Rockets are loud. With engines that can thunder at up to 200 decibels (that's 80 dB higher than the human threshold for pain), rockets roar out sound waves powerful enough to topple buildings, set bystanders' hair on fire and blast rainbows out of the sky.
You can witness that last phenomenon for yourself in a viral video posted to Reddit's space forum yesterday (July 10). [Supersonic! The 11 Fastest Military Planes]
Filmed during the launch of NASA's Solar Dynamics Observatory on Feb. 11, 2010, the clip perfectly illustrates what happens when a rocket's mighty sound waves collide with a semisolid medium — in this case, a wispy sheet of ice crystals draped high in the sky over the launch pad. When crystals like these align in a certain way, they can create an isolated rainbow smear known as a "sundog." You can see one such sundog hovering around the upper-right-hand corner of the video… until the rocket obliterates it, anyway.
"When the rocket penetrated the cirrus, shock waves rippled through the cloud and destroyed the alignment of the ice crystals," atmospheric optics expert Les Cowley explained to NASA shortly after the Solar Dynamics Observatory launch. "This extinguished the sundog."
Despite what the video's title says, the rocket here is not quite traveling at supersonic speeds; if it was, the vehicle's sound waves would be falling behind the rocket in a cone shape, not blasting out in front of it like a ripple. In any case, a rocket doesn’t need to go supersonic to be devastatingly loud. Even during the first stages of liftoff, when a rocket is barely moving but the engines are blazing, visible sound waves can ripple through the exhaust and decibel levels can soar into property- and ear-drum-damaging extremes.
"When the shuttle lifts off, the main engines roar so loudly that a person standing near the pad would be killed — not by the heat of the exhaust, but by the sound of the engines," Rodney Rocha of NASA's Johnson Space Center in Houston said in a 2005 interview with NASA. While there's not much research on how such a death might go down, the general thinking is that such powerful sound pressure could artificially inflate a person's lungs with air, creating an embolism that could travel to the person's heart or brain. Heart attack, seizures and respiratory failure could follow.
NASA engineers take these sonic threats seriously, and try to dampen gargantuan rocket-induced sound waves in various ways. One way is to douse the launchpad in water or mist — hundreds of thousands of gallons of it — right after liftoff. This man-made flood dampens sounds on the launchpad and prevents a rocket's cacophonous pressure waves from ricocheting off the ground and breaking the ship apart with its own noise.
Another way to mitigate rocket noise is to test every single piece of the ship in a special acoustic chamber to see how various parts interact at various frequencies. This testing can involve blasting individual pieces with a 165-dB horn, or staging launches of tiny, scale-model rockets in order to determine which parts of the ship may need more acoustic suppression. Tests like these can save missions and lives. Sundogs, on the other hand, never stand a chance.
Originally published on Live Science.
