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The moving microphone method (MMM) for Dummies using REW

RandomEar

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After getting a couple of weird results on my first MMM measurements, I went searching around the forums & YouTube for what went wrong. To save others from gathering various bits and pieces of info from a dozen sources, I decided to publish this quick write-up here. You can use it as a rough guide and adjust the settings to your liking.

Prepare your measurement
  1. Make sure your volume is at a comfortable level for music listening (to avoid a jump scare later on)
  2. Consider hearing protection - things might accidentally get VERY LOUD if certain settings are wonky
  3. Put your measurement microphone on a monopod or selfie stick, preferrably using a shock / anti-vibration mount
  4. Attach your mic to the PC using a sufficiently long cable
  5. Open REW and configure it to use the correct mic and calibration file (for UMIK's, use the "_90°"-file)
  6. Open the SPL meter using the default settings (dB(Z) S)
    MM Settings_SPL meter.PNG
  7. Open the Generator and configure it as follows:
    MM Settings_Generator.PNG
    • Mode "Noise"
    • Type "Pink Periodic"
    • Target "Full range"
    • Sequence length "64k" (default)
    • Level -12 dBFS
  8. Open the RTA window and configure the graph controls (gear symbol in the upper right) as follows:
    MM Settings_RTA settings.png
    • Mode "RTA 1/48 Octave"
    • Smoothing "1/48 smoothing"
    • FFT length "64k" (default)
    • Averages "Forever"
    • Stop at "30" to "60" (depending on your patience)
    • Window "Hann" (default)
    • Max Overlap "50" (default) to "75" (slightly quicker)
    • Enable "Generator changes restart capture"
    • Click on Appearance and adjust:
      • Disable "Use bars on RTA" (optional)
      • Enable "Adjust RTA levels"
  9. Adjust your SPL:
    • Bring yourself and the mic in position, holding the mic vertical (pointing towards the ceiling)
    • Start the Generator
    • Adjust the volume on your system until the SPL meter shows roughly 80 - 85 dB(Z)
    • Turn off the Generator

You are now ready to measure. Now is a good time to deactivate all EQs and filters you don't want in your measurement, if you didn't get to that before.

Perform the measurement
  • Bring yourself and the mic in position, holding the mic vertical (pointing towards the ceiling)
  • Start the Generator
  • Start the measurement by clicking on the square [record] button in the upper right of the RTA window
  • Slowly move the mic around your measurement position randomly
  • Keep an eye on the "## averages" label in the lower left of the RTA windo. When the number of averages reaches your self-imposed limit, the measurement will stop automatically
  • Turn off the Generator. Your RTA window should now look something like this:
    MM Settings_RTA result.PNG

  • Copy the finished measurement to the main REW window by clicking on the "Current" button in the top center of the RTA window
  • Optional: Save the measurement in the main window to avoid losing the data later on

Congrats, you measured something using the MM method!

From this point on, the next steps depend on your goals and hardware. To create an EQ for your setup, you would go into the "EQ" window, configure your "Target Settings" and then run "Match response to target" in the "Filter Tasks" tab. The results can then be exported into different formats - "Export filter settings as text" for example results in a couple of lines which can be directly copied to an Equalizer APO config file.


Further reading:
 

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Keith_W

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Great thread! Thank you.

BTW, I should mention from my own bitter experience that you should be careful when moving the microphone because it picks up noise. This is the effect of moving the microphone slowly (red) vs. whooshing it around quickly (green), with no sound coming from the speakers:

index.php


The result is not insignificant, it can really mess up the apparent bass coming from your speaker:

image.png.88302c9b8a4ee7c8393a91282caeac0c.png


(Actual MMM from the MLP with pink noise running). After changing my technique and moving the mic verrrrry slowly, the rising bass response has gone away.
 

poxymoron

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Great thread! Thank you.

