Here's why:
* Abstract nouns represent concepts, qualities, or ideas that cannot be physically touched or seen. Examples include: happiness, love, justice, freedom.
* Concrete nouns represent physical objects or things that can be experienced with the senses. Examples include: table, chair, book, dog.
"Loud" describes a quality of sound that can be perceived with our ears. It is a concrete, measurable aspect of sound.
Therefore, "loud" is a concrete term, not an abstract one.