Sigur Rós: Why we're mesmerised by the hypnotic Icelandic band

Each week, along with the basic album and singles sales charts, there are myriad other charts published that track the diverse fortunes of the music industry, including those for the various major download sites, and the number of radio plays each track has secured. The one thing that isn't measured, however, may be the most influential of all: the prominence a piece of music achieves on that most powerful of all media, television.

Television is important in a way that the other charts, by their very nature, ignore: less concerned with immediacy, it can afford to ignore the rapidly changing tastes of a fickle industry like music, but employ the same piece of music over and over again, establishing it as the musical livery of a programme or strand, and confirming it as one of the emblematic musical signatures of its era. For the last year or two, it's been Sigur Rós's heavenly "Hoppípolla" that can't be avoided.

You've all heard it hundreds of times: that tentative piano figure cycling around and around, seeming to climb up and up expectantly like an MC Escher belvedere, until it finally reaches some emotional tipping-point and brims over, cascading in soul-lifting waves of fulfilment as strings and brass crowd round to hymn along.

It's become utterly ubiquitous since its release in 2005, as directors discovered how perfectly it seemed to suit all manner of situations, from baby whales being reunited with mommy whales on nature programmes, to some clueless pleb finally mastering a meaningless task on any of a hundred bogus reality-TV shows. Look! She's managed to run that half-marathon! Cue "Hoppípolla". See! The courting swans entwine their necks! Cue "Hoppípolla". Wow! He's not just conquered his fear of flying, he's enduring barrel-rolls! Cue "Hoppípolla". Gasp! It's the winning goal, in slow motion! Cue "Hoppípolla". And so on.

By last year, it was almost possible to channel-hop randomly and never hear anything else. It was even used in an episode of Doctor Who, and more recently in trailers for Slumdog Millionaire. And Oxfam adverts. And The X Factor, the audio equivalent of sleeping with the enemy. Small wonder that when Sigur Rós were recording it, they gave it the nickname "The Money Song" – they immediately gauged its appeal – before settling on "Hoppípolla" (Icelandic for "jumping in puddles").

You can hear why it's so popular among programme-makers. Because Jónsi Birgisson is singing in his native language, it's not stained by lyrical associations, while it fulfils our current yearning for aspirational sonic euphoria. It's like Coldplay minus the simpering-twatness, Radiohead minus the bitter curmudgeonly aftertaste, U2 minus the overweening egotism. It's the perfect musical soundtrack, it seems, for a UK blinded by vague empathy as it hurtles towards bankruptcy.

But it's not, Birgisson claims, as ubiquitous on Icelandic telly as it is here. "No, that's definitely a British thing," he says. "Everything dramatic and 'Hallelujah', every dramatic ending – cue it up!" (I'm not sure, in retrospect, whether he's referring to that "Hallelujah" or is using the word as an emotional analogue and has, spookily, simply stumbled across TV's new-found replacement for "Hoppípolla".)

"Hoppípolla" thrust Sigur Rós on to an entirely new plane of fame and fortune. The album from which it was taken, Takk..., was their fourth full-length outing, its predecessors having appealed predominantly to a refined art-rock constituency. Their debut album Von, for instance, sold a grand total of 313 copies in Iceland when first released in 1997, only accruing popularity when it was reissued in the wake of subsequent successes with 1999's Agætis byrjun and 2002's (). Since then, they have earned vast sums from their art, a position ironically exaggerated by the recent financial upheavals in Iceland. "Because we get our salaries and stuff from England, and the krone to the English pound has just doubled, it is actually good for us personally," explains Birgisson, with slight embarrassment. "But for the people around us, it is not good."

() still represents perhaps the furthest extremity of aesthetic insularity in pop music. Besides having no title as such (it's usually referred to as "Brackets" or "Parentheses"), and a largely white, albeit elaborate, packaging, its eight tracks lacked titles, and even the songs, sung by Birgisson in the distinctive fallen-choirboy falsetto that has enchanted millions, were written in the band's made-up language of Hopelandic, a meaningless succesion of phonemes that seems as though it ought to mean something, but doesn't. Did meaning matter much to them?

"I think if you want to have lyrics, then meaning has to matter," Birgisson says. "But yes, we have this kind of love-hate relationship with lyrics, because music flows so naturally for us, and when you come to write lyrics you have to put yourself in a different space. We usually start by singing some nonsense over the songs, then I listen to that, and usually, within that gobbledigook, there is often some spark of meaning – so you take out one word and start from there, and find out what the song should be about."

Intriguingly, the band's most recent album, last year's Med sud í eyrum vid spilum endalaust ("With a buzz in our ears we play endlessly"), even contained a track entitled "Gobbledigook" – which with typical perversity made perfect sense in translation, being an ode to the "hair-stroking, hem-blowing prankster-boy" wind ("You make hats fly into the air, you turn umbrellas inside out too often", etc). According to Birgisson, it was inspired by the Eurovision Song Contest, a claim that beggars belief given Sigur Rós's reputation for creating "cathedrals of sound". Surely Eurovision represents the diametric opposite of all they stand for?

"That was two years ago, when we were writing the songs for ...endalaust," he explains. "We had rented a big farm out in the country, and the Eurovision Song Contest was on television one night, so we watched that. The whole competition, all the way through. Then, after the contest, we just picked up instruments straight away and started playing, and this song came out. I don't know where it came from. It's such a crazy contest – it's crazy that you can actually have a contest about music – and it has such amazingly bad 'good' songs!"

