Sound cloaks enter the third dimension | Matter & Energy | Science News: ... the researchers came up with a design made up of 60 rings of various sizes that form a cagelike structure around the sphere. Simulations indicated that sound waves scattering off the sphere and the ringed cloak would interfere with each other and cancel out.
Because the cloak did not need to steer sound waves in complicated ways, Sánchez-Dehesa and his team built it out of plastic with the help of a 3-D printer. They hung their creation from the ceiling of an echo-free chamber, pointed a speaker at it and played a range of sound frequencies. For most frequencies, the sphere scattered an easily detectable amount of sound. But at 8.55 kilohertz — an audible high pitch — the cloaked sphere became imperceptible to the sensors behind it.