Do you watch “Curb Your Enthusiasm?” Did you see the episode in which Larry David began talking to an imaginary companion in a restaurant in response to a single diner at the next table shouting into his cell phone? Do you sympathize with this picture?

I hadn’t heard about this, but I understand the appeal. It’s illegal, but so are radar detectors.

As cellphone use has skyrocketed, making it hard to avoid hearing half a conversation in many public places, a small but growing band of rebels is turning to a blunt countermeasure: the cellphone jammer, a gadget that renders nearby mobile devices impotent.

The technology is not new, but overseas exporters of jammers say demand is rising and they are sending hundreds of them a month into the United States — prompting scrutiny from federal regulators and new concern last week from the cellphone industry. The buyers include owners of cafes and hair salons, hoteliers, public speakers, theater operators, bus drivers and, increasingly, commuters on public transportation.