
Objects larger than the wavelength of sound in an acoustic tractor beam - dnetesn
https://phys.org/news/2018-01-world-powerful-acoustic-tractor-pave.html
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lawlessone
Do lifted objects exert a force back on the hardware producing the sounds?

What happens if we aim these downwards and put a board on top? :-)

[https://www.youtube.com/watch?v=z1ZdMOMUgXE](https://www.youtube.com/watch?v=z1ZdMOMUgXE)

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anfractuosity
I wondered about that too. The professor at Bristol has the following tweet -
[https://twitter.com/sonic_bruce/status/669560944234774528](https://twitter.com/sonic_bruce/status/669560944234774528)

It says you need ~7.6kW of sound power to levitate a human.

That's sound power, so I'm not sure what electrical power you'd need too, I'm
not sure how efficient the transducers are.

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mschuster91
Hmm. 7.6 kW for a human of something around 76 kg (makes calculating easier,
reality should be 80-85). That means you'd need about 100W/kg... or, to lift a
car of 1t, 100 kW. Should be do-able with a modern biturbo engine and by
cutting some weight.

Am I the only one thinking of a Star Wars repulsorlift?

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javiramos
This project reminded me of Pixie Dust, a 2014 Siggraph project that produced
a screen out of suspended particles.

[https://youtu.be/NLgD3EtxwdY](https://youtu.be/NLgD3EtxwdY)

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vidanay
Wouldn't it be more accurate to call this an acoustic suspension beam? Calling
it a tractor beam implies that it can be mounted on the ceiling and pull an
object off the floor. Hence the name (at)tractor beam.

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nannal
Not named after farm tractors then which I'd always assumed had gained their
name from the term traction engine.

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navaati
Well yes and no, traction is the act of "tirer" in French, that is the act of
pulling. So a farm tractor is named as such because it pulls some (usually
plowing) equipment behind it, and a tractor beam because it pulls some stuff
at a distance.

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cyberferret
Also reminds me of an interesting lesson we had in flying school, where our
CFI was explaining that the "propeller" on an airplane should really be called
a "tractor" because it is technically pulling the aircraft along, whereas an
actual "propeller" would push the aircraft along (much as a ship's propeller
does at the stern).

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janekm
A propeller propels air behind it. The traction is a side effect by Newton’s
third law.

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jobigoud
Can this tractor beam approach be used to gently push a finger in mid-air? Or
create the illusion of a solid object where there's none? I'm wondering about
applications to virtual reality.

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yorwba
There's this company using ultrasound for touchless haptic feedback:
[https://www.ultrahaptics.com/](https://www.ultrahaptics.com/)

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j_s
This is awesome! Any further info / alternatives / etc. that you have time to
provide would be greatly appreciated.

found:
[https://news.ycombinator.com/item?id=11025820](https://news.ycombinator.com/item?id=11025820)

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randomdrake
Study: Acoustic Virtual Vortices with Tunable Orbital Angular Momentum for
Trapping of Mie Particles

Citation: Asier Marzo, Mihai Caleap, Bruce W. Drinkwater. Phys. Rev. Lett.
120, 044301 – Published 22 January 2018.

Link:
[https://doi.org/10.1103/PhysRevLett.120.044301](https://doi.org/10.1103/PhysRevLett.120.044301)

DOI: 10.1103/PhysRevLett.120.044301

Abstract: Acoustic vortices can transfer angular momentum and trap particles.
Here, we show that particles trapped in airborne acoustic vortices orbit at
high speeds, leading to dynamic instability and ejection. We demonstrate
stable trapping inside acoustic vortices by generating sequences of short-
pulsed vortices of equal helicity but opposite chirality. This produces a
“virtual vortex” with an orbital angular momentum that can be tuned
independently of the trapping force. We use this method to adjust the
rotational speed of particles inside a vortex beam and, for the first time,
create three-dimensional acoustics traps for particles of wavelength order
(i.e., Mie particles).

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saagarjha
What's the "loudness" of speakers like these? I see that they're ultrasound,
so we wouldn't be able to hear them anyways, but are they something a human
can safely be around without suffering hearing loss?

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rubyfan
Would this work in space?

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cyberferret
I don't think it would without some sort of medium to suspend the end target
in. I am presuming that the articles in the video 'hover' there because they
are caught in an air pressure pocket where multiple opposing waves essentially
phase cancel each other out.

In space, it would mean no air, therefore no medium in which to suspend the
target.

I do wonder about suspending nano particles using light waves though...
conceivably that might work in space?!?

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TFortunato
Not quite what your talking about, but yes we can manipulate small (very!)
objects today with light, using a technique / setup known as "Optical
Tweezers"

[https://blocklab.stanford.edu/optical_tweezers.html](https://blocklab.stanford.edu/optical_tweezers.html)

