Objects larger than the wavelength of sound in an acoustic tractor beam | Hacker News Reader

Objects larger than the wavelength of sound in an acoustic tractor beam

phys.org

103 pointsdnetesn34 comments

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

Better yet, can we make a platform that will levitate and hold whatever size/shape object it wants. The platform would then reflect all the waves back in a direction, away from the source, in a safe manner. I wonder how much force RF waves can really give? How high above 40Khz can we go?

RF? I thought we were talking about sound here...

I wondered about that too. The professor at Bristol has the following tweet - 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.

Be a very amusing way to trap a person. They can wave around all they want, but the tractor beam has them stuck, levitating. Maybe if they had a backpack and threw it sideways very hard?

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?

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

https://youtu.be/NLgD3EtxwdY

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.

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

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.

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).

Sometimes talking about cars someone brings that up the other way around - rear wheels 'propel', the commonly used 'traction' is about the front wheels pulling the car ...

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

From Latin: trahere, past participle tractus. Drag, haul.

Whenever you see "tractor beam" in this context, it most likely refers to "tractor beams" from Star Trek, which are all about pulling stuff towards you.

Besides the farm, also in airplanes. If the propeller is at the front, it's a tractor. If it's at the back, it's a pusher.

I think the idea here is that suspension is easy (fans are enough for that), and the tricky part is pulling from "backwards", which this system does as indicated by the last few seconds of video.

I've only read the posted article, but it seems theoretically possible to move objects, perhaps needing more speakers. For that, shift to a different modulation that sets the trap slightly to the side, at each small step. Analogous to a stepper motor.

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.

There's this company using ultrasound for touchless haptic feedback: https://www.ultrahaptics.com/

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

But if somebody stands between the transmitter and the target, the "object" would disappear. I'm guessing this would make the UX pretty crappy.

Also, what if the tractor beam inadvertently attaches to your eardrum? (Ok, headphones could solve that)

The game could place things that require touch in certain parts of the room (or orient you in that manner)

(Hmm sounds like something that might get patented. Can this be prior art and I donate to you guys?)

Another option would be to prevent another person to stand in the beam. This could be achieved using very strong haptic feedback from another transmitter :)

Or spread multiple transmitters around the room (or even attached to the person's suit) to (try to) prevent blockage. That could work but there are loopholes and thus it's both expensive and not theoretically satisfying.

Yes. Here is a paper from some Japanese researchers in 2015: http://www.hapis.k.u-tokyo.ac.jp/public/hiroyuki_shinoda/res...

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

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).

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?

Would this work in space?

Don't see why not as long as you don't try to use it outside of an environment with an atmosphere.

So, no, it wouldn't work in space.

Mars is in space. Venus is in space. And, I suppose, the Earth is in space, too. :)

But if we're referring to the near-vacuum of space, it's still possible although the forces involved are incredibly small. There is SOME gas/plasma in space, and certain kinds of sound waves with really long wavelengths (called ion acoustic waves) do, indeed, travel through this thin medium. Unlike sound waves traveling in a neutral gas, ion acoustic waves, because they're traveling through a plasma, can also interact electromagnetically.

There's a whole bunch of different waves like this that can travel through the plasma in space a lot like sound waves through the air: https://en.wikipedia.org/wiki/Waves_in_plasmas

...so I imagine that something like this could work under certain conditions (i.e very large and very low-mass objects).

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?!?

But would it work on a spacecraft? (Without gravity?)

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

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