lower vs let down

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

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

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

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

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

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

lower (verb)	let down (verb)
make lower or quieter	fail to meet the hopes or expectations of
set lower	move something or somebody to a lower position
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