Definition of "infrasound"
noun
countable and uncountable, plural infrasounds
(physics) Sound waves having a frequency below the human audible range (below about 20 Hz).
Quotations
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.
2008, BioWare, Mass Effect, Redwood City: Electronic Arts, PC, scene: Elcor Codex entry