# How to use REW for speaker positioning?



## BloomBloom (Feb 15, 2013)

Hello,
I've been using REW and the TRA to get EQ filters on my 2 channel + Sub system.

As I have electrostatic speakers I'm aware that getting the rear wall reflections in sync to the listening position is very important. 

I've been using a laser measurement tool and old school tape to align the speakers and the listening position but I wonder if I could use REW - perhaps the impulse response part of it to confirm the alignment? 

Mt situation is a bit complicated as the rear wall (behind the speakers) is angled so I'm trying to position the speakers and the listening position parallel to it. 

I have a calibrated UMIK but my set up doesn't allow timed loopback measurements. 

Please point me in the right direction?

Thank you!


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## AudiocRaver (Jun 6, 2012)

BloomBloom said:


> Hello,
> I've been using REW and the TRA to get EQ filters on my 2 channel + Sub system.
> 
> As I have electrostatic speakers I'm aware that getting the rear wall reflections in sync to the listening position is very important.
> ...


REW is perfect for that.

Here is our speaker setup guide.

Here is a documented setup that covers your situation.

The solution I recommend is to create reflective surfaces independent of the walls, as in the 2nd article above.


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## BloomBloom (Feb 15, 2013)

Thanks Mate!
Much appreciated. 

Sadly, independent reflective surfaces won't fly past my better half, but I'll see how I could use the walls/room best.

Thank you!


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## AudiocRaver (Jun 6, 2012)

BloomBloom said:


> Thanks Mate!
> Much appreciated.
> 
> Sadly, independent reflective surfaces won't fly past my better half, but I'll see how I could use the walls/room best.
> ...


The overlay panel - impulse response diagram should show you how te be sure the front waves delays are lined up exactly (I look for matching close to +- 20 uS or so, giving about +- 0.2 inch speaker placement resolution; and about the same tight matching, if possible, for the rear waves). In my room, the front wall was irregular enough that it was impossible to accomplish until I went to an adjustable-movable reflection structure. You might fare better, especially if you put up the wall and it is actually straight and flat.


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## BloomBloom (Feb 15, 2013)

Thanks Mate,
I'll give it a try.

Can you please describe exactly where I should look? do an impulse response measurement? With what settings? Do each speakers separately I imagine then both together?

Thank you!


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## AudiocRaver (Jun 6, 2012)

BloomBloom said:


> Thanks Mate,
> I'll give it a try.
> 
> Can you please describe exactly where I should look? do an impulse response measurement? With what settings? Do each speakers separately I imagine then both together?
> ...


Forgot to mention, REW only works for this use if you have an audio interface with two inputs so the second input is used as a loopback input for a timing reference - that is different from loopback for calibration.

You must measure the speakers separately. Then select _overlays_ and _impulse_, select the L and R measurements and you will be able to zoom in and see how closely the two waveforms match up in the time domain.


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## BloomBloom (Feb 15, 2013)

AudiocRaver said:


> Forgot to mention, REW only works for this use if you have an audio interface with two inputs so the second input is used as a loopback input for a timing reference - that is different from loopback for calibration.
> 
> You must measure the speakers separately. Then select _overlays_ and _impulse_, select the L and R measurements and you will be able to zoom in and see how closely the two waveforms match up in the time domain.


Thanks, I guess I'll go find two input audio interface. I use the umik USB microphone, I hope that's not a problem?


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## Talley (Dec 8, 2010)

AudiocRaver said:


> The overlay panel - impulse response diagram should show you how te be sure the front waves delays are lined up exactly (I look for matching close to +- 20 uS or so, giving about +- 0.2 inch speaker placement resolution; and about the same tight matching, if possible, for the rear waves). In my room, the front wall was irregular enough that it was impossible to accomplish until I went to an adjustable-movable reflection structure. You might fare better, especially if you put up the wall and it is actually straight and flat.


2/10ths accuracy huh.

I strive for .05" but unfortunately thats pretty difficult considering room construction and the fact my room is setup about 4" right from center.


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## AudiocRaver (Jun 6, 2012)

More bad news, no I am afraid the UMIK-1 will not work for that application, you will need an analog calibrated mic


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## BloomBloom (Feb 15, 2013)

No worries,
I actually have a Tascam US-122Mk2 - and an analogue uncalibrated mic - I understand it should work...


