What Sonic Detectives Listen for When Rockets Launch

Rocket launches are loud, and big rockets are louder. Launches used to be occasional spectacles, and not many people minded the noise.

But the pace has quickened. SpaceX, the rocket company started by Elon Musk, now sends a Falcon 9 rocket to space at least once every few days from launchpads in Florida and California. Other companies, including Jeff Bezos’ Blue Origin, also have ambitions to send rockets to space at an accelerating pace.

And the noise is no longer just the roar of the rockets heading upward, but also the sonic booms of rocket boosters returning to Earth.

That noise shakes windows and foundations and wakens sleeping people. How the sound waves travel — shifted by wind, bending and reflecting off layers of the atmosphere and the ground below — is complex. Rules and limits that were designed for airports and rock concerts may not suffice.

“There is a gap in the science,” said Kent Gee, a professor of physics and astronomy at Brigham Young University in Utah who is one of the few scientists studying the sounds of spaceflight. “We don’t know what people actually find acceptable.”

And SpaceX is testing Starship, the largest and likely the loudest rocket ever. To fulfill his dreams of sending people to Mars one day, Mr. Musk envisions a steady parade of Starships launching from sites in Texas and Florida.

Triple Boom

A SpaceX booster falling back to Earth faster than the speed of sound generates a triple sonic boom: boom buh-boom.

A simulation of a Falcon 9 booster returning to Earth shows cones of ⬤ high pressure and ⬤ low pressure as it falls through the atmosphere.

The falling booster compresses air into a high-pressure shockwave, which is heard as an initial BOOM.

A second BUH is generated by low pressure from the lower parts of the booster meeting high pressure from the fins that steer the booster’s descent.

A low-pressure wake behind the booster creates the third BOOM as air pressure returns to normal.

The triple boom generates the highest pressure of a Falcon 9 launch, as measured by a microphone placed five miles away.

This image shows a Starship booster on its way back to the launchpad. A Starship launch generates about 11 times the acoustic energy of a smaller Falcon 9 rocket.

A supersonic jet slicing through the air creates only two booms, from the front and back of the plane, because it is more aerodynamic than the rocket booster.

Sources: Kent L. Gee, Mark C. Anderson and Logan T. Mathews, Brigham Young University; J. Taggart Durrant et al., Acoustical Society of America. Starship photo by Eric Gay/Associated Press. Jet photo by NASA/EPA via Shutterstock.

We are having trouble retrieving the article content.

Please enable JavaScript in your browser settings.

Thank you for your patience while we verify access.

Already a subscriber? Log in.

Want all of The Times? Subscribe.