According "Electromagnetic Radiation Spectrum" there is something more in lower frequencies than radio waves. Radio lower frequency is (3–30 Hz)...

The word you are searching for is "infrasonic."  Infrasonic sound waves are in the lower netherworld of radio frequencies, generally from 0 hertz to 20 hertz.  This would put them out of the lower range of human hearing.  The resource I have attached is about elephant communication in the infrasonic radio wave specifications you have listed.  Sound waves in this category are felt, more so than being heard.  Elephants make auditory sounds that are much higher, such as when they trumpet with their trunks.  Much research has been done on elephant's abilities to locate females when it is time to breed.  This research has also explained how elephant families tend to find each other when they have become separated. Elephants also take advantage of a particular time of day, when the temperature on the African savannah increases their ability to hear the low frequency sounds they are using as communication.