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## AudiocRaver (Jun 6, 2012)

Talley said:


> 2/10ths accuracy huh.
> 
> I strive for .05" but unfortunately thats pretty difficult considering room construction and the fact my room is setup about 4" right from center.


Yeah, that's about what I shoot for, too, I was trying not to scare the new guy away.


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## BloomBloom (Feb 15, 2013)

Talley said:


> 2/10ths accuracy huh.
> 
> I strive for .05" but unfortunately thats pretty difficult considering room construction and the fact my room is setup about 4" right from center.


Hey,
no worries, that's Chinese to me as I'm in Europe so working in metric 
;-)


Looks like I almost managed to set this up, using the TASCAM, just need one tiny adapter.

I have a few questions:

1. When placing the mic (ECM8000) for the timed IR measurement, what should it's orientation be? I usually use the Umik in the 90 degrees position - should I point the mic towards the speakers?

2. how do I actually activate the timed impulse response in REW once i set everything up? just do a measurement and look at the IR window?

Thank you! 
I'm getting there:wave:


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## AudiocRaver (Jun 6, 2012)

BloomBloom said:


> Hey,
> no worries, that's Chinese to me as I'm in Europe so working in metric
> ;-)
> 
> ...


Orientation matters little for timing measurements. For speaker measurements, we recommend aiming straight at the speaker and using a 0degree (standard) mic calibration file. For room EQ, we recommend aiming at the ceiling and using a 90degree mic calibration file. For timing measurements, you will be running a standard frequency response plot for each of the two speakers, taken one at a time. The mic orientation really does not matter for this measurement, except I have found it to beneficial to do the following:

Place the tip of the mic at the point in space where the midpoint between the listener's ears would sit. Hang the mic by its cable, pointing downward. Now trust me on the next step: adjust the mic position straight back away from the room's front wall and toward the chair back (not up or down) until it - the very tip of the mic - is just touching the back of the chair. All of the timed signal paths will be equally increased by a few inches by this move. There will be no delayed reflections from the chair back messing up your impulse response plots, so they will be cleaner and easier to work with while matching them up.

Take regular measurement plots of the left and right speakers, so you have two separate plots. Now switch to the overlays view and impulse plot. The impulse plot is calculated from the frequency response measurement plots that you took, and is extremely accurate for this use.

Adjust the horizontal scale and zoom until you see the first impulse for the direct wave. You will see the two plots for the left and right speakers overlaid, and the two impulses should be right on top of each other or at least very close if it two speakers are close to an equal distance from the listening position. You can adjust the speakers to get them more closely matched, or, if the difference is less than 12 microseconds (12 microseconds = 4 millimeters), then don't bother trying to adjust the speakers, you can move the microphone left or right. If the waveform from the left speaker is earlier, or to the left on your computer screen, move the microphone just a couple of millimeters to the right to compensate. Take the measurements again and adjust until the two plots overlay each other exactly. The goal is only a few millimeters off, which equates to about 10 microseconds on the impulse plots.

Once this is done, now you can adjust the time scale so you are zoomed in on the delayed reflections, which come later (to the right). Ideally, they also should overlay each other or be very close to it. The problem is, moving the speakers closer to or further from the LP at will also change the timing of the non-delayed direct waves which you have already made adjustments for. Your best bet is to twist one of the speakers very slightly. Changing only the toe-in, you can actually get a slightly different reflection point off of your front wall for the delayed wave, and change the delayed wave timing vary slightly without changing the direct wave timing.

Talley and I were joking about the extreme matching called for to make this process work. Actually we were not joking. Get it close (4 to 6 mm matching for direct and reflected wave) and you will get great benefits from it. Get closer than that (2 to 4 mm) and there is an impact from the proper timing of those waveforms that really hits you. You'll know when you hear it.


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## BloomBloom (Feb 15, 2013)

Cool thanks,
I'll give it a go!


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## BloomBloom (Feb 15, 2013)

Hey,
one quick question, 

I understand how to set up the loopback (and microphone) via the Tascam - do I also have to play the sweeps through it for this to work?

My set up is a bit weird as I don't use a pre-amp, I have a DAC that goes directly to a 300w power amp - and have 30db attenuators on that connection so that the house won't blow up .

I'd prefer to use that connection (a toslink from my computer to the dac and speakers) than the TASCAM in to the power amp. 

I know it works in REW (I just set up a different playback device and the tascam as the microphone input - my question is will that set up interfere with the timed reference IR? How can I make sure it works?

