plangent vs sibilant

plangent

adj
  • Having a loud, mournful sound. 

  • Beating, dashing, as waves. 

sibilant

adj
  • Characterized by a hissing or hushing sound such as the s or sh in sack or shack. 

noun
  • A consonant having a hissing or hushing sound such as the s or sh in sack or shack. 

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