alarm system vs siren

alarm system and siren both are nouns.

Word	Noun	Adjective	Verb	Adverb
alarm system	Yes	No	No	No
siren	Yes	No	No	No

As nouns, siren is a hyponym of alarm system; that is, siren is a word with a more specific, narrower meaning than alarm system:

alarm system: a device that signals the occurrence of some undesirable event
siren: an acoustic device producing a loud often wailing sound as a signal or warning

Other hyponyms of alarm system include automobile horn, car horn, hooter, horn, motor horn, burglar alarm, fire alarm, smoke alarm, horn.

alarm system (noun)	siren (noun)
a device that signals the occurrence of some undesirable event	eellike aquatic North American salamander with small forelimbs and no hind limbs; have permanent external gills
	an acoustic device producing a loud often wailing sound as a signal or warning
	a warning signal that is a loud wailing sound
	a woman who is considered to be dangerously seductive