Sound Energy: A Beginner's Guide To This Emerging Energy Source

As the world continues to look for alternative sources of fuel to feed its insatiable appetite for energy while reducing reliance on fossil fuels, a new source may lie in an unexpected place: the sounds around us

Investigations into sound energy have been ongoing for many years, but the use of sound as a source of energy is still in its nascent stages. This article will look at what is meant by sound energy, its various uses, and its viability as an alternative energy source

What Is Sound Energy?

There are many different types of energy, and sound energy results from the vibrations of sound waves

But how does this actually happen? When a disturbance causes an object or substance to vibrate, the energy produced by the vibration travels through the object as sound waves. The vibrating object or substance must be either solid, liquid, or gas

Something as basic as waving our hands in a substance like air would cause enough vibrations leading to sound waves. The sound waves produced by these means are not loud enough to be heard by humans, but the principle is the same for sound waves that are audible to our eardrums. The vibrations cause molecules in the substance to bump into each other and create a knock-on effect that leads to the production of sound waves

There are two main types of waves: longitudinal (or compression) waves and transverse waves. Longitudinal waves are formed when the waves move in the same direction as the force acting on the substance. Areas where the waves are most compact are called compressions; those areas where the waves are most extended are referred to as rarefactions

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Transverse waves, on the other hand, move in a direction that is perpendicular to the applied force. Whereas longitudinal waves can occur in any of three substances—solids, liquid, or gases—transverse waves can only move through solids

The vibrations that produce these sound waves are felt by our ears, which then makes sense of them. For human ears to detect a sound vibration, it has to be moving at anywhere between 20 vibrations a second and 20,000 vibrations a second. More sound vibrations per second indicates a higher pitched sound. These sound vibrations are measured in Hertz, denoting the frequency of a sound.

However, the loudness of a sound is measured in decibels (dB), with normal conversation being at around 60 dB

How Is Sound Energy Used?

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Though attention to sound energy as a possible replacement for other power sources is a relatively new phenomenon, there are examples of sound energy that we humans have long used for vital activities

One example of sound energy has contributed to saving lives. When pilots bail out of an airplane that is experiencing engine troubles and they go down at sea, they can release a small explosive charge that emits sound energy.

The sound of a small explosive travels thousands of meters in the deep ocean because it is trapped in the deep sound channel, known as the SOFAR or Sound Fixing and Ranging Channel. The sound can then be picked up by special devices in the ocean and the location of the downed pilot ascertained, enabling their rescue

Another example of sound energy is one that is familiar to many of us—the use of ultrasound. Ultrasound is made up of very high-frequency longitudinal waves that can be used to break up gall and kidney stones. It’s also commonly used for medical imaging purposes during a woman’s pregnancy.

A third example of sound energy is one not normally associated with humans—echolocation. It is common knowledge that animals such as dolphins, whales, and bats use echolocation to help them navigate and hunt. These creatures send out sound waves that bounce off of objects and return to them as echoes, which indicate to them the location of prey or obstacles in their path

But did you know that some humans also use echolocation? Some blind and visually impaired people do so by making clicking sounds with their tongues or tapping with their canes. The sound waves they create bounce off of objects and serve to inform them about their location, size, and texture

Can Sound Energy Replace Fossil Fuels?

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Researchers have been investigating sound energy as one of the renewable forms of energy over the past few decades. Researchers are interested in it because sound energy contains two types of mechanical energy—potential and kinetic—which means it has the capacity to do work. Learn more in our guide that explains the difference between potential and kinetic energy

However, technology for harnessing sound energy as a source of power has not yet reached the stage where it can be used at scale. For example, the sound of a pneumatic drill, something most of us find too intense, generates only as much of a hundredth of a watt of energy per square meter, whereas solar energy over the same area would produce as much as 680 watts

Nevertheless, the pervasiveness of sounds in our environment makes harnessing sound energy for power a tantalizing prospect. Scientists refer to this as acoustic energy harvesting, and research into its potential as a power source has mainly focused on three technologies: Helmholtz resonators, acoustic metamaterials, and thermoacoustic engines. Let’s take a closer look at all three

Helmholtz Resonators 

Helmholtz resonators are used to amplify or absorb sound. In one experiment, researchers drew on this property to capture and amplify sounds at a railway station. They then converted the sound energy into electric energy with the aid of an electricity generator module. The resulting power output was enough to operate small electronic devices.

