mumble vs whisper

mumble

noun
  • A quiet or unintelligible vocalization; a low tone of voice. 

verb
  • To speak unintelligibly or inaudibly; to fail to articulate. 

  • To chew something gently with closed lips. 

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 mumble and whisper occurred in a corpus of books? (source: Google Ngram Viewer )