Foo Fighters guitarist Chris Shifflet leans back on a plush sofa and laughs at his bearded band leader as I explain that, no, BLUNT is a music magazine and Dave and his mates are going to be on the cover.

"Oh, cool," the frontman says, motioning me towards a seat in the plush Sydney hotel the band are staying in during a brief promo tour of Australia in support their fifth album, In Your Honour. "I kinda missed the whole drug thing anyway - I never even got to try ecstasy," he laughs. As we sit down, I explain that, while MDMA is of course illegal, it's not too late to, er, experiment... Dave just laughs again and explains how that's not really his bag. Hasn't been for ages.

"But I took acid and played a show when I was like 17," he remembers with a slight shudder. "It was like the fuckin' scene in Song Remains The Same, with the fuckin' wizard."

Dave's standing up again, happy to be talking about the Led Zeppelin concert film while windmilling his arm around Jimmy Page-style and making a kind of "zgha-zgha-zgha-zgha" sound. "Fast hardcore on acid, bad idea," he adds. I agree with him, but the question's got to be asked: How did the gig sound? "I don't remember. I didn't care, man. I play better on acid - that's why I quit."

He is, of course, lying his arse off, as he proves that night when the band tear through a short acid-free set at Fox Studios in front of several hundred keen fans for Channel [ V] . While they only showcase a few tracks off their new double album, the string of past hits they present - "My Hero", "Generator" and "Monkey Wrench" being just a few - are tight-as. The eager crowd feed off the band's energy and go manic on cue as cameras on cranes pan over them, capturing sweat-soaked footage of the seething pit. Not bad for a low-key Monday evening.

Love 'em or not (you can`t actually hate Dave Grohl), there's no arguing that the Foo Fighters are one well-drilled rock machine. Which makes In Your Honour all the more interesting. While the first disc in the set takes a few chances, building on the band's standard sound by adding rougher - though very well-produced - edges and generally upping the aggression factor, the second disc comes way out of left field. Focusing on a softer acoustic style, the blokes understandably seem a little apprehensive about how fans will respond to it. Nonetheless, it's clear that all four members are well pleased with what they've achieved.

Adopting an exaggerated pompous tone, Dave jokes that, "It was a milestone achievement in the band's career," before getting a little more serious. "I think every band has a double album in them, they just have to figure out the right time to do it," he says. "We couldn't have done this album three years ago or six years ago, it just wouldn't have made any sense."

Drummer Taylor Hawkins, on the other hand, doesn't even like to call it a double-album. After saying that the whole thing overall is "big sounding", he explains that he likes to think of it as two separate records - a loud one and a soft one. "Exactly," Taylor agrees. "I think people who like us will like the rock record a lot," the enthusiastic drummer adds. "And I think the reaction will probably be split with the acoustic stuff. That album takes a little bit of time to sink in with people. Y'know, it's not just a sort of instant listen. But a lot of the rock stuff is pretty fucking direct. It's hard for us to tell though. Who knows? The acoustic record may be our biggest hit ever. But then again, it may just go along as a little hidden gem with the other record."

Dave explains that the idea behind the acoustic record came when he got home after touring the Foo Fighters' last album, One By One. "The band kind of got to a level that I never imagined it getting to," he says of their biggest tour to date. "We came back from that and we were all beat. I kind of crawled home and thought, '' That's the last time I'm doing something like that. I don't want to fucking go out and bust my ass on a festival stage for another two years. "

At a loose end, he looked in another direction. "I thought maybe I would write some acoustic music and find a film to do a score for," he says. "Original songs, kind of like what Tom Petty did with the She's The One soundtrack, or Ry Cooder's Paris, Texas, which is like my favourite fucking album of all time. So I started writing this acoustic music and after a while I listened to it and I thought, You know, this could be a Foo Fighters record. Maybe that's what we do? Maybe we should do something different. Something that's maybe a bit more musical and something that will really fucking reenergise the band."

As Dave pauses for breath, Chris explains that the whole acoustic thing shouldn't be that much of a shock anyway. "It's not like we're Slayer," the laidback lead guitarist jokes. "It's not like we haven't touched on some of those vibes in the past."

Thankfully Dave realised the idea of the Foo Fighters totally abandoning the thing they're best at - rocking the bejesus out of audiences - was a "ridiculous" idea, so he decided it was time for the band to step up to the plate and enjoy the best of both worlds. The end result of that decision has produced some of the band's heaviest work to date. The other record, however, lets the Foo Fighters spread their wings in a very different way. While Dave's original concept for the acoustic album was to have a different guest on each track, that eventually fell by the wayside.

"We'd had a few guests on records before, but I've always taken pride in the fact that when we go in to make a record it's the four guys in the band and we don't fucking outsource and we just kind of do it within the band," he explains. "But with the acoustic album, because we'd never done anything like that before, I thought this is a good opportunity to jam with other people, and it might make it more interesting. But when we went in to record it, we decided to do a song a day and after two weeks we'd finished the record and I listened to it and thought, Fuck, this is done. We don't need Grant Hart from Husker Du or Chan Marshall from Cat Power or fucking Warren Haynes (Mule). There was a long list of all these people." Still , a couple of guests did make the scene, such a s one of Dave's heroes, John Paul Jones of Led Zep (it all comes back to that wizard).

"There was a song that I wanted to have mellotron on and, well, he's the king - whether it's "Rain Song" or "Kasmhir" or whatever, I mean that guy... he was in Led Zeppelin, so he could have come in and played triangle, you know!"

Chris laughs and reminds Dave, "We came in and said that to you at one point beforehand - what if you don't like what he plays? And you were like, ‘‘Doesn't fucking matter. We're putting it on anyway." Also making the guest list was Josh Homme and, interestingly, Grammy award winning jazz chick Norah Jones.

"I wanted to have a male/female duet on this song ["Virginia Moon"] which was an older song," Dave says, his excitement level redlining once again. "And she seemed to make sense because the song is kind of a bossa nova; it was more like something she would do than the Foo Fighters. And she's just unbelievably talented. Musically she's far beyond anyone I've ever played with. She's fucking great."

Still, while the band are enthused about their new direction, with each member saying that they believe it's only set the bar higher for what they can achieve in the future, they're not about to compromise the energy of their regular show by pulling out acoustic instruments. Instead, when they're back in Australia next November the plan is to do some killer rock gigs and offset them with a few acoustic theatre shows.

"We've done acoustic shows here and there and we always really enjoyed them," Chris says. "And we talked about doing an acoustic tour years ago, but it never happened. But now we'll be able to go back and do all the mellow stuff off all the records . " Dave adds that, "Doing an acoustic tour before would have just been acoustic "My Hero" or whatever. Now we have these songs that were written to exist in that vibe. I think the cool thing about the acoustic record, the thing I love about it, is that it was just way more orchestrated than we've ever been. There's fucking mellotron and fucking bells and strings and it's all over the place. I'd love to have a mini orchestra live and just go for it."

Still, for all this soft-rock talk, the band proved they're still a formidable rock beast at a "secret" show at Sydney's Metro theatre two nights later in front of a select crowd of industry wankers and punters lucky enough to score one of the limited tickets released that morning. Watching Dave and the boys tear the roof off at a gig Taylor predicted "may be our best show ever or it could be lousy" it's clear anyone who reckons Foo Fighters have mellowed with age shouldn't worry, they're just looking at a bigger picture.