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Effect of a Comfy Chair on Room Measurements (an example)

MaxRockbin

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I just thought this was interesting. I was doing some room measurements with REW and had read something Mitchco said about getting better results from DSP filters if measurements were taken with some offensive furniture removed (in his experience). My listening spot is a large comfy chair with a back that reaches up just to ear level before curving back. So what do the measurements look like with the chair in place and the chair removed? (actually moved about 4 feet back):
1/48th smoothing, measured at listening position. Right speaker + sub. No left speaker.
GREEN = CHAIR IN NORMAL POSITION
RED = CHAIR MOVED BACK
1633584930074.png


So a pretty apparent difference from about 300hz and up. Audible? Not really something I can A/B.
I had wondered how much damage the large chair was doing to my so very flat Genelec 8030Cs. Probably some of you with big comfy chairs have wondered the cost too.

Here are the same curves with "Psychoacoustic" smoothing applied. It certainly emphasizes different changes. I guess 100lbs of foam and leather has some impact on the lower frequencies when you move it around. I would have moved it out of the room entirely if it didn't require disassembly and a stronger back.

This graph is the better one to look at, I think, for a person who really doesn't want to ditch his/her comfy chair:
1633586267179.png

(again - green is normal positioning and red has the chair moved back)

So maybe this example gives you some idea of the evil that chairs do and don't. My case may exaggerate the mid and higher frequency effects, or at least them appear more prominent, because I sit relatively close to the speakers. So comb filtering adulterates what would otherwise be a signal dominated by direct sound.

For anyone curious, here's the room layout - top view/elevation view
1633585516024.png
 

charleski

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I’d guess the oscillations in the hf response with the chair in place are showing the comb filter caused by reflections off the chair back. In practice your body is going to be blocking most of these. Maybe try measuring with the chair in place and a large cushion on the chair under the mic to simulate your torso.
 

thewas

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I’d guess the oscillations in the hf response with the chair in place are showing the comb filter caused by reflections off the chair back. In practice your body is going to be blocking most of these. Maybe try measuring with the chair in place and a large cushion on the chair under the mic to simulate your torso.
Also our perception is very different than the one from a not gated omnidirectional microphone as above transition frequency we mainly perceive the tonality of the direct sound which of course doesn't change.
Nevertheless could you maybe also repeat those measurements with the moving microphone method as those are more robust to little position changes and show better the sound power with less smoothing needed?
 

Senior NEET Engineer

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Above transition frequency, I notice loss in sound quality when my head is 2 inches away from leather headrest and 1 inch away from microsuede headrest. There is a lot less envelopment and high frequency is boosted. I don't think omni microphone is able to capture this.
 

Hipper

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This has been discussed before to some degree:

https://www.audiosciencereview.com/...e-best-mic-sofa-positioning-to-eq-with.27029/

My post 3 leads to another link where I and others show some experiments.

A lot depends on the chair, leather or fabric. Leather reflects the higher frequencies more.

Then you have to consider that you (and perhaps others) will be sitting in it. To me this means that measuring the effect of a whole chair is not useful but perhaps measuring the effect of a chair with you sitting in it might be. In other words, measure the chair plus you. That's what I tried.

Interestingly my 'body in chair' results look similar to your moving the chair:

F39c 41 Body in Chair.jpg


1/24 smoothing, full range, purple being with me in the chair (I place the microphone where one of my ears would be then carefully got in the chair without knocking it it).

If you are still willing to experiment, can you place your chair completely out of the way and then do a measure? Not out of the room perhaps but say in one of the rear corners.

I never did anything about these measurements. I just thought they were yet another way the sound can deviate before it gets to my ears that was difficult to manage, part of the circle of confusion that Floyd Toole talks about. Indeed it may even be that your brain (psychoacoustics, another part of that circle) can adapt to these interferences.
 

Hipper

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Above transition frequency, I notice loss in sound quality when my head is 2 inches away from leather headrest and 1 inch away from microsuede headrest. There is a lot less envelopment and high frequency is boosted. I don't think omni microphone is able to capture this.
I find that movement of the head, even if you have no headrest, will change the sound. Move it forwards or backwards, sideways, or lower or raise your fore head. All these alter the position of your ears relative to the source. Indeed with lowering the forehead I can get more percussion, as I can with putting my hands behind my ears or bending the ears forward.
 

MakeMineVinyl

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Above transition frequency, I notice loss in sound quality when my head is 2 inches away from leather headrest and 1 inch away from microsuede headrest. There is a lot less envelopment and high frequency is boosted. I don't think omni microphone is able to capture this.
I have a leather couch and if I have my head all the way back, there is some coloring of mids to highs. When I want to listen critically I move to the edge where my head is well clear. I've had a thick damping blanket draped over the back but it doesn't make enough difference to keep it there. Honestly, a lot of the time I lay on the couch anyway when listening. :)
 

Absolute

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This example shows perfectly why you shouldn't correct frequency response based on a single-point measurement. It creates problems to solve issues the graph sees but is irrelevant for our perception of sound.

