Van Halen frontman David Lee Roth has always had musical interests that expand well beyond the scope of rock 'n' roll, and he's proven it once again with a new edition of his YouTube series 'The Roth Show.'

In the clip (embedded above), Roth has met up with New York a cappella group Naturally 7 backstage following their Tokyo show on April 20, and everybody's singing -- but rather than harmonizing on a classic Van Halen cut or perhaps one of Dave's solo hits, they decide to deliver an impromptu rendition of Pharrell Williams' 'Happy.'

While it may not be familiar to some classic rock lovers, 'Happy' is hands down the song of 2014 (so far), having lodged more weeks at No. 1 than any other single this year and selling more than four million copies along the way. Originally released as part of the 'Despicable Me 2' soundtrack last year, 'Happy' was nominated for a Best Original Song Oscar, but lost out to 'Let It Go,' from Disney's 'Frozen.'

Naturally 7 is celebrating its 15th anniversary this year by touring with Michael Buble behind their most recent album, 2012's 'Live.' The group's a cappella style, which they call "vocal play," incorporates beatboxing into an R&B-flavored approach -- in other words, pretty much perfect for a song like 'Happy.' Hear how Roth held his own in this new clip.