infrasound

noun

1. (countable, uncountable)Sound waves having a frequency below the human audible range (below about 20 Hz).

   “Elcor speech is ponderous and monotone. Among themselves, scent, slight movements, and subvocalized infrasound convey shades of meaning that make a human smile seem as subtle as a fireworks display. Since their subtlety can lead to misunderstandings with other species, the elcor often go out of their way to clarify when they are being sarcastic, amused, or angry.”