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So, this is a super-villain origin story. He is a scientist who worked on ultrasound imaging technology. This technology was using ultrasound to "see" underground and into building walls.

The mad scientist had a "They laughed at me, I'll show them, I show them all!" moment and decided to do what all mad scientists do and use his technology for evil. What he decided to do is destroy buildings. He looked into the "Tesla earthquake machine", tested it, saw that it didn't work as promised and got a bit depressed.

As he was sitting in his home, listening to the music on his expensive HyperSonic Sound system (that produces sound from ultrasound) and eating ice cream, he had a eureka moment. Sound is vibration of air molecules in the air. Actually, it could be said that it is vibration of molecules in the medium. You can make sound from ultrasound. If he focused two or three modulated ultrasound sources in the same place, he could produce vibrations in the foundations at the place they met, when the steel of the foundations acts as demodulator. If the wrong part of the building vibrates at just the wrong frequency, the whole thing (or at least a huge chunk of it) goes down.

Then the only thing he needed to do is find the building part's resonant frequency and modulate the signal as it changes and the building goes down.

This is not military technology, but a lone lunatic tech. If the military wants to destroy a building, they use bombs or C4. He has a budget of slightly less than 100,000 dollars and access to industrial equipment. No explosives, he doesn't have those types of legal permissions or contacts.

The question is, how feasible is this technology? Were there any tests or scientific research done for similar things? Would it be portable? Would he need a building-sized ultrasound generator? How much energy would it draw from the power grid?

Edit: The building is a modern building the size of a USSR-style 8 floor tall brezhnevka. He has half an hour before he is detected and stopped. If this is impossible, I would settle for a van-sized reusable device that can break most windows on a skyscraper in five minutes. Edit2: Bolded some parts, because people obviously don't read.

Hard science answers please.

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  • $\begingroup$ How big is this building and how fast does it have to come down, i.e. how long between turning on the device and it collapsing? $\endgroup$ Commented Feb 6, 2025 at 7:49
  • $\begingroup$ @MontyWild I edited the question. $\endgroup$ Commented Feb 6, 2025 at 8:01
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    $\begingroup$ Impossible to do it. brezhnevkas were made from individual prefab concrete panels. Ultrasound propagation will have big troubles to "jump" from one panel to the next and be reflected back by any discontinuity (this is how/why ultrasonic testing works). Therefore you won't be able to make the building behave like a monolith for ultrasound. If you increase the power level, you may get heat/turn to dust the concrete around the point were you applied the source, but that's all you gonna get. Not to mention concrete itself is not homogenous enough $\endgroup$ Commented Feb 6, 2025 at 10:39
  • $\begingroup$ @AdrianColomitchi what if it is a modern building the size of brezhnevka? Or, what if I attack foundations via ultrasound propagation through ground? $\endgroup$ Commented Feb 6, 2025 at 10:41
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    $\begingroup$ @jo1storm Ultrasonic mixer - DIY in the 1kW range. Pay attention what happens when you have impedance mismatches between the transducer (source) and horn (transmission line). Spoiler: serious heating. Now, think how many such impedance mismatches your brezhnevka-sized building would present to one or more ultrasonic beams. $\endgroup$ Commented Feb 6, 2025 at 11:20

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Soil liquefaction

Don't target the building. Target the ground underneath.

Soil liquefaction occurs when a cohesionless saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress such as shaking during an earthquake or other sudden change in stress condition, in which material that is ordinarily a solid behaves like a liquid.

...

The phenomenon is most often observed in saturated, loose (low density or uncompacted), sandy soils. This is because a loose sand has a tendency to compress when a load is applied. Dense sands, by contrast, tend to expand in volume or 'dilate'. (source, emphasis mine)

Long story short: vibrations in the ground shake grains of sand loose, which causes objects on top of the ground to shift. This won't cause direct damage to a building, but it can certainly do damage:

Picture of buildings in Nicaragua fallen over after an earthquake

“Niigata Earthquake, 1964,” Japan National Committee on Earthquake Engineering, Proceedings of the 3rd World Conference in Earthquake Engineering, Volume III, pp s.78-s.105.

The Wikipedia page mentions some mathematical formulas which you can use to assess the potential for liquefaction, which might be useful for back-of-the napkin numbers.

