PHYSICSSection 4.10.45 · Building 10 · ELUSK · College X · Engineering · ELUSKStanding-wave nodes lift small objects against gravity. Frequency, wavelength, pressure interference made visible.
🎈 Physical Sciences · College X · ELUSK
Acoustic Levitation
Standing-wave nodes lift small objects against gravity — frequency, wavelength, and pressure interference made visible.
tuning — drag the reflector—
Ultrasonic Standing Waveno lock
Timebase
1.0×
Gain
0.70
Intensity
0.70
The emitter below and the reflector above trap a standing wave of
sound in the gap. Where the wave is still — the pressure nodes —
the beads can rest, held up against gravity. The scope shows the same wave
as a line of light.
△ up–down: open▷ side–side: opencross-lock: no
Four emitters now — top & bottom drive the up–down wave, left & right drive the
side–side wave. The beads settle where both waves are still at once — the grid
intersections. But one frequency drives all four, so you have to find the cross frequency
that makes both gaps resonate together. Square box — easy. Rectangular — a hunt.
The Experiment
Holding a thing with sound
Sound is a wave of pressure moving through the air. Aim a strong enough
wave at a hard surface and it bounces straight back. The wave going up and
the wave coming down overlap — and where they meet just right, they
lock into a standing wave: a pattern that no longer travels, with
fixed points that heave and fixed points that stay perfectly still.
The still points are the nodes. At a node, the air is calm, but
just above and below it the pressure pushes inward. A small, light object
— a fleck of Styrofoam — dropped into that pocket gets squeezed
from every side and held there, floating, against gravity. This is acoustic
levitation. Over the last century physicists turned it into a real tool;
it is now used to hold droplets and samples in mid-air with nothing
touching them at all.
Why the gap has to be tuned
A standing wave only forms if the round trip fits. The gap between emitter
and reflector must be a whole number of half-wavelengths. Hit that, and the
wave locks; the beads rise. Miss it, and the reflected wave fights the
outgoing one — the pattern drifts, and the beads fall. In this lab you
tune it by hand: drag the reflector and watch for the lock.
The green line
The scope is an oscilloscope — an instrument built around a
tube that paints a moving signal as a glowing line. It is the electrical
cousin of Chladni's sand: a way to take a wave you cannot see and draw it
where you can. On the trace, the nodes are the points that never move.
Why It Matters
The same wave, everywhere
A standing wave is not a laboratory trick. Once you can see one, you start
noticing them everywhere.
Your microwave oven. The waves bounce inside the metal
box and form a standing wave. The hot spots are antinodes; the cold spots
are nodes. That is why the turntable spins — to drag the food
through both.
Every instrument. A guitar string, an organ pipe, a
flute — each one is a standing wave tuned to a length. The note you
hear is which wave fits.
Containerless processing. Acoustic levitation lets
chemists and materials scientists study a droplet or a crystal with no
container touching it — no walls to contaminate the sample. Useful
for pharmaceuticals and for growing delicate materials.
Tuning a space. Concert halls and recording studios are
shaped to control where the standing waves land, so the room does not
boom at certain notes.
And the oscilloscope is in every electronics bench on Earth, for the same
reason Chladni scattered sand: you cannot fix a wave you cannot see.
About This Lab
Acoustic Levitation Lab — v0.6
A browser physics lab pairing an ultrasonic levitation chamber with an
oscilloscope read-out. Tune the gap, find the lock, and watch the beads
rise into the nodes of a standing wave.
The model — what is real
The wavelength is computed honestly: λ = c / f, with the speed of
sound c = 343 m/s. Pressure nodes sit half a wavelength apart. The gap is
"tuned" when it equals a whole number of half-wavelengths — the real
boundary condition for a standing wave between two hard surfaces. Raise the
frequency and the wavelength shrinks, so more nodes fit in the same gap and
more beads can be held. The four drive frequencies (25–40 kHz)
are in the real range used by ultrasonic levitators.
The honest simplifications
The detuned state is modeled as a blend of a standing wave and a
travelling wave — enough to make the scope drift and the beads fall,
which is the true qualitative behavior, but simpler than the full
acoustics.
Beads are shown resting exactly at the pressure nodes. Real beads sit
a hair below the node, where the upward push of the wave exactly balances
their weight.
The scope's swing is slowed to a visible speed. A real 25 kHz wave
oscillates far too fast for the eye — the animation is illustrative.
The scope knobs — timebase, gain, intensity — change how the
wave is displayed, not the wave itself. That is exactly what a
real oscilloscope's knobs do.
What is NOT in v0.6
No variable bead size or mass; no bead-to-bead interaction.
No real audio output.
Lab 2 of the OPA Browser Physics Suite · ELUSK College of Engineering.
Sibling to the Chladni Plate Lab. Built by Travis Jenkins / User Zero.
Educational use. No tracking, no backend, no data stored.
A note from the builder
Single developer. About to be a 49‑year‑old father and solo builder.
If you see something wrong, please let me know — I’ll change it as soon as I can. I’ll credit you if you want the credit.
Some labs are better than others; a few have obvious broken bits I’m still working through. Some were built early, some late — they don’t all look the same, but they share a feel.