stammer vs vocalise

stammer

noun
  • The involuntary repetition of a sound in speech. 

  • A speech defect whereby someone speaks with a stammer 

verb
  • To utter with a stammer, or with timid hesitancy. 

  • To keep repeating a particular sound involuntarily during speech. 

vocalise

noun
  • A vocal exercise performed by singing one or more vowels without actually forming any words. 

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