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post #1 of 9 Old 12-30-15, 09:26 AM Thread Starter
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Ideal Impulse Decay & SS&I improvements - Data Inside

Did a ton of testing last night... all left channel... searching for reflections and concluded a bunch of things that I want to share but first I must ask...

What is the ideal -db drop from the initial peak to adequately control reflections and also does this -db change with time as in the decay. As mentioned in another thread the 0-5ms is the most critical area for controlling reflections when dealing with SS&I so at that -db are we searching for in this timeframe? What about the second most important area of 5ms to 15ms.... what -db are we looking for there and then the third section is 15ms to 40ms.... what -db there? The decay rate?

I'll post my two .mdat for review and please keep this discussion to reflections/impluse. I have alot to post about so this will come over several posts about specific tests. I did a timeline of stuff and put the notes in but will post up screenshots for everyone showing test results and what I did. A few keynotes:

1. I tried to stay consistent as possible always testing with me in the same location (layig on floor near couch to stay out of way unless I was holding a panel up
2. This is only the left channel in full range mode... no subwoofer integration and was disabled
3. This was with dirac processing however the dirac was run in a 8 mic configuration in a 2'x2' square while the mains were SEALED... all of these tests I removed the plug and ran ported to emphasize the low end but it caused my low end ~25-30hz to gain 5-7db from sealed
4. The microphone was elevated on a stand about ear level sitting on the front of the couch at the MLP... the mic was about 37" off the back of the wall.


Some things I learned during these tests:


1. Seems to me that -30db is without a doubt inaudible from peak and possibly -25db too...
2. Moving the mic around and testing 8 different positions to get an average of reflection zone helps find consistency… inches matter
3. Wow does inches matter…. You move treatments around and peaks/nulls move pretty dramatic but I believe this is more associated with microphone/treatment positioning more than nulls in the room. Nulls in the room are harder to locate due to the mic position and this is how I came up with #2… find the average.
4.You really gotta be careful… as you said… a tiny bit of treatment makes dramatic affects to the good and bad
6.I found out that ASTM standards says you take 50 measurements to achieve ONE result (again see #2)

Attached are two .mdat. one is all the testing moving an absorption panel around the room and another was the same exact test ran 8 times just like a dirac test moving the mic around a 2'x2' area so I can overlay all the impulses and find the average.


Let me start with item 1 on the "left channel reflection chasing test 1"

File (50MB): https://www.dropbox.com/s/6yf4clolai...%201.mdat?dl=0

Test 1 vs Test 2
The test 1 is as my room is as in this photo well except my ceiling has more panels than shown but the before and after is simply test 2 laying a 2x4 absorption panel along the back of the couch blocking the diffusion panels or at least whats right behind the MLP it covers the first middle pair of diffusion panels. as you can see it significantly dropped the impulse at 4.74ms but covered the range of 3-5.5ms

Test 2 vs Test 3
The only thing done in this test was layed a 2x4 panel on the floor with the side closest to the MLP elevated by 12" to keep any of the higher frequency from bouncing off the 4lb density rockwool toward the mic... keeping it reflecting up. As you can see at 1.64ms I took the reflection from -14db to -30db and looks like at 9.5ms I dropped the amount by -2.5db. Clearly the first wave took a substantial reduction but is -30db too much?

I'll post more but want to discuss these two. How much decay is good... is there such a think as too much drop? Whats the sweet range to be in for the first 5ms? -20db ish range? any less does it suck the life from the system?
Attached Thumbnails
Ideal Impulse Decay & SS&I improvements - Data Inside-photo-nov-07-5-15-47-pm.jpg  

Ideal Impulse Decay & SS&I improvements - Data Inside-1vs2.jpg  

Ideal Impulse Decay & SS&I improvements - Data Inside-2v3.jpg  


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post #2 of 9 Old 12-30-15, 09:38 AM Thread Starter
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Re: Ideal Impulse Decay & SS&I improvements - Data Inside

Test 3 vs Test 4
In this test I placed two absorption panels at the right wall to see what reflections would be removed vs. the bare all (minus the cabinet I have sitting which only goes up to the 3' high mark) I don't count any reductions less than about 2db just because it could be a number of factors making a 1 or 2db drop but these highlighted are 4db and should be considered improvements.

