Lowpass?is another strange early Airwindows creation!
The top slider is just a frequency control for a one-pole interleaved lowpass, the range covering the full range of sound from silence to Nyquist.
The bottom slider’s the one that gets weird. It adjusts between soft and hard, with the middle position being ‘normal lowpass’.
Soft means, louder sounds will push the lowpass frequency lower. That makes transient attacks sort of get covered up. The body of the sound gives one tone, but then all the dynamics go more muted.
Hard is the opposite: the body of the sound has one texture, but impacts go a little brighter. It’s more punchy, against a muted background, with transients sticking out a bit more.
I’m not sure how often people need a simple one-pole IIR lowpass. That’s an awfully basic sort of filter, and it’s not like it’s steep or anything. Its highpass counterpart is perhaps more obviously useful. But if you do use this sort of lowpass, now you’ve got one that has texture control :)