Sound can be focused like light
With your head in the focal point of the lens you can hear a loud high-pitched sound. Take a step to the side and the sound gets much quieter or even disapears.
Details and Theory
A zone plate is a device used for focussing, but unlike lenses or curved mirrors zone plates use diffraction instead of refraction or reflection. Therefore this principle can be extended to anything that exhibits a wave character, also sound waves. Zone plates consist of a set of radially symmetric rings, known as Fresnel zones, which alternate between opaque and transparent. A wave hitting the zone plate will diffract around the opaque zones. The zones can be spaced so, that the diffracted sound constructively interferes at the desired focus point. If the zone plate is constructed that the opacity varies in a gradual, sinusoidal manner, the resulting diffraction causes only a single focal point. In our case this was realized by adding a sub-structure to the zone plate, with features that are smaller than the wavelength of the sound wave.
Our Lens focusses a single tone onto a single point in space. Keep your head in the right spot to hear the sound. If you move a few centimetres away from this point, the volume decreases drastically.