a cappella singing vs a capella singing

a cappella singing and a capella singing both are nouns.

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

As nouns, a cappella singing and a capella singing are synonyms defined as:

a cappella singing and a capella singing: singing without instrumental accompaniment

a cappella singing (noun)	a capella singing (noun)
singing without instrumental accompaniment	singing without instrumental accompaniment