head register vs head voice

head register and head voice both are nouns.

Word	Noun	Adjective	Verb	Adverb
head register	Yes	No	No	No
head voice	Yes	No	No	No

As nouns, head register and head voice are synonyms defined as:

head register and head voice: the higher ranges of the voice in speaking or singing; the vibrations of sung notes are felt in the head

Other synonyms of head register include head tone.

head register (noun)	head voice (noun)
the higher ranges of the voice in speaking or singing; the vibrations of sung notes are felt in the head	the higher ranges of the voice in speaking or singing; the vibrations of sung notes are felt in the head