The word "rat-a-tats" refers to a fast and rhythmic tapping sound. Its antonyms, however, would be words that describe the absence or opposite of such a sound. These could include the words "quiet," "silent," "still," "calm," or "peaceful." Other antonyms could range from the words "slow," "gradual," "gentle," or "subtle" that do not depict any form of urgency or intensity. Conversely, words like "thud," "boom," or "crash" represent a completely different type of sound. Nonetheless, whatever the antonyms might be, they all describe an opposite of the rapid and pronounced sound of "rat-a-tats.