A subsonic filter is a component on your subwoofer that reduces the intensity of notes which come through at lower frequencies. It decreases the amplitude of those low notes that you feel more than you hear. In other words, it’s what we know as a high-pass filter: a filter that lets through signals above a certain Hz threshold and attenuates any signal below that threshold. It’s the opposite of a low-pass filter, which only attenuates signals above a certain line.
For example, If your subsonic filter is set to 40 Hz, 30 Hz notes will still get through but will be much quieter. Think of the filter as a slope, not an on-off switch.
Subsonic Filters are particularly important if you spin vinyl records as they can have possible warping that causes a phenomenon known as woofer pumping. These warps can produce frequencies well below 20Hz and can introduce distortion into your playback system. It is recommended to set up a very subsonic filter with a very steep slope (attention factor) to combat this issue.
You can set the subsonic filter on your 1723 Series Subwoofers in the Crossover Menu and for the 1961 Subwoofers in the Setup Menu. You can select the subsonic filter frequency (frequency at which you want to begin filtering) and subsonic filter slope (the rate at which you want to attenuate the frequencies).