Despite, or perhaps owing to, the fact that a reunion tour isn't likely to happen anytime soon, Sonic Youth have had the best separation that fans could have hoped for. Much of the time, it hardly feels as if they've even gone away. We are basically living in the state of limbo that we used to inhabit between the "official" studio albums that would pop up every one-to-three years from 1982 to 2009. When waiting for those "proper" records to appear, fans' thirst for all things Sonic Youth would be quenched by various side-projects, live albums, archive releases, reissues, solo works, poetry books, art exhibitions and collaborative performances where Kim, Thurston, Lee or Steve would make an atonal racket while somebody who was still a card-carrying member of the radical underground blew a saxophone like billy-o or sawed a skateboard attached to a contact mic in half. The limbo is now permanent. It still ain't such a bad place to be.

Curated by the ever-reliable Three Lobed Recordings, the material on In/Out/In is not what could strictly be called "previously unreleased". That said, there will be plenty of listeners out there who don't already own it, or at least not all of it, and certainly not in a physical format. 'In & Out' and 'Out & In' were first disinterred in 2011, as part of Three Lobed's limited boxed set, Not The Spaces You Know, But Between Them, alongside offerings from Sun City Girls, Steve Gunn, Mouthus, Comets On Fire, D. Charles Speer, Wooden Wand, Eternal Tapestry, and Bardo Pond. As can be gauged from that kind of company, neither Sonic Youth track sounded very much like, "Dirty Boots, Baby, Dirty Boots".

Nor should it be assumed that those two similarly titled pieces were culled from the same session. In fact, they were recorded a decade apart. 'Out & In', which closes this non-chronological compilation, was made in the year 2000, not long after cult hero Jim O'Rourke had joined the band as its fifth member. It was a controversial time, even for this artiest of art-rock groups. Goodbye 20th Century, a double-album of pieces originally penned by notable avant-garde composers, was not exactly easily listening. I say "penned". The instructions for Yoko Ono's 'Voice Piece For Soprano' read as follows: "Scream. 1. against the wind 2. against the wall 3. against the sky". Kim Gordon and Thurston Moore asked their daughter, Coco, to perform that one for them. She was five years old at the time. That's how you raise a child. A record influenced by the New York beat poetry scene of the 1940s to 1960s, and one which predated by a good 20 years today's craze for all things sprechgesang, NYC Ghosts & Flowers "wasn't a super popular record in its time", as Lee Ranaldo has said so himself. It received a notorious 0.0 rating from Pitchfork's Brent DiCrescenzo (who later changed his mind and, having realised what an "eerie and beautiful" record it is, personally apologised to Moore). There was also the infamous All Tomorrow's Parties incident, which doesn't need re-reporting here.

'Out & In' seems stoned and woozy at first, lumbering along merrily enough. Building up to the four-minute mark, it grows fiercer and distinctly more aggressive. The rhythm cuts out, briefly, and we get a short passage of fusty feedback. Then the original riff kicks back in, this time with greater belligerence, and additional wailing guitar skree. By about its seventh minute, the thrusting groove and jet-plane engine roar propels Sonic Youth closer than they have ever sounded to Hawkwind's Space Ritual. Next, some jazz-noise lead licks tussle for the spotlight. And just when you think the whole darn thing's about to fall apart, the players read each others' minds, like only they can, and draw things to a close with an almost cocky composure. Job done. This has got to be one of the most satisfying jams that Sonic Youth ever laid down on tape. Not sure why they sound so riled. Maybe they'd just finished reading one of their recent reviews.

'In & Out' comes from a soundcheck in Pomona, California, in 2010. This was five years after O'Rourke had left the group and moved to Japan. It would also turn out to be Sonic Youth's final North American tour. In marked contrast, this extract is significantly gentler than its earlier near-namesake. The sound is more skeletal and spacious. The only piece on In/Out/In with vocals included, it has Kim Gordon whispering about light and dawn. She seems fragile. Hindsight will encourage listeners to project onto this track certain interpretations which may or may not hold water. It is definitely one of Sonic Youth's most sedate improvs. Is this the sound of defeat? Of winding down? A sublime whimper to finally wrap things up?

