Unaccompanied song is at once a high-culture vehicle (the intricate counterpoint of a Palestrina Mass) and the primary medium for many ethnic folk traditions. These range from the close-harmony groups of South Africa to the massive choruses of the Baltic states, where the Latvian Song and Dance Celebration, held every five years (the next one is in July), culminates with tens of thousands of people in folk costume singing together.

So it's nice that the Kennedy Center's "A Cappella: Singing Solo" festival, opening today and running through June 6, has not limited itself to Western art music. Certainly that music is well represented -- highlights include the popular Scandinavian group Trio Mediaeval, the British early-music ensemble I Fagiolini and the all-male, San Francisco-based Chanticleer. But also represented are Inuit throat singing (the group Aqsarniit), popular music (the Manhattan Transfer), the popular D.C.-based Sweet Honey in the Rock and folk groups Ladysmith Black Mambazo and Le Mystère des Voix Bulgares, the Bulgarian chorus that has been recording since the late 1950s, whose popularity has sparked wider interest in a cappella folk traditions.

Unaccompanied song occupies a particular slot in our culture: It represents a state of innocence, spirituality, grace. The popularity of Le Mystère des Voix Bulgares is related to the more recent success of the Benedictine monks of Santo Domingo de Silos, whose first album of Gregorian chant went platinum in the 1990s. This image of innocence is more romantic than factually accurate; what groups like the Voix Bulgares offer are contemporary reconstructions or reinterpretations of folk traditions as much as the traditions themselves. Still, the music can be a way for people to reconnect with their heritage, as in Corsica, where a revival of the island's distinctive brand of a cappella singing (notably the three-part paghjella) started in the 1970s and has led to the rise of folk groups such as Les Nouvelles Polyphonies Corses and A Cumpagnia.

These groups are subject to the paradox that faces the whole spectrum of what is today marketed as world music: Once you begin marketing and polishing your product for broader consumption, its relationship to authentic folk music starts to become questionable. For one thing, folk tradition alone may not be enough to sustain success; after establishing yourself in a folk repertory, the next step is the Christmas album, or the arrangements of pop standards. (The King's Singers are poster boys for this chameleon aspect of the unaccompanied singing group; the repertory of the venerable sextet ranges from Thomas Tallis to the Beach Boys.) For another, folk song traditions are often raw or involve unusual harmonies, and a certain amount of prettifying, consciously or unconsciously, is often involved in the process of translating them onto disc.

What recording, and popular success, also remove from the equation is the social aspect of a cappella song. Unaccompanied vocal groups represent some of the last bastions of music as a participatory activity rather than something that is passively apprehended. The point of many a cappella groups is performing rather than performance; even the Santo Domingo monks' Gregorian chant is, at bottom, conceived as a way for the singers to meditate and worship rather than something they put on for the benefit of others. At some colleges, a cappella singing groups have assumed a fraternity-like function (groups from the University of Virginia and James Madison University are on the Kennedy Center festival's program); Yale's Whiffenpoofs represent the acme of that phenomenon.

No musical tradition better represents the junction of high and low culture. For centuries, the West looked on vocal works as the highest manifestation of music; instrumental music was seen as something merely functional. But the rough harmonies and soft burring sounds of, say, Alan Lomax's field recordings of Basque singers would long have been shunned by art music purists. It could be seen as a tacit demonstration of the low-culture repute of a cappella song that so many events at the Kennedy Center festival are being presented free.

The timbres of a cappella song can be an acquired taste. I remember with some bemusement my own violent aversion, as a child, to the original-cast album of "The Sound of Music," because the rawness of the unaccompanied nuns' voices on the first track grated on my ear like fingernails on a chalkboard. I find no trace of that aversion now. It may have been schooled out by years of a cappella singing -- in day camp, in school choruses, or on vacation with my mother's family, who bore tattered copies of "The A Cappella Singer" to evening singing sessions like bibles of a tradition too adult for the children to fully understand.

A cappella song is nothing less than a snapshot of musical experience. It is a topic too large to encompass easily, but the Kennedy Center's festival is a welcome chance to give it attention.