Quadcurve EQ Analyzer Question

Hello Everyone,
 
I don't understand the Analyzer in the Procurve EQ.  Why do I see frequencies showing (in darker grey) that fall outside the scope of my EQ plot (white line)?   Please see image below.  I am concerned with the dark grey under the red arrow I drew in.   Why would that frequency play if I specifically told the HPF to cut off before then?

I was thinking "is that what WOULD be playing if EQ was off?"  but then I thought.. "can't be, because as I move the EQ those "shadows" if you will kind of follow what I am doing, but always stays beyond the HPF point.

It's probably simple.



 
Thanks,
Adam
 
 

 
 
EDIT never mind

I would imagine MEqualizer with a band set to 120db/octave would do it if that is what you really want to do.

No, that is not what I am suggesting. The EQs supplied by SONAR are very good but if one has a requirement to "totally eliminate everything below 40 HZ" there are solutions provided by third party vendors such as the MFreeEffectBundle which one may want to consider. Using a 120db/octave filter is pretty extreme though.

To answer the OP's question what is shown is the "curve shadow". That is the frequencies are shown but the darker part still lets one see the full frequencies but showing that they have been attenuated by the EQ curve. This is a guess on my part but I believe it to be correct.  If you make a more complex curve with more than just one overlapping area you will see that the darker frequencies follow the curve. 

On taking a second look at this, I think the apparent failure of the the lowpass filter to be as effective as expected is likely due to the relatively small "block" size being used to calculate the spectrum using Fourier analysis.
 
Without getting too deeply into it, in order to calculate the amplitude at a given frequency you have to sample a "block" of time that encompasses at least one full cycle at that frequency, and the more you have, the more precise the result is. At these very low frequencies, a typical block size of 2048 samples won't even include a full cycle of 20Hz sampled at 44.1kHz (1/20 * 44,100 = 2205).
 
Voxengo Span uses a default block size of  2048 samples, and the spectrum roll-off with the LP filter enabled looks very similar to your screenshot. But if I crank the sample size up to 8192 or 16384 in the Spectrum Mode Editor, the filter starts to look a lot more effective with a much steeper roll-off of the spectrum "shadow". The downside is that the spectrum can't be updated as frequently with a larger block size so the display isn't as smooth. 
 
So, basically, this is another case where you need to use your ears instead of your eyes. The actual audible result will be much better than what's being displayed.