Linearteam WinISD Pro
A subsonic filter is quite often required in ported systems, due to the fact that driver becomes unloaded below tuning frequency. This is illustrated in graph below. Tuning frequency is 22.65 Hz. Below 20 Hz, cone excursion starts to rise to the sky.
If the program material should contain low frequency signals below 20 Hz, then driver is driven out of it's linear range and it will produce distorted output, or might be even damaged. Let's add 20 Hz second order subsonic filter, go to the EQ/Filter tab, and click on "Add"-button. Choose "filter type" as highpass and subtype as butterworth. Enter order as 2 and cutoff as 20 Hz. Q value does not matter in our case. Then click on "Add". The cone excursion graph should be much better now:
One example might be a subwoofer plate amplifier, which is sold under "Simex"-brand here in finland (type BA-480). Same amplifier is sold by Parts Express (300-794). It's subsonic filter has fc of 27.96 Hz and Q value of 1.5811. Lowpass filter's fc is adjustable, from 56.90 Hz to 195.66 Hz, with fixed Q value of 0.60486.
| filter | fc/Hz | Q |
| subsonic | 27.96 | 1.5811 |
| lowpass (minimum) | 56.90 | 0.60486 |
| lowpass (maximum) | 195.66 | 0.60486 |
We have to use Special second order section-filter, because of the weird Q-value.
Adding these filters to the filter chain and disabling the lower lowpass results following response curve:
