3 Acoustic Metamaterials You Probably Didn't Know About

Metamaterials are incredible feats of engineering and science. By combining various base materials in a specific way, they can be used to manipulate things like light and sound in ways not seen in nature.

For example, do you want to build a cloaking device? You might want to consider using metamaterials.

Here we explore some basic features of metamaterials and showcase three current developments in metamaterial science. 

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What are metamaterials and how do they work?

A metamaterial is an artificially structured material that is able to exhibit extraordinary electromagnetic properties not seen, or available, in nature. They were first developed in the early-2000s, and have since emerged as a rapidly growing interdisciplinary area of research and development.

"The properties of metamaterials are tailored by manipulating their internal physical structure. This makes them remarkably different from natural materials, whose properties are mainly determined by their chemical constituents and bonds." - Encyclopedia Britannica. 

Such materials tend to be made from randomly, or periodically, distributed artificial structures whose size and space are much smaller than the wavelengths of incoming EM radiation. A common existing example of metamaterials is split-ring resonators (SRRs). 

How are metamaterials made?

Metamaterials are a form of engineering materials that have been given properties not normally found in nature. 

"They are made from assemblies of multiple elements fashioned from composite materials such as metals and plastics. The materials are usually arranged in repeating patterns, at scales that are smaller than the wavelengths of the phenomena they influence." - Wikipedia. 

The properties of metamaterials are derived not so much from their constituent materials, but from their final designed and engineered structure. 

"Their precise shape, geometry, size, orientation, and arrangement gives them their smart properties capable of manipulating electromagnetic waves: by blocking, absorbing, enhancing, or bending waves, to achieve benefits that go beyond what is possible with conventional materials." - Wikipedia. 

The structure of each metamaterial is dictated by the natural phenomenon with which they have been designed to manipulate or influence. This could be to affect visible light, sound or electricity, to name but a few.

What is a negative refractive index?

You are likely to be familiar with the concept of refraction whereby the light bends as it passes through a medium, like water. With negative refractive indices, as the name suggests, the light is actually "reflected" when transitioning from a positive- to a negative index medium, rather than being refracted.

Metamaterials with this property tend to be built from periodically-spaced parts called unit cells. These cells are usually significantly smaller than the wavelength of external electromagnetic radiation. 

This property of some metamaterials enables scientists and engineers to create things like a flat lens for optical devices. These are cheaper to manufacture, usually lighter, and can be used for many applications beyond just focussing light. 

What is negative permittivity?

Negative permittivity is the measurement of a material's resistance to an electric field and its interaction with a dielectric medium. The permittivity of a medium describes how strong an electrical field (more correctly, flux) is 'generated' per unit charge in that medium.

It is a fundamental principle that elements like capacitors are based on. 

"More electric flux exists in a medium with a low permittivity (per unit charge) because of polarization effects. Permittivity is directly related to electric susceptibility, which is a measure of how easily a dielectric polarizes in response to an electric field. Thus, permittivity relates to a material's ability to resist an electric field." - researchgate.net.

Therefore, something like a metamaterial with a negative permittivity will have a non-resistance to a given electric field. 

The inverse of negative permittivity is called, unsurprisingly, absolute permittivity. This is the measure of resistance that is encountered when an electric field is created in a given medium.

Some examples of acoustic metamaterials

Acoustic metamaterials, as the name suggests, are artificially created materials that are specifically designed to control, direct, and manipulate sound waves.

Here are some recent developments. 

1. This metamaterial can actually cancel out sound

Researchers at Boston University have recently succeeded in creating a metamaterial that can actually 'cancel out sound'.  Their ring-like structure was able to cut out sounds while maintaining airflow at the same time.

This type of metamaterial could have some interesting applications as a form of lightweight and unobtrusive sound attenuation. It could be used on things like drones to cancel out their noise, or as sound insulation in buildings. 

2. This acoustic metamaterial can actually focus sound

Researchers at the universities of Sussex and Bristol in the UK are working on creating a metamaterial that can focus sound. If successful it could be used to produce super-targeted speakers or microphones.

They work by focussing sound waves much like a lens can focus light.  And the best bit? They are partly constructed from LEGO bricks!

Dubbed "Vari-Sound", this material could also have applications in theatres where the sound quality in the "cheap seats" would be on par with those sitting at the front or center. 

3. This metamaterial can transport and concentrate sound

A group of researchers has successfully designed a metamaterial that can transport sound along its edges and concentrate it in its center. The team from the City University of New York used the mathematical field of topology to design the metamaterial.

"To build their new metamaterial, Alù and his research partners used a 3D printer to create a series of tiny trimers, a ring formed by three acoustic resonators. Scientists affixed the trimers to form a triangular lattice." - upi.com.