The translation of steady-state to how we perceive sound goes from very accurate at low frequencies to moderate in the transition area to very poorly above it.

After "fixing" the response at higher frequencies, take some measurements before and after and compare excess group delay to see a bunch of new problems added.

Don't fix the comb filtering from the chair. Bad idea.
 
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MaxRockbin

MaxRockbin

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With the psychoacoustic smoothing applied I would call the chair beneficial, looks more tonally balanced.

I suspect time domain measurements would support this, most rooms benefit from absorption below 1000Hz.
That's a very good point. Though, oddly the decay times I'm seeing 20-200hz are not really improved (though the frequency response does seem to be):
Chair in normal position:
1633636037354.png


Chair moved back:
1633636110977.png
 

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MaxRockbin

MaxRockbin

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This example shows perfectly why you shouldn't correct frequency response based on a single-point measurement. It creates problems to solve issues the graph sees but is irrelevant for our perception of sound.

The translation of steady-state to how we perceive sound goes from very accurate at low frequencies to moderate in the transition area to very poorly above it.

After "fixing" the response at higher frequencies, take some measurements before and after and compare excess group delay to see a bunch of new problems added.

Don't fix the comb filtering from the chair. Bad idea.
Actually this whole exercise was a precursor to running Dirac. Since Mitchco suggested he got better results moving his furniture out of the way, I thought I'd see how it looked in REW. I would not base any correction on a single position - especially not in the upper frequency range which would likely vary a lot with small mic movement. Comb filtering like in the graph with the chair would probably shift and flip quite a bit with small mic movements. Even the most narrow (e.g. Chair) Dirac measurement option involves 9 mic positions. While the specifics of the comb filtering would vary with small movements of the mic, I don't think the overall magnitude would. I'm seeing about +/- 4dB of oscillation in the higher frequency range. I'll do some more measuring, but I bet as long as the mic has the chair back right behind it, I'll see something on that order.
 

alex-z

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That's a very good point. Though, oddly the decay times I'm seeing 20-200hz are not really improved (though the frequency response does seem to be):
Chair in normal position:
View attachment 157723

Chair moved back:
View attachment 157725
The improvement is not just in the raw decay time, but how consistent it becomes across the spectrum. That dip around 190Hz is mostly filled, potentially a room mode cancellation being tamed.

I would be more interested in the 300-3000Hz region, where porous absorbers are traditionally most effective. Feel free to share the whole .mdat file if you don't want to post more graphs.
 
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MaxRockbin

MaxRockbin

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The improvement is not just in the raw decay time, but how consistent it becomes across the spectrum. That dip around 190Hz is mostly filled, potentially a room mode cancellation being tamed.

I would be more interested in the 300-3000Hz region, where porous absorbers are traditionally most effective. Feel free to share the whole .mdat file if you don't want to post more graphs.
Mdats with and without chair attached...
 

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alex-z

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That improvement is actually larger than I would have expected for a single chair.

Around 200Hz you are picking up over 10dB to your C50 and C80 values, which is major. 23ms group delay spike vanishes down to 2ms. Spike at 450Hz goes from 28ms down to 14ms. That improvement in the 300-1000Hz region is smaller, but overall the difference to vocals should be noticeable.

The only area that became outright worse with the chair is 1500Hz. May want to experiment further, see if that result remains when actually sitting in it.
 
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MaxRockbin

MaxRockbin

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That improvement is actually larger than I would have expected for a single chair.

Around 200Hz you are picking up over 10dB to your C50 and C80 values, which is major. 23ms group delay spike vanishes down to 2ms. Spike at 450Hz goes from 28ms down to 14ms. That improvement in the 300-1000Hz region is smaller, but overall the difference to vocals should be noticeable.

The only area that became outright worse with the chair is 1500Hz. May want to experiment further, see if that result remains when actually sitting in it.
I just haven't figured out how to place a mic while I'm in the chair. I'm where the mic wants to be. I do have some major nodes going on and my seating spot is in a low. Treatment (rockwool in a couple corners) did help a lot.
 
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alex-z

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I just hold the mic underneath my chin. It isn't perfect, but more realistic than measuring an empty seat.

If you are looking for further improvement, I would recommend some absorbers on the ceiling. Tends to help with the vertical reflections being misaligned in 2 way designs.

Are your 8030C's high-passed properly? Just asking because that distortion spike above 10% at 50Hz seems to be an anomaly.
 
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