(electromagnetic radiation, frequency below 3 kHz)
The term subradio is sometimes used for
electromagnetic radiation (EMR) with frequencies below that of radio.
The "lower end of radio frequencies"
is not universally agreed upon, and various frequency
limits have been declared, such as 30 kHz, 3 kHz, and 3 Hz,
and some declare radio to extend to 0 Hz.
In communications, such frequencies are used for only
very specialized applications.
Bands at the lower end of radio or beyond: