SOFT METAL? Isis’s lush swirl of sound and texture is less about banging one’s head than lightly swinging it to and fro.

Aaron Turner, frontman of the formerly Boston-based art-metal act Isis, is not easily swayed by the idea that nurture affects a developing entity more than nature. Over the last few years, Turner and his mates — guitarists Mike Gallagher and Cliff Meyer, bassist Jeff Caxide, and drummer Aaron Harris — have experienced plenty of action. In 2002 they signed to former Faith No More singer Mike Patton’s Ipecac label. In 2003 they relocated from Boston to Los Angeles. And last year they toured with Tool, playing to the largest and most diverse crowds of their career.

Yet Turner (who also runs the successful underground metal label Hydra Head) doesn’t believe that any of these events has played a significant role in shaping Isis. “Frankly, a lot of what we do is for ourselves,” he says over the phone from the band’s tour van as it rattles across Florida in the general direction of Cambridge, where they will play the Middle East this Friday and Saturday. “I mean, it’s certainly possible that our environment has influenced our music. And it’s definitely gratifying to receive the support of our so-called elders; it’s kind of a confirmation of the validity of what we do. But I don’t feel that our music would’ve evolved on a different course if we’d stayed in Boston. And regardless of the encouragement, we’d still be compelled to make the music we make. More than anything else, it’s about our process of internal creation.”

The internal-creation theory is a favorite of innumerable indie bands, many of whom view outside influence as a vehicle of creative dilution. (I could fill pages with examples of scruffy guys telling me, “We just do what we do, and if anyone else digs it, that’s a bonus.”) To Turner’s credit, though, the music Isis make actually does sound like the product of just such a process: on In the Absence of Truth (Ipecac), their latest album, the quintet play metal as a kind of meditative mood music, a lush swirl of sound and texture that’s less about banging one’s head and more about lightly swinging it to and fro. Lengthy cuts such as “Over Root and Thorn” stretch metal’s hard-and-fast elements to slo-mo extremes, transforming distortion into drone.

It’s a sound Isis have been honing methodically since their formation in 1997; delving into their catalogue, you can hear them venture with increasing confidence away from metal-band expectations. “I’m awed by the musical progress they’ve made from record to record,” says Patton from Ipecac’s offices, “which is a difficult thing to do. The remarkable subtlety and beauty in the fabric of In the Absence of Truth is a perfect testament to this. I’m honored to be working with a band that seems to raise the creative bar with each release.”

“Generally with us, the development just happens,” Gallagher says. “But the reason it just happens is because we’re always thinking about how we never want to repeat ourselves. I mean, chances are that someone who’s reasonably familiar with the band could listen to a song they’ve never heard by us and be able to tell that it’s Isis. But we’ve worked hard at not doing the same things over again. We’ve never thought, like, ‘Oh, this worked on an old record, so let’s do something like that again.’ That’s a little bit gross; we couldn’t feel good about what we do if we did that.”

Gallagher says that for In the Absence the band redoubled their efforts to be original, making “shitty two-mic practice-space recordings” of the new songs before they entered the studio with producer Matt Bayles in order to “be able to take the music home and play along with it and mull it over.” The guitarist cites the end of “Garden of Light,” the CD’s nine-minute closer, as an example of the fruits of this labor. “I had certain parts of that song worked out before we went into the studio, but other parts weren’t set, so I just figured it out as we went. And that ended up being one of my favorite parts on the record. I wouldn’t have had the chutzpah to do that when we were recording [2001’s] Celestial. Back then I just concentrated on being able to play as well as I could. As time goes on, I think we’ve all kind of gotten the ability to let things go and see what happens.”

“They were exploring more expansive material than on previous releases,” says Bayles, who also produced 2002’s Oceanic and the 2004 Isis album Panopticon. “That meant spending more time arranging guitar and bass and keys on each song. Songs that might’ve climaxed with heavy distortion on a previous record didn’t necessarily follow that formula on the new one. So we all had to experiment.”