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Sonic booms as weapons? The science behind Peacemaker's helmet
You could do more damage at a rave.
The Suicide Squad's Peacemaker is a study in how characters — or people — with unshakable certainty in the righteousness of their mission can become villains even while they think themselves the hero. The movie, directed by James Gunn, parodied that attitude by having the audience stare it in the face, with lines like "I cherish peace with all my heart. I don't care how many men, women, and children I need to kill to get it."
HBO's new series follows the character more intimately to give us a deeper look into his psyche, motivations, and impressive collection of ridiculously overpowered helmets. He has helmets for breathing under water, for enhanced vision, and a whole slate which weren't explored in the first season, but the sonic blast helmet rises above the rest.
The character uses the sonic blast to absolutely destroy his enemies, turning them into a fine paste spread over dozens of square meters. But, like, could that really happen?
WHAT IS A SONIC BOOM?
First thing's first. What exactly is a sonic boom anyway?
Sonic booms are most closely associated with aircraft traveling faster than the speed of sound, though they can be created by any object moving faster than 343 meters per second. As an object moves through the air, molecules are displaced, creating a trailing wave.
To visualize this, think of a boat moving through the water and the way it generates waves trailing behind it in a V formation. As long as you're moving slower than the speed of sound, the air is able to keep moving out of the way. At supersonic speeds, however, your forward motion happens faster than the air in front of you can get out of the way. The wave trailing you breaks, creating a shockwave which stationary observers hear as a sonic boom.
Sonic booms aren't inherently destructive. In fact, you could create one with nothing more than your arm and a whip. When used correctly, a whip creates a loop that travels along its length at faster than the speed of sound, resulting in a tiny sonic boom. If the result of that pressure change were inherently dangerous, the wild west would have been even more bloody, as every cowboy across the frontier blasted themselves to smithereens. Alas...
So, in essence, sonic booms are a consequence of rapid air displacement and the resulting change in pressure.
MAKING A STATIONARY SONIC BOOM
Now that we know how sonic booms are created, the question becomes whether you could generate one from a stationary object. In the show, Peacemaker's helmet creates a shockwave emanating out from him in a wide radius. There's no apparent movement, so it's unclear where the pressure to displace air is coming from.
Certainly, a helmet could create a sonic boom — if it were launched through the air at speeds faster than 343 meters per second — but creating a shockwave without moving at all is a little harder to reconcile.
The easiest way to create a sonic boom with a stationary object is to cheat. An object, whether its an airplane or a helmet, need only to move at supersonic speeds with respect to the air column. Physics doesn't care if the object is moving, or if the air is. Wind tunnels are a perfect example of this sort of phenomenon.
Objects in wind tunnels are typically mounted in place while air is sent whirling around them at high speeds. Sonic booms are common events inside wind tunnels. Still, that doesn't quite satisfy what we see Peacemaker doing. He could create a change in air pressure using a sufficiently loud speaker, but even then, the sound would only travel through the air at, you guessed it, the speed of sound. No sonic boom.
Ultimately, the sonic boom isn't really important. What Peacemaker wants is the sudden change in air pressure which results from a sonic boom. That, after all, is what's ostensibly doing all the damage.
Let's assume, through some as yet unexplained technology, Peacemaker's helmet is capable creating a rapidly expanding shockwave comparable to a sonic boom. Would it be dangerous?
EXPLODING FOLKS WITH AIR PRESSURE
So, your head is making sonic booms. Now what?
It would actually make a pretty poor weapon. As we discussed above, people frequently stand close to sonic booms anytime they crack a whip. The size of the shockwave is comparatively small, and that's the real kicker.
The change in air pressure which results from a sonic boom is only a few pounds per square inch, and the size of the shockwave is related to the size of the object moving through the air. Basically, the bigger the object, the more air is being compressed, and the bigger the shockwave.
A helmet-sized shockwave wouldn't displace very much air. Even someone holding the helmet, as we saw in the last episode of the season, wouldn't be killed. They might be annoyed, maybe a ringing in the ear, but that's about it.
You'd be subject to a greater pressure change by diving more than a few feet under water. Your body is pretty good at dealing with pressure changes. If it weren't, you might die just by changing altitude.
We're going to place Peacemaker's sonic blast helmet purely in the realm of fiction. It wouldn't work, and even if did, it wouldn't be very effective. Although, given the character's thirst for carnage, maybe that's a good thing.