# Help with REW RTA measurement on generated sweep



## kalohts (Jan 16, 2019)

Hello, 

This is my first post here and first stab at REW after lots of reading and video watching. I wanted to share what I did as a workflow and the graph I got so that someone can help me further as the results do not make sense to me.

1. Installed REW 5.19 with java
2. Connected my miniDSP Umik-1
3. Opened REW, selected 90 degree calibration file
4. Generated Pink Noise and saved at -20 db
5. Generated Measuring Sweep 256k and saved at -12 db
6. Saved those to an external drive so I can play them directly from my AVR and thus engage my full system with the subwoofer and bass management. I simply wanted to avoid to connect HDMI from my laptop to the AVR (display port issue). Thus with this step I was hoping I am still correct in my approach.
7. Played back the pink noise and measured with REW SPL, turned up the AVR volume until SPL showed 71 db (live in a small apartment so wanted to calibrate lower)
8. Played back the Measuring Sweep 20Hz to 20k Hz and used RTA to measure
9. Saved the measurement Peaks and applied 1/3 octave smoothness on the graph.
10. Attached is the result

I think my result looks weird as everything from 500Hz and up goes down very steeply. Obviously something is not right. Can you guys help me out what I am doing wrong? Is measurement only possible by using HDMI and closing the loop between the PC and AVR? I thought playing the file from REW and sending it via HDMI would be equivalent to saving the generated tone and playing it back from the AVR.

Thank you very much in advance


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

You said the apartment is small, so I guess that means you’re sitting fairly close to the speakers? That means reduced highs would sound natural.

A steep slope like this is not unusual in a small room (see the house curve article in my signature).

You might try using the ARV’s treble control to flatten things out on the high end, but if it ends up sounding unnaturally bright, trust your ears.

Regards,
Wayne


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## kalohts (Jan 16, 2019)

Indeed, it is small -- 16.5 x 12 feet. I sit about 8-9 feet away from the speakers. To be honest, I may have gotten accustomed and I do not think I am missing on the highs. So, the way it sounds now is natural to me. I used to tweak my sub with a smartphone and 1/3 octave test signals. Now I got the UMIK mic and can finally attempt at better measurements. To my surprise though, that graph looked nothing like I expected. My KEF speakers are supposed to be fairly linear..

I will try to do some more measurements with higher SPL and recheck levels. Perhaps I can use the AVR's built in Pink noise which should be at -30 db -- so my REW SPL should read 75 db. And then run the measurements at 75 db levels.

Could you verify that my approach is correct though: generating a REW measurement sweep and then playing it back on the AVR (i.e no need for an HDMI closed loop)? Perhaps also use a longer sweep 512k and point the mic directly at the speakers as opposed to 90 degrees?

Also, are the any configurations on the RTA window I should use such as mode, window, check boxes?


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## jtalden (Mar 12, 2009)

It sounds like you may have mixed the 2 measurement process options. If so choose one or the other. I suggest using the sweep method.

*Using a sweep measurement*

You didn't mention including the timing signal with the measurement wav file. Possibly you overlooked that. In your situation, it is necessary to record a timing signal along with the measurement. 

To measure, set REW sweep measurement dialog to 'acoustic timing' and 'wait for timing signal'. Then start the REW measurement process. It will start, but then pause waiting until you play the wav file on your system. REW will then detect the timing signal and properly capture the measurement sweep.

Or:

*Using a noise signal*

Record PinkPN (Pink Periodic) noise at 64k sequence length.
Be sure to set the REW RTA mode to 'RTA 1/48 octave' and set the same 64k FFT length. If the REW RTA mode is inadvertently set to 'Spectrum' mode then a downward slope in the chart will result.

Also: 
Be sure also to measure one channel at a time. Measuring 2 channels can reduce the apparent high frequency output due to comb filtering of the sound.


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## kalohts (Jan 16, 2019)

jtalden said:


> It sounds like you may have mixed the 2 measurement process options. If so choose one or the other. I suggest using the sweep method.
> 
> *Using a sweep measurement*
> 
> ...


I want to do a measurement with sweep files already prepared as opposed to sending through HDMI? I am putting them on my network and playing them via my AVR as regular audio files. Looking at the Generator - there is nothing to include the timing signal you are referring to.


Or are you in other words suggesting that I can either use the Measure tool with sweeps through HDMI input to AVR (where timing signal is needed)

OR

Use Pink PN with RTA tool? And I guess you mean 1/48 octave and checkbox "Use 64-bit FFT"?


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## jtalden (Mar 12, 2009)

kalohts said:


> I want to do a measurement with sweep files already prepared.. I am putting them on my network and playing them via my AVR as regular audio files.


Yes, I understood you are measuring from a played file.



> Looking at the Generator - there is nothing to include the timing signal you are referring to.


In the generator select 'measurement Sweep'. The timing signal option is there.




> OR
> 
> Use Pink PN with RTA tool? And I guess you mean 1/48 octave and checkbox "Use 64-bit FFT"?


Yes that is an option, but the above sweep method provides a lower noise floor and more information.


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## JohnM (Apr 11, 2006)

You can use a saved measurement sweep file but not with the RTA. The generator offers an option to include a timing reference when the Meas Sweep signal type is saved, it's the same as the instructions for saving the sweep in offline measurement.