My math said the speaker to mic was 127" and that the distance to that right wall then to the mic was right around 320" = 193" or around 16 feet of additional travel for this first reflection and this is shown on this image at the 16ms mark the biggest first wave reduction. Dropping about -4db at these various points... the other locations I'm assuming these panels controlled 2nd or 3rd wave reflections again the same 4db mark.
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post #3 of 9 Old 12-30-15, 09:54 AM Thread Starter
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Re: Ideal Impulse Decay & SS&I improvements - Data Inside

Test 3 vs Test 6
For this test I removed the absorption at the floor and layed down a large book to mimic a hard surface. the first wave reflection went up dramatically and also the 9.5ms went up as well which on the Test 2 vs 3 showed the absorption decreased this spot.

This test was for simple validation purposes showing floor reflections are critical to control. If you don't have floor treatment... GET SOME. Carpet is not good enough! even a 2'x2'x2" absorption panel would do alot of justice here or even place a kids tonka truck at this spot or anything to kill that first reflection.

Another item to note: This hard reflection is at -11db where the carpet by itself was at -14db. absorption panel brought this down to -28db. My carpet is a standard run of the mill house carpet w/ a medium grade 1/2" pad. This only accounted for -3db of loss. Carpet is not a significant treatment. If you have thinner and/or harder carpet then the carpet is even less effective.
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post #4 of 9 Old 12-30-15, 10:18 AM Thread Starter
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Re: Ideal Impulse Decay & SS&I improvements - Data Inside

TEST 15 TO 18

I kinda jumped around a bit but the last tests were ones that really stood out for me. I have a ceiling fan in the center of my room (WAF demand) well... test 1-14 was always the same 5 peaks from the 5.5ms to the 7.5ms range.... always around -12 to -18db area and was only reduced as shown in test 10 where I held panel ABOVE the MLP to act as a cloud. As it turns out...

This is my ceiling fan! During test 1-14 the fan was off and the blades stayed stationary. The result was the SAME reflections in this range. I then slightly moved the blades in 15-18 and got different moving around results with the same peaks just moving around in the timeframe. One thing to note is when comparing the peaks before and after when they moved around I saw a -4db to -6db at any given area. I will remove the ceiling fan tonight and test again but I honestly feel this whole range will drop a pretty good amount by removing the fan. WAF factor will get the hammer on this one

What this ended up also telling me was the cathedral peak was causing a double bounce wave issue near the peak. So my proposed fix will be to have some hanging baffles made with the 4" thick 2'x4' panels. I'll start at the peak and drop that down so and then hang another one about halfway down the wall. This will literally KILL all primary reflections from the speaker around the catheral and entering my head.

However... this is a great area to emphasize soundstage height size... considering this reflection is coming at my head from above do I want to kill it or let it emphasize the reflection? It's in the critical 5-7ms range... any thoughts on this? I could try to diffuse it or just attack and destroy with absorption.
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post #5 of 9 Old 12-30-15, 10:41 AM Thread Starter
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Re: Ideal Impulse Decay & SS&I improvements - Data Inside

In the end.... Here is my testing after eliminating some of the reflections I found last night compared to when my system was first fired up and had ZERO treatment on the backwall or the ceilings.

As you can see there is a significant reduction and I'm not even done treating the room... and making me realize I might not have to treat as much as I thought I would.

around 10db reduction since before I started and some peaks as much as 14db. Without a doubt my ears are not lying to me and match what is shown.... the room sounds much better with this treatment than without. Granted I have much more to do... bass traps etc.
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Last edited by Talley; 12-30-15 at 10:47 AM.
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post #6 of 9 Old 12-30-15, 11:02 AM Thread Starter
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Re: Ideal Impulse Decay & SS&I improvements - Data Inside

This is what I plan on doing in my room. Will hang panels in two rows. One at the peak hanging down and two is midway on the back ceiling part hanging down. Then I will remove the ceiling fan and install a cloud above the MLP and put absorption on the back wall instead of diffusion and will report the results when I'm done with that.
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post #7 of 9 Old 12-30-15, 11:22 AM
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Re: Ideal Impulse Decay & SS&I improvements - Data Inside

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Talley wrote: View Post
In the end.... Here is my testing after eliminating some of the reflections I found last night compared to when my system was first fired up and had ZERO treatment on the backwall or the ceilings.

