Part 2 - 2.1 Stereo Calibration - what am I missing?

[Dave the Rave]

Few years back, I started the DSP journey through REW. Advice from experts here and elsewhere brought a real SQ transformation guided by REW measurement capabilities. Tried Dirac but way prefer simpler adjustments using REW.
Below is a snapshot of the measurements years back compared to now.
Key reasons for the improvements are the usual suspects recommended by audiophiles: Speaker positioning, level matching, proper measurements, DSP below 300h only, get source right with minimal latency, decent DAC/amp/speakers.
I am quite happy with the PEQ applied and only use cuts. I always found dynamics/clarity could be compromised when trying to smooth the FR towards the curve (with PEQ gains or convolution DSP).
However, i just wanted to know if there are any other techniques I can use to further improve the bass side esp tackling the nulls?
I have attached the measurements. Note, I can't go with room treatment; Most of the time the impulse graph align at zero when I place the mic accurately in the middle (hence no issue on impulse response).

Measurements 3 years back

Measurements NOW

PEQ Applied

Impulse Response

 

[Keith_W]

Look at that huge forest of early and loud reflections (ETC taken from measurement of left speaker alone). Are you using one of those mini tripods that come with the UMIK-1? This is typically what you see when you use a mini tripod and perch it on a chair, or wedge a microphone between books to take a measurement, etc. That first reflection at 0.6ms is coming from something that is about 20cm delayed from the microphone. For e.g. a wall that is 10cm away, or a sofa, etc. Your measurement needs to be taken from where your head is when you listen. Ideally you should purchase a proper mic stand, or at least duct tape your microphone to a broomstick and try to get it in a proper position. Mic tripods are cheap, my first tripod cost $50 and lasted me for years until I broke it. My new tripod is much nicer, but it still cost me only $200.

As for those dips, some of them will be real, and some of them will be artefacts from microphone positioning. You can't tell which is which from a single sweep from a fixed position.

Which Australian city are you in? I'm in Melbourne.

 

[ddude003]

You can get a nice Pro Tripod here in the States for under $100.00 USD... Try Guitar Center or Sweetwater .com...

 

[Dave the Rave]

Thx mate am from Sydney. Yep the cheapo tripod in play but taken at head position from the listening sofa.
What are the correction options with the better measurements? I will definitely get another standing tripod if that would help.

 

[Keith_W]

Hi Dave. I love Sydney. In fact I have a rental property over there in Zetland.

You do not want to correct artefacts from microphone positioning. If you mount a mic on a tripod and take a single sweep, that mic is capturing sound from a very specific point in space. If there are a lot of local reflections, there will also be a lot of comb filtering. Comb filtering causes peaks and dips in the frequency response.

Remember that your microphone has a 1/4" (6.35mm) capsule, and your ears are about 15-18cm apart. In addition, you also move around in your listening chair. Your ear "averages out" the peaks and dips in the response which change very quickly. What your ear does not average out are actual peaks and dips that do not change. For example, if your room had a massive 80Hz peak that does not change much when you move around your seat, you would surely hear that. It is the same story if you made an inappropriate EQ adjustment. The measurement would look fine (from that very specific point in space) but it would sound off.

So the real question is what to EQ, and what to leave alone. Some people will make a blanket rule that you should not EQ a single mic measurement, and you should only EQ an MMM or averaged measurements from several positions around your listening area. I think that it depends - if your frequency response does not change much over a listening area, then it's OK to equalise from a single position.

The presence of copious early and loud reflections in your ETC suggests that you are measuring (and potentially correcting) a lot of "local" phenomena. If you want to investigate this, place your microphone at the MLP (Main listening position) and do a sweep. Label it MLP0. Then move the mic 10cm, 20cm, 30cm to the left and take sweeps. Do the same for the right. You will end up with 7 measurements: MLP-30, MLP-20, MLP-10, MLP0, MLP+10, MLP+20, MLP+30. You can choose a larger area if you want to correct for a larger sweet spot.

Overlay all of them and examine the bass response closely. You will see that some of the curves "move" by quite a bit. Others may be relatively static. Then average all of them (Vector Average) and use that as basis for your EQ. The alternative is to do an MMM, but I don't like MMM's because I prefer to study the peaks and dips in detail. The advantage of the MMM is that it is quick, repeatable, and you don't have to purchase a mic tripod. Just duct tape your mic to a broomstick and you're done! The outcome is the same though, so do whichever makes more sense to you

I know you said you can't install room treatment. But ... looking at your ETC, I think you should consider it.

