What is a subsonic filter on a car amplifier?

A subsonic filter on a car amp is a filter that cuts out the frequencies that a human ear can't hear. A subsonic filter is important for a ported subwoofer enclosure because the ultra low frequencies can cause damage to the subwoofer's spider. If you have a sealed enclosure, having a subsonic filter is useless because this type of box design uses the air inside the box to filter the unwanted frequencies. Most car amplifiers should have a settings to turn on and turn off the filter for your subwoofer. If your car amp does not have it built in, then you can buy it or make your own by creating a circuit from scratch.