modem vs acoustic modem

modem and acoustic modem both are nouns.

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

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

modem: (from a combination of MOdulate and DEModulate) electronic equipment consisting of a device used to connect computers by a telephone line
acoustic modem: a modem that converts electrical signals to telephone tones and back again

modem (noun)	acoustic modem (noun)
(from a combination of MOdulate and DEModulate) electronic equipment consisting of a device used to connect computers by a telephone line	a modem that converts electrical signals to telephone tones and back again