infrasonic

adjective

1. noting or pertaining to a sound wave with a frequency below the audio-frequency range.

adj.

1. Generating or using waves or vibrations with frequencies below that of audible sound.

1. Sound whose wave frequency is too low (under 15-20 hertz) to be heard by humans. Some animals, such as elephants and whales, emit calls at infrasound frequencies. See Note at ultrasound.