Acoustic Metamaterials 

These types of materials are designed to manipulate sound waves to achieve a particular objective. Those being developed to harness sound energy both amplify incoming sound waves and concentrate the energy from the sound waves to enable their conversion into electrical energy. Piezoelectric structures are commonly used for designing acoustic metamaterials

Thermoacoustic Engines 

These types of engines convert heat into sound energy from which electric energy is then harvested. Thermoacoustic engines are considered an easy and reliable technology to use. However, it does rely on an external power supply to drive its energy harvesting process, unlike the other two. Among its applications is the recovery of automobile exhaust waste heat to reduce emissions, which is then converted to sound energy from which electric energy is harvested

However, these methods of acoustic energy harvesting are limited in their real-world application. One reason is that some methods work with only a narrow band of frequencies, whereas many of the sounds available for sound harvesting in modern life are in the broadband frequency range. An allied limitation is that the areas used for collecting sounds, as in the case of acoustic metamaterials, are typically very small, so not much energy can be collected

Is Sound Energy a Renewable Energy Source?

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The use of energy from acoustics as a renewable energy source is still more in its experimental stages. One such experiment captured the noise generated in three bus stations. With the aid of acoustic metamaterials, namely piezoelectric, it converted this noise to electric energy.

The researchers said their aim was to reduce the use of non-renewable energy sources while creating “green sustainable electric energy” that could be used to power streetlights. The locations where they captured sound routinely had noise levels of around 80 decibels and the energy caught yielded about 0.024 watt hours from an area around 1.5 meters squared

However, a few companies seem to have made headway with the use of this form of energy on a commercial scale. A Department of Energy-funded technology, known as ResonantAcoustics Mixing (RAM), developed by Resodyn Corp. is marketed for use in industries that do mixing as a major part of their production process.

Sound vibrations are used to mix liquids and slurries, such as in the manufacture of ceramics, food, and pharmaceuticals, and for water treatment. So far, the company has sold the mixers both in the U.S. and overseas and has introduced mixers of widely varying capacities

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Another commercial application of sound energy as renewable energy is being made by the aircraft manufacturer Boeing. The company filed a patent in 2015 for a technology that would harvest the sounds from jets taking off at airports and would convert that sound into electricity for use as power

Yet another patented sound energy application was developed by researchers at RMIT University in Australia. The scientists there have patented a technology that uses high-frequency sound waves to deliver vaccinations by inhalation rather than using needles.

An even more down-to-earth application of sound energy technology involves a sonic cooker developed by University of Nottingham researchers, which uses sound energy to produce electricity for powering stoves. The stoves, which produce 36 watts of electricity, have been tested in Asia. It’s considered a transformative technology since many people in sub-Saharan Africa and some parts of Asia lack an electricity supply

Can Sound Energy Be Stored?

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The same issue that hindered the uptake of most forms of renewable energy for several years arises with sound energy: How do you store the energy after you have captured it? 

Typically, any sort of mechanical wave, including sound waves, will change its form upon coming into contact with an object. Some of it will be absorbed by the material it comes into contact with or it will be scattered. When the sound waves are absorbed they become another form of energy

Researchers have developed a way to store sound energy until it is needed so that it will be converted to electrical energy only on demand. Known as coherent virtual absorption, the technique basically disrupts the way sound waves typically interact with materials to allow the energy to be stored rather than being lost or transformed into electrical energy before it’s needed

Almost two decades ago, a patent was filed for a technology to store energy from sound. It involves the use of flywheels that would store electrical energy over long periods and was said to be a particularly useful device for storage in remote areas

Whether any of these technologies and patents will eventually take off and become mainstream or developed for use at scale remains to be seen. But they highlight that researchers are engaging with the pressing challenge of finding alternatives to polluting fossil fuels that are driving climate change and natural disasters worldwide. Any such efforts are, therefore, to be applauded

Is Sound Energy Only For Your Ears?

As this article shows, sound energy does far more than meet the needs of us humans to communicate, whether it be through the use of musical instruments or speech. The article has shown some of the many possible applications of the science behind the way sound energy travels because of air molecules and the technological innovations being achieved. Such research will perhaps lead to devices using acoustics that afford visually impaired persons even greater independence. There may also be discoveries of new ways to locate lost people or objects, or to harness this renewable resource for electricity

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