Finnish vocal duet Vox Silentii, which has been performing since 1992, specializes in monophonic medieval plainsong. Johanna Korhonen and Hilkka-Liisa Vuori have voices of exceptional purity and such an uncanny ability to blend that they can sound like they are singing with a single voice. Their combined tone occasionally produces ghostly but clearly audible overtones, and the effect is awe-inspiring. The unity they project also extends to their beautifully formed vowels, and even the subtle shifts in their diphthongs are perfectly matched. Their impeccable unanimity is highlighted at the rare moments when they break into two parts, with one voice singing a drone under the melody. The effect could hardly be more dramatic; it feels like a seismic shift of great magnitude. The album's subtitle is "Medieval Chants for Christmas," and all of the selections emanate a stillness and sense of wonder, sometimes hushed and sometimes quietly ecstatic. This album would make an excellent background for quiet meditation, but the subtlety and ravishing purity of the performances also reward close listening. Although there is no great variety in the moods presented here, the singers' focused intensity keeps the music continually fascinating; there's not a dull moment on the CD. The sound of the SACD is appropriately resonant and accurately captures the details of the performance and the acoustical characteristics of the church in which the recording was made. The singers are miked at a relatively high level, so listeners may want to lower the volume to keep some of the pitches from being too piercingly intense.