The phrase "quiet down" is often used to ask someone to reduce the noise they are making or calm down their behavior. However, if you want to express the opposite meaning of "quiet down," you can use antonyms such as "louden up," "amp up," or "energize." These words are often used to encourage someone to become more energetic, expressive, or vocal. Other antonyms for "quiet down" might include "stimulate," "excite," or "incite," all of which convey a sense of invigoration and excitement rather than restraint and relaxation. Whatever your preferred antonym might be, be sure to use it appropriately in context to convey the right tone and meaning.