code: protected long cutOffToFrequency(long cutOff, int flt_modifier, boolean isExFilterRange)
  	{
  		double fac = (isExFilterRange)?(20.0d*512.0d):(24.0d*512.0d);
  		double fc = 110.0d * Math.pow(2.0d, 0.25d + ((double)(cutOff*(flt_modifier+256))) / fac);
  		long freq = (long)fc;
  		if (freq<120) return 120;
  		if (freq>20000) return 20000;
  		if ((freq<<1) > sampleRate) return ((long)sampleRate)>>1;
  		return freq;
  	}
docs: calc the cut off
  @param cutOff
  @param flt_modifier
  @param isExFilterRange
  @return