silence vs pipe down

silence is a noun but pipe down is not a noun.

Word	Noun	Adjective	Verb	Adverb
silence	Yes	No	Yes	No
pipe down	No	No	Yes	No

silence (noun)	pipe down (noun)
the absence of sound
a refusal to speak when expected
the trait of keeping things secret
the state of being silent (as when no one is speaking)

silence (verb)	pipe down (verb)
cause to be quiet or not talk	become quiet or quieter
keep from expression, for example by threats or pressure