a cappella singing vs singing

a cappella singing and singing both are nouns.

Word	Noun	Adjective	Verb	Adverb
a cappella singing	Yes	No	No	No
singing	Yes	Yes	No	No

As nouns, singing is a hypernym of a cappella singing; that is, singing is a word with a broader meaning than a cappella singing:

Other hypernyms of a cappella singing include vocalizing.

a cappella singing (noun)	singing (noun)
singing without instrumental accompaniment	the act of singing vocal music
	disclosing information or giving evidence about another

a cappella singing (adjective)	singing (adjective)
	smooth and flowing