headroom vs high fidelity

headroom and high fidelity both are nouns.

Word	Noun	Adjective	Verb	Adverb
headroom	Yes	No	No	No
high fidelity	Yes	No	No	No

As nouns, high fidelity is a hypernym of headroom; that is, high fidelity is a word with a broader meaning than headroom:

headroom: the capacity of a system to reproduce loud sounds without distortion
high fidelity: the reproduction of sound with little or no distortion

Other hypernyms of headroom include hi-fi.

headroom (noun)	high fidelity (noun)
the capacity of a system to reproduce loud sounds without distortion	the reproduction of sound with little or no distortion
vertical space available to allow easy passage under something