By their own standards, "Gobbledigook" was a bizarre song, its stomping beat, static structure and light-heartedness operating at a sharp tangent to the slow, steadily developed sense of anthemic yearning for which they had become celebrated. If one were to assemble all the reviews and features written about Sigur Rós, the adjective used most often would probably be "glacial", and the critical stratagem most frequently employed would discuss their music in terms of the imposing Icelandic landscape – lazy clichés, of which the band themselves have become thoroughly sick and tired.

So how would they themselves describe their music? "I think the words that come to my mind are, like, 'organic', maybe," Birgisson eventually concedes. "There's something quite natural about it, and we think a lot about soundscapes when we are doing it. Basically, when you strip everything away from the music, at its quietest it's normal pop songs; but it's the way that you produce it that puts the meat on the bones of what you do. But it's always hard for us to describe how we sound."

Most bands, if pressed, will make similar claims on inexplicability, but in Sigur Rós's case there's more justification than most, their music being less permeable to descriptive, physical comparisons than abstract, emotional comparisons. And even then, they seem to have the gift of finding the gaps between emotions, sometimes leaving the listener adrift on a sea of conflicting moods and vague yearnings. In terms of instrumentation, however, they have shifted more towards using acoustic sources than electronic ones, particularly on ...endalaust. This, it transpires, was more a matter of convenience when they found themselves in unfamiliar surroundings.

"When we rented the farm in Iceland, we started out using mainly acoustic instruments, and it just developed from there," explains Birgisson. "But the basic structure of most of the songs is just acoustic, that is always our starting point."

Do you have a favourite sound?

"My favourite sound, ever?"

Yes, a sound source, such as marimba, piano or violin...

"Definitely," he decides, "it would be something like wind in trees, a nature sound of some sort. But as regards instruments, I like piano, celesta... there are so many beautiful-sounding V C instruments around. It depends how you play them."

This observation leads into a discussion about Washington Phillips, a gospel singer and songwriter from the Twenties, of whom we are both fans. Like Robert Johnson, Phillips recorded only a handful of songs (16 in total) but he accompanied himself on a mysterious instrument – either a dulceola, dolceola, celestaphone, phonoharp or fretless zither – related to the hammer-dulcimer. But, as with Johnson, the lack of documentary evidence and the unique sound of Phillips's instrument have provoked feverish debate among enthusiasts ever since.

"Is it a dulcimer?" queries Birgisson. "But it seems like he's strumming it! It sounds amazing, like some form of harp-guitar." His fascination with Phillips makes obvious sense, both musicians' work exhibiting a haunting blend of certitude and vulnerability – what might best be called a fragile majesty, especially when the band's sound is swelled by the addition of the Amiina string quartet, or the subtle lowing of horns which, on ...endalaust, relates more to the British brass-band tradition than the American R&B tradition. This may or may not have something to do with their collaboration on that album with the British producer Mike "Flood" Ellis, best known for his work with indie and goth acts such as Nick Cave, Nine Inch Nails, U2 and The Killers."We had never worked with a producer before, and it was a good learning experience for us," Birgisson says. "Before, it was always just the four of us together, doing everything for ourselves. When he first came, it was a weird situation, because he has his own way of working, and we have ours. He is so focused, and such a hard worker. He became like a father figure to ...endalaust: he was always there, from 10 in the morning to 10 in the evening. When you have your own studio, like we do, and no pressures of time, it's easy to just have a coffee and decide to do it tomorrow! That had been happening quite a lot with us. And it was good for us to go to other studios: we recorded the basic tracks in New York, and basically, you're just locked in a room all the time. It was fun, though."

Their tentative outreach programme for ...endalaust did encounter one stumbling block, however, when they decided to commission the Berlin-based artist Olafur Eliasson, who created the Sun installation in the Tate Modern turbine hall, to do the album artwork, an alliance that didn't work out as well as hoped.

"We had long talks with him and met a couple of times to discuss ideas with him, but basically it just didn't work out," Birgisson says. "We had just totally different characters in our working methods – he is so methodical and mathematical, so well-thought-out and correct, and has definite meaning, and we are so spontaneous and rough, and everything we do has a huge amount of soul, but no meaning."

Instead, they opted to use a picture by the photographer Ryan McGinley of naked youths running across a road, which the band felt captured their spontaneous quality. Even that caused problems in America, where bare buttocks – at least those not belonging to porn stars – seem to offend the sensibilities.

"That was so weird!" recalls Birgisson. "When we played in America, we would arrive at venues and there would be posters outside advertising the gig, and they would be blacked out! And the CD would have stickers put over the asses! Why should they be embarrassed by naked bodies? There's nothing offensive about it. Look at rap album covers and you see, say, 50 Cent, and he's posing with guns and stuff, flexing his muscles – what a role model! That should be censored, surely? It's crazy! But hopefully, times are changing."

Oddly, for such a guileless, reserved band, Sigur Rós now seem to be the favourite band of every A-list celebrity, from tattooed rocker Tommy Lee of Mötley Crüe, who appears to use them as some form of meditative chill-out refuge from his racy lifestyle, to megastar Brad Pitt. More queasily, the band's music was apparently playing when Gwyneth Paltrow produced her little Apple. It all seems a million miles away from their lives in Iceland, as Birgisson confirms.

"It's nothing to do with us," he says. "We just live our normal lives in our small Reykjavik, we have our own apartments, our own families and kids, and our fame doesn't affect us at all, I think. We never think about it, we never talk about it, and we don't make a big thing out of it – we don't play the media game, stuff like that. We just like to be able to walk down the street and go to a coffee-house, things like that."