NEW YORK, July 26 (Reuters) - Are pop-under ads, which open new Web browsing windows that promote companies like X10 Wireless Technology, too annoying to be effective?

According to research firm Jupiter Media Metrix Inc the X10 ads reach a third of global Web users, but few users actually buy the wireless cameras and other products being advertised.

Nearly two-thirds of people who see X10's inescapable ads close the browsing window within 20 seconds, according to a Jupiter study released on Thursday.

"Such a dramatic drop-off rate is pretty compelling evidence consumers are getting annoyed," said Jupiter analyst Marissa Gluck.

The study found that the number of users who enter a secure Web-browsing mode from X10's ads -- indicating they are about to use a credit card -- is infinitesimal, smaller than the margin of error.

While Web users could once ignore banner ads online like drivers averting their eyes from highway billboards, the new pop-up ads may require the online equivalent of a television remote control.

"Consumers may sit through a couple of ads they find entertaining," Gluck said, "But it's like when Crazy Eddie comes on screaming on TV -- they'll change the channel."

Web advertisers may shoot themselves in the foot with the ads, Gluck warned, as consumers become increasingly annoyed with ads they cannot escape.

"The data implies that advertisers may be buildingawareness, but they're not building affinity," she said.

Consumer annoyance aside -- Gluck said one irate Web user tracked her down and demanded to know how to stop X10's pop-under ads -- the Web sites that make money from the ads are unlikely to cut off a lucrative source of funds amid an advertising slowdown.

"When this opportunity came to us, we weighed it against what we wanted to achieve in diversifying our ad formats, and it seemed to be a good choice," said Christine Mohan of the New York Times Co, one of the most prominent content sites to display X10's ads.

"We've had some negative comments, but nothing disproportionate to anything new we've launched," she said.

X10's sky-high Jupiter rating -- the firm ranked it the fourth-most visited Web site in June, ahead of well-known names like eBay  and TerraLycos  -- is not without controversy.

Rival research firm Neilson/Net Ratings does not include pop-up ads in its measurements. But Jupiter says it is trying to objectively report the Web user's experience.