 

[Dave the Rave]

@Keith_W Thanks for that elaborate explanation. You did well having a foot in Zetland (prices gone through the roof).
I am not an expert at analysing all the REW components other than FR but your proposed actions are easy to understand and makes a lot of sense.
Excited to get onto the new measurements and will report back. I did manage to find a proper standing mic in the meantime as @ddude003 suggested (thx for calling it out).

 

[Dave the Rave]

G'Day,
Ran a new set of measurement over the week-end with a standing mic and MLP. Attached is the updated file and showing the mic in the listening room.
- Found that I was using the wrong cal file for the Umik mic since i have 2 !!!! Resolved that issue.
- Moved the mic as advised above. Found the R channel (ref) Impulse Response consistent unlike L channel which moved. I am assuming this is ok.
- Vector average is not the same as RMS average - not sure if this is OK as I am assuming the IR derives the Vector Average
- Applied PEQ cuts per the Target (with some manual adj). The net effect yielded BETTER results than the previous PEQ. Applied < 300h only.
So one good win through that long process. Happy to get any other feedbacks.

 

[Keith_W]

Let me show you something.

These are the impulse responses for all your left sided measurements.

If you select them and click "Align IR start", this is the result. The reason why this matters:

Compare the vector average you obtained without aligning IR start (green) with mine (purple). In this case there is very little difference between them below about 500Hz or so. If you were planning to DSP the high frequencies above 500Hz then this kind of summation error matters.

I was telling you about the difference between "real" dips in the response (room related) vs. "local" dips (caused by proximity of the measurement mic to furniture). This overlay of all your left sided measurements shows you the difference between the two quite nicely. If you checked over a wider area, you would observe bigger differences. Not that you need to, if this is where your head is positioned when you listen, that's all you need to correct for.

Anyway it appears that in your case, your measurement does not seem to change much over a +/- 30cm listening area. So you can go ahead and correct it with PEQ.

 

[ddude003]

Hi @Dave the Rave... I hope you don't mind if I am a little picky... I have looked over your room photo and have some comments... First, I corrected the photo tilt a bit to align with the back wall... Then I noticed that the center of the room seemed to relate via the back window... I try and show this in the photo below...
So, speaker placement as well as mic placement are pretty important to get speaker time alinement and mic perfectly centered...
And your left speaker seems to be toed in more than the right... Maybe its just the photo?
I love those hard wood floors... And they play havoc in a small room... I would suggest some/more physical room correction to complement your digital room correction... Some Bass absorbers and some wide band absorbers would be of great benefit...

 

[Dave the Rave]

Hello, Thanks heaps for the feedback. Appreciate those much better.
I used the following DSP steps:
1. IR time aligned to generate vector averages. Generated LR (Vector average) from these R & L channel averages.
2. Applied PEQ at 1.0 slopes above and below 200h at 66db with Var graph. Applied to L, R and LR
3. Vector averaged EQ-L, EQ-R and compared with LR Average in step 1.
4. Found out I was cutting too much from PEQing L & R channels for some frequencies. Adjusted L & R PEQs accordingly to align closer to EQ-LR
Net result - BEST PEQ effect in my system so far. Comparing the same tracks, bass is firm and taunt. Mid and High sound more open.

Understand I have more room for improvement for the dips with some Bass absorbers. If these can be placed with WAF considerations, will look into it (and re-measure with the proper center placement).
Would it be of any benefit placing the absorbers behind the speakers (hidden treatment = less WAF negociation = less spend on shoes shopping for misus)?

 

[Keith_W]

You don't need bass absorbers. I'll show you why.

First, this is the vector average of all your measurements after aligning IR start for left (red) and right (green). I then Vector summed both of them to get an idea of what your overall bass response would look like (purple). I staggered the purple curve for clarity. And bear in mind this is without ANY DSP to improve the response.

I then applied ERB smoothing to the vector sum. Here it is in red compared to the original vector sum in faint purple. Notice that all the dips disappear.

If you want to understand what ERB smoothing is, watch this video by Amir from ASR. Start from 3:55.

Basically, your hearing is nowhere as "hi res" as your microphone. Yes, those dips look ugly on a graph, but you won't hear them. The exception is if you have very wide and deep dips, which you don't. The reason we correct graphs is so that we can correct beyond the limits of our hearing.

If you REALLY wanted to do something about those bass dips, room treatment is the wrong approach. Bass absorbers need to be 1/8 as thick (at a minimum) of the longest wavelength you wish to absorb, and its effectiveness depends on very specific placement and how much surface area you treat. At that thickness it is really intrusive and will have all sorts of undesirable side effects - like killing off all your high frequencies. I can guarantee that some thin foam behind your speakers will do next to nothing.

If obtaining a super-smooth bass response is your goal, you would be better off with multiple subwoofers and careful DSP.