Engineers Want To Muffle Roar Of Jets

Researchers at the University of Cincinnati are working to muffle these sounds without sacrificing engine performance.

"Jet noise is a big problem," said Kaurab Gautam, who is studying aeroacoustics in UC Distinguished Research Professor Ephraim Gutmark's Gas Dynamics and Propulsion Laboratory.

The leading causes of military disability claims are hearing loss and tinnitus, sometimes from working around chronic loud noise. The disabilities affect more than 3.6 million former service members.

Sound waves produced by jet engines can even damage the aircraft itself, Gautam said.

"The goal is to make quieter jets," Gautam said. "But you have to think about performance as well."

Sponsored in part by the U.S.Office of Naval Research, the lab is developing new jet engine nozzles that can dampen the sounds produced by supersonic aircraft.

In the lab, Gautam measures the noise generated by various types of jet engine nozzles. Jet engines can produce roars, rumbles and shrieks based on the sound's frequency (or pitch) and amplitude (or loudness). The most obnoxious noises from many commercial aircraft combine several of these sounds, Gautam said.

Gautam works in two anechoic chambers in Gutmark's lab, which feature noise-absorbing foam padding on the ceiling, floor and walls to dampen sound. With no hard edges to bounce off, the human voice sounds unnaturally flat like when the pressure in your ears hasn't equalized at high altitude on an airplane.

This lack of auditory feedback can be so disorienting that some people who work in these spaces report feeling disoriented or even claustrophobic. But Gautam said it doesn't bother him.

Researchers set up two arrays of microphones around the jet engine nozzle to measure near and far acoustic waves generated by an engine bolted to the floor that produces enough propulsion to reach the equivalent of Mach 1.5 or 1,150 mph.

Researchers also capture images of the air flow thanks to the diffraction effect of the high-density air as it squeezes through the engine nozzle. A high-speed camera that can shoot 1 million frames per second captures this air flow using an array of mirrors and UV light. The resulting video resembles ripples in water.

Since visible disturbances in the air are associated with different types of noise, the images practically allow researchers to "see" the sounds created by different shapes of engine nozzles, Gautam said.

"We have to understand how the nozzle reduces the noise," he said. "So we have quantitative measurements of the jet flow and the qualitative description of the flow."

A rounded nozzle creates a bigger heat signature, making fighter planes more susceptible to enemy missiles, he said. So Gautam is experimenting with numerous rectangular nozzle designs in more than a dozen iterations, similar to the kind used in advanced fighters such as the F-22 Raptor.

"They're not perfectly rectangular. It's more of a trapezoid," he said. "We're measuring the jet flow and capturing the sound."

UC students such as Gautam use high-end 3D printers to create precise models of their nozzles for use in jets that blow cold air. But they can be printed in metal for testing in high-temperature jet engines.

Gautam said a lot of science goes into the project, from signal processing and flow diagnostics to rapid prototyping. His work with propulsion was recognized with a 2025 graduate award from the American Institute of Aeronautics and Astronautics.

Featured image at top: UC doctoral student Kaurab Gautam is developing new designs to dampen the sounds of jet engines. Photo/Michael Miller