Some people know the Scottish band Capercaillie mostly by the voice of its lead vocalist, Karen Matheson, who sang a stark Gaelic lament in the film Rob Roy. Matheson's singing is impossibly beautiful — Sean Connery has called hers "a throat surely touched by the hand of God" — but that's not all it is.

Coming up on its twentieth year, Capercaillie ("capper-KAY-lee" — a kind of grouse) has constantly evolved, and its members have challenged each other with new sounds and innovative arrangements. The group started out with traditional material; Matheson, a native Gaelic speaker who would still rather sing in Gaelic than in English, grew up in a small town in the Outer Hebrides where pub musicians on Saturday night were the main source of entertainment. Gradually, though, they grew dissatisfied with what their liner notes call "kilt culture" and began to modernize their sound.

Synthesizers, electronic percussion, digital samplers, and even occasional hip-hop-style scratching made their appearance (in concert all neatly boiled down to a keyboard), and Matheson kept pace with them every step of the way. Capercaillie does a traditional number called "The Tree," in which she artfully weaves the song's nonsense refrain into a background of mostly electronic sounds. Vocal purity is only one arrow in her quiver, and the range of music the group makes as a whole is wide. Capercaillie does contemporary songs of romance and social comment, and the group has worked with West African musicians in fusion pieces for which the homogenizing term worldbeat doesn't seem precise enough.

The reflexively antimodern sector of the folk music press has sometimes criticized Capercaillie as a commercial sellout, although the results to date (one song in the lower reaches of the UK Top 40) hardly support that charge. A more plausible concern would be whether it all amounts to mere novelty — but Capercaillie's arrangements seem inevitable, not novel. A traditional core of instruments headed by a fiddle anchors each song, and the music wouldn't make sense without it. Electronic percussion seems to expand the palette of sounds that are possible with acoustic instruments, not to define the essence of a piece, as it often would in a pop song. And when pure traditional tunes burst forth, as they often do in the middle of a piece or in a lengthy final jam, they are more than just touchstones — they bring the music to a keen pitch of intensity.

Indeed, one thing that's kept Capercaillie growing for so long is that every so often it circles back to its roots, not discarding sonic experiments but incorporating them unobtrusively into pieces learned from older Scottish musicians. The group's latest album, Choice Language, so far released only in the UK, marks such a moment. Its luminous, timeless soundscapes will be featured when Capercaillie comes to the Ark on Monday, September 29.