Help with REW RTA measurement on generated sweep - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 14 Old 01-16-19, 07:42 AM Thread Starter
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Question Help with REW RTA measurement on generated sweep

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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Help with REW RTA measurement on generated sweep-mv-60-sub-8.5-xover-80hz-meas-sweep-12db-1600.jpg  

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post #2 of 14 Old 01-16-19, 09:36 AM
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Re: Help with REW RTA measurement on generated sweep

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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post #3 of 14 Old 01-16-19, 10:07 AM Thread Starter
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Re: Help with REW RTA measurement on generated sweep

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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post #4 of 14 Old 01-16-19, 10:51 AM
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Re: Help with REW RTA measurement on generated sweep

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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post #5 of 14 Old 01-16-19, 11:00 AM Thread Starter
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Re: Help with REW RTA measurement on generated sweep

Quote:
jtalden wrote: View Post
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.
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"?

Last edited by kalohts; 01-16-19 at 11:07 AM.
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post #6 of 14 Old 01-16-19, 11:35 AM
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Re: Help with REW RTA measurement on generated sweep

Quote:
kalohts wrote: View Post
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.

Quote:
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.


Quote:

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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post #7 of 14 Old 01-16-19, 11:43 AM
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Re: Help with REW RTA measurement on generated sweep

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:

Name:  rtapnsettings.jpg
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post #8 of 14 Old 01-16-19, 12:25 PM Thread Starter
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Re: Help with REW RTA measurement on generated sweep

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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post #9 of 14 Old 01-16-19, 02:04 PM
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Re: Help with REW RTA measurement on generated sweep

Quote:
kalohts wrote: View Post
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.

Help with REW RTA measurement on generated sweep-capture.jpg
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post #10 of 14 Old 01-17-19, 02:24 PM Thread Starter
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Re: Help with REW RTA measurement on generated sweep

Quote:
jtalden wrote: View Post
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)

Last edited by kalohts; 01-17-19 at 02:32 PM.
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