I hope that's clear - thank you!


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## AudiocRaver (Jun 6, 2012)

BloomBloom said:


> Hey,
> one quick question,
> 
> I understand how to set up the loopback (and microphone) via the Tascam - do I also have to play the sweeps through it for this to work?
> ...


So, the question is where is it best to run the timing loopback from? It can be run from the output of the audio interface, from the output of the DAC, or from the output of the power amp, with appropriate attenuation of course. I have done it all three ways, it works fine from any of them. And it really makes no difference at all. All you are trying to measure accurately is relative timing, which will be stable and repeatable and accurate from any of those points. So do it the easy way, use the output of the audio interface. For your purposes that will be as accurate as anything else.


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## BloomBloom (Feb 15, 2013)

AudiocRaver said:


> So, the question is where is it best to run the timing loopback from? It can be run from the output of the audio interface, from the output of the DAC, or from the output of the power amp, with appropriate attenuation of course. I have done it all three ways, it works fine from any of them. And it really makes no difference at all. All you are trying to measure accurately is relative timing, which will be stable and repeatable and accurate from any of those points. So do it the easy way, use the output of the audio interface. For your purposes that will be as accurate as anything else.


Perfect, thank you!

I'll go try this. How van I tell the timing loopback works?


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## Talley (Dec 8, 2010)

what is loopback timing

/noobquestion


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## BloomBloom (Feb 15, 2013)

I tried this, I think I set the loop back properly - this is the result - two channels, notice the green one starts a few milliseconds before 0??

What am I missing?

Thank you!


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## jtalden (Mar 12, 2009)

The way to confirm if the loopback timing is setup properly is to measure the same speaker several times. The IR will fall exactly on the same timing (greater than 0ms) if all is working properly. It helps to zoom in all the way on the time scale so small errors will be easy to see. I usually use '%FS' setting for graph format.


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## AudiocRaver (Jun 6, 2012)

BloomBloom said:


> Perfect, thank you!
> 
> I'll go try this. How van I tell the timing loopback works?


Let's say your speakers are 3 m from your listening position (LP) and you run the timing loopback from an output of your audio interface. The speed of sound is 343 m/sec, or 2.9 millisec/m. So the delay time for the sound to get from your speakers to your LP (3 m away) is 8.75 millisec. The timing loopback reference will eliminate the latency in the computer & interface. So your measurement at the LP will show the impulse delayed by almost 9 millisec from the beginning of the plotted curve.


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## BloomBloom (Feb 15, 2013)

jtalden said:


> The way to confirm if the loopback timing is setup properly is to measure the same speaker several times. The IR will fall exactly on the same timing (greater than 0ms) if all is working properly. It helps to zoom in all the way on the time scale so small errors will be easy to see. I usually use '%FS' setting for graph format.


OK,
Thanks for this tip! 
Here's what I got, what's wrong here? How to fix it?
Thank you!


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## jtalden (Mar 12, 2009)

I am unsure of your setup so you may need to adjust for your situation. Hopefully this will be clear enough. The same signal path is probably needed in your case for both the loopback and the mic. Typically only one soundcard is used.

Connect the R/L output channels of the Tascam to the R/L inputs of the DAC. If the Tascam doesn't have optical out then maybe you can use the analog inputs in the DAC for L/R?
Connect the output of the of the loopback channel (left?) from the DAC output to the Tascam left input.
The measurement channel (Right?) of the DAC goes to the P-Amp.

The option is to drop the DAC for this timing exercise. Just connect the Tascam to the P-Amp directly for the measurement channel and then the loopback cable can be between the input and output of the Tascam. 

The 10ms difference you show is more than I would have expected however even with the current setup so there may be another issue.


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## BloomBloom (Feb 15, 2013)

jtalden said:


> I am unsure of your setup so you may need to adjust for your situation. Hopefully this will be clear enough. The same signal path is probably needed in your case for both the loopback and the mic. Typically only one soundcard is used.
> 
> Connect the R/L output channels of the Tascam to the R/L inputs of the DAC. If the Tascam doesn't have optical out then maybe you can use the analog inputs in the DAC for L/R?
> Connect the output of the of the loopback channel (left?) from the DAC output to the Tascam left input.
> ...


Thanks,
I'll try to connect directly to the poweramp. I wonder if I set the Tsacam correctly, using the calibration measurement seem to work well but maybe there's a gain issue somewhere? I'll try only the Tascam directly to the power-amp...


