Kestukui teorijos
"A subsonic filter is nothing more than a highpass filter. When a highpass filter is set low, such as 30 hz, and used on a subwoofer, people tend to call them subsonic filters."
"subsonic filters block frequencies below which will be audibly heard. low frequencies that could potentially damage the speaker if reproduced. these are often used for ported enclosures. sealed enclosures use the air sealed inside as a spring/cushion to keep it from being damaged. so you don't typically use filters for sealed boxes"