What sonic detectives listen for when rockets launch

SCIENCE

In an image provided by Marshall Space Flight Center/NASA, a test of a Saturn V rocket at the Mississippi Test Facility is shown in 1966. Physicists who record rocket launches and landings, most often by SpaceX, are learning important facts about the acoustics of spaceflight. Marshall Space Flight Center/NASA via The New York Times

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By Kenneth Chang

The New York Times

STARBASE, Texas — 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 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, Musk envisions a steady parade of Starships launching from sites in Texas and Florida.

The best time to address rocket noise is before problems become pervasive and entrenched, Gee said. He is not calling for an end to launches, but says data could point to how to lessen disruptions.

“If we wait 10 years, it’s too late,” he said.

Boom buh-boom

As an undergraduate at Brigham Young, Gee liked physics and music, so the field of acoustics, and noise in particular, intrigued him. For his doctorate at Pennsylvania State University, he studied noise propagation from F-22 stealth fighters. He then returned to BYU as a physics professor and started working on rockets.

In recent years, Gee and his team have mostly studied SpaceX’s rockets because the company’s Falcon 9 launches far more frequently than any other rocket.

One of the questions they have studied is why a Falcon 9 booster falling back to Earth at supersonic speeds generates a three-part sonic boom. Instead of the “boom-boom” made by a supersonic jet, spectators hear a “boom buh-boom.”

Gee’s team has also found that not every launch sounds the same, not even when it is the same type of rocket launching from the same location.

Falcon 9s lifting off from Vandenberg Space Force Base in California, for instance, sometimes generate sonic booms heard about 100 miles away. The most recent defense policy legislation, signed by President Joe Biden in December, acknowledges the disturbances and calls on the Department of Defense to minimize them.

At a meeting of the Acoustical Society of America this month in New Orleans, Makayle Kellison, a member of Gee’s team, reported that the uneven topography around the Vandenberg site seems to focus sound waves into certain areas.

And the corridor where the booms were loudest changed between summer and winter.

“During winter, we’re finding that it shifts up closer to Santa Barbara,” Gee said. Vandenberg has conducted rocket noise studies for years, and in 2024, it added Gee’s measurements there after complaints about the sonic booms.

Gee and his team have also listened to the noises of other rockets, including the small Alpha rocket from Firefly Aerospace, the first launch of NASA’s giant Space Launch System rocket, the last flight of the Delta IV Heavy from United Launch Alliance and ULA’s soon-to-be retired Atlas V.

The SLS rocket and the Delta IV Heavy were loud, but not as loud as would be expected given the magnitudes of their thrust. That data could give insights for designing quieter rockets in the future, similar to how jetliners today are not nearly as loud as those flying decades ago.

Indeed, the SLS launch was almost soundless for one team of observers about 20 miles west of NASA’s Kennedy Space Center in Florida. Yet the roar was heard at another recording station farther away in the same direction.

Gee suspects weather conditions in the atmosphere played a role.

The researchers have also debunked a science fairy tale, that loud vibrations from the Saturn V, the rocket that took NASA astronauts to the moon, melted concrete and set grass on fire more than a mile away.

The researchers calculated that it generated 203 decibels. That shock wave would have heated the air by about 40 degrees Fahrenheit — not nearly hot enough to melt concrete or ignite grass.

Then there is Starship.

This month, SpaceX received approval to launch the behemoth rocket 25 times a year from its Starbase site in Texas, up from five. The company is seeking even more Starship launches from Florida.

The city of Cape Canaveral last month hired scientists from the Florida Institute of Technology to monitor sound levels, vibrations and air quality changes for every launch in Florida for at least a year. That will provide a baseline for comparisons with Starship in the future.

The ninth test flight of Starship is to occur as early as Tuesday evening. Gee and colleagues are in South Texas, setting up their recording stations.

disappointing Thump

The last time they were there was in November, for the sixth test flight.

Their equipment, deployed at 21 sites around Starbase, is unimpressive in appearance. A microphone on a stand low to the ground inside a cylinder of black foam to block wind noise, all sitting on a white disk. Everything else — the digital recorder, a GPS receiver and a battery to power everything for a few hours — sits within a metal suitcase.

The microphones have to capture a swath of frequencies wider than what humans can hear, including low-frequency rumbles that are not so much heard as felt. They also have to accurately record volumes ranging from the almost inaudible to the loudest roars. The data is gathered at a rate of more than 100,000 times a second. That’s about 60 times faster than what the human ear can accomplish.

“It’s hard to record a rocket accurately,” said Grant Hart, a BYU physicist who teamed up with Gee a few years ago.

On the morning of the Starship launch, Hart and Mark Anderson, a doctoral student, set up recording stations on South Padre Island, to the north of Starbase.

Other microphones had been deployed within a mile of the launchpad, in the nearby town of Port Isabel, on the beach to the east of the launchpad — the BYU team had spent much of the previous evening dodging waves of the incoming tide — and to the west in Brownsville.

The researchers were looking to understand how sound varied with distance and direction. “We’re just here to get the physics right,” Anderson said.

The recordings do not sound very impressive unless one listens to them through high-end headphones or a music system that includes a large subwoofer to generate the low-frequency thumping. Even then, they cannot reproduce the full-body experience of watching a launch in person.

A trip to record the fifth test flight of Starship, which took place a month earlier, was financed by BYU, to offer its undergraduates the chance to take part in hands-on research, and a private organization, which remains anonymous.

The researchers said it was not SpaceX, a competitor of SpaceX or a politically motivated critic of Elon Musk.