Podbridge is a valley company officially launching (actually relaunching) with the aim of bringing advertisers and podcasters together. There is a ton of activity in this space as advertisers awaken to this new medium and seek ways to efficiently buy “air” time and track their ads’ effectiveness.

As far as we can tell, Podbridge appears to be taking a unique approach. Here’s how it works. Advertisers buy into the Podbridge ad network using a self-serve online tool that is similar in some ways to Google’s AdWords. They can choose the category their ad falls into, set a daily ad budget of how much they want to spend, and upload the mp3 of their audio ad. Advertisers can also buy specific podcasts, if they want.

On the podcaster side of things, the publisher wraps his podcast feed in some special Podbridge code. When a potential listener clicks on the podcast feed, it downloads a small plugin to their computer that hooks into their podcast player, such as iTunes. It also spawns a one-time pop-up that asks the listener a few demographic questions – age, gender, zip code and whether there are any children in the household.

Once the plugin is installed, Podbridge can download the advertising audio files and invisibly splice them with downloaded podcasts. Typically, the ads will be appended to the beginning and end of podcasts. But if the publisher gives Podbridge time stamps, it can splice the ads inside of the podcasts. CEO Murgesh Navar tells us that once the plugin has been installed, the ad insertion happens seamlessly and behind the scenes. “As a user, you notice nothing,” Navar said.

Navar says this method allows Podbridge to serve ads dynamically on the fly. In other words, Podbridge can rotate ads in and out of podcasts depending upon when they are downloaded and/or listened to.

The plugin aims to solve another problem that has loomed over the podcasting ad space – listener metrics. It monitors when a person actually listens to a podcast, not just downloads it, allowing advertisers to know if and when their ads are actually getting heard. Armed with this data, and the demographic info it collected from the users, Podbridge is able to give advertisers an analytical profile about who heard their ads, how many times and when.

Podcasters also get access to this data so they can know their audience and listenership, something that’s hard to track with current tools.

Podbridge’s business model is a pretty straightforward revenue share of the ad dollars with the podcaster and its ad serving partner, Ronning Lipset Radio.

For now, Podbridge is seeking deals with big media companies. It already has accounts with Sports Byline USA, Sporting News Radio, BBC, Military.com and others. Down the road, Navar said, it hopes to open its service to a wider range of podcasters.

Poddridge is a reincarnation of Audio Feast, a venture-backed company that launched in late 2004 as an Audible competitor. It offered downloadable subscription programming of news, sports, entertainment and music. But the company ran into a ”headwind” of competition in the music space, Navar says, namely a company called Yahoo. “We couldn’t sustain the competition with Yahoo.”

Navar is the only founder left from the Audio Feast days. But the original venture backers are still around, and the 13-person Mountain View company is still drawing on the original $10 million it raised from the Mayfield Fund and Worldview Technologies under its previous name. The company will go out for a second round in May or June.