Acoustic cloak silences nuisance noise

Here is an discovery we wish we knew about a few years back. There is a Cosi coffee shop near the HS Daily Wire’s office in the DuPont Circle neighborhood of Washington, D.C. Before it became Cosi, it was called XandO — and then, as now, it was, during the morning hours, home to writers, graduate students, and acadmeics sipping coffee while reading their books and working on their laptops. Trouble was, the staff and servers used to turn up the volume of the background music to such deafening levels that many of the regular patrons decided to do something about it. A University of Maryland’s philosphy professor — we will not use his name here, but only say that he has written on (ir)rationality, intentionality, and qualitative experience, arguing that a computational/representational theory of mind seems to present a promising way of dealing with the first two issues, and even a weakened version of the third — took the lead in the effort to restore a measure of calm to XandO, and organized the regulars in a group he called the ALF (for the Acoustic Liberation Front). The management realized they had a real insurgency on their hands, and the staff was told to keep the volume of the music down to allow patrons to read and write in peace.

ALF members no doubt wished they had access to an acoustic cloak. Researchers in Spain have proven that metamaterials, materials defined by their unusual man-made cellular structure, can be designed to produce an acoustic cloak — a cloak that can make objects impervious to sound waves, literally diverting sound waves around an object. The research, “Acoustic Cloaking in Two Dimensions: A Feasible Approach,” was published two weeks ago in the New Journal of Physics (NJP), builds on recent theoretical research which has sought ways to produce materials that can hide objects from sound, sight and X-rays (we have wrtiten several stories in HS Daily Wire about invisibility cloaks, but this is the first story about acoustic cloaks). Daniel Torrent and José Sánchez-Dehesa from the Wave Phenomena Group, Department of Electronics Engineering at the Polytechnic University of Valencia, cite theoretical work published early last year in NJP by researchers from Duke University in North Carolina as the starting point for their more practical approach. To realize the cloak physically, the Spanish research team calculated how metamaterials constructed with sonic crystals, solid cylinders in a periodic array that can scatter sound waves, could be used in a multilayered structure to divert sound completely around an object. The researchers performed multiple simulations to test their theory. They investigated the optimum number of layers required completely to divert sound and how thin the materials could be made to maintain their use but also ensure that they are easy to implement. Results were very encouraging, showing that optimum cloaking requires approximately 200 layers of the metamaterial but that there is scope for much thinner materials to be used than technology can currently produce. Sánchez-Dehesa, one of the lead researchers, writes, “We hope that this proposal will motivate future experimental work demonstrating the materials’ performance.”

One of the first uses of the material is likely to be warships, hoping to avoid sonar radars which pick up on the noise that ships emit, but if developments continue apace it could be used in concert halls to direct noise away from problem spots or even as a way to deal with noisy neighbours.

- read more in Daniel Torrent and José Sánchez-Dehesa, “Acoustic Cloaking in Two Dimensions: A Feasible Approach,” New Journal of Physics 10 (13 June 2008) (doi:10.1088/1367-2630/10/6/063015)