The concept of using sound as a weapon is pretty old. I would say it goes back (at least) to the legend of the Israelites' attack on Jericho, where the priests were ordered to blow trumpets, and the soldiers to shout at the top of their lungs, and thus (according to the legend) did the walls crumble. Now, Erich von Daniken claimed this as evidence of the presence of aliens with sonic weaponry, but he said that everything was evidence of aliens, so never mind him. I think it's just a neat legend.
But it starts a long history of the idea. Sound as a weapon. Wow. Use not only the lightning but also the thunder.
So I will talk about that today.
The idea of using sound to destroy fortified emplacements is... well, hard to engineer, to say the least, but also popular. Westwood Studios, and after them Electronic Arts, were fond of the idea of sonic cannons as vehicle-mounted weapons that could hit the resonant frequency of any object in their field of effect, and cause them to shatter or crumble. And resonance sort of does work that way. Who here is familiar with the story of the Tacoma Narrows bridge? Built to withstand tornado-force gales, it nonetheless crumbled when exposed to a mild breeze that came in occasional gusts. See, suspension bridges sway gently in the breeze, and if you give them a slight push every time they oscillate back and forth - timed just right - well, the structure will accumulate so much energy that it destroys itself.
But while a resonator tank might be awesome, it would be remarkably difficult to build and use - you'd have to find a resonant frequency for any one of a huge variety of targets (bunkers, vehicles, human beings in or out of ballistic armor, etc.) and broadcast it precisely in phase, making sure there's no interference, etc., etc. Sound does not work that way.
Well, what else do we know about sound that could be put to use? Obviously, you can use it to hail people at a distance. "Hey you! Sit down in front!" or some such message. Sure, you can shout. Or you can use loudspeakers, if you're willing to bust everyone's eardrums.
You could also use a directional loudspeaker. Examples include the LRAD, basically a circular array of sound emitters that produces, well, a relatively narrow beam of sound. The LRAD can be "fed" with any acoustic information you like, such as a microphone, but it comes with a few noise templates built in. One of them, supposedly, is the sound of an infant crying played backward, overlaid with two unpleasantly dissonant sirens. If you're in the center of the beam, being hit with this incredibly disgusting noise at 100+ decibels, you'll want to get out of the beam really, really fast. Hence, the LRAD is marketed to police and military forces as a method of crowd dispersal. A US Marine speaking on NPR mentioned that soldiers have sometimes replaced the programmed noises with AC/DC or other rock or metal, which has the combined effect of dispersing crowds and boosting unit morale.
The LRAD has also found use as a defense for ships against pirates. I believe it was the Seaborne Spirit, a cruise liner, used an LRAD to drive off pirates near the coast of Somalia. The pirates fired a single RPG-7 shell and then retreated, injuring only one member of the crew. (The maximum effective range of the LRAD is slightly less than the maximum range of the RPG-7. The shooter would certainly have been hearing an unpleasant amount of noise, but it wouldn't have been unbearable.) Overall, I think this is a good thing.
What about something a little more direct than simply being annoying? I believe it is the mantis shrimp, a small undersea arthropod, that snaps its claw at potential prey (and potential predators, like divers) with such speed and force that it creates a shock wave capable of stunning small critters and seriously annoying larger ones (like divers) - any potential of that? Well, one possible technology like this turned out to be a hoax. It was called the Sqwark Box - supposedly it projected a pair of ultrasonic frequencies, inaudible by themselves, that interacted to produce a beat frequency that would do terribly uncomfortable things to the internal organs. (A real device that does this is called Inferno, although it works on the ears and vestibular organs, and it is not ultrasonic. Nor is it any more directional than the LRAD.)
There had been a plan to use something like the Sqwark Box for more peaceful purposes - like advertising or giving prompts to public speakers, wherein a pair of low-intensity ultrasound beams would be pointed at a person's head. One would be a carrier wave; the other would be the signal frequency, which would vary precisely from the carrier, creating a beat at audible frequencies. You could transmit words to it. Like, say, "It's 'remittances', Mr. McCain, not 'emissions'." Or, if you were walking past a soda machine, "If you're feeling thirsty, how about a nice cold Pepsi?" (I'm thirsty now. I'll be right back...)
(Okay, I'm back.)
There were at one time some concerns about whether such technology would be usable to stalk and harass someone, or whether the experience of having one pointed at you - it's been described as hearing a voice inside your head - could push a vulnerable person over the edge into psychosis. Does such a thing exist? Well, I know someone who would vehemently deny it - he would insist that the voices he hears are just ethereal voices that come out of nowhere.
Certainly the most interesting depictions of sonic... devices, is in fiction rather than in reality. The mantis shrimp writ large most dramatically (remember, in fiction) is probably the Noise Marine, a member of a band of militant and heavily-armed religious fanatics who devote themselves to hedonism and overstimulation in service of a god they call the Prince of Excess. Among other peculiar stories about them, they are said to have weapons that use incredibly loud sound to destroy their enemies. Here is a picture of a member of that faction, and notice what appear to be loudspeakers on his shoulder plates - and in his face.
In any case, being hit by a blast from any sonic weapon, real or imagined, will really make you appreciate the value of peace and quiet. I mean that.
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Previous MSPW diaries can be found as follows (and don't read them if you're trying to preserve your unwarped mind):
MSPW 8: Rapid prototyping brings engineering to the masses
MSPW 7: Putting Mentos and Diet Coke to good use
MSPW 6: Why Bjorn the Fel-Handed is probably unhappy
MSPW 5: Combining the latest concepts in farming and power generation
MSPW 4: Project Orcon, or why pigeons make good pilots
MSPW 3: Can cuttlefish drive?
MSPW 2: The hafnium bomb
MSPW 1: Building a better skunk