# ETC/reflection measurements and correction advice?



## patchesj (Jun 17, 2009)

I've been looking for a sticky or some information on using REW to detect and then treat reflections. I've done the more "basic" steps of treating first reflection points, installing bass traps, etc. Is there a recommened method for taking the measurements when looking for reflections? Correct way to post the graphs? Thanks!!


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## JohnM (Apr 11, 2006)

Try Nyal Mellor and Jeff Hedback's article at http://www.hifizine.com/2011/12/listening-room-reflections-and-the-energy-time-curve/


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## patchesj (Jun 17, 2009)

Interesting article... So I'm looking at a decent sized reflection at 2.59ms and another around 8.35ms, and those should be my initial focus?

This measurement was taken with both speakers (L+R), so perhaps I should step back and take some new measurements using each by itself?


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## patchesj (Jun 17, 2009)

Also, assuming 1.13 feet per ms I have major reflection points at 2.9 and 9.4 feet of the speaker?

Is this a better way to graph the data?


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## patchesj (Jun 17, 2009)

I think it just hit me that the points I'm looking for are somewhere that would create a path 2.9 feet longer than the direct path... I'm guessing this is the first reflection point on the side wall. So I need more toe in and/or more absorbtion on the walls in front of the speakers.. More measuring tonight!!


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## patchesj (Jun 17, 2009)

Also cool little trick in REW in case anyone doesn't know it:

Originally Posted by JohnM 
You can't show a distance axis instead of (or as well as) a time axis (on my list of things to do though), but if you use the delta/zoom box it will show both time and the equivalent distance. To bring that up press the Ctrl key, hold down the right mouse button and drag.


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## localhost127 (Jan 2, 2011)

you need to be utilizing the hardware loopback - and UNCHECK T=0.

you cannot simply subtract the spike on the ETC from the direct signal when you have the direct signal set to T=0.

once you change this and identify the total time of flight of the reflection, then you can work backwards.

SAC's string test and the blocking method are a good start:
http://www.gearslutz.com/board/6133764-post8.html

and do not band-limit your ETCs.


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## patchesj (Jun 17, 2009)

localhost127 said:


> you need to be utilizing the hardware loopback - and UNCHECK T=0.
> 
> you cannot simply subtract the spike on the ETC from the direct signal when you have the direct signal set to T=0.
> 
> ...


So I need to have my left channel output connected to my left channel input (assuming right channel is selected as the "input" channel in REW?

If it shows "no filter" on the filtered IR tab, then I'm not band limiting?


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## patchesj (Jun 17, 2009)

I've retaken some measurements one speaker at a time (but not yet with the hardware loopback).

Left









Right










Center


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## dannut (Oct 20, 2011)

patchesj said:


> ....
> I've done the more "basic" steps of treating first reflection points, installing bass traps, etc....


According to what you have just posted, your basic steps aren't working as intended. So - get the first measurement as a baseline and work from there - get all your treatments out of the room and get the hardware loopback working.


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## patchesj (Jun 17, 2009)

dannut said:


> According to what you have just posted, your basic steps aren't working as intended. So - get the first measurement as a baseline and work from there - get all your treatments out of the room and get the hardware loopback working.


Could you explain how you reached that conclusion? Just trying to learn what I should be looking at in the graphs that indicate issues. Will get a baseline soon.

Thanks.


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## patchesj (Jun 17, 2009)

So I've got the loopback hooked up and did some sweeps. REW was giving me errors regarding clipping detected, but I'm pretty sure this is related to the loopback and no amount of adjustment on the soundcard inputs, outputs, pre-amp volume, etc. made this go away.

I'm doing the image capture in REW and only looking at the first 40ms. Should I open this up some, or does it give enough info for analysis?

Left








Right


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## dannut (Oct 20, 2011)

Are these your baseline measurements with no treatments in the room?
Is your listening position and speaker position fixed and the bass response optimized? Can you also post pictures and floor-plan of your room?

Firs concentrate on finding what is causing the peaks ~4ms after the direct signal for each source. These are only 5dB down and will cause image smear, coloration and loss of intelligibility. First peak has ~1ms more delay than the direct signal, could be from speaker mounting or some nearby object, or possibly a poorly designed speaker. Try to find the exact position of each protruding spike with string or blocking method, SAC-s tutorial above is excellent. Take your time, it is a slow process first time.


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## patchesj (Jun 17, 2009)

To add to the mix, I'm running the speakers behind an AT screen. I'm running Thiel MCS1s. I'm thinking about pulling the screen and running a test to see what impact that has. The entire "real" front wall is covered in 4" Rockboard 60, should be absorbing any reflections off the backside of the screen.


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