Here is an example of the RTA settings to use Pink PN, FFT length is the one John was referring to:


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## kalohts (Jan 16, 2019)

Thank you all for the guidance thus far! I understand a bit better now how to redo my RTA measurements with a Pink PN.

Regarding the Meas Sweep recorded offline method I was able to follow through up until I had to import a recorded impulse response. The documentation for offline measurement does not explain how to do that and I got stuck at that point. I get the sense it might be a bit convoluted, so I might focus on the RTA for now.


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## jtalden (Mar 12, 2009)

kalohts said:


> Regarding the Meas Sweep recorded offline method I was able to follow through up until I had to import a recorded impulse response. The documentation for offline measurement does not explain how to do that and I got stuck at that point. I get the sense it might be a bit convoluted, so I might focus on the RTA for now.


Import is not involved in this method.

Maybe this will explain:
In the main REW window select the 'Measure' button. Then set the other circled settings in the resulting 'make a measurement' popup. Then select 'Start Measurement'. REW will start the measurement, but will then pause waiting on you to play the sweep file. When REW hears the sweep it will finish taking the measurement.


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## kalohts (Jan 16, 2019)

jtalden said:


> Import is not involved in this method.
> 
> Maybe this will explain:
> In the main REW window select the 'Measure' button. Then set the other circled settings in the resulting 'make a measurement' popup. Then select 'Start Measurement'. REW will start the measurement, but will then pause waiting on you to play the sweep file. When REW hears the sweep it will finish taking the measurement.


John, thank you for breaking it down for me. I did exactly as you said and configured as in your screenshot. However, when I hit measure, the sweep plays through my laptop speakers and does not wait for me to play the sweep as you describe. Is it possible that in the latest version I have installed this is no longer the case? (the waiting does not happen)


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## kalohts (Jan 16, 2019)

Hi guys,

I am super excited as I managed to do my first hopefully proper measurement using the Pink PN + RTA method.

Here is what I did:

1. Used a Pink noise to set the volume of my AVR until the SPL showed 75 DB (that was 65 on the Master Volume for me)

2. Generated and saved a PINK PN sample 20Hz - 20k Hz -12 DB, Stereo, 24-bit

3. Ran a measurement with RTA with the following parameters (thanks John!)
- RTA 1/48 octave
- FFT - 65536
- Max Overlap 93.75
- Adjust RTA Levels


Here are the results with the 1/48 smoothing and the 1/3 smoothing. What do you think?


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## jtalden (Mar 12, 2009)

kalohts said:


> John, thank you for breaking it down for me. I did exactly as you said and configured as in your screenshot. However, when I hit measure, the sweep plays through my laptop speakers and does not wait for me to play the sweep as you describe. Is it possible that in the latest version I have installed this is no longer the case? (the waiting does not happen)


I can't provide a specific answer as I do not know your entire equipment setup and capabilities.

Basically, use any method you have available that prevents the REW output signal from going out through the system; route the REW output to an unused channel, unplug the REW output wire, mute the REW output channel, etc. 

If the output of REW is shared with the music player on the same computer and you have no other direct way to mute the REW output maybe try setting the REW output volume initially to minimum (-90 dB). REW not be able to detect the generated signal and will have to wait until the file is played. You can then play the file with the music player using the appropriate volume. My setup does not have this problem so I have not tried this. 

I just set the input mode of my AVR to my music player rather than the input that is connected to REW.


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## kalohts (Jan 16, 2019)

THANK YOU!

I finally figured it out how REW waits for the timing reference - very smart! I basically turned all the way down my laptop volume, so as REW starts it does not hear any sound coming out. Then I play the file from my AVR.

So here are my results. I used the measuring sweep in the Measure tool.

I do not understand much (yet), but I think on my stereo measurement my speakers output is generally flat up until 500Hz. Interesting is that my speakers measure quite differently... which I sort of expect. My left speaker is next to an open door - so an open space. The right speaker is limited to the right wall of my room which is about 9 feet away from it and probably some other living room stuff.

What do you guys think about the graphs? I have Graphic EQ on my AVR with 9 bands (63 Hz / 125 Hz / 250 Hz / 500 Hz / 1 kHz / 2 kHz / 4 kHz / 8 kHz / 16 kHz) but I am hesitant to tamper with them as I know that's totally different than Parametric EQ.


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## jtalden (Mar 12, 2009)

That looks very good to me.
The individual speakers 500-20k response looks good. It is normally not recommended to make any adjustments there based on LP measurements anyway. The room influence changes the relationship of the SPL vs our perception of the sound. Our perception is more based on the direct sound portion of the total SPL measured. Both channels measured together is completely unusable as comb filtering is an additional big problem that impacts the upper end of the SPL response. It is responsible for the large sag centered on 7kHz on the L+R measurement.

Any EQ you want to try is best limited to the 20-200 Hz range. That should be viewed using no smoothing. You can used 1/24 octave smoothing to help decide the 1/3 octave GEQ setting, but then switch back to no smoothing to confirm the improvement. There is usually little that can help as it takes some luck for the GEQ to be able to help with the problem. If you can reduce the 65 Hz peak without causing other issues then that would help a little. 

Basically is very good as is. The L+R 20-200 Hz measurement is ideal and that may be better than with any adjustment to the 65 Hz individual peaks. It depends is the bass of a particular recording is mono or in just one channel.

Just the thoughts of this hobbyist.


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