head tone vs head register

head tone and head register both are nouns.

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

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

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

Other synonyms of head tone include head voice.

head tone (noun)	head register (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