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## AudiocRaver (Jun 6, 2012)

BloomBloom said:


> OK,
> Thanks for this tip!
> Here's what I got, what's wrong here? How to fix it?
> Thank you!
> ...


I agree with John, the 10 mS difference is surprising. According to the diagram, the path from the left speaker front to the measurement mic is about 3 m shorter than the path from the right speaker front to the measurement mic. Could that possibly be?

Also, it is not clear where the negative numbering on the horizontal scale is coming from. Those should be positive numbers, going from zero at the left.


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## BloomBloom (Feb 15, 2013)

OK guys,
new IR direct to amp - I think its looking better? still not sure how to read this, what do you think?

Thank you!


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## jtalden (Mar 12, 2009)

That looks normal for L/R measurement when the mic is almost perfectly equidistant from the 2 speakers. If the distances were exact the peaks would be on top of each other. It's a little tricky to get the mic centered any better than shown however. 


I expected to see 3-4 repeat measurements of the same speaker to assure that the IRs all fell exactly on top if each other. That would assure that the loopback is working as expected. You may want to do that just to be sure, but It appears to be a very safe bet that all is now working well. You are good to go. 


I will let AudiocRaver help you with the setup process. I have never done that using panel speakers.


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## BloomBloom (Feb 15, 2013)

jtalden said:


> That looks normal for L/R measurement when the mic is almost perfectly equidistant from the 2 speakers. If the distances were exact the peaks would be on top of each other. It's a little tricky to get the mic centered any better than shown however.
> 
> 
> I expected to see 3-4 repeat measurements of the same speaker to assure that the IRs all fell exactly on top if each other. That would assure that the loopback is working as expected. You may want to do that just to be sure, but It appears to be a very safe bet that all is now working well. You are good to go.
> ...


Thanks mate,
I did take several measurements that came out the same, just put two in the image to make it easy to interpret.

Thanks for the help, we're getting there!


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## AudiocRaver (Jun 6, 2012)

The two curves in the measurement in post #23 are separated by 200 uS, which equates to a 3 inch (1.2 cm) path length difference. That is not a bad starting point, but I am hoping that you will be able to hear the benefits of much tighter matching than that (2 to 3 mm ??).

Edit: math error, see below.


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## BloomBloom (Feb 15, 2013)

AudiocRaver said:


> The two curves in the measurement in post #23 are separated by 200 uS, which equates to a 3 inch (1.2 cm) path length difference. That is not a bad starting point, but I am hoping that you will be able to hear the benefits of much tighter matching than that (2 to 3 mm ??).


Thanks mate,
I'll try improving the speakers positions. 

Maybe a silly question, but where do you see 200u in the graph I posted? Is there a way to see the distance in cm?

thank you, we are getting there


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## AudiocRaver (Jun 6, 2012)

Oops, made a math error:

The vertical grid lines around the waveforms are at 8.3 and 8.4 ms, or 100 us apart, and the two peaks are about 1/5 of that distance apart, or 20 us, not 200 us. That 20 us difference is roughly a 0.3 inch or 7.6 mm, and you are going to have a hard time getting much closer than that in your matching.

An REW tip. Want to measure that difference directly? Ctrl+Shift+[hold down R mouse button] and drag the mouse cursor from one graph point to another and - for impulse timing - you will be shown the difference in seconds, feet, and meters!


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## BloomBloom (Feb 15, 2013)

AudiocRaver said:


> Oops, made a math error:
> 
> The vertical grid lines around the waveforms are at 8.3 and 8.4 ms, or 100 us apart, and the two peaks are about 1/5 of that distance apart, or 20 us, not 200 us. That 20 us difference is roughly a 0.3 inch or 7.6 mm, and you are going to have a hard time getting much closer than that in your matching.
> 
> An REW tip. Want to measure that difference directly? Ctrl+Shift+[hold down R mouse button] and drag the mouse cursor from one graph point to another and - for impulse timing - you will be shown the difference in seconds, feet, and meters!


