extremely low frequency (ELF)
The Extremely Low Frequencies (ELF) frequency range includes frequencies between 3 Hz and 30 Hz. The ELF range is below the SLF range, Super Low Frequency (SLF), which starts at 30 Hz. The ELF range involves ultra-low frequencies, some of which are below the human hearing range. In the ELF range lies the frequency range known in acoustics as infrasound.