PENSACOLA, Fla. -- Low-frequency noise such as the whomp-whomp of helicopter rotor blades can be more than just annoying. Research indicates it can raise blood pressure and hasten fatigue.
Navy scientists assigned to develop ear protectors for helicopter crews found no material on the market that would adequately block the sound, so they invented a new technology.
Their patented solution shows so much promise that commercial interests are lining up to use it in appliances, automobiles, airplanes, spacecraft, ships and buildings as well as for ear cups, said Capt. Bob Hain, science and technology director at the Naval Aerospace Medical Research Laboratory.
"It's a technology as opposed to a material," said Gerry Thomas, head of acoustics for the lab at Pensacola Naval Air Station. "So far every material that we've attempted to apply the technology to, we've been successful."
That includes epoxy resins, silicon gels, polyurethane foam, plastics and silicon- and carbon-based rubbers, said Thomas, who has a Ph.D. in acoustics from the University of Florida.
Tests show, depending on the application, the technology can improve sound absorption by 50 percent to 1,000 percent. A quarter-inch thickness can block as much sound as a foot of concrete.
Mold-Ex Rubber of nearby Milton is licensed to use the process in certain products and over the past two years sold Ford Motor Co. more than 1 million exhaust recirculation hoses that reduce engine noise. Production, however, has discontinued because Ford made a design change that solved the problem another way, said Bruce Guy, Mold-Ex vice president and technical director.
Later this year, he said, the company, a subsidiary of Norfolk, Va.-based Southland Technologies Inc., will begin using the formula in rubber and plastic protectors as well as foam insulation for boat builders.
Thomas and co-inventor Bill Cushman, who worked on the project on a contract basis, relied on basic physics for their sound solution.
"We knew that laminates of different materials were better than homogeneous materials in blocking lower frequencies," Thomas said. Sound waves lose energy at the point where dissimilar materials meet, but such layering would be impractical for ear cups.
"We wondered if it would work using very small particles that were very dissimilar since progression of sound requires molecular collisions," Thomas said.
The answer was yes, but it took 41 attempts before they arrived at a correct balance of high- and low-impedance particles. "The formula is very specific," Thomas said. "If you are off by a couple parts per hundred, you lose the effect."
The technology works across the frequency spectrum but is particularly prized for its low-frequency performance because nothing else has been effective in that range, Guy said.
Molded products such as the ear protectors represent only a small fraction of potential uses so the lab now is focused on perfecting materials in sheet and spray-on forms.
"That's the big enchilada," Thomas said. "It's just been this past summer that we've had a sufficient number of breakthroughs because the science involved with large sheets is very different than the science involved with small surface areas and airborne sound."
The lab also has approached Florida State and Florida A&M universities for help in applying the technology to high-strength composites such as those used to make aircraft. A chemical company wants to try it with fibers, including Kevlar, a main component in helmets, Thomas said.
Since initial research in 1993-95, Cushman's contract has expired but he is working independently to advance the concept in what he termed a friendly competition with the Navy lab.
He has patented the use of discarded tires, frozen and broken into crumbs, as a low-cost raw material and a more expensive approach combining various technologies.
Cushman, who has a Ph.D. in sensory psychology from the University of Maryland, also has joined with Intelligent Automation Inc. of Rockville, Md., to develop headsets for the Navy that can be used in noisy environments. They are in the first months of a two-year contract to combine soundproof ear cups with microphones that digitally cancel background noise.
The Navy lab has spent nearly $2 million developing its technology, but if sheet materials prove successful that investment will be returned within months in manufacturing royalties, Thomas said.
The military will reap other benefits just from better hearing protectors that can be worn around jet engines, big guns and other noisy environments.
The Navy spent $37.5 million in one year -- 1995 -- on hearing disabilities and the Department of Veterans Affairs paid out $3.68 billion from 1968 through 1995, Hain said.
Noise "not only affects someone's hearing," he said. "If you can't hear what's going on, it affects your ability to accomplish your mission."