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# Sonic Wall

Use standing wave to trap or repel objects.
 (+1, -6) [vote for, against]

Take directional speakers to create a standing high or low pressure wave. I'm thinking something that generates two or more waves that travel out some distance then maximally reinforce each other then cancel out so that the target can't detect it until it's too late. Apparently you can generate practically any wave shape using both even and odd frequency scales. Would require huge amounts of power to run though, a kilowatt or two I'm guessing for a small one that can move small objects to many megawatts for missile/torpedo defense (you could probably tune it for under water defense to).
 — Spaceman Spiff, Feb 22 2008

Haptic interface http://www.gizmag.c...ion-holodeck/29360/
Like this, except really powerful? [neutrinos_shadow, Jan 05 2016]

Newton's impact depth theorem https://en.wikipedi...g/wiki/Impact_depth
[snictown] is almost certainly referring to Newton's impact depth theorem. [Custardguts, Jan 05 2016]

I think you are badly underestimating how much energy it would take to stop a projectile I suspect by at least a power of ten likely a hundred.
 — WcW, Feb 22 2008

just thinking air defenses are pretty good these days and this idea is not very efficient in air anyways. You could still use it under water as it's better at transferring sound energy. As for underestimating power usage very likely but I was thinking this system could be mounted onto nuclear powered ships and subs and the entire output of the reactor coupled with a capacitor bank could provide the required power long enough to destroy the torpedo. [UB] If you are going to post that something is bad why don't you also post why it is bad so that I at least know that you understand the concept of the idea.
 — Spaceman Spiff, Feb 22 2008

Standing waves *are* used to trap or repel things, but mostly we're talking dust particles here, or things the size of dust particles. Larger things are a little more intractable.
 — DrCurry, Feb 22 2008

Such a system would be huge Spiff. It would only work over very small distances making it a poor defence against explosives (power requirement more than squaring with distance). Furthermore in air even a massive standing wave would not be able to stop a missile. It might damage it or send it off coarse but at the distances we are talking about (less than 10 feet) that wouldn't help much. In water you cannot impart enough energy due to a property called cavitation the same property that limits prop speed. I'm not sure what purpose trying to muffle the system would serve in either medium. That sort of thing works in the lab and in headphones but not on the scale that you are imagining. Systems like this are far better at putting people out of commision than weapons due to our tender inner bits.
 — WcW, Feb 22 2008

 Let's do some sums here.

 Bullets penetrating something soft are stopped once they push through their own mass of the soft material.

 The area where the waves from your speakers constructively interfere would be a region where the air pressure goes from really high to really low over and over.

 Sonic booms break windows at ~100psi pressure. Let's assume you've built this thing to have a 4000psi pressure standing wave. This is a higher pressure than a scuba tank, and would cause massive injuries to anything absorbing a wave of this magnitude.

 4000psi is about 275 atmospheres. Air density would thus increase to 275x its density at STP, or about 0.013 lb per in^3, or 0.2 oz per in^3.

 A .223 rifle slug weighs about 0.4 oz and has a cross- sectional area of about 0.05 in^2.

 In other words, the bullet would need to pass through 4 inches of crazy-high-pressure air to be stopped.

 But, a standing wave from speakers isn't constantly high pressure; it's an area where the pressure goes from very low to very high as the waves pass by. So the pressure would average about half the above, and the bullet would need to travel through 8 inches of this high- pressure air to get there.

 I'm envisioning a high-volume 10,000psi air source driving a dozen whoopie-cushion-like devices arranged in a ring, such that the volume adds in the center of the ring. Since the blast from the brrrzt-ers would not be in phase, the intensity at the center would increase with the square root of the number of brrzt-ers, not linearly. Still, this would be quite the cacophany.

 It would sound like an ammunition depot being detonated continuously, more or less.

 Plus, it would need to be suspended in a hoop-like structure that could be rapidly moved to place the region of high-pressure air in front of the incoming bullet.

 Unfortunately, even if the noise sources could be placed appropriately, the cacophany would render it unusable for security at public events (presidential speeches, etc.).

A better idea for political speeches might be to encode the speaker's words into the standing-wave generators. The speaker would need to speak continuously to maintain protection.
 — sninctown, Jan 05 2016

 //Bullets penetrating something soft are stopped once they push through their own mass of the soft material. //

Is that true? Only it would mean that a lead bullet fired in air would stop after about 11,000 bullet- lengths, which is maybe a few hundred metres.
 — MaxwellBuchanan, Jan 05 2016

If 11,000 bullet lengths isn't far enough, just shoot it out of a bigger gun.
 — the porpoise, Jan 05 2016

So it only depends on the mass of the bullet, not how fast it is going?
 — pocmloc, Jan 05 2016

 [poc] - [sninc] is referring to Newton's impact depth theorem, which ignores silly things like velocity and kinetic energy. It's purely based on density ratio.

 Funny thing is, it works, as far as approximations go. Clever dude, that Newton.

 — Custardguts, Jan 05 2016

Wow. Well, I will now put on two hats, in order that I may take one of them off to [sninc] and one to Ike.
 — MaxwellBuchanan, Jan 05 2016

I don't think this is a new idea. Bionicle had this in 2002 or 2003—ISTR one of the Bohrok or Bohrok-Kal, I think the ice one, had this power. And I don't think that was the first time I'd seen it.
 — notexactly, Jan 11 2016

Very good, thank you.
 — pocmloc, Jan 11 2016

 // It would sound like an ammunition depot being detonated continuously, more or less. //

<deranged cackling>
 — 8th of 7, Jan 11 2016

 Back to the idea... I can't help wonder if standing wave nodes be produced to coincide with where a projectile 'will be' in-line with it's direction of travel, so that the projectile does not pass through a single screen of cohesive sound so much as it travels a gauntlet tunnel of cohesive interference?

Surely a bullet could at least be redirected from its path using this approach if it could be tracked in real-time.
 — 2 fries shy of a happy meal, Jan 12 2016

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