It's been three years since Alabama Shakes first brought their soulful roots rock to T in the Park - three years in which they have played for a president, been nominated for a Grammy Award, contributed to the soundtrack of Steve McQueen's Oscar-winning film 12 Years A Slave and travelled the world performing their powerful debut album, Boys & Girls.

It's no surprise, then, that singer and creative engine Brittany Howard has a little trouble remembering that first visit north of the Border.

"T in the Park?" says the 26-year-old, slowing her Southern drawl to a crawling pace as she dredges the memory banks. "Muddy," she says finally. "Yeah, that's what I remember the most."

The mud may yet put in another appearance when Alabama Shakes take to the main stage at Strathallan Castle on July 12 for their second visit to Scotland's premier outdoor rock festival. But if it does, no matter.

The group's appearance on Glastonbury's Pyramid Stage last weekend took place in a downpour which did nothing to lessen the spectacle as Howard, her four bandmates and the backing singers who make up the nine-piece touring ensemble ran through a typically muscular set built around one of the most powerful and emotional voices in rock.

Howard has grown weary of the comparison - "For the record, I don't think I sound anything like Janis Joplin," she tells me. "I think I just sound like me" - but the notion persists, and for good reason.

But some things have changed for Alabama Shakes. In April, they released their second album, Sound & Color, and showed they have taken a significant step forward sonically.

Howard's distinctive vocals remain to the fore; she and fellow guitarist Heath Fogg still trade guitar licks which alternate between languid and jagged; idiosyncratic drummer Steve Johnson maintains a propulsive beat; and heavily-bearded bassist Zac Cockrell still anchors the sound. But in has come a more textured and nuanced approach to songwriting and recording, one more in keeping with Howard's vision for the band.

"The first record was made in a rush because we all had day jobs and we didn't have very much money, so there were limited resources," she explains.

"We would spend a day and do three songs then maybe not go back into the studio for three weeks. On this record we had time to fill in the details which is something we've always wanted to do. That's how I prefer it. It made it feel more representative of us as players and song-writers."

The word Howard uses to describe the recording of Sound & Color is "studious". Among the "details" she wanted to "fill in" was a sense of texture and among the instruments she used to do it was a vibraphone.

It features on the album opener, also the title track, and gives the song a near minute-long entrée of warm, slowly pulsing sound. The overwhelming impression, then, is of a band moving confidently into that less-is-more territory where silence is a tool and the space between the notes is as important as the notes themselves.

"That's what we like," says Howard. "We've all grown as players and our tastes have also grown. Something I've noticed as I've been listening to music over the years is that the space is really beautiful. It gives your mind time to let everything sink in, including the lyrics and the sonics and the movement of the song, whereas being bombarded by compressed noise is not something I enjoy. I never have."

The sound has also become more orchestral. Howard grew up listening to a vast array of different music - "I was absorbing everything," she tells me.

"Prince, Top 40, Elvis, Chuck Berry, Guns'N'Roses, The Rolling Stones, The Beatles, Little Richard" - and at her Alabama high school she bonded with Shakes co-founder Zac Cockrell over bands like Black Sabbath and Texas rockers At The Drive-In. But if there's a single influence in play on Sound & Color it's David Axelrod, the veteran American composer and arranger whose genre-bending soundscapes have seen him much sampled by hip-hop artists over the last couple of decades.

"I've been listening to him for years," says Howard. "Composers are something I'm interested in. When I was growing up, classical music was always intriguing to me because there's no words and it's not in a song format. It's not pop. The fact is it's just really beautiful and David married those two things. He married pop and rhythm with the classical and jazz worlds."

Howard's musical erudition is impressive though not surprising given her background: she was playing guitar before she was in her teens and had always been fascinated by music.

Neither of her parents played, but her uncle and great-uncle did and there was a piano in her grandmother's house. Relatives and neighbours would perform and when they did, the young Brittany Howard was always listening intently.

"Any type of music was intriguing to me because I was interested in how it was put together, in much the same way an engineer would be interested in how a cassette player was put together when they were a little kid," she says. "There's always these signs of having this endless interest in one thing, and that's how it was when I was coming up."

Born to a white mother and a black father, Howard's "coming up" was done in Athens, Alabama. A city of 21,000, it's close to the Tennessee border and midway between Nashville, where she now lives, and Birmingham, the state's largest city.

She met Zac Cockrell at high school - "I don't know that me and Zac would have had very much to talk about it if weren't for music," she laughs - and after graduation she famously worked as a truck driver and a postal worker. But, as she says, everything was a means to an end.

"I'd wanted to be in a band since I was about 11 and everything from that point on was about me trying to get one together," she says.

"I was tirelessly driven just to be in a band. But it took a long time because where we're from there's not a lot of people who want to sit down and write and dedicate themselves to it. They're all like 'Yeah I might do this today but tomorrow I won't care any more'. But that wasn't for us. We wanted to see how far we could go."

Howard had never left Alabama before she started that band, so her journey is as much about geography and travel as celebrity and accolades.

Perhaps more so: when I ask her which experience has made the greatest impression so far she doesn't say appearing at the Grammy Awards or seeing Sound & Color top the US album charts or performing for Prince in his Minneapolis mansion. And, though she says Barack Obama "seemed like a fine guy. It was nice of him to welcome us into his home", she doesn't say it was the day she played in the White House for the president and his wife.

Instead she talks about the first time she crossed the Alabama state line, "the first time I saw the desert, the first time I saw the ocean in California, the first time I saw the peaks of mountains ... That to me was the most memorable thing I've ever done, emotionally. We saw so much of America together, right there in the beginning."

And does distance now lend her a new perspective on her homeland and her home state? "Definitely," she says. "One thing we were talking about the other day was that a lot of countries have an Arts Council which tries to take care of artists.

We don't have anything like that. It would be nice to have some kind of affordable schooling for a kid that's talented. I think artists should be more highly regarded in our country than they are, is what I'm trying to say. They should be nurtured". And as for music, "it would be better if it wasn't just left up to really, really rich dudes."

Where Alabama Shakes are concerned, it isn't. Dudes they may be and rich they may become, but as much as their music is subtle and nuanced and "studious" (to use Howard's word), so is it rootsy and authentically Southern. And perhaps that's its strongest appeal: its palpable sense of place and its ability to sound both vintage and freshly-minted.

But Brittany Howard is taking nothing for granted. She's still a little dizzy with wonder at the journey she has made, still surprised to "be where I am, to have all these people coming to the shows, to be selling out all across the world", still eager to return to Nashville to "decompress" at the end of it all. "But," she adds, "you have to take it in your stride. You got to."

Alabama Shakes play T in the Park on Sunday July 12