Without backing, in a way: Var

• Like many doo-wop sessions (var.)
• Like many doo-wop groups: Var.
• Without any musical accompaniment (var.)
• Like some choirs: Var.
• Like many doo-wop groups: Var

Wikipedia Word definitions in Wikipedia
Acapella is an historic trimaran sailboat , designed and built by Walter Greene in 1978 and later renamed Olympus Photo . Walter and his wife Joan raced in the 1978 Round Britain race, coming in 1st in class and 4th overall. Later that year, Canadian Mike ...

Wiktionary Word definitions in Wiktionary
adv. (alternative spelling of a cappella English)