subradio

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: