Unaccompanied, perhaps

• Lacking backing
• Without instruments
• Barbershop quartet style
• Singing without instruments
• Like Pentatonix numbers
• Like singing sans instruments
• Music style endured in "Pitch Perfect" or "Your Friend's College Concert"
• Like some college singing groups
• With nobody playing, say

Wikipedia Word definitions in Wikipedia
Returning to their roots, Johnny Maestro and The Brooklyn Bridge released Acappella , a collection of notable doo wop covers.