Naval research grant leads to a bubbly solution to hearing protection

Above 85 decibels, the threshold for harmful sound levels set by the U.S. Occupational Safety and Health Administration, prolonged exposure to extreme noise can permanently damage a person’s hearing. But naval personnel who work on aircraft carrier decks regularly encounter levels greater than 150 decibels.

Just the 180-decibel roar of a plane’s afterburner at launch exposes workers to more than 200 percent of the recommended dosage of noise.

To develop new solutions for protecting naval personnel, the Office of Naval Research established a dedicated Jet Noise Reduction Program, which has awarded $1.1 million in funding to a Virginia Tech-led team to tackle it. Joseph Meadows, associate professor of mechanical engineering, and Brigham Young University Professor Kent Gee came up with an elegant solution that implements a wall of water infused with air bubbles to block damaging acoustic waves.

A curtain of bubbles

Jets on an aircraft carrier need to achieve high speed quickly to take off. Prior attempts to reduce noise have focused on changing the jets themselves but had an adverse effect on engine performance.

“The big challenge is reducing engine noise without reducing engine performance,” Meadows said. “As engines get more powerful, they generally get louder. Our idea is to put a physical barrier between the engine exhaust and naval personnel working on deck; however, a stationary, rigid barrier is not practical for aircraft carrier decks.”

What would this physical barrier be made from? What material is plentiful around an aircraft carrier and has the potential to block acoustic waves and lower the amount of sound transmitted? Water.

Meadows and Gee set about finding a way to turn the ocean of resources around an aircraft carrier into a physical barrier to protect the hearing of deck workers. A water curtain would take advantage of acoustic reflection at the air/water interface with one additional trick: Meadows and Gee proposed adding air bubbles, which in marine applications have been shown to reduce transmitted acoustic waves propagating in water.

“The hard part is maintaining a sheet of bubbles,” Meadows said. “Gravity causes the bubbles to separate, so we’re working on determining the optimal operating parameters to minimize the amount of acoustic energy transmitted.”

First the noise, then the noise solution

With previous funding from the Office of Naval Research, Meadows’ team members had already designed an afterburning jet noise rig capable of reproducing the acoustic spectra of from tactical aircrafts. That gave them a relatively low cost way to test their noise reduction technology.

“This is the first time these conditions have been produced in a laboratory environment,” Meadows said. “These measurements have generally been gathered from full-scale engine measurements.”

Now they’re developing a fundamental theory for operating parameters, bubble density, and diameter distributions with the aim of designing a system for deployment of the bubble shield, which is a secondary objective of their work. Ultimately, they plan to propose a design to the Navy for implementing the shield on an actual aircraft carrier.

“We are hoping to test in early spring,” Meadows said. “The next step will be testing the prototype in a relevant acoustic field to demonstrate its effectiveness.  We are also developing the fundamental modeling approach to design such systems, which will be validated against experimental data. Finally, we will recommend a design for the aircraft carrier deck.”