# REW accuracy?



## bjs (Jun 12, 2008)

I've just discovered REW. Looks like a great program.

Has anyone compared it's accuracy against other programs (such as ETF, RDplus, ARTA, Praxis etc) or indepently confirmed it's validity? 

I hope I'm not burned at the stake for asking such an impertinent question...but curiosity knows no limits...!


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## Wayne A. Pflughaupt (Apr 13, 2006)

I'll leave the technical confimation to JohnM, the prolific creator of the program. I'll just say: Thousands of satisfied users, does that count? :jiggy: At least one manufacturer is using REW graphs on their website. :T

Regards,
Wayne


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## JohnM (Apr 11, 2006)

Yes, haven't played with Praxis but I have checked against the others mentioned and some more besides.


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## Bob_99 (May 8, 2006)

I use RplusD and REW and found that the biggest differences are in the features. FR measurements are very close and the waterfalls appear to be just a bit different. I have leaned more to RplusD only because I have used it longer but now that I haven't been able to run it under MS Vista (others have no problem with that issue), I have been using REW more. As I originally said, each have some very good features that would be nice to have combined in one package.

Bob


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## JohnM (Apr 11, 2006)

Bob_99 said:


> the waterfalls appear to be just a bit different.


You may not have come across this in the REW help: _ETF users should note that ETF gate times are specified in a different manner to REW window durations, to convert an ETF gate time to the approximately equivalent window width, multiply by 1.4._ That also applies to RplusD.


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## Bob_99 (May 8, 2006)

Thank you, John. I've read the manual but obviously missed that information. Also, let me add my thanks to all the others for an amazing program. The fruit of your labor is very much appreciated.

Bob


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## DrWho (Sep 27, 2006)

I've taken my measurement rig into the Klipsch anechoic chamber over in Indianapolis and my measurement was within 0.5dB from something like 100Hz to 10kHz.

They're using Leap and TEF.

I also get comparable results to Praxis.

Ultimately, the accuracy of REW depends on the accuracy of your soundcard. If your noise floor and distortion are low, then you'll get a fairly accurate measaurement. Usually the microphone is going to be the limiting factor in this case...


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## jlohl (Jun 6, 2007)

> You may not have come across this in the REW help: ETF users should note that ETF gate times are specified in a different manner to REW window durations, to convert an ETF gate time to the approximately equivalent window width, multiply by 1.4. That also applies to RplusD.


Thanks, this is written in the REW manual but could JohnM explain why are those gate times not equivalent ?


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## JohnM (Apr 11, 2006)

jlohl said:


> Thanks, this is written in the REW manual but could JohnM explain why are those gate times not equivalent ?


You'll have to ask Doug Plumb, I don't recall there being a way to plot what window extent a particular gate time gives in ETF or R+D so can't tell how the figure is used there. In REW you can see the window shape on the Impulse Response graph, the widths of the windows are the times you set.


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## Doug Plumb (Mar 16, 2007)

JohnM said:


> You'll have to ask Doug Plumb, I don't recall there being a way to plot what window extent a particular gate time gives in ETF or R+D so can't tell how the figure is used there. In REW you can see the window shape on the Impulse Response graph, the widths of the windows are the times you set.


The widths of the windows in RPlusD is the actual square part of the window, but RPlusD adds some rounded truncation to this. A 2 ms gate has a value of 1 between -2 ms and 2 ms and it is truncated with an additional time of 1.2 X gate time using a hanning gate. 

Most measurement software uses just a hanning or hamming gate, RPlusD uses a square one to not de-emphasize the effect of reflections over the gate period.


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## Doug Plumb (Mar 16, 2007)

Doug Plumb said:


> The widths of the windows in RPlusD is the actual square part of the window, but RPlusD adds some rounded truncation to this. A 2 ms gate has a value of 1 between -2 ms and 2 ms and it is truncated with an additional time of 1.2 X gate time using a hanning gate.
> 
> Most measurement software uses just a hanning or hamming gate, RPlusD uses a square one to not de-emphasize the effect of reflections over the gate period.


ERROR - the additional truncation is 0.2 X gate time.

The software uses a symmetrical gate in anticipation of some processors using a small amount of FIR filtering. For this reason, the gate does not start at t=0, but at the equivalent gate time before the time reference of 0 ms.

(I don't like FIR filtering myself)


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## Ricci (May 23, 2007)

+ whatever.

It is accurate. If you find that there are notable differences between ARTA, HolmImpulse, or whatever you are using comared to REW it is usually a difference in the internal settings of the 2 programs and not either program itself.


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## Doug Plumb (Mar 16, 2007)

Accuracy is a non issue with these programs. 

You can run into problems with non linearities and the programs typically don't work well with non linearities. Frequency response is a linear concept.

All you have to do to see if non linearities or noise are affecting a result is lower or increase your SPL levels a little bit (a few dB) and see if your frequency response is the same with a second measurement without the mic pos changed. OR you can change from MLS to sweep and re take the shot and compare the frequency response between the two since the affect of distortion on these two different test signals is radically different.

The concept of frequency response only applies to a system without non linearities, but our measurements are close enough with the typical non linearities.


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