Why is the ninth month called September?
infrasonic in·fra·son·ic (ĭn'frə-sŏn'ĭk)
Generating or using waves or vibrations with frequencies below that of audible sound.
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.