Acoustic Levitation Works On Small Animals
20mm wavelength seems to be 17KHz (at sealevel in air), which isn't very "ultra" sound. To levitate the 3 meters radius of adult humans (with extended arms/legs), we'd need 6m wavelength.Which is about 57Hz.
How much power...well lots. Area of human (one side) about 1sq. m. mass, (order of magnitude) 100Kg, say 1KN force required = 1Megapascal.
That's 10bar pressure, implying an acoustic pressure of 10dB above atmospheric..or 203dB into 4pi space.
Imagine you had a 'regular' hifi speaker, radiating into half space..and that it behaved in a perfectly linear manner to power input (no chance!) at the typical 1% efficiency of electrical-to-acoustic conversion, you' need to hit it with, oh, about a half megawatt. Really. Which means poor sorry-assed-but-levitated-human falls apart.
(assumptions: 87dB/watt for reasonable speaker; +3dB gain for half-space; power required = 10^ [(200-87)/20] )