also infra-sonic, 1920, on the model of supersonic, etc., from infra- + sonic. Or perhaps modeled on French infra-sonore.
Entries linking to infrasonic
infra-
word-forming element meaning "below, beneath," from Latin infra (adverb and preposition) "below, underneath, on the under side, beneath," also "later than; smaller than; inferior to," related to infernus "low, below," from PIE *ndher "under" (source also of Sanskrit adnah "below," Old English under "under, among;" see under). Modern popular use of it dates from the 1920s, as an opposite to super-, often in science fiction. "This use of infra- is scarcely a Latin one" [OED].
sonic adj.
"done by means of sound waves," 1923, from Latin sonus "sound" (from PIE root *swen- "to sound") + -ic. Sonic boom "shock wave from an aircraft exceeding the speed of sound" is attested from 1952.