Cell-phone callers, do not panic. If, someday soon, you start hearing voices in your head, it may just be your mobile, trying to cut through the noise that keeps you from making sense of your conversation.

Crowded rooms and fast-revving cars have a way of overwhelming even the loudest of phone calls. Quiet chats turn into shouting matches. And every other sentence can quickly reduce to, "Hunh?"

But electronics-makers may have found a way to cut through the clatter.

It's called bone conduction. Instead of pouring sounds into the ear, the system converts them into vibrations, and sends them through the skull. Audiologists have used the technique for decades — to test ears, and to build specialized hearing aids. And now, bone conduction is slowly starting to make its way into consumer gadgets.

"The only thing more annoying than getting a call in a noisy, clamory place is not being able to actually take the call if you need to. Bone conduction could help fix that," said Joel Johnson, editor of Gizmodo, the online gadget bible.

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High fidelity
But it'll be years until a wide array of bone-conducting electronics are available in the United States, skeptics warn. And even when the gear is ready, the fidelity may not be quite right. The sounds absorbed through the skull are rarely as crisp as the ones taken in by the ear.

Ordinarily, a chain reaction is responsible for what we hear, explains

Richard Danielson, an audiology professor at Baylor College of Medicine. Sounds vibrate through the air, which rattles the eardrum, which shakes the three small bones of the middle ear. That sets off a series of vibrations within the cochlea — the series of spiral-shaped,

fluid-filled chambers containing the small sensory cells that are ultimately responsible for hearing.

Bone conduction short-circuits the process. The fluid in the cochlea stays stable. But the bones around it shake — stimulating those sensory cells and, in effect, tricking the hearing mechanism.

Late last year, Sanyo Electric in Japan unveiled what it claimed to be the world's first bone-conducting mobile phone. Plug up your left ear with your finger, and place the

TS-41 on your cheekbone, jaw, or the top of your head — and your phone calls will start rattling through your skull, Sanyo promised. "It's a perfect fit here in Tokyo with so many crowded noisy areas," said Sanyo spokesman Ryan Watson.

But at least one American reviewer was less than impressed with the phone, priced at 7,800 yen, or about $73. "It just sounded like a speaker phone vibrating more than normal," said Michael Oryl, editor of Mobile Burn, a cell-phone enthusiast site. "Way, way too loud."

Can you hear me now?
Other bone-conducting gadgets are in the works in Japan. Mobile-phone giant NTT DoCoMo, for one, is developing a wristwatch-shaped device that turns the index finger into a phone receiver. On the inside of the band of the "FingerWhisper" phone are actuators, which turn speech into vibrations, which are then sent up the hand. Stick a finger inside the ear, and the conversation becomes clear, NTT promises.

The FingerWhisper isn't commercially available, yet. But Temco, a Japanese walkie-talkie maker, is selling tiny, bone-conducting ear buds that serve as both receiver and as microphone for its line of radios. A Korean electronics firm has also been selling headsets based on bone conduction. And a British company, Feonic, is developing a series of bone-conducting gizmos, including a pair of swimming goggles with a vibrating MP3 player built in at the temples.

In the United States, Brisbane, Calif. start-up Aliph is relying on a bone-conduction-like system for its new Jawbone cellular headset. A sensor in the headset picks up vibrations in the wearer's head. Those vibrations indicate when a person is talking or not. That helps Aliph's algorithms sort out conversations from background noise — and eliminate the racket, notes company co-founder Alex Asseily. The Jawbone will also tweak the signal as it is coming into the headset, boosting volume and changing frequencies in order for the talk to be heard.

Military matters
The Jawbone came largely out of studies funded by the Defense Advanced Research Projects Agency, or DARPA, the Defense Department's futuristic research arm. The Pentagon has been interested in bone conduction and related activities for years. The reason: No matter how loud a G.I. shouts into his radio, it's hard to be heard over gunfire and mortar explosions.

So U.S. Special Forces have been testing out new helmets in Iraq with bone-conducting microphones, instead of the standard headsets. And in the next generation of soldier uniform, "Future Force Warrior" (FFW), bone-conducting gear will come standard.

The dime-size receivers, placed near the top of the helmet, were originally made out of metal, notes Jean-Louis "Dutch" DeGay, with the Army's Natick Soldier Systems Center. But, during testing, it became clear that some soldiers' heads "were literally too thick for that to work," DeGay says. Now, the FFW helmet comes with a bone-conducting gel-based sensor so sensitive that it picks up pulse and breath rates as well.

In the civilian world, bone conduction isn't about to replace standard speakers — pushing sound through skin and skull causes the signal to lose as much as 40 decibels, Danielson points out.

But on a windy street or in a jam-packed bar, when noise is filling up your ears, hearing a voice in your head — no matter how quiet — sure beats hearing nothing at all.

Noah Shachtman writes about defense, technology, politics, and geek culture for The New York Times, The Chicago Tribune, and Wired magazine. He is the editor of DefenseTech.org, a Web log covering the future of national security.