The other tracks on this compilation have only ever existed in digital form, once obtainable as MP3s from the band's official website and more recently as part of the treasure trove that is their ever-expanding Bandcamp page. 'Basement Contender' and 'Machine' were recorded in the run-up to The Eternal, Sonic Youth's final studio album (not that anyone knew that yet, hence its now-ironic title). There is a warm, cheerful and contented feeling to the former instrumental piece. Well, by this point they had recruited smiley Pavement bassist Mark Ibold. The separate guitar lines twist around each other delicately. It gets a little more vicious, eight minutes in, just before another tidy conclusion. "Are you tuned to anything? Are you tuned to a tuner?", Lee Ranaldo asks at the end. Doesn't seem to have been a problem.

'Machine' is a shorter, spikier and skronkier affair, driven along by Steve Shelley's forceful drumbeats. Lack of vocals notwithstanding, it appears to hark back to the Sonic Youth sound of the mid-90s when Butch Vig was producing, David Geffen was king, and flannel shirts were all the rage. It'd be great to hear Kim growling one of her ambiguous character studies over the top, but it's a hoot nonetheless.

'Social Static' is another cut from the Jim O'Rourke era, recorded to soundtrack Chris Habib's short film on Spencer Tunick, who specialises in photographing large groups of people without their clothes on. You don't get much more arty than that. Fittingly, this one is the most abstract banana in the bunch. It is sheer noise music, essentially. Imagine a traffic jam where most of the cars are pumping Metal Machine Music and the collected works of Z'EV from their dashboard cassette players, across the thick fog of exhaust smoke.

As should be clear by now, do not expect some long-lost sequel to Daydream Nation or anything as directly pretty as the exquisite 'Sunday'. In/Out/In has more in common with the musical sketches compiled on The Destroyed Room, the band's swansong from their Geffen contract, or the SYR (Sonic Youth Recordings) series which was used to showcase an alternative side to what some writers like to call the group's more "song-based" material.

The run of SYR albums encouraged other musicians to release works-in-progress, or show-your-workings recordings, in the run-up to their non-self-released output. Often they did this via CD-Rs. The likes of Wolf Eyes and Magik Markers, both endorsed by Sonic Youth, are a case in point. More recent racket-makers have been doing this through Bandcamp, often to coincide with those regular Fridays when the host site waives its fees. "THIS IS NOT AN OFFICIAL RELEASE THIS IS NOT A FULL LENGTH ALBUM THIS IS NOT PART OF THE OFFICIAL RELEASE LIST STOP EQUATING THIS WITH OUR REAL ALBUMS," scream the virtual sleevenotes to Oozing Wound's Blech, two "sides" of muddy run-throughs from their "practice space dungeon". It opens with, of all things, a thrashing rendition of 'Dammit' by Blink-182. No wonder they don't want it considered part of the formal oeuvre.

Sonic Youth benefitted from the alternative music boom of the 1980s/90s and were in large part responsible for it. It is unlikely a band like that – strangely tuned guitars, a cut-up approach towards song structure, prone to placing long passages of feedback in the middle of songs about the plight of Karen Carpenter – will be as popular as they were, ever again. Thurston Moore doesn't think so, and can barely believe it even happened the first time round. As he told me in one interview, "I see footage of us online and we're playing in front of tens of thousands of people. What?! It's like this other world I was living in. We were doing that, to that many people, and they were actually okay with it?"

Yet Sonic Youth do not belong in the past. They are with us still, albeit in dispersed and distanced form, like Tom Joad or E.T. the Extra-Terrestrial. "Whenever someone wonders what unusual and unpredictable sounds could be made if a random piece of cutlery were to be shoved under the instrument's strings, they'll be there. Wherever a band has three different vocalists who all take turns to 'sing', they'll be there. Why, they'll be there every time an amplifier shrieks when a guitar is held really close to it, and in the way kids laugh when the volume's too loud and they know they should be wearing earplugs. And when a band on a big label declines the typical, already overhyped act as their tour support, handpicking instead an iconoclastic gang of noiseniks who normally perform to fewer than 15 people, why, they'll be there. See?"

Thinking beyond such obvious legacies, you could say that Sonic Youth have also had an impact, perhaps via osmosis, on those who have, in the past, been most resistant to their experimental charms. "I don't think music should be clever, or be avant-garde, or artistic," Noel Gallagher, one of the most conservative figures in modern pop, boasted during a 1996 MTV special. "I hate art in music," he continued. "You know, all this pompous art-rock like Sonic Youth and all that, sticking guitars in dustbins and playing them with screwdrivers and that. I mean, fuck that. Let's rock."

Years later, Noel's High Flying Birds employed Charlotte Marionneau of Le Volume Courbe to provide French voiceovers and play percussion with a pair of scissors. Hits are for squares, man.

In/Out/In is released today digitally via Three Lobed