Synonyms for Quieten (same or very similar meaning)
WordNet sense 1 (become quiet or quieter):
hush, quiet, quiesce, pipe down, quiet down
WordNet sense 2 (make calm or still):
calm, lull, still, tranquilize, tranquillize, tranquillise, calm down
WordNet sense 3 (cause to be quiet or not talk):
silence, shut up, hush up
From the ODE community, based on WordNet
add/edit