It's just a test. Only a test. Do not freak out.

Federal officials will try out the Emergency Alert System at 11 a.m. Wednesday for the first time.

The 30-second message will be similar to other emergency broadcasts that have taken place for more than half a century.

But this one will be different. It'll be the first time the alert system will be used at the national level, and the test will be beamed simultaneously to more than 20,000 TV and radio stations.

No matter where you tune in, or what satellite TV channel you're watching, you'll hear the same message and tones: several beeps followed by a message, then more beeps.

The test will allow the Federal Emergency Management Agency, the Federal Communications Commission and the National Oceanic and Atmospheric Administration to assess the system for real emergencies.

The test is not expected to affect mobile phones or landlines.

A national alert system, formerly called the Emergency Broadcast System, has been around for decades, and could, in theory, be used by a president to warn the populace of incoming missiles or other national emergencies.

"It predates the Cold War," said Barb Graff, director of Seattle's Office of Emergency Management.

The system is tested in the Puget Sound area on a monthly basis. It was once used to broadcast a warning before the eruption of Mount St. Helens in 1980, Graff said.

It can also be used to issue Amber Alerts, or notices about missing children. It cannot be used for anything minor, Graff said.

"We think a national test is a great idea, and probably overdue," said Mark Layhew, director of the State Patrol's communication division, in a news release. "But we don't want people wondering about it and calling 911 for information."

Calling 911 should only be for police, fire or medical emergencies, Layhew said.

Seattle Mayor Mike McGinn urges everyone to spread the word.

"Tell your neighbors, co-workers, friends and family," he said. "And remember not to stress. It's just a test."

Information in this article, originally published Nov. 8, 2011, was corrected Nov. 8, 2011. A previous version of this story said the alert would occur at 11 p.m. The alert is scheduled for 11 a.m.