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This is a bit of a continuation of my previous question, could a form of acoustic levitation be used as artificial gravity?

In that question, I referenced how some scientists use acoustic levitation to suspend substances in the air, and wondered if that method could act as artificial gravity by pushing down on people. However, the answer I got was that the ultrasonic pressure would simply push down on exposed areas, not effect the person's entire mass like gravity does.

But that got me thinking: since they confirmed that ultrasonic frequencies push down on objects, would it be possible for the speakers producing the sound to push off the ground using ultrasonic waves, propelling the speaker (and the object it's connected to) into the air?

To specify, could this method be used to create a vehicle that levitates off of the ground?

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  • $\begingroup$ You should probably be searching for the energy density required to levitate a given mass. Currently the max size levitated is some few grams. Possibly with 100's of kg lifted you would need an energy density that was impractical. $\endgroup$
    – puppetsock
    Commented Jan 7, 2020 at 18:12
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    $\begingroup$ Sound is a pressure wave. An explosion creates a pressure wave strong enough to throw something heavy away like a car. To continuously levitate it through that sound you need to continuously create an explosion below it strong enough to keep it airborne. I dont know what is a better question: how do you generate a continuous explosion capable of lifting the average car? Or the question of how do you keep the car and occupants in one piece? Ignoring the fact that using the explosion directly as exhaust would be much more efficient. $\endgroup$
    – Demigan
    Commented Jan 7, 2020 at 18:43
  • $\begingroup$ @Demigan “how do you generate a continuous explosion capable of lifting the average car?” That’s simple. You build a rocket engine. $\endgroup$
    – nick012000
    Commented Jan 9, 2020 at 7:34
  • $\begingroup$ @nick012000 That would be using the explosion directly as exhaust (like I said) rather than the shockwave of the explosion carrying the ship, which would mean you have to somehow throw the explosive underneath you where it explodes and creates a continuous shockwave to keep the car aloft. $\endgroup$
    – Demigan
    Commented Jan 9, 2020 at 9:18
  • $\begingroup$ @Demigan So, an Orion drive-style pulse rocket, then. $\endgroup$
    – nick012000
    Commented Jan 9, 2020 at 21:26

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I think the answer is no, and maybe.

! atm exerts 101 $\frac{kN}{m^2}$

Since sound is a time-varying pressure value about a mean value -- let's assume 1 atm -- the rising pressure would exert an equal force on the ground as on the vehicle by displacing the air.

To get enough force to lift an object you need to counteract gravity and then some. As @Slarty observed that would require a huge impulse to achieve. But, it would also displace the air, leaving (we'll assume for sake of simplicity and illustration of the argument) 0 atm or a vacuum beneath the vehicle. Atmospheric pressure would push the vehicle back down and the sonic transducer wouldn't have any air to create the next impulse until the vacuum dissipated.

The sonic impulse would generate such a force that the air would accelerate to speeds faster than the speed of sound -- you are talking tremendous power here. But the filling rate of the vacuum can only proceed at approximately an average of the speed of sound. This because the speed of sound in gases medium is geometrically related to the average distance a molecule can travel before colliding with another and its average velocity due to temperature and pressure.

So if your vehicle (transducer, power source, payload, etc) weighed 1 kg. You'd need to generate an average positive 10 N force to keep it aloft. (rounding to avoid math) Assume the pressure under the vehicle varied between 1 atm 0.9 atm, you have a 10 $\frac{kN}{m^2} force pushing you down on the negative or off cycles of the sonic impulse. You can't really get the values close to equal unless you make the area microscopic or the mass very tiny.

But, if you did have the power to do it. YOu could easily use it to heat the air like a ramjet and give yourself a continuous lift.

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Yes and no. Sound theoretically could be used to levitate a car, as ultimately sound consists of pressure waves in the air, but the energy needed would mean that extreme sound levels would have to be used. So extreme that anyone riding the car would be immediately deafened. In fact so extreme that their bodies and that of the car would probably be disrupted by the violent sound shockwaves.

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Only inthe same way that you could levitate with an explosion -- the problem is the sheer power levels involved. The air pressure would certainly blow out eardrums, and probably do plenty of other damage was well. And the dogs for miles around would be very unhappy!

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