Rocket is like a guiter, they both resonate.
When you pick up an acoustic guitar, one of the first things you’ll notice is the body is basically an air chamber. The shape of the chamber is designed to be ‘in tune’ with the sound from the strings. When one plucks the E string and the body of the guitar vibrates, producing “sympathetic” E-frequencies of its own. It is called ‘resonance’ and it’s a big part of what makes a great guitar.

Resonance may be great for guitars, but it can be disastrous for a spacecraft. When the shuttle lifts off, the main engines roar so loudly that a person standing near the pad would be killed—not necessarily by the heat of the exhaust, but by the sound of the engines. The engines “strum” the spacecraft with incredible force. Rumbling sound waves penetrate the shuttle and its cargo, seeking, probing, shaking.

The engines aren’t the only source of sound. After liftoff, the rocket rips its way through the atmosphere en route to space. Rushing air creates strong aerodynamic noise, which rattles the ship.

Even in space, the noises don’t stop. Vibrations can ripple through a spaceship when it docks with another ship, or when it fires its maneuvering thrusters. The goal of engineers is to make sure these vibrations die out quickly, before they do any harm. When one strums his guitar, the sound lasts a long time, that’s the sustain. Long-lasting vibrations are encouraged by the fabric of the guitar itself. Notice how the guitar is made of lightweight, flexible wood—a material that likes to vibrate.

Rockets are made of stiffer, heavier materials, that damp resonances and reduce sustain. But that’s not the only trick spacecraft designers use. Sometimes they modify the shape of the rocket, adding supports or filling in empty spots. The purpose: to detune the rocket from itself.

Detuning rockets isn’t easy because, as instruments, they’re much more complicated than guitars. A guitar is constructed from dozens of parts. A typical rocket, on the other hand, is made of thousands of parts. The space shuttle famously contains more than a million components. All these pieces vibrating together produce a cacophony of frequencies ranging from subsonic waves that only an elephant could hear to high-pitched whines akin to fingers scratching a blackboard.

Which frequencies might do the most damage? What parts of the spacecraft are most vulnerable to resonance? And how do you de-tune this complicated instrument?

To answer these questions, NASA engineers have developed “sound studios” for spacecraft. By observing the response of “test articles” to the sounds, engineers can discover resonances and make changes to squelch them.
Acoustic testing has been a regular part of rocket design since the Apollo program four decades ago.