Roars to whispers: NASA's quest to mute jet noise pollution
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Roars to whispers: NASA's quest to mute jet noise pollution

Sep 24, 2023

NASA/Quentin Schwinn

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In the perpetual symphony of progress, there's one discordant note that communities around airports, and possibly even aviation enthusiasts have long wished to mute: the thunderous roar of jet engines.

Hidden within the history of noise reduction is NASA's Glenn Research Center, an unsung hero in the pursuit of quieter skies. For more than seven decades, this research hub has been on a mission to silence the once-deafening jet engine roars, resulting in groundbreaking innovation.

Venture inside the NASA Glenn Research Center, and you'll uncover a story of relentless pursuit. Nestled within its campus is the Aero-Acoustic Propulsion Laboratory (AAPL), a distinctive hemispherical structure colloquially referred to as "the dome."

This unique 130-foot diameter and 65-foot-high hemispherical structure is not just an architectural marvel; it is a testament to years of dedication to quell the cacophony that once characterized jet propulsion.

Amid its fiberglass-clad interior, intricate microphone arrays capture every decibel, offering insights into the tumultuous marriage of hot exhaust gasses and frigid atmosphere that birthed the ear-splitting roars.

This facility is no isolated experiment; it is a chronicle of history. From the clamor of the first jet airliners to the hum of turbofans that brought respite, the AAPL has been a guardian of progress.

NASA/Bill Bowles

Decades ago, when aviation pioneers first conquered the skies with jet engines, a new challenge emerged: the cacophony these machines created. Communities around airports voiced their discomfort, sparking NASA Glenn's inquiry into the roots of the ruckus.

The team mounted jet engines on steel stands placed microphones around them, and soon stumbled upon a surprising discovery: it wasn't the mechanical elements of engines but the turbulent tango between scorching exhaust gasses and icy air that composed the ear-piercing symphony.

This discovery led them to test various innovative nozzle and ejector designs to balance the effects of the additional sound-deadening materials on performance and weight.

Fast forward to the era of turbofans, an innovation that muffled the roars while boosting efficiency. Despite the increased efficiency and lower noise of these turbofans, the pursuit of serenity persisted.

With the Federal Aviation Agency’s new standards and an increased interest in vertical and short-takeoff and landing (V/STOL) aircraft in mind, NASA Glenn constructed new outdoor test stands, including the Powered Lift Facility (PLF) in 1986.

The PLF would, however, suspend testing three years later, following complaints from the local community concerned about its noise levels.

This would translate into NASA building a structure to cover the PLF and a newly introduced Nozzle Acoustic Test Rig (NATR), part of the High-Speed Research program aimed at supersonic transportation, in 1990.

AAPL embraced change and expanded its arsenal with the Advanced Noise Control Fan (ANCF) rig, the Small Hot Jet Acoustic Rig (SHJAR), and an acoustic arena that transformed it into a hub of sonic exploration.

The latest endeavor involves bridging the gap between scale-model tests and real-world flights, promising a future where roaring skies give way to tranquil whispers. Through collaboration with Learjet 25 flight tests, the AAPL is on the cusp of revolutionizing the prediction of takeoff noise for forthcoming supersonic aircraft.

The once-thunderous skies are evolving, and so too is NASA's Glenn Research Center, turning the tide against jet engine noise pollution.

This article was written and edited by a human, with the assistance of Generative AI tools. Find out more about our policy on AI-powered writing here.