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The U.S. podcast audience hit 18.5 million in 2007 and is on track to increase 251 percent to 65 million in 2012, eMarketer estimates.

Out of the 65 million listeners, 25 million will be active users who tune in once a week. "As the US podcasting industry matures it is unquestionably creating a listening audience," says Paul Verna, eMarketer Senior Analyst and author of the new report, Podcast Audience: Seeking Riches in Niches, "And along with a larger audience comes increases in advertising spending tied to podcasts."

Because of the audience growth, U.S. spending on podcast advertising will increase to $435 million in 2012, up from $165 million in 2007.

The reason for the growth of the podcasting audience can be attributed to a number of factors. Growing awareness of podcasting along with radio's use and promotion of the medium. In addition the ease of consumption for podcast content, more portable players and smart phones with reasonable mobile data plans.

"No one will argue that mobile devices and communication are becoming widespread," says Mr. Verna. "Even so, the majority of podcasts are actually experienced on PCs, not portable devices."