Listen to Ira Kaplan describe Yo La Tengo’s impetus for going into the studio to record its 10th album, Summer Sun (Matador), and you get a pretty good sense of how the band operates as a whole. "We basically decided when we wanted to have a record out -- we were sort of tired of not being on tour -- so we set a deadline for ourselves, and wanted to see if we could meet it," he explains by phone from Columbus, Ohio, the first stop on the tour that brings them to The Trocadero Friday night. "We’d been working for a long time, kind of slowly and frequently, and by design aimlessly, but finally we decided to force this deadline on ourselves and see what happened."

"Aimless by design" isn't quite a fair description of the Hoboken trio's 18-year career, but it'll do in a pinch. The very idea that you can plan not to have a plan seems central to the band's intuitive, utterly organic evolution. It's been a long road from the modest folk-rock of 1986's Ride the Tiger to Summer Sun's hushed squall, but one with few signposts along the way. With the exception of 1990's standalone Fakebook, each album seems in retrospect like a natural progression from the one before. It's only when you skip over a few records that the differences become apparent: Put Summer Sun's sprawling, jazz-tinged "Let's Be Still" up against, say, "Blue Line Swinger" from 1995's Electr-O-Pura and you can hear how Kaplan's once-trademark feedback has been replaced with a swirling mist of electronic loops, where objects appear and vanish without ever coming fully into view.

As a tradeoff, though, Summer Sun continues the trend toward lyrical lucidity begun with 2000's And then nothing turned itself inside-out. Songs like "Last Days of Disco" and "The Crying of Lot G" surprised fans and awed critics with their (seemingly) transparent references to Kaplan and YLT drummer Georgia Hubley's marriage, and the album's mix pushed the vocals further to the front than ever before; suddenly, you could not only hear the singers (Kaplan, Hubley and bassist James McNew take turns at the mic), but understand them as well.

Kaplan, who's always been reluctant to discuss his lyrics, says he was "definitely surprised" by the ado over Nothing's supposed confessionals and cautions against reading any of them too literally. (It's worth noting that, on the lovers' duet, "Our Way to Fall," it's McNew, and not Hubley, singing the high harmony against Kaplan.) But the move toward simpler lyrics and more intelligible vocals is a conscious one. "We've all, probably starting with [1997's] I Can Hear the Heart Beating as One, tried to sing better, and sing more creatively," Kaplan says. "Part of that was making the singing louder, not trying to bury things, not being afraid to have it heard. Lyrically, there's been that thought as well, not to hide what I think [by] being vague, and not shy away from ," he searches for the word, "clarity." Such heart-on-sleeve sentiments are in evidence on Summer Sun's lilting "How to Make a Baby Elephant Float," which, ironically, is a paean to inside jokes: "I'm not averse to pillow talk/ But I prefer a private joke/ Because it's our punch line."

Since Electr-O-Pura, every song has been credited to the group as a whole, marking an increased reliance on improvisation as the basis for songwriting. These days, says Kaplan, "no song is really created in the studio, but no song is really finished before we get there either." Lyrics typically come last and only with great effort, Kaplan says. "Writing lyrics is always the hardest part, I think for all of us. It's something we always save till the last minute -- If we go into the studio with one complete set of lyrics, we're really way ahead of the game. There were a couple times on this record we shut down recording just so Georgia and I could finish stuff we had to do."

It's unlikely that Yo La Tengo will ever climb to the top of the charts -- although there's a page on their website dedicated to the week the band's single of Sun Ra's "Nuclear War" cracked the Billboard Hot 100 -- but then, how many bands manage even to exist for 18 years, let alone experience a "noticeable jump" in sales with their ninth album, as Kaplan says the band did with Nothing. "Noticeable to us, anyway. We're playing a bigger place tonight in Columbus than we've ever played before. There have definitely been changes, and noticeable ones, but the overnight-success option has been closed." But when you can sell out all eight nights of Hanukkah at Hoboken's storied Maxwell's, as the band did for the second time this year, and get such luminaries as Ray Davies, Ronnie Spector and David Byrne to step on stage with you, who needs overnight success?

Fans who are worried that the band has gone soft needn't fear, an EP featuring three rock songs from the Summer Sun sessions will be along in the fall. They were cut, Kaplan says, not out of a desire to keep the album quiet, but to keep it short. "One thing we really wanted to do was to make a shorter record. The last two were way over 70 minutes, and I think we really wanted to prove to ourselves that we could cut some songs; it's funny to think of a double album, 60-plus minutes, as being short, but it is short-er. It was a different way for our band to work to cut those songs off. We had the option of making a more striking break with the last record, which has generally been the way we'd do things, but to cut off the rock songs and make it more of a piece with the last record -- I think there was some intrigue for us, that it would be a different way of working." In other words, the way Yo La Tengo defies expectations is by not doing the unexpected. Does that clear everything up?

Yo La Tengo plays Fri., April 18, 8 p.m., $16, with Portastatic, The Trocadero, 1003 Arch St., 215-922-LIVE.