The challenge is figuring out the frequency and strength for the sound to be considered ultrasonic and which causes grains of sand to move. According to Wikipedia, that puts you at more than 20,000 Hertz to be considered "ultrasound". You would need some hardcore number crunching to figure out the strength.

I found a vibrating back massager capable of "3500 pulses per minute" - whatever a "pulse" is. I'm thinking most consumer grade "things that vibrate" will be an order of magnitude lower in frequency that you'll need to experiment with a box of sand. Unless you have a friend who is an ultrasound technician, but even then I'm willing to bet an ultrasound device is too weak. Tissue damage, and all.

A cursory search on Google scholar for soil liquefaction hertz gives a little information. Mostly I'm seeing papers related to earthquakes with frequencies less than 1 Hertz, which is certainly not ultrasonic. Then again, researching earthquakes gives you some physical constraints on the frequency, which means only a mad scientist would experiment with soil liquefaction at ultrasonic frequencies. Do you know any?

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    $\begingroup$ Great idea. If wet, the sound should conduct better through the soil. OP should pick a rainy day. $\endgroup$ Commented Feb 6, 2025 at 23:19
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    $\begingroup$ "The wise man built his house upon the rock." $\endgroup$ Commented Feb 7, 2025 at 11:43
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    $\begingroup$ If you're sincerely seeking hard science, be aware that the amount of energy needed to induce soil liquefaction on the scale required is dramatically more than the energy needed to crush/explode any building on top of the soil in question, and a device capable of doing so will not be subtle or portable using any known methods. Isolating the effect to a single building will also pose challenges. $\endgroup$ Commented Feb 8, 2025 at 17:56
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All the rumbling about ultrasounds and bla bla bla made by your mad scientist is just a very very very very convoluted way of describing what earthquakes do and why certain buildings collapse during quakes: the ground shaking creates vibrations close to the resonant frequencies of those building and they snap.

The bad news is that seismic waves (and building oscillation frequencies) have periods of the order of seconds, while ultrasounds are way higher than that.

Converting ultrasound into sound waves capable of forcing building resonance seems quite ineffective.

That aside, it's possible: many research centers have systems built to test in real life the effect of oscillations on building, here is one example.

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    $\begingroup$ Reading this makes me realize that most large buildings in developed countries, at least where there's any risk of actual earthquakes, are probably constructed specifically to resist this kind of attack. $\endgroup$ Commented Feb 6, 2025 at 11:25
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    $\begingroup$ "Replicate effects of earthquake on a building with ultrasound": Not without actually inputting comparable amounts of energy... $\endgroup$ Commented Feb 6, 2025 at 12:40
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    $\begingroup$ +1 Bombs have similar effects to earthquakes too. If a machine could be made that could collapse a building cleanly with ultrasound then the military would be 2nd in line to use it. The first group to take up the technology would be people doing controlled demolitions in crowded neighbourhoods - if there was a way to not use high explosive then they would use it. Instead they do use HE, which is a pretty good indication that ultrasound either doesn't work or no one has developed it yet - either way, no hard science favourable answers. $\endgroup$ Commented Feb 6, 2025 at 13:23
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    $\begingroup$ @KerrAvon2055 I just assumed that bombs are much cheaper and the production facilities are already in place for the bombs and explosives. This is the mad scientist not caring about the cost, he just wants to prove them all wrong! And they called him mad! Mwahahaha! So my question is, was there any testing done with that technology by anyone? $\endgroup$ Commented Feb 6, 2025 at 15:58
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    $\begingroup$ It's not just research centers, when the older mid-sized office building I worked at needed seismic retrofit >30 years ago, there was uncertainty about how things were built and connected and the properties of the materials. They did a study where they shook the building, measured the frequency response and modes, compared that to models, did the retrofit construction, and shook it again to make sure it worked. $\endgroup$ Commented Feb 6, 2025 at 20:26
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I like your idea but the settings are not in your favor.

Bad resonance match between buildings and ultrasound
The frequency of objects depends on the size (see the formula at Mechanical Resonance), larger objects require higher frequencies.
The needed resonance frequence for buildings is far out of the ultrasound range.

It is possible to resonate objects by lower or higher frequencies (multiples of the same resonance frequency) (see Wiki), but in best case you want to match the frequency exactly to get the best result.
Simply said, give the object a push at every interval.
Lower frequences are weaker as they push only every n-th interval. Higher frequences are weaker as they carry less energy per single interval. (1)

(1) It is the amplitude which applies the force. As of relation-between-amplitude-and-frequency, higher frequencies tend to have a lower amplitude. Thus higher frequencies are worse to push something.

