Not a comment on the question as such, but the use of this Italian phrase has always seemed more than somewhat pretentious to me when there's a perfectly good English word that means exactly the same thing. Folk singers have been singing 'unaccompanied' for centuries, and unless I'm mistaken the same word has been used in classical music.
The first time I heard this type of singing described as a capella was around 1983, when the Flying Pickets had a No. 1 hit with Only You.
Like I said: pretentious – or what?
© Macclesfield Quiz League 2019