Anyone who thinks the music of Sigur Rós is indistinguishable from the relaxation Muzak sold in new-age, tie-dye shops will find much to support their argument in the Icelandic quartet's sixth album. It has none of the upbeat pop-guitar irreverence that erupted from its predecessor, 2008's Með suð í eyrum við spilum endalaust, while its tendency to restraint limits those overpowering crescendos that, on earlier albums, jolted the listener from their flotation-tank reverie. Varuð has some of the scratchy elegance of Dirty Three, but drifts into portentousness, as does the shape-shifting Rembihnútur. All too easily, the songs sink into the background – which is a shame, because there is also great beauty here. The slow pulse, squiggling electronic notes and warped, whispery vocals of Varðeldur suggest extra-terrestrial communication: at the end the UFOs disappear, leaving an ineffably lonely piano. Similarly, the glitchy, gently frosted Ekki Múkk could be sung by spectres, trapped outside the window and murmuring: "I love you."