At the same time, the shift from PC-based display ads to mobile audio ads is driving down the blended CPM.

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Banners (often hidden behind at-work listeners' spreadsheets and documents) back up audio ads with, for example, telephone numbers or links to the advertisers' sites.

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Pandora currently serves about 8 to 12 ads per hour which can consist of 7- to 8 interaction-based display ads and 3 to 4 audio ads.

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