Metamaterials have been the focus for many researchers for years. Researchers have been attempting to design structures that have abnormal properties with the ability to control acoustic or optical waves. There have been major developments made in metamaterials, but they are always in fixed geometries, which also make their abilities fixed. The newest development in 3D printed metamaterial changes this. The 3D printed material has the ability to change between active control and passive states.

This is a sample of the 3-D printed acoustic metamaterial. Source: Qiming Wang

The new 3D printed metamaterial can block sound waves and mechanical vibrations. The metamaterial stands out from other metamaterials because it can turn off these abilities with a magnetic field. The uses of this material are noise cancellation, vibration control and sonic cloaking.

"When you fabricate a structure, the geometry cannot be changed, which means the property is fixed. The idea here is, we can design something very flexible so that you can change it using external controls," said Qiming Wang, USC Viterbi School of Engineering assistant professor of civil and environmental engineering.

Metamaterials have been used to manipulate wave phenomena like radar, sound and light in many technologies. The new metamaterial controls environmental sounds and structural vibrations through the shape. It is made of iron particles in a lattice structure so it can be compressed by a magnetic field.

"When you fabricate a structure, the geometry cannot be changed, which means the property is fixed. The idea here is, we can design something very flexible so that you can change it using external controls," said Wang, an assistant professor of civil and environmental engineering.

The magnetic field compression method is preferred because it doesn’t constrain the material like a physical force would. When magnetically compressed, a wave runs into a material, enters it and generates the properties to block waves from certain frequencies from passing through.

"Material with a negative modulus or negative density can trap sounds or vibrations within the structure through local resonances so that they cannot transfer through it," Yu said.

The team maintained control over the metamaterial while switching through double-positive, single-negative and double-negative thanks to the negative properties and the magnetic field.

With a little more development, the team may be able to demonstrate negative refraction. Negative refraction goes through the material and bounces back at an unnatural angle. The team wants to further study this when the material is created in a larger structure.

"We want to scale down or scale up our fabrication system," Wang said. "This would give us more opportunity to work on a larger range of wavelengths."

Currently, they can only 3D print the material with a beam diameter between a micron and millimeter, which restricts the uses considerable. Smaller beams control higher frequency waves and larger beams control lower frequency waves.

"There are indeed a number of possible applications for smartly controlling acoustics and vibrations," Yu said. "Traditional engineering materials may only shield from acoustics and vibrations, but few of them can switch between on and off."

The paper on this research was published in Advanced Materials.