ILLUSTRATION: MICHAEL KUPPERMAN

Limor Fried got the idea when a friend with whom she was eating dinner broke off their conversation to answer her cellphone. Fried got mad. Then she got even, in the way a graduate student at the MIT Media Laboratory, very well might. She built a gadget.

She calls it the Wave Bubble because it creates a cellphone-free bubble of silence 4 meters in diameter. It does so by jamming the phones' radio-frequency bands with a junk signal of a few milliwatts.

She's not the first to make a cellphone jammer. They are for sale over the Internet as well as on the streets of New York and other big cities. Restaurants, hospitals, and schools reportedly have been buying them. In the United States, however, Federal Communications Commission regulations forbid using or even making a frequency jammer. And although enforcement has been lax to nonexistent, an FCC spokesman says the agency has begun to notify infringers of the rule that they could be fined and even jailed.

The widespread interest in jammers suggests an unslaked need that legal and technical refinements may one day allow legitimate companies to satisfy. The process is already under way in other countries. Brazil and Spain use jammers in prisons. Japan allows them in concert halls. In India, legislators have actually hung them on the walls of Parliament. France has authorized jammers in movie theaters as long as they don't block emergency calls, even though no technology has achieved that degree of discrimination.

Fried says she isn't sure that the FCC rules forbid her from building her own jammer. In any case, she says, the rules say nothing against documenting in detail how to make one, as she has done, at http://www.mit.edu/~ladyada/thesis.pdf (13.6 mb download).

Fried made the Wave Bubble for her master's thesis, "Social Defense Mechanisms: Tools for Reclaiming Our Personal Space," which argues that electronic devices increasingly distract and annoy people and that the electronics industry has had little incentive to address the problem. She concludes that citizens must therefore explore methods of self-defense.

Against television she has built Media-Sensitive Glasses, which darken their lenses upon detecting the characteristic 59.94-hertz flicker of a TV set. Perhaps it's not a very practical solution, as the wearer wouldn't be able to see other things, but Fried is making an almost artistic statement, not laying out a business plan.

It took her two months of work to design, prototype, and test the Wave Bubble. The parts cost less than US $100, and she says she thinks she could get that sum down to $70 by using cheaper components.

The device works by generating a range of voltages in a circuit that tunes an oscillator. This voltage-controlled oscillator's amplified output, in turn, spews out signals between 800 megahertz and 2.5 gigahertz, a range wide enough to cover the bands for CDMA and GSM cellphones, radio frequency identification tags, Wi-Fi networks, and the Global Positioning System.

"I tried it out on people around the lab, and it worked pretty well," she says.

Some call Fried's device a blunt instrument. "I believe that there are better ways of expressing one's artistic values than disrupting the safety and well-being of others," says Steve Mann, an electrical engineering professor at the University of Toronto and an MIT alum himself. Mann says the jammer could disable critical devices, such as health-monitoring equipment, or perhaps one of his inventions, EyeTap, a wearable device that projects information into the user's eye.

But again, the Wave Bubble was meant to provoke thought, and in that role it seems to have succeeded. Fried says that when employees of the cellphone giants Motorola and Nokia visited the Media Lab, they showed great interest in the gadget. One employee from Samsung even wondered out loud about incorporating its technology into a cellphone that could make calls while blocking those of nearby phones. Now we're talking! Or not.