let down vs lower

let down is not a noun while lower is a noun.

Word	Noun	Adjective	Verb	Adverb
let down	No	No	Yes	No
lower	Yes	Yes	Yes	No

As verbs, let down and lower are synonyms defined as:

Other synonyms of let down include bring down, get down, take down.

let down (noun)	lower (noun)
	the lower of two berths

let down (adjective)	lower (adjective)
	the bottom one of two

let down (verb)	lower (verb)
fail to meet the hopes or expectations of	make lower or quieter
move something or somebody to a lower position	set lower
	cause to drop or sink
	move something or somebody to a lower position
	look angry or sullen, wrinkle one's forehead, as if to signal disapproval