Say, you're near a tractor and worry the noise is loud enough to cause ear damage. Simply press your thumb on the handheld device's sound port to see whether it's too loud. Likewise, you can press your iPod's ear buds against the port to know whether you ought to turn the volume down.

Three staff members with the Hollins Communications Research Institute, a nonprofit organization that does research on speech and hearing, spent about a year developing the device, called Ear3. The group is selling the device for $50 at ear3.info.

"We're so concerned about eating properly, exercising and getting physical checkups, and we're idiots about our ears," executive director Ronald Webster said.

People can lose half of their hearing range before they notice it, said Mr. Webster, a retired Hollins University psychology professor who founded the institute in 1972.

Of the 33 million American adults with some degree of hearing loss, about 22 million suffered permanent damage from loud noise, according to the National Institute on Deafness and Other Communication Disorders.

Young, healthy ears can recover from limited exposure to loud noise, Mr. Webster said, but "repeated insults lead to hearing loss."

Mr. Webster said the institute team developed the measuring device because they were concerned about the sound that music players such as Apple Computer Inc.'s iPod can emit. A sound level of 85 decibels is a danger zone for hearing loss, he said, and music players can exceed that.

"We're beginning to see more and more younger people showing up" with hearing loss, he said.

Earlier this year, Apple updated its iPod software so listeners can set how loud their players can go. But the controls do not specify decibel levels, leaving listeners guessing.

With the Ear3, the danger signal flashes slowly at 85 to 90 decibels, increasing to steady red at 90 and rapid-fire flashes at 100.

"We're just so absolutely ignorant about sound," Mr. Webster said. "We expose ourselves to all kinds of threats and never even think about it."