Ultra Low Frequency (ULF) is the frequency range between 300 Hertz and 3000 Hertz. There are no manmade communications transmitters in this frequency range.
See also
Earthquakes
Ultra Low Frequency (ULF) waves are electro-magnetic waves that might consistently appear before earthquakes. Especially in California many groups are trying to devise an early-warning system using the occurence of ULF and ELF (extremely low frequency) waves to foretell when and where an earthquake will happen.
