Single-transducer acoustic levitator with equidistant vertical positioning capability

We present an acoustic levitator experiment in air, in which objects can be vertically positioned to several equidistant locations. The setup consists of only one single-element transducer and one reflector. The main objective of this work was to investigate whether it is possible to move and position an object vertically in such a setup with only one narrowband single-element transducer and only one reflector, without moving the transducer or reflector at all. The key idea is that we exploit gravity to let the object fall from one pressure nodal node to one at a lower position. This is achieved by amplitude modulation of the transducers driving signal with varying gap times in order to match the free-fall time of the levitated object from one nodal plane to one of the nodal planes below. The levitator is able to generate sound pressure levels of up to 163 dB SPL, and, thus, capable of levitating lightweight objects over longer distances of several centimeters and high density samples such as water drops, glass or steel spheres over shorter distances. Our results prove that the levitated objects can be precisely and reproducibly transported from a higher pressure nodal plane to the next lower nodal plane position.