Vox Silentii consists of two women from Finland who have developed, or probably rediscovered, the technique of singing the chant in such a way as to excite the natural harmonic resonances of the building. In this way, the church itself functions as an additional voice. Where this is a large medieval space like the Abbey Church at Vadstena, the sound is out of this world.
I have never heard of such a style of singing before but on reflection, it seems likely that this was the normal way of singing the chant in medieval times, since the spaces were so obviously designed for the purpose. The Brigittine church, with its long echo, was an excellent setting for reciting and prayer chants. Birgitta had issued explicit instructions on the construction of churches, based on her views of the requirements of the liturgy. The proportions of the church (width two times the height of the vault; length two times the width) and sound-reflecting materials seem to favour the resonance of a standing wave.