How 'Serial' is making podcasts mainstream

Almost as long as there has been digital music, there have been podcasts – regular downloadable audio programs on everything from hip hop to high finance.

But after a decade or so, a podcast has broken through into the media mainstream, and experts are claiming that new podcast "Serial" could mark a sea-change in listening and advertising habits.

The first episode of "Serial"—a spinoff of weekly public radio show "This American Life"—was published in October, and although the first series is still ongoing, it's already been dubbed "the world's most popular podcast".

In the series, co-producer and host Sarah Koenig investigates a 1999 Baltimore murder, exploring different aspects of the case in weekly episodes.

Serial website
Serial
Serial website

The team behind the podcast told CNBC that they were averaging 1.5 million downloads an episode, and Apple confirmed on Friday that the podcast was the fastest ever to hit 5 million downloads and streams through iTunes. At the time of writing, "Serial" was top of the iTunes podcast chart in the U.S., U.K., Germany, Italy, Canada, Australia, according to iTunesCharts.net.

Read MoreWhat can Nielsen do for streaming?

Viral popularity

For those with knowledge of the podcast world, a high-profile success story like this has been a long time coming.

"Podcasting has been bubbling just below the mainstream for some time," Larry Rosin, president of market research firm Edison Research, told CNBC. "It's a low-reach, high-engagement medium that's been waiting for something to burst through."

He compared "Serial"'s viral popularity to that surrounding Internet TV some 18 months ago. Then, he said, everyone was talking about Netflix-only series "House of Cards" and "Orange is the New Black".

"Now the buzz is all about 'Serial'. It's following the same pattern; all of a sudden, everyone is interested in podcasts," Rosin added.

Read MoreGoogle's 'risky' YouTube bet still paying off

Podcasts had been growing in popularity before "Serial"'s arrival on the scene. Edison Research found earlier this year that monthly podcast consumption in the U.S. had grown 25 percent year-on-year in 2014. Around 39 million Americans had listened to a podcast in the last month, according to the research.

'Barriers to entry'

For Michael Wolf, a market analyst and podcaster himself, it's no surprise in the sudden increase in interest in the podcast.

"The barriers to entry have come down," he told CNBC, highlighting how smartphones have made it easy to subscribe to and download podcasts. "And the biggest names in the business have really started to innovate."

Because "Serial" isn't the only popular podcast. Other series with massive audiences include "The Adam Carolla Show", Stephen Dubner's "Freakonomics" and Roman Mars' design podcast "99% Invisible". Mars' show proved so popular that he quit his job on public radio to work full-time on the podcast—which is funded solely by fans and sponsors.

Because most podcasts are a one-man operation, the hosts often also read out their sponsors' messages.

For these sponsors, podcasts offer something different to more traditional media, according to Adam Sachs, CEO of podcast advertising network Midroll Media, which specializes in host-read ads.

Read MoreSony's big plan to shake up TV biz

"We know that podcasts are a very intimate medium – the hosts are literally in your ear on a regular basis," he told CNBC. "They feel like your friends. So it can be very powerful when they read an ad."

#mailkimp

Podcast sponsors have traditionally been technology companies appealing to a tech-savvy listener. The sponsor of "Serial", for instance, is Mail Chimp – an email marketing service provider.

"Podcasts' listeners are online—maybe they're creating their own websites. You can see why it appeals to Mail Chimp," Wolf said. But this could be about to change.

Sachs said non-tech companies such as Random House and HBO were currently among the company's 250-plus advertisers. "We are seeing interest from more and more bigger brands."

And advertiser interest has increased since "Serial" was released, Sachs added. "It's made podcasting more mainstream. In the U.S. at least, I'd say it's talked about more than any TV show," he said. "That's attractive to advertisers."

In fact, Mail Chimp's sponsorship of "Serial" has spawned its own meme and Twitter hashtag - #mailkimp – based on a mispronunciation of the company's name in the pre-roll ad. Online news service Quartz called it "the year's biggest marketing win".

For Edison Research's Rosin, what was "hugely significant" about "Serial" was the interest from "Hollywood and the creative classes". That, he said, is where the innovation – and money – was.

"Podcasting was a germinating form of content that was waiting for a hit. It finally has one," Rosin said.

- By CNBC's Katrina Bishop