A NUMBER of American string bands have recently evolved into chamber-folk ensembles that apply the rigor and focused power of classical chamber arrangements to their Appalachian folk instruments and songs without spoiling their front-porch informality.

At the core of these groups is the contrast of the ringing, percussive hammered dulcimer against a humming, sustained fiddle. As this contrast between ringing notes and sustained lines is elaborated with voices and other instruments, these pastoral songs capture the complexity that lies beneath the seeming simplicity of country life.

Three such groups perform together Friday night at Georgetown University's Gaston Hall, along with two other hammered dulcimer masters, Paul Van Arsdale and John McCutcheon.

TRAPEZOID -- "Cool of the Day" (Sugar Hill SH/PS-1132). On its fifth album, this quartet transforms the most intricate complexity into fully satisfying harmonies. Chicago's multi-instrumentalist, Howard Levy, functions as a fifth "Zoid" on the album, deepening the already thick harmonies with reeds and keyboards.

Drawing from contemporary songwriters such as Si Kahn and Townes Van Zandt, Trapezoid tackles its most substantial material ever. Ferron's existential dreamscape, "Never Was," is given a pulse-music arrangement. On "Nightsong" and "Rainfall," two glowing nature hymns by Trapezoid member Lorraine Duisit, the instruments imitate the sounds of a meadow at dusk and a farmhouse in the rain. The best song, though, is Barbara Keith's "Bramble and a Rose," in which Paul Reisler's prickly hammered dulcimer and the voices of Duisit and Freyda Epstein entwine like the title images.

METAMORA -- "Metamora" (Sugar Hill SH/PS-1131). Metamora is the new name for an old trio: Malcolm Dalglish, Grey Larsen & Pete Sutherland. This album marks their final maturation from a trio of virtuosos into an ensemble with a unified, identifiable sound.

The three multi-instrumentalists, who wrote all 16 pieces on the album, have developed into fine songwriters who capture a genuine sense of wonder at an infant daughter (Dalglish's witty "Sweet Potato"); at the sources of music (Sutherland's a cappella "Fiddler's Hymn"); at the new day (Dalglish's slowly awakening "Morningtime"); and at the art of quiltmaking (Sutherland's glorious "Endless Chain").

WALT MICHAEL AND CO. -- "The Good Old Way" (Front Hall 033). Michael and Co. are closer to the bluegrass and old-timey tradition. A medley of fiddle tunes, "Gray Cat on the Tennessee Farm" and "Cindy," kicks off the album. The quartet also essays two a cappella quartet numbers, "Down in the Valley To Pray" and "I'm Lookin' for the Stone."

Neither Michael nor John Kirk is a particularly compelling vocalist, which weakens good songs. The best moments come on "The Old Oak Cabin," a folk baroque instrumental; and on the Celtic lilt of "Arthur's Rose" and "Fanny Power," which is based on an O'Carolan melody. On this last cut, Michael's liquid hammered dulcimer is gently cushioned by the twin fiddles of Kirk and Frank Orsini and the cello of Mark Murphy.