Thanks, so I'm done? 
How do I see the rear wall reflections and how to align them - the graphs I mean


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## AudiocRaver (Jun 6, 2012)

BloomBloom said:


> Thanks, so I'm done?
> How do I see the rear wall reflections and how to align them - the graphs I mean


Zoom out (button in lower-right corner of REW plot area), click cursor just to the left of the reflected/delayed impulses, zoom back in again. Your direct impulses are tightly matched, if your reflected impulses were as tightly matched or you could get them there (the ugly panels I suggested) and you could get rid of other disruptive reflections (sound treatment on front wall I mentioned), the impact and soundstage and imaging would knock your socks off!


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## BloomBloom (Feb 15, 2013)

AudiocRaver said:


> Zoom out (button in lower-right corner of REW plot area), click cursor just to the left of the reflected/delayed impulses, zoom back in again. Your direct impulses are tightly matched, if your reflected impulses were as tightly matched or you could get them there (the ugly panels I suggested) and you could get rid of other disruptive reflections (sound treatment on front wall I mentioned), the impact and soundstage and imaging would knock your socks off!


Thank you, I'm getting there!


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## BloomBloom (Feb 15, 2013)

Hello Wayne,
I keep progressing and slowly achieving great results, I think.

I want to try to reach your 'ultimate setup' even though I won't be able to place the massive reflectors you used.

Here's a photo of my listening room from the LP. You'll notice that the wall behind the speakers is almost entirely reflective and I wonder if there's a way to angle the speakers so that the back sound wave is reflected at me?My limited knowledge of geometry suggests that to achieve this I'll need to toe-out the speakers...? Am I missing something?

your thoughts are welcome - thank you!


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## AudiocRaver (Jun 6, 2012)

Your logic is correct, however, I do not believe you will get good soundstage and imaging with the speakers toed out. There is enough width to the rear wave dispersion that you will get a good rear reflection even with some toe in.

Also, trying to accomplish the ideal timing without movable reflective surfaces, you might end up with too many variables to get there. If your room is perfectly symmetrical, maybe. If not, you will end up moving a speaker to improve one timing and will make another worse.

However, it is worth a try. Do not use the windows or blinds as one of the reflective areas, there is too much variation there. If you refer to my article about Sonnie Parker’s setup in his home theater, it will refresh your memory about the reflective surfaces and absorption on the area in between to improve imaging. That is hard to achieve with windows and blinds in the middle.

Again, it is worth a try. I would start with the speakers spaced wide, toed in part way, and using reflective areas on the walls just outside of the window/blind areas.

edit: Realizing it is not a permanent solution, you might experiment a little with absorption in between those reflective areas just to see how it helps. A couple of boards leaning on the blinds (gently!) with a blanket draped in between will give you an idea.


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## BloomBloom (Feb 15, 2013)

AudiocRaver said:


> Your logic is correct, however, I do not believe you will get good soundstage and imaging with the speakers toed out. There is enough width to the rear wave dispersion that you will get a good rear reflection even with some toe in.
> 
> Also, trying to accomplish the ideal timing without movable reflective surfaces, you might end up with too many variables to get there. If your room is perfectly symmetrical, maybe. If not, you will end up moving a speaker to improve one timing and will make another worse.
> 
> ...


Thanks Wayne - I'm grateful for your help!, 

I'll give your idea a try, it basically mean angling the speakers with much higher toe-in angle, in fact listening on-axis... I'll give it a try.

Another option is to completely change the orientation of the speakers/LP along this wall (see attachment - ignore laundry ;-)
Then I'll have the book shelves directly behind the speakers, it's not reflection it's diffraction (?) would that be a better placement? 

Thank you!


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## AudiocRaver (Jun 6, 2012)

BloomBloom said:


> Thanks Wayne - I'm grateful for your help!,
> 
> I'll give your idea a try, it basically mean angling the speakers with much higher toe-in angle, in fact listening on-axis... I'll give it a try.
> 
> ...


I guess I favor the bookshelves a little, although I am not sure why. More mass, more solid? My devious mind envisions the large movable reflectors somehow clamped to the bookshelves, easily removable for when company is over,, and can be replaced with precision (1/16th inch?) later without having to re-run calibration. I know, I am incorrigible.


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## BloomBloom (Feb 15, 2013)

AudiocRaver said:


> I guess I favor the bookshelves a little, although I am not sure why. More mass, more solid? My devious mind envisions the large movable reflectors somehow clamped to the bookshelves, easily removable for when company is over,, and can be replaced with precision (1/16th inch?) later without having to re-run calibration. I know, I am incorrigible.


I like your thinking ;-)

I'll try repositioning everything... will take a while...


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