# Has anyone explored using sound as a heat shield?

I edited original post to better reflect question. I was looking around to see if anyone had explored the idea of using (Ultrasonic or Hypersonic)sound to deflect heat from a ship. Specifically sound that is directed like a laser at a point about 4 meters ahead of the ship on re-entry. This could maybe create a plasma like shield that would create a cooler boundary layer between the ships body and the heat of re-entry.

• There are some inherent difficulties in projecting a focused, coherent beam of sound ahead of a vehicle moving at 25 times the speed of sound. Some great, big, stonking, enormous difficulties. Starting with the need to make the sound move faster than the sound moves. Oct 22, 2021 at 1:00
• There already is an atmosphere-based compression wave ahead of capsules during reentry, and there are still massive temperatures involved - is this what you are talking about? en.m.wikipedia.org/wiki/Atmospheric_entry
– Moo
Oct 22, 2021 at 1:23
• @Mikael Long I don't understand how sound wave acts as a protective shield? Can you post down the link of paper where laser beam is used? Oct 22, 2021 at 2:24
• I think your idea is a non-starter, powerful sound waves compress air, raising its temperature which is exactly what you are trying to avoid.
– GdD
Oct 22, 2021 at 7:56
• youtube.com/watch?v=aBdVfUnS-pM (this video uses ultrasonic speakers to make sound which can be focused to a specific point. I am hypostasizing that if you could focus a sound wave far enough in front of the incoming craft that colder air would flow between the ship and the pressure plasma wave in front it would still be hot but maybe 1200 degrees enough that Stainless steel would not suffer. I have not done any math or even contemplated the extent of power you would need to produce a sufficient sound wave. but it is a compressed pressure wave that I would be trying to create. Oct 23, 2021 at 21:42

# TL;DR: No, but sortof yes

It's hard to prove a negative, but as PcMan's comment observes, this isn't worth pursuing since it falls apart on physical first principles.

The first, and obvious, problem is that "sound" is a compression wave. Gas molecules bump into other molecules, causing the wave to propagate. The informal "power" or "loudness" is how hard they do that. The wave propagates at a particular rate (the "speed of sound"), which varies with temperature/pressure/etc. but is quantifiable. In particular, if the air molecules are moving, in bulk, faster than this value, then the bulk motion of the air will carry the whole sound wave backward faster than the wave makes forward progress within the air itself.

This situation is fundamentally the same as what happens on re-entry: the spacecraft travels through the atmosphere much faster than the speed of sound. You could make a sound at the spacecraft's surface, but that sound wave, no matter how powerful, will be hit by the spacecraft itself before it gets anywhere, just because the spacecraft is traveling faster than the sound.

The second problem is that sound waves don't actually move / deflect air beyond the extent of the vibration itself. This makes it useless for deflecting incoming hot air, per-se, for example.

. . . that said . . .

In a twisted sort of way, a sound shield is kindof what actually already happens. Here are shadowgraphs showing testing of re-entry vehicle shapes:

See the black lines? Those are shock fronts. To the left, the air is supersonic, and to the right it is subsonic.

To the left of the shock front, the air is approaching the spacecraft at supersonic speeds, and therefore "doesn't know" about the spacecraft yet, and is cold. However, when it hits the shock front, it is compressed. This concentrates the air's thermal energy within a smaller space (which is to say, it becomes a higher temperature).

With a blunt surface, there is a space between the shock front and the vehicle. The way this happens is that the air "piles up" in front because it can't get out of the way.

The heat flux that compressed air creates is related to the amount of air that gets compressed, so by letting some of the air pile up on the front of the spacecraft—and mostly stay there—less new air, and therefore less heat, is brought in to touch the spacecraft's hull. Therefore, the spacecraft's hull need withstand much less thermal power. Meanwhile, the shock front is very very hot because lots of supersonic air is being compressed and bringing lots of energy, but with a blunt design the subsonic air cushion makes this happen farther away from the hull.

So . . . in some sense, (the speed of) sound does play a role in protecting spacecraft during re-entry.

• This is interesting and it does help but it does not completely answer the question. I am talking about hypersonic sound. This does not move a membrane and those the air but instead uses the more solid Plasma to act as a force to act against. For objects traveling in dry air of a temperature of 20 °C (68 °F) at sea level, this speed is approximately 343.2 m/s (1,126 ft/s; 768 mph; 667.1 kn; 1,236 km/h). Speeds greater than five times the speed of sound (Mach 5) are often referred to as hypersonic. An incoming ship seems to come in at over 24 times the speed of sound. Oct 27, 2021 at 12:03
• So buy creating a bigger pocket of air Or creating a boundary layer between the ship and the Hypersonic or Ultrasonic barrier acts as a force to push air away from the ship.(the question is can I calculate the force needed) Or makes a bigger area for colder air to exit. This uses a form of speaker with no moving parts. Not a diaphragm that has to move but air around a solid or semi solid body(like your head) this avoiding the problem of a sound barrier buy using the sound barrier. Oct 27, 2021 at 12:15
• @MikaelLong Possibly the key point is that there is no sound barrier - there are several phenomena that have a discontinuity at local mach 1, drag being one of them but there is no 'barrier', just more complicated math. Oct 27, 2021 at 12:47
• @MikaelLong It's not clear what you're asking. The phrase "hypersonic sound" is an oxymoron, for one—sound by definition travels at the speed of sound. You are correct that this value is 343ish m/s under basic conditions. If you push air faster than this, the air piles up and it can't "communicate" anything (i.e. make a sound wave) ahead of it. Now, the piling up indeed creates some cushion of air (which I talked about in my answer). However, these closer-together particles have a different speed of sound, sound itself doesn't really enter into it, and a shock front is the main dynamic . . . Oct 28, 2021 at 20:26