“The behavior of listening to music, at the end of the day, is almost entirely audio driven,” said Eric Ronning, co-president, sales of the Internet Radio firm TargetSpot. But many music playlist sites have been hesitant to push audio ads, instead building their ad business around display advertising and sponsorships. Ronning predicts that may change as these businesses evolve. “You can argue that playlists are highly engaging, but they are also an iPod like. I don’t expect an ad so much in that experience…and almost none of that is visual.” Yet many Web music purveyors see visual ads as better suited for such an interactive medium. For example, when users listen to free CDs on AOL Music, “they may be focused on other things, but there’s lots of natural engagement moments that bring you back to the site,” said Mike Rich, AOL’s senior VP, AOL Entertainment. “For us, context and curation are key to keeping users engaged.” (AOL Music’s audience surged by 24 percent to 28 million uniques this past May, per comScore). That’s true even for a seemingly background-relegated music product like the popular Web radio platform Pandora. Its users actively rate songs 7 million times a day in aggregate. “That’s seven million times people come in contact with your ad,” said chief revenue officer John Trimble. Still, Pandora has introduced audio ads in the past year.