Acoustic Cloaking

AndrewNorrisAcoustic Cloaking? Is this the sci-fi section?  Perhaps it is, but the maths of physics says we can hide sound behind a cloak of invisibility, at least theoretically.  Professor Andrew Norris, from Rutgers University created more than a ripple of interest around 2008, when he revealed his theoretical ideas.

Here is the introduction to one of his papers.

It might drive bats batty, but there is no fundamental physical limitation on developing acoustic cloaking devices.

Introduction

An acoustic cloak is a shell surrounding an object so that sound incident from any direction passes through and around the cloak, making the cloak and the object acoustically “invisible.” We do not experience acoustic cloaking because the materials required are exotic and, as far as we know, not found in nature. Yet there is no fundamental physical restriction on acoustic cloaking. Implementation is a matter of developing metamaterials with very unusual properties. Acoustic cloaking is in fact more likely to be achieved before its electromagnetic (EM) counterpart. The reason is that the cloaking material must have structure on sub-wavelength scales. Specific examples will be explained below. Acoustic wavelengths are typicallyorders of magnitude larger than optical wavelengths, meters vs. microns, which makes the acoustic problem easier, in principle.


LINKS
Andrew Norris website
Acoustic Cloaking article


Image from Rutgers University