pipe vs pitch pipe

pipe and pitch pipe both are nouns.

Word	Noun	Adjective	Verb	Adverb
pipe	Yes	No	Yes	No
pitch pipe	Yes	No	No	No

As nouns, pitch pipe is a hyponym of pipe; that is, pitch pipe is a word with a more specific, narrower meaning than pipe:

pipe: a tubular wind instrument
pitch pipe: a small pipe sounding a tone of standard frequency; used to establish the starting pitch for unaccompanied singing

pipe (noun)	pitch pipe (noun)
a long tube made of metal or plastic that is used to carry water or oil or gas etc.	a small pipe sounding a tone of standard frequency; used to establish the starting pitch for unaccompanied singing
a tube with a small bowl at one end; used for smoking tobacco
the flues and stops on a pipe organ
a tubular wind instrument
a hollow cylindrical shape