limit freq to below Nyquist (make it work at 44.1KSPS)

even with this tweak, the filters still comply with
'class 0' of the IEC octave band specs (Annex B).

spectr.lv2: band f: 20158.74Hz (17824.72Hz -> 22492.75Hz) exceeds nysquist (44100/2)
spectr.lv2: shifted to f:19937.36Hz (17824.72Hz -> 22050.00Hz)