An observation from a reader sent me on a search for a good photograph of a sonic boom, and inevitably, I ended up chasing facts and cool tidbits down a few rabbit holes.
First cool tidbit: Sonic booms are created by shock waves, and those shock waves were first photographed in 1888 by Ernst Mach, who was studying the pressure waves produced by bullets traveling at supersonic speeds.
What he discovered was that there are two pressure waves, one at the front of the projectile and one at the back. Stanford University has a great write-up about these famous photos and the technique used to produce them.
(Mach’s research produced the equation that immortalized his name: a Mach number is the ratio of the speed of a traveling object to the speed of sound in the medium through which that object is traveling.)
The bullets create those shock waves by shoving molecules aside with the force of their passage, much the way a ship creates a wake as it moves through water. The bigger and heavier a traveling object is, the bigger the shock waves it produces.
Which brings us to the shock waves we all know about: sonic booms.
NASA has a spiffy little fact sheet on sonic booms, offering this succinct explanation:
The shock wave forms a “cone” of pressurized or built-up air molecules, which move outward and rearward in all directions and extend all the way to the ground. As this cone spreads across the landscape along the flight path, it creates a continuous sonic boom along the full width of the cone’s base. The sharp release of pressure, after the buildup by the shock wave, is heard as the sonic boom.
The change in air pressure associated with a sonic boom is only a few pounds per square foot — about the same pressure change experienced riding an elevator down two or three floors. It is the rate of change, the sudden changing of the pressure, which makes the sonic boom audible.
The closer the source of the cone, the louder the boom. As the cone travels farther down through the air, it widens and weakens, so the boom created by a high-flying supersonic aircraft won’t be perceived as loud on the ground, while a boom from the same aircraft screaming by 500 feet overhead will knock your socks off.
(Seriously, check out that fact sheet. Especially the part about overpressure.)
Back to those bullet photos up above. Remember the two pressure waves? Those form on jets, space shuttles, and rockets, too. So they all create double sonic booms, but we often hear the two booms as a single sound because they’re so close together. For instance, a supersonic fighter jet 50 feet long will generate nose and tail shock waves that are about one-tenth of a second apart. We can’t hear that difference. But the Space Shuttles, famous for their double booms, were 122 feet long. That meant their nose and tail shock waves were about a half-second apart, and that is distinguishable to our ears.
Though the Space Shuttles are retired, sonic booms have returned to Florida’s Space Coast with the advent of SpaceX and their rocket launches, with one spectacular difference: the SpaceX rockets produce triple booms as they land. That’s because of their shape. As explained in Space Flight Insider:
“[The] first boom is from the aft end (engines),” said John Taylor, SpaceX’s Communications Director. “[The] second boom is from the landing legs at the widest point going up the side of the rocket. [The] third boom is from the fins near the forward end.”
Want to hear that? Check out this video. I’ve cued it to just before the booms, but you really should watch the whole thing, with headphones on, because it is super cool.
Now for a bit of urban mythbusting. You’ve probably seen this photograph, which was taken by Ensign John Gay of the US Navy in July 1999:
This is not a sonic boom, though practically every site/blog/article that has ever featured it claimed it was. It’s actually a phenomenon called flow-induced vaporization. Atlas Obscura has a wonderful article on when and how this photo was taken, how shocked Ensign Gay was to return from his tour at sea to learn that the photo had become world famous, and what he really photographed.
I did, however, find a great photo of an actual sonic boom produced by a jet. It will look very familiar to you:
It’s been more than a century since Ernst Mach took those images of the bullets, but the technique used for this jet photo is exactly the same. The New York Times has a write-up on how it was produced. For most people, it wouldn’t have the visual punch of the wrongly-labeled photo by Ensign Gay. But having been down the various rabbit holes, we know what this image is actually showing, and that what Mach did in 1888 with a bullet is the exact same thing a NASA scientist did in 2015 with a supersonic jet.
And that is cool.