Synonyms for Placidness:

n.

quietude, calmness, peacefulness. lull, untroubledness, stillness, hush. calm (noun)
peace, repose, stillness, tranquility, lull, quiet, rest, placidity, peacefulness, quietude, calmness, serenity, hush.
peace (noun)
calm, order, armistice, neutrality, amity, repose, nonresistance, peace, concord, rest, quiescence, cease-fire, nonaggression, quiet, tranquility, orderliness, accord, friendliness, quietness, serenity, truce.
placidness (noun)
placidity.
stillness (noun)
untroubledness, calm, quietness.

Other synonyms:

untroubledness, peacefulness, quietude. calmness, stillness. Other relevant words:
lull, hush, placidity, peacefulness, quietude, stillness, untroubledness, calmness.