A subsonic filter is basically a High Pass Filter for very low frequencies, generally 45hz or lower. That is, whatever you set it at; only frequencies above that setting go through. Anything below that setting is attenuated (made quieter).
As only frequencies higher than your setting pass its a high pass filter.
A subsonic frequency is very low, so low the human ear can barely detect it, but your body can feel it. Because you cant hear it, and they're prone to damaging woofers, filtering subsonics is a desirable characteristic in subwoofer amplifiers.
A subsonic filter is used differently in sealed enclosures than in ported.
In a sealed box: The lower the frequency, the more excursion your subwoofer exhibits in order to play it loudly and accurately. A sealed box is tuned by enclosure volume, larger enclosures tune lower, smaller enclosures tune higher.
Subsonic frequencies potentially damage your subwoofer because they make it expend a lot of energy, travel right to the limits of its excursion and play below the enclosures tuning.
So for safety, we want to cut those frequencies out. For a sealed enclosure you adjust the subsonic filter to 25-35 Hz, to filter the extremely low bass frequencies your woofers unable to play.
- 45 Hz / 2 = 22.5 Hz (one octave lower) So we take 45 - 11.25 = 33.75 Hz (rounded up to 34 Hz)
In a ported box: You tune the port to a certain frequency, the enclosure is then capable of playing all frequencies above that tuning without an issue. The enclosure can also play below that frequency, but only half an octave, before the cone starts over-excurting and there's potential for damage.
So for safety, we set the subsonic to 1/2 an octave below our tuned frequency. Whats half an octave you say ? Lets do some maths ! One octave up is double the frequency, One octave down is half the frequency.
Lets say you are tuned to 45 Hz:
- 22.5 Hz /2 = 11.25 Hz (half an octave lower)
- 45 Hz / 2 = 22.5 Hz (one octave lower)
So we take 45 - 11.25 = 33.75 Hz (rounded up to 34 Hz)
In other words, you set your sub-sonic to 34 hz. So that way, if there's music below the ports frequency, it gets filtered out protecting your woofer.
Always remember the subsonic filter is NOT a cut-off. It has a roll off "slope" where whatever frequency its set to will be attenuated, and the attenuation effect increases as the frequencies get lower. Thus the power to the woofer decreases at filtered fequencies, which reduces the excursion and risk of damage.
As a final note for the audio geeks: Subsonic filters have steep slopes such as 3rd or 4th order(18 or 24dB/Oct) this is so u can set it nice and close to your 1/2 octave frequency, or 25-35 Hz sealed, without losing power in the neighboring frequencies.