a capella singing vs a cappella singing

a capella singing and a cappella singing both are nouns.

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

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

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

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