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[MJT]

I am running 3 way active speakers using the DSP studio PEQ and am gettin an odd result from the woofer low pass filter.  The DSP studio analizer shows a very shallow rolloff for the woofer filter only, the mids and tweeter filters look normal.  The attached image 1 is with a pink noise file playing.  Image 2 shows the PEQ 1 filters that I am using.  The speakers actually sound very good with this setup going from MC to an Echo Audio Gina3G to a Denon AVR-3805, the jriver version is 19.0.104.  

Testing today with REW I am also seeing a shallow rolloff for the woofer low pass, the mid and high filters look good.  I tried dragging the low pass filter up and down the PEQ list and it didnt make any difference (as long as was below the channel mapping).  Next, I unchecked PEQ 1 and set up a test filter in PEQ 2 with only the needed mapping and a single low pass - see image 3.  I set the EMC8000 mic about 1 inch from the woofer and muted the other woofer, the mids, and the tweeters.  Image 4 shows the RTA traces when playing a pink noise file, for lowpass unchecked, and lowpass checked at 6 dB/octave, 12 dB/octave, 24 dB/octave, 36 dB/octave, and 48 dB/octave.  The lower line shows ambient noise from the same loction at the woofer cone, the filtered traces all look basically the same to me and rolloff really start well past 400hz .  Have I missed a step somewhere?

Thanks,
Mike

[mwillems]

That's all very odd.  Your primary settings look like they should work fine, and I have no idea why the analyzer is showing that shallower roll off.  I run a pair of bi-amped speakers and the low-passes have always worked exactly as expected (even at low frequencies).  I really don't know what might be causing it, but here's a few things to check:

When you engage your PEQ 2 filter, what does analyzer show?  Does it show different rolloffs for the different filters, or does it look like the REW measurements?

What is your channel output setting?  Are you using 2 channel in a 5.1 channel container or something else?  Do you have any kind of upmixing or down mixing enabled? I don't think that's it, but it's worth checking.

Does your sound output device have any built in EQ that might be getting applied?

This is a puzzler for sure.

[MJT]

I added a second hi pass filter to test PEQ 2 and the result is about the same.  I have attached screen shots Image 5 and image 6 with the pink noise file playing.  I'm using internal volume at 100 with the loudness unchecked for this test.  The lowpass roll off looks about the same to me at 24 and 48 dB/octave, I did not yet try to re-run the REQ tests.  I also see the long low slope in the low pass filter when playing music - its harder to get a good screen capture that way.  The Echo audio sound card has 6 analog outputs (with no EQ) that I have connected to the AVR-3805 external inputs.  No other up or down mixing.  The External inputs bypass the Denon DSP in the AVR and only go through the volume control and amps.

Thanks,
Mike

[mwillems]

I added a second hi pass filter to test PEQ 2 and the result is about the same.  I have attached screen shots Image 5 and image 6 with the pink noise file playing.  I'm using internal volume at 100 with the loudness unchecked for this test.  The lowpass roll off looks about the same to me at 24 and 48 dB/octave, I did not yet try to re-run the REQ tests.  I also see the long low slope in the low pass filter when playing music - its harder to get a good screen capture that way.  The Echo audio sound card has 6 analog outputs (with no EQ) that I have connected to the AVR-3805 external inputs.  No other up or down mixing.  The External inputs bypass the Denon DSP in the AVR and only go through the volume control and amps.

Thanks,
Mike

So I did some testing myself to see if I could replicate your results.  Here's what I found:  

When I dialed in a lowpass (or a highpass for that matter), and looked at analyzer, it showed me a very similar rolloff pattern to the one you're seeing (sharp immediate rolloff, which gradually becomes more shallow).  Different filter settings showed the shallowing at different levels of attenuation, but when I used a loud enough pink noise signal all high or low pass filters I tested (I tried many different frequencies and slopes) showed that flattening in analyzer at a certain point.

However, when I measured the filter effects in Holm Impulse, the filters I tested behaved correctly (i.e. it did not show a similar shallowing of the rolloff, but instead showed the expected filter slope down to the noise floor of my measuring rig at around -95dBFS).

For that reason, I think the analyzer issue is a bug in the way the analyzer shows signals, not a bug in the lowpass or highpass filters.  Each of the cross lines in the analyzer represents about 5 dB, so in your first example the filter doesn't show "flattening" until it's 30 or so dB down from the passband.  

I think the analyzer is just poor at showing large differences in volume.  If I play an 80Hz pure sine tone, the analyzer shows a much wider area of signal playing with a long tail off in either direction, whereas an external 1/32 octave RTA just shows much narrower 80Hz-ish tone with some very small sidebands.

So that's my explanation for the analyzer issue; I'm still baffled about your REW measurements.  All my measurements show the filters working exactly as expected.  
Sorry I'm not more help.

[MJT]

Thank you for taking a look into this issue and for your test results, I appreciate your time.  I will try some tests in REW over the weekend with sweeps or sign waves and post if I get something interesting.  I'm not up to speed on Holm Impulse, I took a look and will give it a try when I have some time.  I also did some testing on the DSP Studio analyzer on a different Win 7 x64 system and had the same outcome in the display, it seems the lower the low pass crossover point the more the shallow the displayed rolloff in the window.  For now the speakers sound great so I'll keep listening to music!