Here's why:
* Adverbial comparison uses adverbs to compare the degree of something. In this case, the adverb "quiet" describes the degree of the audience's noise level.
* The sentence compares the audience's noise level before the character spoke to the noise level after the character spoke. It implies that the audience was louder before and quieter after.
Example: The sentence uses the comparative form "quieter" to show that the audience was less noisy than before.
Let me know if you would like to explore other types of degree comparisons!