There's a moment early on in 'Ég Anda' – the opening track on Valtari, the sixth album from Iceland's Sigur Rós – when a series of solemn chants transcend gently whirring strings, and take on the form of multiple sighs of relief being unleashed at once. Its almost as if one can hear the group's devoted fanbase brought to an anticipated hush, and who could blame them? Since their 1999 breakthrough Ágaetis Byrjun first caused a stir on a global level, Sigur Rós have – for better or worse – followed their own unique trajectory, irrespective of passing cultural ephemera; a highly commendable trait in itself.

But, just as quickly as 'Ég Anda' builds expectation, it abruptly cuts it short. The droning suspense bubbling under the surface swiftly vanishing, only to return again as a majestic string section underpins frontman Jónsi Birgisson's falsetto vocals. It gradually descends this time though and – much like the floating ship depicted in the album's artwork – moves gracefully into the mists of uncertainty that surround it. Eventually, it collapses under its own tension, hitting the water with a series of foreboding sub-bass pounds – louder than cannons, more resounding than heartbeats.

Single 'Ekki Múkk' is a classic exercise in the more minimalist style of composition cultivated by the band on 2002's ( ). It sees scraping bows and the rustle of static grounding the ethereality of Birgisson's voice, before reaching a coda upheld by a solitary piano figure, accompanied only by its own echo. Coupled with the preceding 'Ég Anda', it's a wonderfully subdued start; more tangible and honest than 2005's Takk and more developed than 2008's Med Sud Í Eyrum Vid Spilum Endalaust.

It's on 'Varúd' that Valtari achieves its first true moment of transcendence though. Beginning with a series of submerged, underwater chords, it surfaces to the sound of an army of string crescendos that are only matched in intensity by Birgisson's siren call. Once again however, it quickly declines into softer territories dominated by found sounds, yet these too radiate a certain beauty. Every frailty in the song's quieter moments – each squeaking string and incidental noise – is a strength in itself, a blemish of the realism and irreparability of the modern world cutting through the otherworldly escapism of the music.

'Rembihnútur' and 'Daudalogn' continue the record's transitional movements between cathartic release and carefully cultivated experiment. The former sees the chaos of another monumental chorus cut through by distorted xylophones, like little rays of moonlight in the long Scandinavian winter. The latter is a far more removed affair, and perhaps appropriately considering the death implied by its title – roughly translated it means 'dead calm' – it's almost dirge-like in its funereal approach. Structurally inconsequential, the enjoyment's in the musical journey; the procession being led to the graveyard by muffled choral vocals and string glissandos.

If 'Daudalogn' marked the key moment of death on Valtari then the record's next three tracks – 'Vardeldur', 'Valtari' and 'Fjögur Píanó' – act as its understated, sympathetic epitaph. The first is dominated by a slowly descending xylophone figure repeated ad nauseam. It's sombre but bittersweet, simple and organic, wanting for accentuation yet concurrently complete. 'Valtari', on the other hand, sees a sense of gravitas begin to return to proceedings, crafting an eerie soundscape of resonant organs and twinkling music boxes.

After all the impalpable tension mounting through 'Vardeldur' and Valtari', 'Fjögur Piano' enters the fray with a satisfactorily summative coda. Its unaffected titular pianos encapsulating all that has come before it, marking the moment at which Valtari becomes consolidated as an experience. Its subdued ending being perfectly evocative of the personal drama and realism prescribed by Valtari; there are often no big finishes to real life, just an appreciation of the wonder found in the most minuscule of things. Much like Valtari, the delight is in the details and as the last whir of sound leaves its sonic space, we too are left empty, alone and craving more.

Mirroring Sigur Rós' career as a whole, Valtari flows through a series of peaks and troughs, each as beguiling as the last. The only visible faults are those we find in ourselves – the incidental, the noisy, the unapologetic – and these are often the source of our greatest beauty too.