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The dogs barks got louder and louder
No, loud is the positive degree. The comparative is louder.
how much the stereocilia of the hair cells are bent
Louder can be used as both an adverb and an adjective.
I shouted louder than him. (adverb)
My voice is louder than his. (adjective)
Dual is usually louder due to less back pressure. The amount it is louder depends on how it's built.