As you can see there is a significant reduction and I'm not even done treating the room... and making me realize I might not have to treat as much as I thought I would.

around 10db reduction since before I started and some peaks as much as 14db. Without a doubt my ears are not lying to me and match what is shown.... the room sounds much better with this treatment than without. Granted I have much more to do... bass traps etc.
A really worthwhile improvement to the reflections to a point where they are now probably just fine. Now onto the bass traps.

Regards

Paul
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post #8 of 9 Old 12-30-15, 11:35 AM Thread Starter
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Re: Ideal Impulse Decay & SS&I improvements - Data Inside

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A really worthwhile improvement to the reflections to a point where they are now probably just fine. Now onto the bass traps.
Thank you. Those are coming quicker than later. (this weekend).

You say probably fine but that is my whole desire of this exercise... correcting the reflections to the point of inaudible. To me this is time dependent and at what magnitude is necessary?

In some research I found that Nasa performed just the task... what db loss becomes inaudible for reflections. A pdf can be found here: http://human-factors.arc.nasa.gov/pu..._Inaudible.pdf

This PDF shows a couple of testers and found that at 10ms anything lower than -18db could not be detected and that at 20ms anything lower than about -20 to -22db could not be detected showing that with further time... a need for further reduction is a must obviously as this is decay. What's interesting is it shows that anything at the 180 degree mark could not be as easily detected in the way of sounds behind the head. at 5ms is where my head to the back wall reflection is happening and my current diffused setup shows -8db to -12db in that era but according to that PDF sounds at 180 degrees at 10ms even could not be detected below -5db meaning that even though they are there and look bad they are inaudible.

With the asorption in place instead of the -8 to -12 I'm at -24db. To me the sound is better with the absorption IF and only IF I keep my head perfectly straight... if I turn my head I'm so close to that absorption panel that it sounds funny like my ear is being covered up thats closest to it. So maybe the diffusion is better since it promotes "space" and not dead.

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post #9 of 9 Old 12-30-15, 12:16 PM
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Re: Ideal Impulse Decay & SS&I improvements - Data Inside

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Thank you. Those are coming quicker than later. (this weekend).

You say probably fine but that is my whole desire of this exercise... correcting the reflections to the point of inaudible. To me this is time dependent and at what magnitude is necessary?

In some research I found that Nasa performed just the task... what db loss becomes inaudible for reflections. A pdf can be found here: http://human-factors.arc.nasa.gov/pu..._Inaudible.pdf

This PDF shows a couple of testers and found that at 10ms anything lower than -18db could not be detected and that at 20ms anything lower than about -20 to -22db could not be detected showing that with further time... a need for further reduction is a must obviously as this is decay. What's interesting is it shows that anything at the 180 degree mark could not be as easily detected in the way of sounds behind the head. at 5ms is where my head to the back wall reflection is happening and my current diffused setup shows -8db to -12db in that era but according to that PDF sounds at 180 degrees at 10ms even could not be detected below -5db meaning that even though they are there and look bad they are inaudible.

With the asorption in place instead of the -8 to -12 I'm at -24db. To me the sound is better with the absorption IF and only IF I keep my head perfectly straight... if I turn my head I'm so close to that absorption panel that it sounds funny like my ear is being covered up thats closest to it. So maybe the diffusion is better since it promotes "space" and not dead.
Research and testing has shown that once reflections drop below -20dB they become inaudible to the listener unless there are special test circumstances. Reflections in the Hass region, up to 20mS, are generally not heard as such and can blur the imaging. Reflections after 20mS will be heard as discrete echoes. So if you keep all reflections below -20dB in the first 20 mS you are doing well. The audible effect of the reflections during the Hass window is well documented and even in my case where they are only -15dB at 9mS due to a rear wall QRD (see graph 19D in my thread PDF ), they do not cause any apparent adverse effects to the stereo imaging. Removing them makes the room sound too "dead".

The issue you noted regarding the audible effects of putting an absorber on your rear wall I also noted during my tests, even though my rear wall is 4'10" behind me. However, all QRD's have a critical distance that if not met the diffuser has no real effect or can cause sever lobing issues.

Regards

Paul

Last edited by Digione; 12-30-15 at 01:01 PM.
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