Hello, can anyone clarify what's the principle (or physics law) according to which small objects are kept suspended in air in acoustic levitation devices? I understood that the device essentially creates an acoustic standing wave inside a container. This means that, in some specific regions of the container air remains "compressed" while in other regions it remains "rarefied". Such regions are located in correspondence of nodes/antinodes of the standing wave. So far, so good. Now what should I do if wanted to compute the force exerted upon a small object placed inside the container? Actually, what IS the force that is supposed to lift the object upwards? What is this force due to? Thanks.