buoy vs acoustic buoy

buoy and acoustic buoy both are nouns.

Word	Noun	Adjective	Verb	Adverb
buoy	Yes	No	Yes	No
acoustic buoy	Yes	No	No	No

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

buoy: a float attached by rope to the seabed to mark channels in a harbor or underwater hazards; typically brightly-coloured
acoustic buoy: a buoy that can be heard (at night)

Other hyponyms of buoy include can, can buoy, conical buoy, nun, nun buoy, spar buoy.

buoy (noun)	acoustic buoy (noun)
a float attached by rope to the seabed to mark channels in a harbor or underwater hazards; typically brightly-coloured	a buoy that can be heard (at night)