Bad energy transportation
Well, I spare the proof here and simply say: Sound spreads and will go into many directions, so be prepared to require more energy than can be delivered to the target building.

You need a lot of energy
Anything that bends and moves has friction and thus will turn the movement into heat.
You need to make sure to add more energy than is consumed by the movement to be able to inflict a resonance disaster.
I am not really able to give a good proof here but stone or concrete are not very flexible and should consume quite a lot of energy. (I assume there are some numbers out there, e.g. from towers or so)

Don't forget, you try to shake a building (even a pretty sturdy looking building). It is not like the energy comes from a mighty earth quake, no. The entire energy needs to be emitted by your sound generator and transmitted through sound waves.

The problem for the energy is not only for how to emit the energy by a generator and how to get enough energy.
I am not able to proof this but I assume the sound might be so loud to turn everyone in the area deaf (including the villain himself) (see the table in Sound Power)

Result
I am not able to do the calculations or find all related formulas, but I am sure the energy transportation through ultrasound to emit a resonance disaster on a building is terrible, terrible enough to be not feasible when calculated.
My impression is that even a building-sized ultrasound generator would not be able to do that or would rather destroy itself before destroying something else.

This answer may not have all details to meet the hard-science tag, but my findings should give enough indications that the involved physics make it not feasible in several ways.
So even when properly calculated, the result will be the same.

A small frame challenge:
To shake large objects through sound waves, an Infrasound generator with a matching or a fraction of the resonance frequence may be better suited than an ultrasound generator (though probably still bad).
Also longer wavelengths are better to penetrate solid object and don't get reflected so easily.
I found this part quite fitting for a villain attack:

One study has suggested that infrasound may cause feelings of awe or fear in humans. It has also been suggested that since it is not consciously perceived, it may make people feel vaguely that odd or supernatural events are taking place.

The only problem is that the device to emit an infrasound needs to be quite big.

On 31 May 2003, a group of UK researchers held a mass experiment, where they exposed some 700 people to music laced with soft 17 Hz sine waves played at a level described as "near the edge of hearing", produced by an extra-long-stroke subwoofer mounted two-thirds of the way from the end of a seven-meter-long plastic sewer pipe.

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The problem with using anything based on resonance to destroy large structures is that you can't build up energy forever, and something like a building isn't a great energy battery.

This video by AlphaPhoenix is a great dive into the practicalities of resonance: https://youtu.be/WCKt405t9V8?si=G9M5_QunVmreZNts

The first half explains resonance and a bit about Tesla's theoretical device. The second half gets into the limits of resonance.

First is that the energy built up by resonance decays exponentially due to losses to sound, friction, and heat. That's going to limit the maximum amplitude (energy) of the waves your villain can build up in this building. It doesn't really matter how fast you vibrate, the limiting factors on how much energy you can build up is how much energy you can put into the system and how fast.

Second is that the building won't fail catastrophically—your villain can't build up that much energy in one go. Instead, the “best case” scenario here is that resonance builds up until a bolt or rivet breaks, or a small crack appears in the concrete, or some other weakest link fails. That mechanical failure absorbs a lot of energy, which essentially resets the resonance. It has to build back up, until it can cause another small crack…

You villain is going to have to sit there, destroying the building bit by bit. If the building is up to code, it should be able to withstand many small failures like these before it even becomes unsafe, let alone collapses. Forget C4—they could probably take the building down faster with a sledgehammer.

The input waves from something like an earthquake have a lot of energy, which takes longer to decay. So resonance can build to higher amplitude (energy) before it plateaus. Your villain simply can't put energy into the system fast enough to do any real damage.

Edit to add: I feel that this answer does address the original question. I know OP asked about ultrasound and targeting pieces of a building, but the goal was still to cause large amounts of damage using resonance. The method of delivering the energy doesn't matter (except by introducing more losses), and starting resonance in one small part of the building isn't going to stay contained—it's still attached to the rest of the building. So this is equivalent to standard resonance.

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    $\begingroup$ Great answer but I picked soil liquification one. It solves my problem. $\endgroup$ Commented Feb 7, 2025 at 8:01

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