A million albums shifted or not, Alabama Shakes have evidently clocked that you can’t just keep mimicking the past indefinitely, however good an impressionist you are; or rather, you can, but it’s an artistic dead end.

Sound and Color finds them audibly trying to break out of the self-imposed restrictions of their debut, Boys and Girls.

If there’s nothing here to upset the kind of person who feels faint at the very mention of hip hop rhythms or dubstep bass drops, some of their new ideas are surprisingly radical. Not least is the deliberate muffling of Brittany Howard’s voice, presumably to try to make it seem less like the band’s focal point and more like part of their palette of sounds — which, amid the spectacularly potent riffs of Dunes, expands to include fizzing synthesiser, a muted orchestral arrangement that might even be a sample, and our old friend, the tripped-out guitar effect.

But that voice is still the most arresting thing about their sound, even when it sounds as if they’ve locked Howard in a cupboard. She somehow manages to make a falsetto sound lascivious on Guess Who, and introduces Don’t Wanna Fight by issuing a remarkable noise somewhere between awheeze and a scream. And the song is fantastic: tense funk decorated with a rubbery bassline, and guitars that keep breaking out of an edgy little riff into something more clangorous and spacey. It shows how great Alabama Shakes can be when they’re not just paying homage to the past. The same goes for Gimme All Your Love, which takes an old-fashioned Southern soul ballad and amps up the melodrama until it sounds cathartic and abandoned, everything crashing and flailing around Howard’s voice — and Future People, its tricksy guitar part overlaid with slabs of fuzz bass, its distorted, reverb-heavy climax thrillingly fierce.

At those points, Alabama Shakes sound like a band cutting loose: there’s a real joy and abandon here. But that’s not the whole story. Some of the song names sound like temporary titles, scribbled down in the studio to differentiate sections of a jam session — Shoegaze, This Feeling — and there are moments when that’s pretty much how the music on Sound and Colour feels: like half-sketched ideas, improvised in the studio in lieu of substantial new songs.