tone arm vs mechanical device

tone arm and mechanical device both are nouns.

Word	Noun	Adjective	Verb	Adverb
tone arm	Yes	No	No	No
mechanical device	Yes	No	No	No

As nouns, mechanical device is a hypernym of tone arm; that is, mechanical device is a word with a broader meaning than tone arm:

tone arm: mechanical device consisting of a light balanced arm that carries the cartridge
mechanical device: mechanism consisting of a device that works on mechanical principles

tone arm (noun)	mechanical device (noun)
mechanical device consisting of a light balanced arm that carries the cartridge	mechanism consisting of a device that works on mechanical principles