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Air Displacement?

  1. Apr 14, 2008 #1
    Hello all... first time poster here.

    I have a logistical question I'm hoping someone can help me with.

    Now, everyone knows that sound travels in waves. We also know that these waves can have various physical effects on objects as they pass through, around, over, etc. My question is this: is it possible to use sound waves to displace air molecules in a certain space?

    The idea that I'm driving for here is a way to quickly move air out of a certain area (imagine quickly and efficiently extinguishing a fire by rapidly removing all of the oxygen from around it - that's the basic concept, anyway).

    Is something like that theoretically possible?

    Thanks in advance for any help. :)

    -- ossuary
  2. jcsd
  3. Apr 14, 2008 #2

    Andy Resnick

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    Sure- even better, the concept is already in use in medical instrumentation- a lithotripter.

    What you are imagining would require substantially more energy than alternative methods already in existence- the volumes you are thinking about are thousands to millions of times larger than kidney stones.
  4. Apr 14, 2008 #3

    Thanks for the quick response. That's fascinating... so how does a lithotripter work, exactly? Understanding that the power use would have to be much larger, is the technology scalable, or does it have physical limitations in terms of effective strength / range?
  5. Apr 14, 2008 #4
    I think that it is impossible to remove air with the use of sound waves. Air depression in a sound wave is quite negligible. But may be shock waves can be used for air removing...
  6. Apr 14, 2008 #5
    They can be. In fact, in some cases, fires have been extinguished using an explosion to create a shockwave. But the problem with a shockwave is that it is not inherently sustainable, it is not easily controlled, and it can cause a lot of damage.

    The removal of air from a specific area to douse a fire was just one example of my question, to illustrate the point. But in order for what I am thinking of to be functional, it would have to be controlled and sustainable.

    Sound waves were the first thing that jumped into my head when I first came up with the concept, but really anything that could be used to quickly displace the air molecules from a controlled area and be sustainable would suffice - that's what's I'm actually trying to find out about the possibility of.
  7. Apr 14, 2008 #6


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    What you're trying to achieve is commendable, but I'm not sure that there's a lot of room for improvement over current systems that replace the ambient air with a substance that doesn't support combustion (carbon dioxide, argon, halon, etc.). It sounds as if you want to create a vacuum in the fire area, which is very problematic and unsustainable.
  8. Apr 14, 2008 #7


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    Isn't lithotripsy the use of sound waves to dissolve kidney stones? Is this accomplished by quickly removing the air from a particular area?
  9. Apr 14, 2008 #8
    Well, yes, but again, the fire was just one example. Replacing the air with some other non-combustable material is not the goal. I suppose you could say that I am indeed trying to create a vacuum at a specific point (theoretically, I mean - I'm not actually trying to accomplish this). It doesn't need to be sustainable for a LONG time, but it also cannot be a one-shot like you would get from a shockwave.

    It's sounding more and more like sound waves probably could not accomplish this, but I'm not giving up on the theory quite yet. :)
  10. Apr 14, 2008 #9


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    No, it involves setting up vibrations in the stones that overcome their structural integrity and cause them to shatter.

    Good on ya, mate. One should never give up on an idea until all possibilities have been eliminated. While I don't know exactly what it is that you're up to, it's interesting. Keep us in the loop. :smile:
  11. Apr 14, 2008 #10
    Ah, in that case, that particular device doesn't do what I would be looking for.

    I can't say TOO much about it, because this is something still very much in the early stages of conceptualization, but I'll definitely be talking more about it as I get more details on feasibility. The first thing I need to do is figure out whether or not what I am trying to accomplish is even theoretically possible... the second step is then finding someone who might actually be able to help accomplish it. ;)

    The basic idea is like I said in my last post - I am looking for a way to temporarily (but sustainably, at least for a short period of time) displace all of the air molecules from a controlled area, essentially creating a temporary vacuum in that space.

    There would be several potential applications for this, if it could be done cleanly, safely, and on demand. Extinguishing a fire would be one. Another possible application would be to create a sort of "slipstream" area where there was almost no wind or air resistance (no air molecules to push through means less resistance for the object in motion, and therefore less friction and higher speeds).
  12. Apr 14, 2008 #11
    Consider the implications of detonating an explosive device in an area where the combustion of air results in a vacuum of the space where the explosion took place--or natural phenomena such as lightning incinerating a 'column' of air. While sound does result from these events, the sound is not the direct cause--rather the air collapsing back into the vacuum that the original even took place in.

