“loud”的反义词是“quiet”.
1.quiet
英 [ˈkwaɪət] 美 [ˈkwaɪɪt]
adj.清静的;僻静的;不起眼的;不激动的
vt.& vi.安静下来，使…安静，平静
vt.减轻（病痛、疑虑、恐惧等）;[法律]确定（产权等）
prep.克制的，稳重的，不张扬的
n.宁静;寂静
adv.安静地，平静地
例句：
1.Tania kept the children reasonably quiet and contented.
塔妮娅把孩子们哄得安安静静而又心满意足。
2．She was received in a small, quiet office.
她在一个安静的小办公室里受到接待。
3．It is very quiet without him.
他不在的时候这里非常清静。
4．He called for quiet and announced that the next song was in our honor.
他叫大家安静，然后宣布下一首歌要送给我们。
5．I told them to be quiet and go to sleep.
我叫他们不要说话，马上睡觉。