ELF is from 3 HZ to 30 Hz.
VLF is from 3 Khz to 30 Khz
ULF is from 300 Hz to 3 Khz
ULF and VLF are most often picked up by the same receiving equipment. ELF requires special circuitry and is not often found in the frequency range of receivers that emphasize a range that starts at 300 Hz or higher. On this site, VLF and ELF are separate bands requiring different circuitry, and ULF is not specifically mentioned.
The Ionosphere acts as a waveguide for electrical and magnetic waves caused by lightning, solar activity, and other atmospheric phenomena. When there is a lightning strike somewhere in the world, waves are generated. These waves are not sound (acoustic) waves. They can only be “heard” by special receivers.
It can be confusing as you read material on various web sites on the subject of VLF and ELF. But all the material can be divided into 3 categories:
1. Solar activity can be monitored indirectly by measuring the signal strength of VLF transmitters. For example, there is a 2 megawatt VLF transmitter in Maine, USA, which transmits information to submarines underneath the ocean. When solar activity occurs, or when dawn or dusk occurs, the strength of this type of transmitter is affected. A VLF receiver & recorder can pick up these transmissions and record the amplitude (signal strength). The resulting charts can be compared to solar activity readings from NASA and other sources to see if they correspond to each other.
2. ELF receivers are designed to monitor natural events, such as earthquakes, tremors, or other phenomena in the Earth or atmosphere that create extremely low frequencey waves.
3. The third category is a receiver that will pick up Whistlers, Sferics, and tweeks. These waves are caused by lightning. Lightning can strike clear around the Earth, and electric and magnetic waves are generated. The waves are picked up by receivers which convert the waves into sound.