflow? It’s a tricky question, but new work out of Boston University shows a promising advance.
The tricky part comes from the fact that we call “sound” is actually a series of vibrations, a long chain reaction of air molecules knocking together to pass along an acoustic signal, creating a constantly changing pattern of high- and low-pressure air for our ears to register. That’s why the most straightforward method to silence sounds, like a noisy piece of machinery or the constant rumble of cars, is to prevent the air molecules from jostling each other, stopping the signal in its tracks.
Most sound dampeners are a variation on the same theme, whether they use foam or a solid wall, and for the most part they can be quite effective. However, because they rely on blocking waves from passing through air, they’re completely useless in any application that requires unimpeded air flow—for example, industrial fans or jet engines.
A few researchers have developed ways to circumvent this problem using metamaterials, which derive their properties not from their composition but from their structure; with a careful arrangement of sound-reflecting and sound-scattering structures, it’s possible to manipulate a sound wave into destroying itself. Although there has been some progress in creating sound dampeners for applications like air ducts, currently existing models have still been severely limited by the small amount of air passage afforded.
That is, until Dr. Xin Zhang, Professor at Boston University and her Ph.D. student Reza Ghaffarivardavagh unveiled their new design for a metamaterial silencer. Their design leaves a full 60% of the surface area free for air to pass through—yet it blocks 94% of the acoustic energy for a specific frequency range!
They’re able to achieve such impressive results by taking advantage of what’s called Fano-like interference, a wave-destroying phenomenon that arises when a wave is scattered into continuum and discrete states, then joined back together. Though the theoretical framework is complicated, Zhang and Ghaffarivardavagh explain that Fano-like interference is fairly straightforward in the context of their design, which resembles a lightweight ring with a spiral groove around the inside, much like a threaded bolt. This allows sound waves two different ways to pass through the metamaterial. “The first pathway is through the open area (center part)”, they say, “and the second pathway is through the helical metamaterial section.”
The helical portion is specially engineered so that, relative to sound waves that take the first pathway, certain frequencies are phase-shifted by precisely half a wavelength—effectively flipping the wave so that it’s perfectly out of phase with the wave passing through the opening. When they meet just at the other side of the metamaterial, the two waves cancel each other out and the sound is blocked. In practice, this isn’t too different from the mechanism that’s used in noise-canceling headphones, except that it’s entirely passive. While active noise cancelling uses microphones to detect the incoming sound wave, and a speaker to generate the counter-wave, this technology has no moving parts and doesn’t need power to operate.
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| This metamaterial sample measures 7cm (2.75″) in diameter and was 3D printed for Boston University as a proof-of-concept. Image Credit: Cydney Scott for Boston University |
The drawback of this approach is that, once created, the metamaterial can’t be tuned in real-time to silence different frequencies. This makes it useful primarily for applications which have a single dominant tone to be muted, like the hum of a fan. In the future, Zhang and Ghaffarivardavagh want to investigate a more dynamic design that can be adapted to specific pitches in real-time.
On the other hand, the advantages of this design are numerous. To begin with, the equations that govern the shape of the metamaterial have several free parameters that allow the researchers to engineer a functional material around several constraints. Practically speaking, the size of the central hole, the thickness of the final material, and the frequency selected for cancellation can all be taken into account when designing the perfect shape for a specific job. The range of frequencies that it can cancel is theoretically limited only by production capabilities, and since the metamaterial’s properties depend on only its shape and not the constituent material, it can be made quite cheaply. The team used a 3D printer to make their prototype, but Zhang and Ghaffarivardavagh see the product adapting well to plastic injection molding for mass production. Since it’s so much more versatile and has a better airflow than any currently existing sound barrier, it may be just a matter of time before we start seeing it on the market!
—Eleanor Hook