Tortoise don't lend themselves easily to categorisation. They rose out of the fertile Chicago jazz scene of the early 90s, but since their debut almost eighteen years ago they have hopped from genre to genre, underpinned by a jazz sensibility that drove them to strike out in unexpected directions, but producing music that could hardly be considered alongside even the most avant garde of the genre, 1998's TNT excepted.

Released for Record Store Day, Lonesome Sound is, however, the closest the group have come to conventional modern rock music. Diametrically opposed to its sister 7 inch, 'Mosquito', which is very much business as usual for Tortoise, 'Lonesome Sound' is striking for its use of vocals, traditional guitar hooks and structures which are relatively close to traditional song. Don't be alarmed, this isn't quite Kings of Leon territory. The title track, however, has a lilting brushed snare rhythm and vocal motif that find Tortoise come closer to American folk territory than you would ever have expected. It's an eerie four minutes and the subject matter, the death of a child, is dark: "Hear that lonesome sound, a baby got put in the ground." Given that this is a relatively new foray for the band, it is all the more impressive that it works.

Next up is 'Reservoir'. Its tightly woven math rock guitar and drums and its muted spoken word vocal could have come straight out of Slint's Spiderland sessions. As the asymmetrical rhythms intertwine with mechanical accuracy, the confessional reflection on life, knowledge and wisdom is touching: "One time I thought I had everything in my pocket … Sometimes you stumble through your life any way you can. I don't think about it much as long as I've still got my face and I've still got my hands."

The closer of the eight-minute suite is 'Sheets'. Another dark affair, its one-note bassline throbs with precision, propelling the track unerringly through a landscape of detuned guitar strings and bursts of feedback. This, allied with the understated, menacing vocal – "The rain comes down in sheets" - evoke Liars at their foreboding best. Again, it works beautifully.

Make no mistake, this isn't standard rock fare, but Lonesome Sound is conspicuous for its relative conformity within a heavily innovative oeuvre. There is a warmth to the record that is lacking in Tortoise's back catalogue and it's refreshing to hear something truly new from a group who have become master purveyors of post-rock exactitude. It should be remembered that succeeding here is succeeding in uncharted territory for Tortoise, and for this they should be applauded.