Very low frequency (VLF) is the term used to describe a range of sounds that are transmitted across a particular radio band. Specifically, very low frequency refers to any radio waves that have frequencies between 3 kilohertz (kHz) and 30 kHz. These are called very low, as the frequencies that are used for communicating span from 3 hertz (Hz) to 300 gigahertz (GHz).
The International Telecommunications Union regulates the radio band frequencies and has a standard for the each frequency band designation. VLF is not the lowest of the frequency bands. There are three frequency bands that are even lower than this designation, which are ultra low frequency (ULF), super low frequency (SLF) and extremely low frequency (ELF). These three frequency bands cover the spectrum from 3 Hz at the bottom of the usable radio frequencies up to 3 kHz, which is the lowest frequency included in the very low frequency band.
Each of the different frequency bands has sounds with a range of wavelengths. Within VLF, the wavelengths can vary from about 62 miles (100 km) for sounds with a frequency of 3 kHz to about 6.2 miles (10 km) for sounds with a frequency of 30 kHz. The different frequency bands for radio waves are used in different ways by the government, military and public use.
The frequencies that are transmitted in the very low frequency range are rarely picked up by antennas that are used for most communication. This is because most radio communications, including radio station broadcasts and telephone communication, are within much higher frequency ranges. The antennas used for higher frequencies are enhanced to better pick up those signals and make the sounds clearer.
There are VLF antennas and stations across the globe, and these types of signals can be transmitted worldwide. Very low frequency messages are used in a range of different applications due to the behavior of the sound waves found in this radio frequency band. The fact that the sound waves found within VLF emissions have long wavelengths means that the sounds can be heard deep in the water and also far underground.
Although very low frequency signals are used for many different reasons, probably the most common is to provide a means of communication with submarines that are underwater. The amount of salt found within the sea water affects how deeply the VLF sounds can travel when sent from the surface. The depth within water that can be penetrated by VLF ranges from 30 to 130 feet (10 to 40 meters). As well as providing a communication method, VLF can also be used to help submarines navigate when they are submerged in the ocean.