BY JENNIFER KELLY

"I have been thinking about this beautiful note, a note that we begin every show with," says Múm's co-founder Å�rvar Póreyjarson Smárason, as his seven-person band finishes setting up drums, keyboards, pedal boards, cellos, violins, guitars, two melodicas and one ukulele. "It's such a beautiful note. I can't stop thinking about it."

And then Iceland's long-running electro-pop band drifts into the ethereal opening to "Illuminated," into a note grows and wafts upward in breathless, scale-climbing "ooh ooh oohs." It's the kind of note you could, indeed, get lost in, and both the band and the audience is simultaneously transfixed by its startling purity, its soft dreamlike intensity.

Indeed, tonight's show is all about the jaw-dropping, sudden beauty of experimental pop. Múm, along with opener Sin Fang Bous, both traffic in gentle, delicately arranged pop songs, billowing with soft harmonies and given spine by hard, dance-floor-thudding rhythms. Both are from Iceland. Both have new records out on German electro-label Morr Music, and both surpass these records decisively in live performance.

Sin Fang Bous, the opener, is a new project from Sindri Sigfússon of the Icelandic country folk outfit Seabear. (You can hear both Sin Fang Bous and Seabear on Morr's New Zealand pop tribute compilation Not Given Lightly, released earlier this year.)  Begun largely as a solo, electro-infused experiment, Sin Fang Bous has lately been evolving into a full-band effort. And in addition to a rhythm section, Gunnar Å�rn Tynes, the guitarist/keyboard/effects maven from Múm, also sits in with Sin Fang Bous.

Still, it's Sigfússon's show, and you can tell because he stands surrounded by three microphones, all pointing directly into his face. (This makes him easy to hear but rather difficult to photograph.)  You can also easily infer that he's from somewhere in Scandinavia, because he is wearing a sweater decorated with alternating bands of hockey players and skiers. And also, perhaps, because he and his band play a glitchily rhythmed, melodically entrancing kind of pop that you associate with northerly bands like Loney Dear, Fredrik and Boy Omega.

Sin Fang Bous songs tend to open with whispery tenderness, jangled tangles of picked guitars, then open out mid-way into big galloping choruses. Sigfússon's voice is the main focus, soft, cool, unstrained, wrapped in the band's harmonized counterpoints. It would all be quite weightless and elusive, I think, except that the songs are so firmly grounded in rhythms. The drummer, wearing a Miles Davis shirt, alternates between slapping toms and cymbals with his hands for the quiet bits and laying down a thundering four-four for the louder ones. Tynes crouches in the back, fiddling with switches and fine-tuning overlays. At one point, during "Clangour," I spot him holding a small electric fan over his guitar strings, eliciting a wavery tremolo...and, perhaps, a nice breeze as well.

Sin Fang Bous' set closes with three of the best songs on Clangour. The (semi) title cut "Clangour with Flutes," pits somewhat disturbing lyrics ("I will be the forester and you will be the tree") against effortless pop melody and handclaps. The pop side wins, but not without taking on a little shadow. "Catch the Light" turns into a driving, drum-driven rock song live, Sigfússon's voice breaking a little at the edges, so that he sounds more like Jeff Mangum than David Bowie in the chorus. And "Lies" starts in big shimmering washes of guitar, and cymbals, its absolute serenity ruffled, but only a little, by anxious verses about wanting to scream but only closing one's eyes.

Sin Fang Bous' set is good. Múm, however, edges over into great. In fact, the last time I was this unexpectedly blown away by a live show was with Tunng, and the comparison is fairly apt. It's not just the songs, which are lovely, or the musicianship, which is quite good. It is the way these bands become absorbed, entranced, enraptured by their own music that makes the difference. The image that will stick with me for now, forever, is of cellist/violinist/singer Hildur GuÇ�nadóttir jumping up and down like a grade schooler, hair in her eyes, mouth open in a wide smile, hands a blur of rapid fire clapping. Or of ukulelist Sigurlaug Gisladóttir, arms extended, neck craning, eyes half-closed as she flutters effortless angel harmonies over the band's steady droning groove. Or of Smárason, clutching a square guitar-like instrument, his whole being vibrating like a tuning fork in the sound of that one beautiful note that opens the show.

After "Illuminated," from the 2009 album #Sing Along to Songs You Don't Know# Mum plays "Marmalade Fires," the single from 2006, with two melodicas, one conventional, the other seemingly played through a bubble pipe, for an eerie, watery effect. A trumpet solo, later on, lends a jazzy buoyance to the song, which is surreally bright on the surface, but with shadowy dissonances and discords lingering underneath. Multiple vocals, instruments and samples rub together in frictive harmonies, slipping sideways against each other like smooth rocks in the tide.

"Húllabbalabballúú," next, is pure joy, GuÇ�nadóttir holding her violin like a banjo and picking it, the drummer pounding out massive rhythms with mallets. The chorus rises on stair steps, each progressive iteration of "in these words we drown," a bit more elated and effervescent, and just when you think it cannot go any higher, it backs down into quiet, breathy trumpet notes and swathes of lullaby strings.

Smárason  introduces the next song as a song about "flora and fauna." The fauna he has in mind, he explains, is the grass that grows up in the cities, between the cracks in the sidewalk, and though I can't quite catch the title, I think now it must have been the band's 2002 single "Green Grass of Tunnel." The song itself reflects its subject matter, softer organic sounds enmeshed in machine-like rhythms, its delicacy enhanced rather than obscured by rough, abrasive, nearly industrial elements. Then it's back to the current album with "If I Were a Fish" whose breezy pop is embellished with soft breathy trumpet (Erikur Orri Ólafsson) and a lovely interval of slide guitar (Tynes again), and "Blow Your Nose," my candidate for the H1N1 virus' personal theme song ("Blow your nose right on my sleeve...").

The main set ends with a long driving, improvisatory piece, with the band's front line switching from recorders to harmonicas and finally to kazoos. And then they dedicate the final song of their main set to the opening band, saying that since Sigfússon sings "like a young David Bowie" all they can do is sing along. The song, not surprisingly, is "Sing Along to Songs You Don't Know." It is lovely, soaring and over all too soon, not one beautiful note, but lots of them, and like Smárason, I can't stop thinking about them.