When you of think of the phrase ‘string quartet’, what comes to mind? Standing at a wedding reception, perhaps, while four overqualified musicians play “Eine Kleine Nachtmusik” to a crowd of dapperly attired individuals who are far more interested in the open bar than the music, most likely.

For forty years now, the Kronos Quartet have been forcing audiences to revise their expectations of the string quartet. They have, almost singlehandedly, overhauled the mainstream perception of chamber music, transforming it from light luncheon entertainment into a challenging, experimental genre. These two forthcoming releases celebrate the group hitting the big four–oh with a retrospective Explorer Series box-set and new material in the form of the album A Thousand Thoughts.

The Kronos Explorer Series is comprised of five quintessential Kronos Quartet releases: Pieces of Africa (1992), Night Prayers (1994), Caravan (2000), Nuevo (2002) and Floodplain (2009). Each of these discs is designed to represent Kronos’s forays into music from various geographical regions; Africa, Central Asia, Eastern Europe, Mexico, the Middle East and even California.

For Kronos aficionados, the additional material accompanying the release – including an interview with and new liner notes by David Harrington (violinist and founder of the quartet) – will no doubt be a draw. More interestingly, perhaps, the amalgamation of these seemingly disparate collections provides a great ‘beginners guide’ to Kronos. It’s never easy to engage with group that has a mammoth back-catalogue behind them, particularly a group that is as singular and cerebrated as this one. This release offers an insight into the palette of rhythms and timbres that have coloured the quartet’s output for four decades, displaying a broad selection of the many musical paths that the group has trodden in its long, fruitful career.

The diversity in the collection is astounding. “El Llorar” (Nuevo) is one example of the quartet’s exploration of Mariachi music, with some eye-wateringly impressive vocal falsetto accompanying the quartet. “White Man Sleeps, I – V” (Pieces of Africa) demonstrates just how potent the quartet can be as an ensemble, contrasting its ability to playfully employ exposed rhythmic lines with the delicate tapestry of timbres created by the group as whole.

Although World Music elements feature heavily in the work, Kronos, almost by the very nature of being a string quartet, are not a conventional World Music group. In addition to recording movie soundtracks (notably Requiem for a Dream) the quartet always has one foot in the pop music camp. Take for example, their appearance at the December 2012 ATP festival, where they stunned a hung-over crowd (of which I was one) with their rendition of Steve Reich’s WTC 9/11. “Mini Skirt” is an affectionate send-up of 60s psychedelia, with the occasional, whispered ‘groovy’ thrown into the score. The Explorer Series also nods to the group’s penchant for pop covers with “Misirlou Twist”, famously the first track used in Pulp Fiction. Sadly, their previous renditions of Bob Dylan, Jimi Hendrix and Sigur Rós are not included in the set.

A Thousand Thoughts continues to add to patches to the Kronos Quartet blanket, assembling pieces from 14 different nations. The album is book ended with two emotive folk melodies, “Tusen Tankar”, the Swedish folk song for which the album is named, and a reworking of “Danny Boy” with an Appalachian twist. It would seem that, even forty years on, the quartet is still brimming with dynamism and inventiveness.