Tapping Into Sound Waves

The next time you pick up a wine glass or glass bottle, try timing how long you can keep it ringing from a single tap. A group of Yale researchers have recently discovered that this “lifetime” of sound in glass can be extended by building on top of pre-existing sound waves. Glass has become the center of study for many modern scientists due to some of its strange interactions with sound waves. By investigating the interesting characteristics of sound waves traveling in glass, the researchers were able to improve our understanding of “acoustic atoms,” a mysterious phenomenon of sound. They developed a model to explain the life extension of sound waves in glass, demonstrating an important intersection between theoretical and applied research in the area of quantum physics.

Acoustic atoms, also termed “absorbers” by the Yale researchers, are not completely understood in the field of quantum physics. Generally speaking, they are similar to atoms—they can absorb particles of light, or photons, which causes their inner electron particles to vibrate and absorb that light energy. Acoustic atoms are involved in the lattice structure of a material such as glass. “Inside of glass, absorbers behave like atoms,” says Ryan Behunin, one of the lead researchers in this discovery. “Saturation is the term that describes this phenomenon of waves in glass tubes,” he said. Absorbers have limited conformations and respond to energy in quantized amounts. This leads to the formation of sound waves if the energy is large enough to vibrate the absorbers between the conformations very quickly.

Some older publications on the physical properties of glass materials describe a wave-like potential for sound release. Ryan described the shape of these potentials as “bowl-like,” or containing stable local minima that rise on either side. A trio of rolling hills most accurately illustrates this phenomenon. A hill in the middle separates the first low point from the second stable low point, creating a two-point potential system, or “two-level system”. The acoustic atoms settle in either low point, but can move around if they overcome the high energy of the hill.

If the acoustic atom finds itself in the middle of these two points, at the top of the hill, it may fall into one of the low stable points and release energy as a result. This released energy is what one observes audibly as a sound wave.

Behunin and fellow researchers discovered that by inducing other sound waves first, like playing music in the background as the wine glass is tapped, the waves from these acoustic atoms continue to live for a longer period of time, and louder background musics increase the period of time that the glass resonates.

One application of this Yale study is the improvement of optomechanical sensors from a better understanding of quantum physical properties. This could aid in developing remote sensors and physical link layers of information technology, such as Ethernet cords used by computer systems. The researchers also expanded the current base of knowledge on the quantum mechanics of sound. Still, although their discoveries elucidate the physics of producing sound in glass, the concept of acoustic atoms remains largely a mystery and further research is needed to understand its strange properties.

The purpose of this study was not just to observe a phenomenon, but also to determine a model to explain this phenomenon. Their model shows the importance of theoretical research. Behunin said, “From this, we can see the applications toward the performance of whole classes of systems.” The idea behind their research was twofold: focusing firstly on finding results and applying them elsewhere in optomechanics, and secondly on strictly discovering a theoretical principle. With this mindset, the Yale team developed a model and demonstrated the impact of their research on others in the quantum and optomechanical fields.