Two opposing ultrasonic transducers set up a real standing wave between an emitter and a rigid reflector, exactly as in a lab acoustic levitator (e.g. a TinyLev-style device). The cavity length is tuned to a whole number of half-wavelengths so the wave resonates, with pressure antinodes forced at both rigid boundaries:
λ = c / f (c = 343 m/s, speed of sound in air)
p(y,t) = 2·A·cos(k·y)·cos(ω·t), k = 2π/λ, ω = 2πf
pressure nodes at y = (m+½)·(λ/2) — velocity antinodes
A small object placed in the field is denser and far less compressible than the surrounding air, so the acoustic radiation force (the Gor'kov mechanism used in real ultrasonic levitators) pushes it toward the nearest pressure node, not the antinode. Near a node this restoring force is well approximated by a periodic potential well:
F(y) = K·sin(2·k·y), K ∝ A²·V·k (V = object volume)
m·dv/dt = F(y) − m·g − b·v b ∝ damping · radius (Stokes-like drag)
The proportionality constant K is an illustrative scale factor — real Gor'kov coefficients depend on the density/compressibility contrast between object and air — but it preserves the correct physical scaling: trap stiffness grows with the square of drive amplitude and with object volume. That's why dropping the amplitude slider far enough makes the maximum trap force fall below the object's weight: it simply can't be held any more and falls through the cavity, just as a real levitator loses its payload if driven too weakly.
- Frequency — sets λ = c/f directly; node spacing (λ/2) shrinks as frequency rises, visibly compressing the standing-wave rings.
- Cavity length n — number of half-wavelengths between emitter and reflector; more cells means more available trap nodes along the axis.
- Damping — a Stokes-like linear air-drag coefficient (scales with object radius) that determines how quickly a displaced object settles back into its node.
- Drag the skull along the vertical axis to displace it from its node — release it and watch the radiation force pull it back, oscillating and settling under drag, or overshooting to the next node if the trap is weak relative to the push.