subsonic filter

Infrasound is the frequency range that lies below 20 Hz, i.e. is no longer audible to humans. To protect woofers, woofer and subwoofer against overloads and damage caused by such subsonic frequencies, the low frequency signal is filtered out with a high pass filter at frequencies below 20 Hz. These high-pass filters are called subsonic filters.

Subsonic filters are built into the power amplifier, but they also exist as a stand-alone component that is placed immediately before the subwoofer. They have a lower cutoff frequency between 15 Hz and 20 Hz and are extremely steep-sided. Their slope can be between 12 dB/ octave and 24 dB/octave. These high slopes cause signal distortion by changing the group delay.