Acoustic Energy Harvesting

Introduction

When it comes to energy innovation, a great deal of attention has been focused on sources that are already abundant within the environment – wind and solar power are two prominent examples that come to mind. These intermittent sources fluctuate with time and cannot be controlled or generated spontaneously. Instead, people came up with unique ways to harness them, like wind turbines and solar panels. These tools are capable of capturing and converting energy that would have been lost to the environment anyway.

What if acoustical energy could be similarly harvested? There is already an entire industry (noise control engineering) dedicated to reducing unwanted sound. Instead of simply redirecting it or addressing it at the source, is there some tool akin to the wind turbine or solar panel, but for sound?

In fact, many engineers and other researchers have been studying that question (as well as the analogous question applied to mechanical vibrations). The conversion of acoustical energy to electrical energy is already commonplace – it happens every time someone uses a microphone to record or amplify sound. Acoustic energy harvesting (AEH) is a research field that seeks to explore this concept further and implement it in other useful applications.

This entry will outline some of the approaches researchers have used to develop AEH technology, including Helmholtz resonance, acoustic metamaterials, and thermoacoustic engines.

Helmholtz Resonance

In mechanical engineering, the spring-mass system is a classic model that is often used to illustrate the physics of harmonic motion. The basic idea is that when a mass stretches or compresses a spring to which it is attached, the spring in turn exerts a restoring force on the mass, guiding it back toward its equilibrium position. This principle applies to many other systems apart from just a literal mass and spring. One such example in the field of acoustics is known as a Helmholtz resonance.

The standard Helmholtz resonator consists of a cavity and a neck with an opening at the end. When air is forced into the resonator, the air inside the cavity acts as a “spring” that exerts a restoring force on the oscillating “mass” of air inside the neck. This vibrating air generates sound waves at a resonant frequency that is a function of the geometric features of the apparatus. If you’ve ever blown a stream of air across a soda bottle and listened for the resulting sound, you’ve used a Helmholtz resonator!

When incoming sound waves have the same frequency as the Helmholtz resonator, they excite the cavity, causing acoustic energy to accumulate within. For AEH applications, the goal is to capture and store this energy for later use.

In a 2018 paper, a group of engineers from China described how they used this principle to develop a prototype of a barrier capable of harvesting sound energy from high-speed railways. The basis of their design was a honeycomb-structure with hundreds of Helmholtz resonators, each one sized so that the resonant frequency would match the primary low-frequency component of passing trains.  When a passing train induces excitation of the resonators, a strip of specialized plastic film (which produces a voltage when deformed) housed within the cavities vibrates along with it, allowing the sound energy to be converted into electrical energy.

The low energy density of sound power means that the output of each honeycomb “unit” is rather low – in this study, the researchers found that a sound pressure level of 110 could produce only 74.6 mV. But that electrical energy, stored in supercapacitors, is enough to power autonomous wireless sensors, monitors, and other devices along the railway. When considering a multitude of honeycomb units and trains operating over a longer stretch of time, the cumulative effect of the energy contribution becomes more significant.

Acoustic Metamaterials

When we want to predict the behavior of sound, one of the most important factors to consider is the medium through which it is traveling. Most people are aware that sound travels at different speeds in air vs water vs steel, for instance. There are also differences in behavior when sound waves encounter a new medium: some materials reflect the sound, while others absorb it. The effect of different material properties on sound is what drives the research of acoustic metamaterials, which are designed to manipulate sound waves for some specific purpose.

Recently, AEH has emerged as one promising application of acoustic metamaterial technology. Prototype development requires designing and shaping a material to not only amplify incoming sound waves, but also to concentrate or localize the energy to facilitate its conversion into electrical power.

The image above shows an example of an acoustic metamaterial designed for AEH. Though it can be hard to see with the naked eye, sound waves cause slight deformations to materials when they collide with them. As a result of this deformation, “strain energy” is stored in the material, and various materials tend to differ in how they maintain this stored energy. The researchers in this case designed their material to be particularly effective at confining the energy stored within it when it gets deformed by low-frequency waves. After the acoustic waves hit the surface, the strain energy excites a piezoelectric diaphragm at the center of the plate, causing it to accumulate an electric charge that can then be harnessed to provide electric power.

Thermoacoustic Engines

In architectural acoustics, when we consider adding absorptive treatment to a room, it’s based on the concept of converting sound energy into heat. Thermoacoustic engines, another approach to AEH, operate on the opposite principle. Thermoacoustic engines facilitate the conversion of heat to sound energy, which in turn can be harvested for electricity.

A basic thermoacoustic engine apparatus is shown in the diagram above. It consists of two heat exchangers near the closed end of a resonant tube, with a porous material situated between them. As gas flows between the two exchangers, the difference in temperature causes it to periodically expand and contract. This oscillation leads to the generation of acoustic waves along the open end of the tube, which can then be converted into electrical energy through the use of a transducer.

There is a simple elegance to the design of thermoacoustic engines, and they have the benefit of being efficient, reliable, and relatively easy to maintain. However, in contrast to the Helmholtz resonator and acoustic metamaterial examples described above, thermoacoustic engines rely on an “active” rather than “passive” approach to AEH, meaning that they require an external power supply to operate the heat exchangers.

Conclusion

The examples discussed above represent just a small sampling of the efforts researchers have made to bring acoustic energy harvesting to fruition. Many designs are still in the research and development phase, and it will likely be some time before they are implemented more broadly. Since the sound power density of acoustic energy is quite low, its application is generally limited to small-scale electronics and the powering of sensors, and as a result it is often overlooked compared to other intermittent sources like wind and solar. That being said, when it is integrated into existing systems, acoustic energy harvesting can be considered among the many strategies used to maximize energy efficiency and sustainability.