BTW, I should mention from my own bitter experience that you should be careful when moving the microphone because it picks up noise. This is the effect of moving the microphone slowly (red) vs. whooshing it around quickly (green), with no sound coming from the speakers:

index.php


The result is not insignificant, it can really mess up the apparent bass coming from your speaker:

image.png.88302c9b8a4ee7c8393a91282caeac0c.png


(Actual MMM from the MLP with pink noise running). After changing my technique and moving the mic verrrrry slowly, the rising bass response has gone away.
Was this done with a microphone windshield? Just wondering how effective windshields are with MMM. Thanks.
 

theREALdotnet

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Is there a particular reason to use Hann windowing during capture? I thought the main benefit of using pink periodic noise is that one can use a rectangular window (same sequence length) and avoid the approximation?
 

Keith_W

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Was this done with a microphone windshield? Just wondering how effective windshields are with MMM. Thanks.

If you are talking about those fluffy things that looks like the bearskin fur hat of a Buckingham Palace Royal Guard ... I didn't use one. I have a much thinner foam cover which I don't use.
 

JohnPM

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Is there a particular reason to use Hann windowing during capture? I thought the main benefit of using pink periodic noise is that one can use a rectangular window (same sequence length) and avoid the approximation?
Rectangular can be used when input and output share the same clock. If they don't (if they are different devices) a window is required as there will be small differences in clock rates. Hann at 75% overlap with at least 4 averages or other windows at 87.5% overlap with at least 8 averages, for example.
 
OP
RandomEar

RandomEar

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Was this done with a microphone windshield? Just wondering how effective windshields are with MMM. Thanks.
I used the stock foam tip supplied by miniDSP for the example measurement in my post. I would assume that this is more than enough for the typical use case (indoor measurement, no wind, fans or AC blowing directly onto the mic). As pointed out in the instructions and by @Keith_W, it's important to move the mic slowly. This also avoids excessive handling noise being injected into the measurement.
 

Tom C

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What’s the advantage of using MMM? Is it just time savings? If so, I think I’ll just stick to 8 or so discrete points.
 

Blumlein 88

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What’s the advantage of using MMM? Is it just time savings? If so, I think I’ll just stick to 8 or so discrete points.
Yes ease of use and time saving with pretty much identical results to doing 8 discrete measurements. Plus much easier and quicker to do averaging over different sized areas if you wish.
 

theREALdotnet

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Rectangular can be used when input and output share the same clock.

Thanks John, I suppose that’s the case when using the REW signal generator and a USB microphone with REW?
 

Draki

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Are you measuring both channels playing at the same time?
The "play" output setting on the generator screen you attached are set to "L+R".
I would think channels are measured separately?
 
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RandomEar

RandomEar

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Are you measuring both channels playing at the same time?
The "play" output setting on the generator screen you attached are set to "L+R".
I would think channels are measured separately?
For the example above, both channels were measured simultaneously. Of course you're free to measure them one by one. I'm pretty sure that there are arguments for and against both methods and it also depends on what you actually want to measure.
 
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Fred H

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I saved my test output to files (48kHz, 440 repetitions, 32-bit Float => 600.747 seconds) for mono, left-only, and right-only. This avoids attaching my laptop to my Flex to take a reading.
 

Mister.Mole

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Thanks John, I suppose that’s the case when using the REW signal generator and a USB microphone with REW?
No, this is one of the cases where input and output do not share the same clock.
A USB microphone brings it's own clock, the other one is in your soundcard.
 

theREALdotnet

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Hm, this isn’t in the manual. It only says that the sequence length between generator and RTA must match, then windowing and averaging are not required with pink periodic noise.

Periodic Noise (PN) sequences are ideally suited for use with spectrum and real time analysers (RTA's). They contain every frequency the analyser can resolve in a sequence length that matches the length of the analyser's FFT. Their great benefit is that they produce the desired spectrum shape without requiring any averaging or windowing

If this also requires synchronised clocks between DAC and ADC, and that isn’t usually the case with a USB DAC and a UMIK-1, then this caveat should be made clear.
 

JohnPM

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Hm, this isn’t in the manual.
Read further than the first few lines, the help shows settings to use including:
example RTA settings for a 64k FFT length for use when the input and output are on different devices. Windowing and specific averaging and overlap settings are required to counter the effects of any clock rate difference
 
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