40 Hz High Pass does not show in Spectroscope

I’ve just noticed that if I put a HPF at 40 Hz, even a steep one like 45 db per octave (or even brick wall), Spectroscope doesn’t show any real reduction below 40 Hz. I can hear the difference so the HPF is working.

Its only when I move the filter up to 100 Hz that I see any real reduction in 30-40 Hz, but still not much. With the cut at 200 Hz there’s significant reduction below 40 Hz on the meters, but there’s still quite a bit there. And according to Spectroscope there’s very little reduction at 100 Hz. Yet my ears tell me there’s very little audio left below the cut off of 200 Hz.

By contrast a LPF shows complete or almost complete cuts immediately above the cut off frequency on Spectroscope.

First I’m wondering why it’s different between HPF and LPF in how Spectroscope displays them.

Second I’m wondering if there is a way to increase the resolution of Spectroscope. If I use Voxengo Span set to max block size it shows the HPF cuts very accurately, but a lower block sizes it displays similar to Spectroscope.

Thanks for any insight into this.

Please attach pictures to illustrate your case.

Here’s a screenshot showing a HPF at 60 Hz 96 db/oct with both Spectroscope and Melda analyser (set to hi-resolution). You can see how Spectroscope is showing a good deal of level below the cut off, where as Melda is correctly showing that it has been high passed. Voxengo Span gives the same results as Melda when it’s set to high res.

By contrast, if I use a LPF Spectroscope displays those cut frequencies as expected.

I’ve also included a screenshot with no filters applied

Start to use a more compressed scale, as seen in the other product spectroscopes.
Secondly, if you want a more accurate display, use the Spectrometer, not the Spectroscope.


Ok thanks, I’ll try that. I was wondering why the high frequency display was more accurate than the low frequency. Is this some property of waveform size/physics?

Indeed, by nature, FFTs possess higher resolution for high frequencies than for low frequencies. Literature is abundant on this subject.