Joby in late 2023 conducted a demonstration flight in New York City with this S4 prototype. Credit: Joby Aviation
AIAA AVIATION FORUM, Las Vegas — The ambient sounds of daytime construction, chirping birds and city traffic would be louder than a Joby Aviation S4 electric air taxi flying 1,000 meters overhead, likely masking any noise from the aircraft, according to a paper presented here Monday by company engineers.
Take an average city dweller lounging at a cafe near busy streets: “Our aircraft is on the order quieter than that ambient noise, so it’s likely to be imperceptible, or you’d have to really listen for it and know what the aircraft sounds like to identify it,” Austin Thai, the paper’s lead author and the acoustic senior engineer for Joby, told me in an interview prior to the forum.
For more about advanced air mobility, receive the True Mobility newsletter in your inbox.
Thai and his co-authors concluded in their 19-page paper, “Contextualizing the Acoustics of Joby Aviation Aircraft Operations with an Ambient Noise Model,” that the sound of an S4 overhead was “negligible when accounting for the ambient – even in relatively quiet noise-sensitive locations such as parks, and are orders of magnitude lower than that of a helicopter with similar payload.”
For the paper, they obtained from the U.S. Navy the noise signature of a Bell Sea Ranger helicopter carrying a similar payload and compared it to the noise generated by an S4. Joby captured this noise signature during 2021 flight tests conducted in partnership with NASA, in which an S4 prototype was remotely piloted near Big Sur, California, over open fields arrayed with microphones.
When the aircraft flew at 185 kph at an altitude of 500 meters, the microphones registered 45.2 dB(A) — a form of decibel measurement adjusted to reflect how sound is perceived by the human ear. The Hearing Health Foundation in New York City describes 60 dB(A) as “background music” and 70 dBA as “average office noise.”
When NASA published the results of those flights in 2022, Joby indicated that S4s “will barely be perceptible” in cities, a finding that Thai said his recent study confirmed.
He told me that he’s received a high level of interest about this noise research from other aviation professionals. “I think that’s going to be the challenge in the next few years: trying to communicate the noise level in realistic environments and contextualizing it beyond just showing a decibel level,” he said.
In the new study, the noise data from the S4 and the Sea Ranger were overlaid on topographic maps that depicted a theoretical commute from downtown Los Angeles to John Wayne Airport in Orange County, a journey of about 65 kilometers. Even when the S4 passed over two park areas, “the sound exposure level of the Joby Aircraft was 10 to 15 dB(A) less than that of a helicopter,” the paper reads. “At the same two locations, the Joby aircraft is never above the daytime ambient” sound levels.
Joby’s six-propeller, five-seat aircraft — which has not yet been type certified by FAA — is designed to fly relatively short urban trips averaging 45 kilometers. Helicopters today conduct similar-length flights in some places, but they have been restricted in locations such as downtown San Francisco, except for emergency flights, due to noise complaints. Joby and other electric air taxi developers believe their designs would be much quieter, with the added bonus of not producing any fossil fuel emissions.
In addition, a helicopter such as the Sea Ranger tends to idle for a while before takeoff and after landing, the paper notes. In contrast, the S4 electric motors are generally spun up and down almost immediately before and after flight, decreasing the length of time any perceptible noise could be generated.
Joby has spent well over a decade developing its aircraft and measuring the noise level from it, but most of those tests were performed in remote areas to isolate the noise of the aircraft and gather pristine data. The company said that up until this study, applying that data to urban environments was a challenge.
Going forward, Joby is continuing to study aircraft noise with NASA, including how the sound is perceived by the human ear and what types of noise are most annoying to people on the ground. For one of the future studies, plans call for characterizing how tall buildings scatter, amplify or shield the aircraft noise.
Related Posts
