THE CATASTROPHIST

After more than twenty years the music of Tortoise is still incredibly hard to pin down. So I turn to guitarist Jeff Parker for an insider's description of the band's eighth and arguably best studio album, which he calls "progressive experimental music with pop sensibilities". You won't find a more succinct analysis of the elusive Tortoise sound than that, and it's almost true.

Some of the material here was first aired in embryonic form as part of a 2010 Chicago commission to celebrate the Windy City's musical heritage. But the much later album far exceeded that original mandate, becoming the band's widest ranging and richest effort to date.

The title track kicks off with some of the nerdiest analog synthesizers heard in forty years: a perfect audio illustration of the album's unfortunate (but eye-catching) cover portrait. After that the stage expands far beyond the shores of Lake Michigan to embrace, and then surpass, all the usual Tortoise detours: lush post-modern grooves; eclectic after-hours lounge bop; jagged ethno-funk with a touch of Krautrock grunge. And, in "Yonder Blue", unexplored territory for this otherwise strictly instrumental group: a truly romantic pop song.

Robert Wyatt was invited (but declined) to be the guest vocalist, which should be a clue to the style of the music. The job eventually went to Yo La Tengo's Georgia Hubley, to these ears a better fit for such an unexpected change of pace.

And then there's the album's other happy surprise: a left-field interpretation of the 1973 David Essex chart-topper "Rock On". The new version presents a suitably reptilian Tortoise re-imagining of an already oddball novelty hit, miles ahead of the similar but less effective avant-pop covers heard on the 2006 album "The Brave and the Bold" (quod vide, but not a true Tortoise recording).

Even their best fans (let's see a show of hands) would have to admit the music of Tortoise can sometimes resemble academic exercises in Post Rock engineering. But the band has never sounded more relaxed, or been so focused. After two decades of unqualified cult success, is it fair to say the Tortoise quintet has finally emerged from its collective shell?