CHICAGO — Peter Hubert keeps a mirror on his desk so he can see people walking up behind him, even if he can't hear them.

Headphones clamped on, MP3 player plugged in, the 28-year-old draftsman has fashioned a virtual office using invisible walls of sound. Listening to heavy-metal rock doesn't distract him from the precise computer-based drawings he creates for Bigelow Homes in Aurora, Ill.

To the contrary, "it puts my head somewhere else so I can concentrate on what I'm doing," he said. "It helps keep me focused."

Technology makes it easy for workers like Hubert to bring their music collections to the office and to retreat, at least some of the time, into audio worlds of their own making. Listeners say their music energizes them, tunes out distractions and makes them more productive.

But as more employees march through the day to the beats of their own drummers, personal music players are changing the ways in which workers interact and collaborate while posing challenges for some bosses who consider the devices unprofessional and disruptive, employers and workplace experts say.

"Companies are struggling" with all kinds of personal technology, said Richard Chaifetz, chief executive of employee assistance firm ComPsych Corp. "A lot of employers just don't want to have a personal playground atmosphere in the workplace."

Music at work is not new. Factories have piped in tunes for decades on the theory that music makes workers more productive. In offices, workers listened to portable cassette players or popped CDs into their computers long before the Internet expanded their choices by offering streaming radio and downloadable music files.

What is changing workplace behaviors is the popularity of devices such as iPods, which allow workers to carry around thousands of songs — draftsman Hubert brings 3,500 to work — and to surround themselves with soundtracks of their choosing.

Eighty percent of technical and creative employees — programmers, engineers and graphics designers, for instance — listen to music more than 20 percent of their working hours, said Tom Nolle of CIMI Corp., a New Jersey-based research and consulting firm.

"It's only been within the last 15 months or so that MP3 players have become the main source of workplace music," said Nolle, who listens to Internet radio when he works on demanding projects.

The technology is ushering in new social conventions at companies like Chicago's Closerlook Inc., a strategic communications firm where 35 employees work in loftlike spaces.

Wearing earbuds or headphones telegraphs the message, " 'Unless it's urgent, please do not disturb,' " said David Ormesher, the firm's founder and chief executive. "It's almost like you're in an office and you have a closed door or an open door. There's new sensibilities around when you can interrupt and when you can't."

Rather than isolating co-workers, music offers common ground, he added. "They share playlists on the office network and everyone winds up playing each other's music — jazz, classical, hip-hop, world music. You learn a lot about each other just by checking out playlists."

iPods are the rule rather than the exception at Chicago advertising and marketing agency Two X Four Communications.

"They're pretty much wired into these kids' heads," said David Stevenson, president and chief creative director, who carries an iPod loaded with TV spots the agency created for the Chicago White Sox.

"Most of our employees will sit at their desks and listen to their iPods all day long," Stevenson said. "Once you're on a network you can see everybody else's music."

There's one stricture: "No pirated music," he said.

At World Business Chicago, a quasi-government economic development agency, research director Glen Marker doesn't object when his assistants put on headphones.

"I can say their names and they hear me," Marker said. "While they may be rocking out, they're not going hog wild."

The debate in Marker's office centers on whether Internet radio listeners are slowing the office's high-speed network. "If our IT guy had his way, he'd clamp down on that," Marker said. "But there tend to be more pro-(radio) people, so I think we would win out."

At SBC Communications Inc., a crackdown on personal appliances that was part of an effort to trim utility costs did not extend to MP3 players, which use batteries that sometimes need to be recharged.

"Coffee pots, radios, fans, heaters, fish tanks — they actually put fish tanks on the list — those are discouraged," but personal listening devices were not banned, said John Hunter, a manager at SBC's sales and support office in Oak Lawn, Ill., for government, medical and education accounts.

Hunter added he had "no issues" with employees using headphones in private areas where they don't deal with customers. "People who are doing behind-the-scenes work and are working efficiently, that's fine."