Image copyright iStock Image caption Around 9% of adults in the UK say they download podcasts

S-Town, the gripping saga about life and death in Alabama, is the latest podcast to have notched up impressive listening figures. But podcasts on the whole still don't seem to be breaking through to the mainstream.

Have you ever downloaded a podcast? And, if so, did you actually listen to it?

Podcasts have long been seen as the future of radio, a great way to pass the time on a long commute or catch up on a radio show you've missed.

They've been growing in popularity since the early noughties, when Apple's iPod first hit the market ("podcast" is a cross between the words "iPod" and "broadcast").

But, 15 years on, they remain a relatively niche pursuit.

"I don't know whether podcasting is a mainstream proposition," says Matt Hill, co-founder of the British Podcast Awards.

"Its core strength at the moment is in narrowcasting. It creates audio content for niche groups of people, but it does so really effectively."

Image copyright iStock Image caption Podcasts have been growing in popularity over the last decade or so

According to Rajar, the body that monitors radio listening, 9% of adults in the UK say they download podcasts per week - around 4.7 million people.

Which is a fair few - but not much compared with the 90% (or 48.7 million adults) who listen to live radio every week.

Kate Chisholm, radio critic for The Spectator, says: "Podcasting is arguably something for metropolitan people, maybe in their 20s and 30s.

"I don't think it's something that particularly seeps out to the mainstream. On one level I would say that's changing, but then how many people who live on my street would be downloading podcasts? I'm not sure it would be very many.

"They'd listen to Classic FM or Radio 2... but a lot of people look at me blankly when I mention Serial."

What is a podcast?

Image copyright iStock

A podcast is a piece of audio made available online, which the listener typically downloads to their smartphone or laptop

They are very often speech-based because of restrictions on the use of commercial music

Podcasts differ from live radio because you can choose to listen to them when you want, rather than in an allotted timeslot - kind of like how Netflix or iPlayer differ from live TV

They can be bespoke specialist content or simply a downloadable version of a previously-broadcast radio show

Serial, of course, is the biggest podcast success story to date - its makers say it has had more than 250 million downloads.

That certainly sounds like an impressive figure - albeit perhaps not as much as it might first seem.

It doesn't mean 250 million different people have downloaded Serial, but rather that its 26 episodes have been downloaded a total of 250 million times.

Plus, the RAJAR figures show only about two thirds of downloaded podcasts are actually listened to.

Image copyright Getty Images Image caption Serial, hosted by Sarah Koenig, won a Peabody award in 2015

"Serial made 2015 the year of the podcast," says Julia Furlan, podcast producer for BuzzFeed.

"Everybody was saying at that time that podcasting had finally made it, but it's still hard for a lot of people to find and download a podcast, hard to share it, it's still something we're figuring out as medium."

But, she says: "Since Serial, you do see different names on the top 10 podcast chart, you see larger media companies and brands investing significant money in making new content.

"And I do think those are indicators that there is growth, that Serial did something really big."

S-Town, released in March and made by the team behind Serial, is the latest podcast to hit the headlines.

Image copyright iStock Image caption The term "podcast" comes from "iPod" and "broadcast"

The documentary begins with a suspected murder in Woodstock, Alabama, and unfolds around its central character - an eccentric local named John B McLemore.

It was downloaded 16 million times in its first week - although again that number is spread across seven episodes, which were all made available at once.

Other recent podcast success stories include Russell Brand's new show on Radio X - which marked his return to radio after an eight-year absence.

The high listening figures of the few breakthrough hits are what make podcasts a very attractive prospect to advertisers.

Hill says: "Even though the audiences are quite small, those shows do very well with advertisers because those listeners are interested in one specific area - it's exactly who they want to market to.

"Podcasting is starting to educate advertisers that there is an upmarket audience that would be interested in intelligent speech programming and would be happy to hear advertising alongside it."

Image copyright PA Image caption Russell Brand's first podcast after his return to radio this month was hugely popular

Many of these advertisers offer podcast listeners discount codes, because then they can monitor where their new customers are coming from.

Which means many podcasts are effectively working on commission - and only become financially viable if companies can see a demonstrable boost in customers.

But few podcasts become popular enough to attract advertisers at all. There are just so many of them around - with no quality control.

"I think podcasts are very different from mainstream broadcasting, it's like the difference between blogging and print," says Chisholm.

"Like blogs, the quality of podcasts is variable. There's a big difference between people who blog and people who actually get published."

Image copyright Getty Images Image caption Advertising is becoming more common on podcasts

Part of the problem facing podcasts is that, in general, audio doesn't tend to go viral.

Have a scroll down your Facebook feed, and the chances are there will be several videos of dogs, cats, babies, pranks, fails and Kermit the Frog memes.

But people rarely share a great radio programme they've heard.

"The internet is a place that you take in with your eyes, it's a visual medium," Furlan says.

"I also think that downloading a podcast is quite hard, people think, 'Oh, I'm subscribed to this, what does that mean? How long is a season?' All of these things are unhelpful for the industry at large."

Image caption Desert Island Discs is one of the BBC's most popular podcasts

With such a slow rate of growth, podcasts may become the minidisc of the radio industry - sold as the future but eventually becoming redundant. Or they may just take time to become established.

"Every year the listening figures creep up, but they haven't done a Netflix and exploded, it's slow burn," Hill says.

"But the thing about a slow burn is it's not a flash in the pan - those are the things that stick around."

Furlan goes further: "I think absolutely podcasts will break through in the years to come.

"If you take into account how everybody has a smartphone now, smart cars are on their way, the more technology opens up, the more we are going to see podcasts in our daily lives."

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