alarm vs siren

alarm and siren both are nouns.

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

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

alarm: 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

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

alarm (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
a clock that wakes a sleeper at some preset time	an acoustic device producing a loud often wailing sound as a signal or warning
an automatic signal (usually a sound) warning of danger	a warning signal that is a loud wailing sound
fear resulting from the awareness of danger	a woman who is considered to be dangerously seductive

alarm (verb)	siren (verb)
warn or arouse to a sense of danger or call to a state of preparedness
fill with apprehension or alarm; cause to be unpleasantly surprised