An emergency alert test will sound Oct. 4 on all U.S. cellphones, TVs and radios. Here's what to expect.

Your electronic devices may alarm you on Wednesday afternoon — but there's a reason for that.

A nationwide test of the federal emergency alert system will be broadcast at approximately 2:20 p.m. EDT to cellphones, televisions and radios across the United States at around the same time. Most Americans with wireless cellular devices will receive an emergency alert message, as will most whose televisions or radios are on when the test occurs.