40KHz xducer, I finally Get It

I guess if you have a surface that's vibrating hard enough, air can be compressed more on one sweep than on the other, since air can't go below -1 atm. Sort of a rectifier.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Exactly that. This also explains the "sound laser" or "hypersonic loudspeaker," the Woody Norris LRAD, where AM or DSB-modulated ultrasound will spontaneously transduce itself into the audible spectrum. Air itself is the detector diode? That means we can bounce a small air-jet off a mirror surface, by bouncing the ultrasound.

I wonder how much of "sonic levitation" is from this fluid-streaming effect, rather than exotic "acoustic pressure." When underwater, a transducer can actually provide significant propulsion, via the water jet coming off the vibrating surface. I've seen a patent for a "stomach camera" micro-drone that free-swims in a water-filled gut, propelled by tiny sonic-streaming transducers. Build a very confusing RC boat with no visible props or even jet orifices, with ultrasound propulsion through flexible solid surfaces.

Anyway, if you have any of those 40KHz transducers ($1 cheap from various surplus mailorder,) just poke at styrofoam to get some low-mass frag- ments. Then blow them around on the desk. Also: electrostatics! The styro-bits will easily roll around at first, but then they charge up and start attracting to the table. Same with aluminum surface. Maybe with extremely clean pcb copper they'll keep rolling under acoustic power. That, or extremely damp weather. (Also, I cheated by using the extremely spherical 4mm beads from a 70s-era beanbag chair.)

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((((((((((((((((((((((( (  (    (o)    )  ) ))))))))))))))))))))))) 
William J. Beaty              Research Engineer 
beaty a chem washington edu   UW Chem Dept,  Bagley Hall RM74 
billb a eskimo com            Box 351700, Seattle, WA 98195-1700 
ph X3-6195                    http://staff.washington.edu/wbeaty/

20 years ago I worked manufacturing an ultrasonic transducer and amplifier. One of our demos was putting the transducer in an aquarium face up with about 5" of water above it. Then pulsing the transducer with ultrasound. We could get about a 6" plume of mist/water above the water. The piezo was a 2" ceramic on a 4" faceplate. The amp pulsed it at 660kHz. We had two models 250Watt and 1000watt. Plume pictures, > Should have been better pictures, but the best I got.

Mikek

Related:

It's not always easy to experience or appreciate, but waves do indeed carry momentum. A wave is the medium moving out of the way of something, after all! With the right phase and polarization (give or take the multiple modes and dispersion that acoustic waves support), the momentum can be forwards or backwards, or rotating!

I don't know if nonlinear effects (jetting, rarefaction) apply to the first example, but they aren't necessary to explain the observation. And also, they surely do occur at some intensity, which probably helps even more!

Tim

-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website:

Stick your little ultrasound transducer on a sig generator, and at 10Vrms or so, it starts to create detectable "levitation" forces. Feeble though.

But instead, sprinkle tiny styrofoam beads on the bench, and fire the sound beam sideways. Wind! You can cause the fragments to scatter. It's clearly not just radiation pressure; the beam is moving the air as well. (Same as water jets created by ultrasonic humidifiers.)

((((((((((((((((((((((( ( ( (o) ) ) ))))))))))))))))))))))) William J. Beaty Research Engineer beaty a chem washington edu UW Chem Dept, Bagley Hall RM74 billb a eskimo com Box 351700, Seattle, WA 98195-1700 ph X3-6195

So could we focus ultrasound and make a spot of air glow, like the soniluminescence thing in water? Cavitate air?

Probably not. Bummer. Maybe some less dramatic optical effects.

Perpendicular sound beams could mix and scatter and make sum and product frequencies.

There are no longitudinal (compression) electromagnetic waves, so no equivalent rectification of light, but aren't gravitation waves longitudinal?

Nothing worse than a million charged styrofoam bits all over the room. The only cure for that is to burn the building down.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

There's an equivalent gravitomagnetic effect, which is (IIRC) a 3+1D equivalent (albeit possibly a low-rate approximation?) of the 4D (spacetime) warping. (This explains why g-waves can travel at the speed of light without orbits being unstable.)

