'There are only two kinds of music: Good, and the other kind," Duke Ellington is said to have said. (The quote has also been attributed to Richard Wagner.)
The Icelandic corollary is that, in the Land of the Midnight Sun, there are only two kinds of bands: Metal, and those that conjure whimsical, diaphanous sounds that reflect the otherworldly landscape of that geologically fantastical land.
Sigur Ros, who played a rapturously received sold-out show at the Academy of Music on Saturday night, are supposed to represent the apotheosis of the latter.
And they do. The two nearly hour-long sets played by the band fronted by Jón Þór Birgisson - known simply as Jonsi - were full of shimmery textures and mystical atmospherics, with songs that put the audience in a luminous trance as their slow build was accompanied by an inventive, tripped-out light show.
Not to mention Sigur Ros' chief special effect: the 41-year-old Jonsi himself, singing in a bewitching countertenor pitched skyward to the 19th-century opera house's uppermost balconies and enticingly entwined with his sweeping electric guitar lines, which he most frequently plays with a cellist's bow.
All of that contributes to the experiential strangeness of a Sigur Ros show, the sense of entering an alternate universe populated by fairies and woodland sprites.
That feeling is further perpetuated because Jonsi most often sings not in English - though on one song, "Festival," he did - and not even in Icelandic, but in a mythical language called "Hopelandic" akin to a post-rock version of scat singing. It's about the sounds the band makes, not what the words mean. Though FYI: Sigur Ros, who named themselves after Jonsi's sister, born a few days before the band was formed in 1994, means "Victory Rose."
So yes, it's true: Sigur Ros sound like elves, and sing in a made-up language. How wimpy is that, right? Wrong. Because along with the ethereal, the band - which on this tour consists of Jonsi plus bass player Georg Hólm and drummer Orri Páll Dýrason (all of whom play other instruments, too) - also brings the noise.
On previous trips to Philadelphia - at the Tower Theater in 2005 and the Mann Center's Skyline Stage in 2012 - Sigur Ros have toured as a big band, augmented by horns and strings. Those configurations, perhaps counterintuitively, tended to emphasize the delicate, gossamer qualities of the music.
This time, though, without multi-instrumentalist Kjartan Sveinsson, who departed in 2013, Sigur Ros is a power trio. At the Academy, both sets started with brand new songs - "A" and "Oveour" - and most pieces began quietly with hypnotically repeated instrumental phrases that guided listeners toward a hypnotic fugue state.