car horn vs alarm system

car horn and alarm system both are nouns.

Word	Noun	Adjective	Verb	Adverb
car horn	Yes	No	No	No
alarm system	Yes	No	No	No

As nouns, alarm system is a hypernym of car horn; that is, alarm system is a word with a broader meaning than car horn:

car horn: a device on an automobile for making a warning noise
alarm system: a device that signals the occurrence of some undesirable event

Other hypernyms of car horn include alarm, warning device.

car horn (noun)	alarm system (noun)
a device on an automobile for making a warning noise	a device that signals the occurrence of some undesirable event