They can cloak objects from sound.

Scientists' recent efforts to render objects invisible to the eye are based on the fact that our perception of the world depends on the scattering of waves. We can see objects because waves of light strike them and scatter. Similarly, the Navy can detect faraway submarines because they scatter sound waves (sonar) that hit them.

So for the last several years scientists have been developing cloaks that prevent scattering by steering light or sound waves around an object.