 The opera begins with a short, "strenuous" instrumental prelude, which forms the basis of the musical accompaniment to the opening congregational hymn; its first phrase is founded on a descending bass E-B-F, itself to become an important motif.Humphrey Carpenter notes that throughout the hymn the bass line is out of step with the singing, an effect which, he says, "suggests an adult world where purity is unattainable".Following the hymn, the Voice of God is accompanied, as it is in all his pre-flood warnings and declamations, by the E-B-F notes from the opera's opening bass line, sounded on the timpani.