low-pitched vs alto

low-pitched is not a noun while alto is a noun.

Word	Noun	Adjective	Verb	Adverb
low-pitched	No	Yes	No	No
alto	Yes	Yes	No	No

low-pitched (noun)	alto (noun)
	the pitch range of the lowest female voice
	(of a musical instrument) the second highest instrument in a family of musical instruments
	the lowest female singing voice
	the highest adult male singing voice
	a singer whose voice lies in the alto clef

low-pitched (adjective)	alto (adjective)
used of sounds and voices; low in pitch or frequency	(of a musical instrument) second highest member of a group
set at a low angle or slant	of or being the highest male voice; having a range above that of tenor
	of or being the lowest female voice