    Fire prevention through electric shock of the surrounding area of detonation of an explosive device to starve it. Is that something more akin to what you're describing?
  13. Apr 14, 2008 #12
    I seem to recall the oil fires in Kuwait being put out by shockwaves from explosives.
  14. Apr 14, 2008 #13
    No... again, extinguishing a fire is just ONE of many examples of possible applications of the idea I am talking about. And as I have specifically stated (three times now), extinguishing a fire using sound waves is NOT my actual goal. It was merely an example to demonstrate the basic concept of what I am talking about.

    Please review my previous post, wherein I state exactly what I am trying to accomplish.

    I am aware that creating a temporary vacuum in which no air exists will lead to an eventual (usually very quick) collapse of said vacuum as the displaced air rushes back in. That fact is not particularly relevant to my concept (though it is certainly a ramification that may need to be taken into account).
  15. Apr 14, 2008 #14

    Andy Resnick

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    It's accomplished by tightly focussing sound waves, fomed by a shock, into a small volume to deposit energy. This is exactly like what you want to do... or what exactly do you think "removing air from a particular area" is?
  16. Apr 14, 2008 #15
    So the energy that the sound waves generate and dump into a focused location displace the air molecules from the same space? That sounds at least somewhat like what I am thinking of (functionally, anyway). Are the sound waves generated by a lithotripter sustainable for a short period of time, or is it just a quick blast that almost instantly dissipates? And going back to one of my earlier questions to you, would the technology that operates a lithotripter be (at least theoretically) scalable?

    What would be the ramifications for any physical object that was in the same area of space upon which the sound waves were being focused? Do the sound waves actually shatter the kidney stone, is it the vacating air that does it, or are the stones physically broken up by the vacuum's presence / creation / destruction?
  17. Apr 14, 2008 #16


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    While I suppose that there might be intermittent small pockets of vacuum created by accident, that has nothing to do with how the thing works. Think of the sound waves as tiny jackhammers; just watch some construction worker breaking up conctrete with one, then scale it down.
  18. Apr 15, 2008 #17

    Andy Resnick

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    Sound waves are density fluctuations in a continuous medium, no more or less.

    I really don't have an idea about what sort of device you are trying to invent- a portable vacuum beam?
  19. Apr 15, 2008 #18


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    That lightning statement sounded like it might actually be useful. There was once a concept being developed for launching vehicles into space that used a laser beam aimed at a reflective surface on the underside of the vehicle. Several beams, in fact. The curvature of the reflective surface focused all the energy of the beams to a very small point just under the craft. The resulting heat caused a rapid expansion of the air, and this propelled the craft upward. Now, at the center of that rapidly-expanding pocket of air, there must be an area of very low air density. Not exactly a vacuum, but maybe close enough for some of the applications you have in mind?

    In fact, a quick websearch reveals that the designer was planning to use this effect to reduce wind-resistance ahead of the craft at higher speeds:

    http://science.nasa.gov/newhome/headlines/prop16apr99_1.htm [Broken]
    Last edited by a moderator: May 3, 2017
  20. Apr 15, 2008 #19
    Well, I thought "hyper death ray" sounded cooler, but... ;)

    I suppose that creating a miniature vacuum on demand is the basic principle, though I may not be explaining myself properly, not being a science-y type person. That's the basic idea, in any case, though the vacuum itself is not actually the goal, so much as moving the air molecules out of the area to reduce friction and wind resistance.
  21. Apr 15, 2008 #20
    Now this is extremely interesting. It seems like it is at least similar in concept to what I am talking about, though on a much larger scale... and the exact process they are using would actually function as propulsion for the craft, which is not particularly what I was thinking of, but is a fascinating idea. I wonder if they can get it to work?
  22. Apr 16, 2008 #21


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    It is possible to create air movement with a electrically charged sharp object (like needle).
    I remember that one of my teachers once told us that it is possible to extinguish fire in a fuel tanks by using something like a electrically charged grid , which creates air movement which separate the fire from the fire.
    I don't remember how exactly the air movement is created, but i guess it's related to ionizing the air.
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