Could use some help with Impulse response (ETC)

[pratul]

Hello REW and acoustics experts: I am hoping some one here can help me with the right way to measure, interpret and address impulse response in my theater.

As you can see in the chart below, the impulse response (ETC) chart looks horrible. I spent the entire day today trying to figure out why I am getting these strong reflections. I tried the following to understand what is going on:

• I placed absorption panels on the side walls where the reflections would happen: No change
• I placed absorption panels right behind and besides the speakers: No change
• I placed absorption panel on the floor in front of the speakers where the reflections would happen: No change
• My front speakers are M&K S-150THX which are tripole speakers (drivers on the front and both sides) I covered the side speakers with thick blanket: Very little change but not the the reflection at around 2ms.
• I covered the MLP chair where I was taking measurements with thick blankets. Very little change but not the the reflection at around 2ms.
• I tried measuring using the ARC microphone I got with my pre-processor and it should similar reflections. There were minor differences but I am attributing them to the mics themselves
• I then tried replacing the M&K S-150THX speakers with Definitive Tech tower speaker (bi-polar) that I use elsewhere and the impulse response (ETC) graph was almost identical.

According to REW the first reflection adds 2ft 7.46 inches to the sounds path resulting in a 2.33ms delay. I can't think of any way this could be a reflection from any of the surfaces around the speaker.

A brief description of my theater room:

• Size: 33' 10" (L) x 13' 10" (H) x 8' 9" (H)
• Walls: The lower 4 feet of all the side walls and the back wall are covered with 1" thick fiberglass panels covered with GOM fabric.
• The points where the sound would reflect from the ceiling has 2" thick absorber. There is a 2" gap between the absorber and the ceiling.
• The front and rear corners have 16" x 20" bass traps (Rockwool 60 8" of each exposed side)
• The front speakers are on stands 2.5 feet tall and are right in front of the bass traps
• The speaker stands are on a 3.5" platform made with 2 x 4 lumber and 3/4" thick plywood and filled with fiberglass insulation. The lumber frame is glued to the concrete floor and the plywood using construction adhesive. The plywood is also screwed (not nailed) to the frame. Basically it is a solid platform.
• My seats are leather theater seats. The front row are never used and completely covered with soft fabric covered cushions. The second row is where we usually sit and I covered the seat where I was taking measurements with thick soft throws.

I can use your help with the following:

• Am I measuring the impulse response the right way?
• What else can I do to figure out where the reflections are happening?
• Could my speakers being tri-poles be the reason for these reflections? I don't think so based on the tests above but still curious.

Anything else I am missing?

Thanks for your help guys!

 

[sm52]

If you measured both sides of the speakers together - right and left, then the peak at 2.33 ms is the IR side farthest from the microphone by 2ft 7.46 inches. Very much like that.

 

[Sonnie Parker]

Yeah... those 2, 4, 6ms reflections are pretty close and very apparent at -1dB to -8dB or so. Gotta be something close by to the mic... from 2.5 to 7ft or so.

 

[welldun]

Hello,
Have you tried using a different view for the Impulse Response. By this I'm referring to perhaps switching the window view to % instead of DBFS. you can do this from the drop down menu near the top left corner within the graph itself. Once there if you want to zoom in to the graph just click on the screen where you want to zoom in and then use the respective +/- button to zoom in. If you do all of this in the Impulse Response window inside of the OVERLAY tab, you will also be able to view multiple IR at the same time.
I've attached a screenshot for reference. the areas with the yellow circle are the ones I mentioned above. (actually, after reading your post again, the Overlays window might not help your search, sorry).

 

[pratul]

Yeah... those 2, 4, 6ms reflections are pretty close and very apparent at -1dB to -8dB or so. Gotta be something close by to the mic... from 2.5 to 7ft or so.

That is interesting you say that.. I was focused on finding a reflecting surface close to the speakers and right around the mic. Guess I have a few more things to check off the list. I was actually going to get a string between the speaker and mic, add 2.5 feet to it and then see what I could touch with it Project for this weekend or sooner if I can find time.
Thanks!

 

[welldun]

I also found a pretty good tutorial/discussion about using the ECT in REW to find these reflections. It's worth reading if you haven't already done so. I especially liked the tip about using the IR smoothing to help read the graph better.
Here is the link: https://www.gikacoustics.com/unpacking-etc-time-domain-measurements-early-reflections/

 

[pratul]

I also found a pretty good tutorial/discussion about using the ECT in REW to find these reflections. It's worth reading if you haven't already done so. I especially liked the tip about using the IR smoothing to help read the graph better.
Here is the link: https://www.gikacoustics.com/unpacking-etc-time-domain-measurements-early-reflections/

Thank you! That link was very useful and the smoothing did really help. Thank you!

 

[ddude003]

Just a thought... Do you have your mic positioned with a real mic stand/boom? That little one that comes with the mic leads one to want to set it on the couch propped up with a pillow or set on the back of the couch causing issues...

 

[pratul]

Just a thought... Do you have your mic positioned with a real mic stand/boom? That little one that comes with the mic leads one to want to set it on the couch propped up with a pillow or set on the back of the couch causing issues...

YEs, I have a mic stand. I modified the one that came with my pre-processor to work with one of my studio light stands (photography is my other hobby )

 

[welldun]

I hope you've made progress with your measurements and room treatments. Also, it would be a great contribution to this thread and forum if you share your findings and method, and approach once you've sorted it all out. I'm sure that I'm not alone in wanting to see some real-world examples of a person taking measurements, interpreting the graphs, and then turning that info into physical changes in the room to address the issues detected via the measurements. Too often when I see examples of treated rooms, they all seem to follow a similar design with bass traps and reflection treatments in the "usual spots". But I sense that your quest goes beyond just doing the "usual", and you truly want to pinpoint the trouble areas and fix them. Good luck and I look forward to reading about your results.

 

[pratul]

I hope you've made progress with your measurements and room treatments. Also, it would be a great contribution to this thread and forum if you share your findings and method, and approach once you've sorted it all out. I'm sure that I'm not alone in wanting to see some real-world examples of a person taking measurements, interpreting the graphs, and then turning that info into physical changes in the room to address the issues detected via the measurements. Too often when I see examples of treated rooms, they all seem to follow a similar design with bass traps and reflection treatments in the "usual spots". But I sense that your quest goes beyond just doing the "usual", and you truly want to pinpoint the trouble areas and fix them. Good luck and I look forward to reading about your results.

Thanks! Will do.

 

[DanDan]

You need to run just one speaker to acquire a useful ETC.

 

[pratul]

You need to run just one speaker to acquire a useful ETC.

Dan: I am running it for one speaker at a time.