This lego-like wall creates 3D soundscapes that put your surround sound to shame

This is a close up look at the metamaterial device that can create acoustic holograms. Each grid or block contains a spiral of one of 12 various densities, each of which slows sound waves by a different amount.
Steve Cummer/Duke University
This is a close up look at the metamaterial device that can create acoustic holograms. Each grid or block contains a spiral of one of 12 various densities, each of which slows sound waves by a different amount.

Researchers are promising intricate 3D soundscapes that are to audio what holograms are to images – all coming from a single lego-like source. You probably never felt like it should be possible to get more out of your home stereo, but it is starting to look like we have been missing out.

It is a relatively simple concept – Research Triangle engineers are moulding the sound waves to make it seem like the audio source is much more intricate than it really is. Just imagine what the difference is between listening to a live orchestra, and a recording coming from the speakers.  The fact of the matter is that sound doesn’t only carry notes and volume – it also carries spatial information – and anyone could hear and feel the difference.

“We show the exact same control over a sound wave as people have previously achieved with light waves,” said Steve Cummer, professor of electrical and computer engineering at Duke University. “It’s like an acoustic virtual reality display. It gives you a more realistic sense of the spatial pattern of the sound field.”

The comparison to visual holograms is quite apt, since they manipulate light to make it appear as though a 3D object is sitting in empty space. These optical tricks work by shaping the electromagnetic field so that it mimics light bouncing off an actual object.

Sound also travels in waves. But rather than electromagnetic energy traveling through space, sound propagates as pressure waves that momentarily compress the molecules they are traveling through. And just like visible light, these waves can be manipulated into three-dimensional patterns.

A computer rendering of a sound wave that traveled through an array of acoustic metamaterial and was shaped into a pattern like the letter A one foot past the array. This pattern could not be seen, only heard. (Source: Duke University)
A computer rendering of a sound wave that traveled through an array of acoustic metamaterial and was shaped into a pattern like the letter A one foot past the array. This pattern could not be seen, only heard. (Source: Duke University)

Duke and North Carolina State University researchers have demonstrated that they can produce any three-dimensional soundscape they want with sound waves with metamaterials – synthetic materials composed of many individual, engineered cells that together produce unnatural properties. Each individual block is made of plastic by a 3-D printer and contains a spiral within. The tightness of the spiral affects the way sound travels through it — the tighter the coil, the slower sound waves travel through it.

While the individual blocks can’t influence the sound wave’s direction, the entire device effectively can. For example, if one side of the sound wave is slowed down but not the other, the resulting wave fronts will be redirected so that the sound is bent toward the slow side. By calculating how 12 different types of acoustic metamaterial building blocks will affect the sound wave, researchers can arrange them in a wall to form any wave pattern on the other side that they want. With enough care, the sound waves can produce a specific hologram at a specific distance away.

Possible uses for an acoustic hologram extend beyond home entertainment – to advanced aerial sensing and imaging technologies – but there is a good chance that sound companies and speaker manufacturers will be very interested. Not to mention virtual reality enthusiasts. You may want to keep an eye on this one if immersive virtual escapism sounds like something that floats your boat.

Michal Dudic

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