(A reminder that, in Einstein tensor notation, Maxwell's equations are practically trivial, a single expression. So a lot of functionality is encoded in that format.)

G-waves are also quadrupole or higher, although I don't know if that has any relevance for apparent direction of the field.

L-waves can't be polarized, right? G-waves are, AFAIK. That would be a problem.

I'm not aware of any linearity quirks in g-waves until relativistic levels, i.e., until you're in the Planck energy scale, or orbiting a small black hole, everything works the same. Same with E&M, except the energy levels are rather more easily accessible thanks to QM (pair production and such, on the scale of ~1 MeV).

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
Website: https://www.seventransistorlabs.com/

Or maybe yes, because "bubble resonance" is a real effect in fluids. Is there an air analog? Maybe a gas-cavitation pocket would slam shut and then bounce repeatedly (so try driving it at resonance for better efficiency.)

I have an old 1920s book on "supersonics" which shows it.

They photographed a little silvery donut shape hanging in space in front of a compressor-powered ultrasonic whistle. It's a cavitation in air, and reflects light just like a thermal mirage, when viewed at grazing angle.

I've often wondered if the giant subsonic meter-scale version might exist, using an empty room as a 100Hz resonator. Try to pass your hand through it, and find a back-pressure, an invisible wall? (While maybe your ears bleed, and also your lungs cave in.)

A PVdF piezo-membrane, if bent, makes a good audio transducer. If slathered on density-neutralized mylar balloon, we might propel the balloon by ultrasonic gas-streaming, by driving a section of the membrane at 100KHz or so. Sell it to NASA (who had plans for using little fans to propel their zero-gee drones around the ISS in free fall.)

I have a cubic meter of 2mm styrofoam spheres. Once they get loose on wood floors, you can see all the room air motions and traveling vorticities (and 40KHz ultrasound beams if more than a few tens of mW.) Anybody want a sample?

PS

Goldmine is selling at $1 until midnight:

This so-called "receiver" is really just a standard transceiver, but slightly de-tuned from 40KHz, for use as security prox sensors when paired with a "transmitter" version. Use as rocket engines for propelling very tiny low-mass vehicles?

--
((((((((((((((((((((((( (  (    (o)    )  ) ))))))))))))))))))))))) 
William J. Beaty              Research Engineer 
beaty a chem washington edu   UW Chem Dept,  Bagley Hall RM74 
billb a eskimo com            Box 351700, Seattle, WA 98195-1700 
ph X3-6195                    http://staff.washington.edu/wbeaty/

I just wanted to thank you for existing and starting threads like this one.

You've been talking mostly about micropower uses, but the topic reminded me of the SciAm article about sticking large artificial quartz crystals into microwave cavities to transduce microwaves into high-power "kilomegacycle" ultrasound beams in air.

So maybe those lab-scale "levitation" tricks can actually be scaled up to picking up cars and stuff?

Mark L. Fergerson

CL Stong will never die! :)

Coooool! Parametric microwave audio amplifiers. In an old 70s-era physics dictionary, they called this spectrum "hypersound" rather than "ultrasound." At some point the wavelength comes close to lattice spacing of solid objects, and the speed of sound slows down to the speed of heat conduction.

Besides the "wind," wouldn't the nonlinearity cause an acoustic standing wave to have slightly higher net pressure than ambient? A "sound pillow" trapped between floor and object then lifts the object.

Also, ultrasound in water, at optical wavelengths, can form off-axis holograms on the water surface, producing images of submerged objects (such as animals with invisible flesh, moving skeletons.) Need laser ref beam, also acoustic ref beam.

((((((((((((((((((((((( ( ( (o) ) ) ))))))))))))))))))))))) William J. Beaty Research Engineer wbeaty a uw edu UW Chem Dept, Bagley Hall RM74 billb a eskimo com Box 351700, Seattle, WA 98195-1700 ph X3-6195

At 40 kHz, wavelength in air is 8mm or so; a standing wave might easily have higher air temperature at antinodes than at nodes. Even light solids might dissipate energy as heat.

So, maybe your 2mm beads can become hot-air bouyant?