croon vs falsetto

croon

verb
  • To hum or sing softly or in a sentimental manner. 

  • To make a continuous hollow moan, as cattle do when in pain. 

  • To say softly or gently 

  • To soothe by singing softly. 

noun
  • A soft or sentimental hum or song. 

falsetto

verb
  • To sing or utter in falsetto. 

noun
  • A person who sings in falsetto. 

  • The "false" (singing) voice in any human, usually airy and lacking a purity of vowels; created by utilizing the next highest vocal folds above those used for speech and normal range singing. It is commonly confused with the head voice register. 

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