a cappella vs whirring

a cappella

• A vocal performance with no instrumental accompaniment. 

• In a manner of a choir with no instrumental accompaniment; literally, "in the style of the (Sistine) Chapel (in Rome)", such as a musical Mass done a cappella. 

• Singing solely or mainly without instrumental accompaniment. 

• Related to a form of purely vocal music mostly associated with American college performance groups. 

whirring

• A whirr, the sounds of a whirr. 

• Producing a whirr; giving off a whirring noise. 

• Spinning rapidly.