Google spokesman Michael Mayzel said this week that the company will begin a public test of Google Audio Ads by the end of the year. Advertisers will be able to go online and sign up for targeted radio ads using the same AdWords system they use to buy Web search ads.

Google is generally testing its ability to move into offline media, this week saying it would help customers buy advertisements in 50 U.S. newspapers.

It made a clear move into radio in January when it agreed to pay more than $1 billion, depending on performance, for dMarc Broadcasting Inc., which connects advertisers to radio stations through an automated advertising system.

It's all part of what Google Chief Executive Eric Schmidt has said is an investment in radio advertising that could grow over time to include up to 1,000 Google employees -- not just in ad sales, but also in engineering and operations.