a cappella vs whisper

a cappella

noun
  • A vocal performance with no instrumental accompaniment. 

adj
  • Singing solely or mainly without instrumental accompaniment. 

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

adv
  • 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. 

whisper

noun
  • The act of speaking in a quiet voice, especially, without vibration of the vocal cords. 

  • A private message to an individual in a chat room. 

  • A low rustling sound, like that of the wind in leaves. 

  • A rumor. 

  • A faint trace or hint (of something). 

verb
  • To speak softly, or under the breath, so as to be heard only by one near at hand; to utter words without sonant breath; to talk without that vibration in the larynx which gives sonorous, or vocal, sound. 

  • To speak with suspicion or timorous caution; to converse in whispers, as in secret plotting. 

  • To make a low, sibilant sound. 

  • To mention privately and confidentially, or in a whisper. 

How often have the words a cappella and whisper occurred in a corpus of books? (source: Google Ngram Viewer )