There’s no doubt that Anonymous 4 are the reigning superstars of the medieval sacred music world. That may be a limited market, but they pack a punch.</p><p>Their concert, presented Saturday night at Visitation Church by the Friends of Chamber Music, was an extraordinary musical event.</p><p>Composed of 4 vocalists — Ruth Cunningham, Marsha Genensky, Susan Hellauer and Jacqueline Horner-Kwiatek — the ensemble displays an uncanny vocal blend.</p><p>Their program, entitled “Secret Voices: The Sisters of Las Huelgas,” referred to a historic community of nuns living in Burgos, in north-central Spain. The convent was called “Las Huelgas” (“Place of Refuge”) and welcomed women from noble families who sought a religious life.</p><p>The Codex Las Huelgas was a famous manuscript of medieval music copied in the early 14th century and still studied by serious students of the middle ages. It contained nearly 200 sacred works.</p><p>Anonymous 4 postulates that the original singers of the works contained in the manuscript were the nuns from the community. From the sounds the ensemble produced, sisters sing on!</p><p>In the opening work, the sequence “Virgines egregie” (“Illustrious virgins,”) the quartet’s characteristic blend on the unison chant caught the ear; but so did the artful manner in which they negotiated the free rhythms and melodic ornaments.</p><p>Anonymous 4 organized their program around music that the nuns might sing throughout their day of prayer and labor. In the “Morning” section they sang “O Maria Virgo/O Maria maris stella/In veritate.” (Medieval music often calls for singers to sing different texts simultaneously, so all opening words are included in the title.) Textural expansion from two to four parts and impeccable tuning were highly impressive.</p><p>Several excerpts from masses were included, such as the “Gloria: Spiritus et alme.” The work offered a juxtaposition of traditional liturgical text sung in plainchant with added texts set polyphonically. The use of added polyphonic sections gave a glimpse into the medieval mind. Confronted with required traditional texts and music, the interpolations offered an opportunity for reverent creativity.</p><p>One very noticeable flaw came during “O monialis conscio” (“O assembly of nuns,”) a lament on the passing of one of their members. While this featured some of the most moving singing of the evening, the highest note proved a bit too high for one singer.</p><p>The most impressive work came near the end — “Mater patris et filia” (“Mother and daughter of the father,”) — where fine technique and superb musicality coincided.