The Oakland-based site Personalized Web radio service Pandora has added 15-second audio rolled out advertisements that will occasionally play between songs.

The new ads, which Pandora announced last night via its Twitter account, began running yesterday when users reported hearing a short, 15-second ad for the Fox TV show Lie To Me.

The ad plays after streaming 10 songs. Users can expect to hear 15 seconds of advertising approximately every 2 hours.

Listeners were not happy with the news and a number of Twitter posts were very opinionated in their objections. In response, Pandora said so you know, we did not take on audio ads lightly. We try to be extremely respectful of your listening experience, & promise to be prudent.

Pandora founder promised the site will never carry as many ads as broadcast radio, despite the fact it pays substantially higher royalty fees to the recording industry.

The fears people have about it overtaking the listening experience are unfounded. It's going to be a fraction of what you hear on broadcast radio, Pandora Chief executive Tim Westergren said in statement.

The changes affect the 21 million users of its Web site and 3 million people who use its iPhone application, which was the most-downloaded app of 2008.

Pandora is recommending that users who do not want to listen to these audio ads can sign up for their commercial plans costing $36 per year.