Imagine a giant tank that can roll into town emitting literally no sound. This could be the future of stealth warfare. Scientists have recently devised a "sonic black hole;" any sound that passes its edge can never come out again.
The Technion in Israel developed this "black hole," creating a well into which air flows faster than the speed of sound. That means that any sounds trying to pass through this moving air just can't keep up with the flow of the air into the "hole." Any sound wave in this air is like someone trying to run up the down escalator. But in this case, the down escalator is four times faster than the person can run.
This is achieved by two clouds of atoms (called a Bose-Einstein condensate) cooled to almost absolute zero, with a pool of very low density between them. Atoms can flow very rapidly, pretty much unhindered, into this area of low density at speeds over four times the speed of sound.
The result is a well into which all sound falls and cannot escape. Any sound that passes close enough to the "black hole" essentially ceases to be.
This sonic black hole offers scientists a method for testing their theories about black holes in general. But imagine putting one of these in your car instead of a muffler, or in a rocket. A battalion of armed soldiers equipped with a few devices like this could march anywhere silently. These sonic devices wouldn't just dampen the sound; they would virtually obliterate it. It raises the old question: if a tree falls on the Bose-Einstein condensate sonic black hole, does it make a sound?