A couple of years ago I built a sub-woofer hidden in the floor (See here for more info on that). It sounds great instantly, but I have also put in a digital filter system to allow the response to be equalised to make it close to perfect. I played with the set-up when I built a test-driver, but this weekend I finally got round to setting the system up properly. For the geeks out there (I know it’s not just me ;-)) the frequency response is shown below. Uncorrected response shows below in brown, with the original filters in blue (not doing a lot to be honest). The end result is pretty good, as you can see from the purple line (if all output is within +-3 of a centre line then it is usually judged fine).
There is a material drop at about 85 Hz that is a room effect, and broadly uncorrectable – you can put all the power you want to at that frequency but the room will cancel it back out again. And, it gets a little flakier at about 100Hz where the main speakers are also playing, and aren’t corrected by the filter system (it is in the circuit after the man amp and before the power-amp, so can’t fix anything but the bass response). But, in raw bass terms, it’s pretty flat all the way down to about 13Hz, which in my book will do fine (most sub-woofers are dropping away by about 35Hz).
For the non-geeks, what that flat bass line translates into is awesomely smooth, deep and powerful bass – pretty much as good as it’s possible to get, no matter how much money you spent – and this cost me well under £500 all up (plus time, if I’m honest – but I enjoyed it).