Acoustic “cloaking device”
Back in 2008, a hypothetical cloaking device for sound waves was proposed, and an initial prototype has apparently been built. It uses a similar principle to “cloaking devices” that work in the visible-light spectrum – yes, some scientists are working on something like Harry Potter’s Invisibility Cloak.
The problem is that most of them require materials that don’t exist in nature, known as metamaterials. Some of these have properties such as negative indices of refraction, which quite obviously don’t occur in nature. Here is a picture of what THAT would look like. Another problem is that it would have to take ALL frequencies of visible light at ALL angles into account, which just adds more potential problems. And on top of that, I bet you’d still be visible in the infra-red part of the electromagnetic spectrum.
So I’m not expecting this to come onto the market for a very long time – I’d say at least 50 years.