iMM-6 and REW readings

Discussion in 'Official REW (Room EQ Wizard) Support Forum' started by Alex Smith, Feb 11, 2019.

  1. Alex Smith

    Alex Smith New Member
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    Hello everybody,

    I purchased an iMM-6 mic and was using REW or ARTA proggs but didn't get a line/curve in a graph for IEMs I was expecting. Instead I got something fur-tree like.

    On one PC with built-in sound card I got the graph like in pic.1.
    On another PC I had something with a lot of noise. See pic.2

    What is the correct way of measuring FR for IEMs?
     

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    #1 Alex Smith, Feb 11, 2019
    Last edited: Feb 11, 2019
  2. Matthew J Poes

    Matthew J Poes Staff Writer
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    I’m not totally sure what you are asking or what this is showing. The second graph with “noise” simply looks like an unsmoothed graph. Try adding smoothing.
     
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  3. Wayne A. Pflughaupt

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    You can’t get a usable graph if it has a 200-300 dB + vertical axis span. Re-adjust the axis for 45-105 dB.

    Regards,
    Wayne
     
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  4. Matthew J Poes

    Matthew J Poes Staff Writer
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    Oh wow I didn’t even notice that. That’s got to be the widest scale I’ve ever seen. Good catch.
     
  5. Alex Smith

    Alex Smith New Member
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    Thank you for fast responses.
    I am absolutely new in this field.
    Could anybody explain how I can get "smoothing" and re-adjust the axis for 45-105 dB?
    Sorry for asking dumb questions.
    The idea is to compare frequency responses for different In-Ear-Monitors.
     
    #5 Alex Smith, Feb 11, 2019
    Last edited: Feb 11, 2019
  6. Matthew J Poes

    Matthew J Poes Staff Writer
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    You might want to look into how in ear monitors need to be measured to get accurate results. You can’t just stick a mic at the tip of the monitor and measure. I have the MiniDSP EARS and had worked with folks from large labs as well as fellow reviewers with proper gear to validate my measurements. My IEM measurements in particular were inaccurate. The problem goes beyond the typical response errors that we normally think of. Headphones and especially IEM’s interact with the head by setting off resonances, reflecting down an ear canal, and creating a seal in the ear canal. All of these impacts a bunch of response issues. Headphones always have a spike in the response and one of the things my EARS rig gets wrong is where that rise in the high frequencies is located. It’s not just the wrong level, which is correctable, it’s in the wrong place. It can’t be corrected and as such gives you an inaccurate sense of the response.

    I’ve been playing around with solutions, but what I’ve learned is that you need an accurate ear simulator. For IRM’s you don’t need an accurate Pinna model, but you do need an accurate ear canal and head resonator chambers. Without that the response will be inaccauraye and uncorrectable. At this point I’m hoping to find a used ear simulator on eBay, because I haven’t found a cheap DIY solution.
     
  7. Alex Smith

    Alex Smith New Member
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    I use a shell. I put IEMs to one side of the shell and the mic to the opposite side. It's almost air tight. It's similar to this
    Is there a way to measure FR for headphones?
     
    #7 Alex Smith, Feb 11, 2019
    Last edited: Feb 11, 2019
  8. Matthew J Poes

    Matthew J Poes Staff Writer
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    But is it an accurate simulation of the ear canal and head resonances?
     
  9. Alex Smith

    Alex Smith New Member
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    No, it is not precisely accurate. But at least something. How do the manufacturers make the FR graphs of IEMs they sell?
     
  10. Matthew J Poes

    Matthew J Poes Staff Writer
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    This is actually getting at the issue I was trying to explain, and what has been a big lesson for me. When you say, “at least it’s soemthjng.” You are really getting at the common idea that it may not be exactly right but you can at least compare. That implies that the measurement is consistently in error. But it’s not and that’s the problem.

    Imagine you want to track how fast a car is using a radar gun. But imagine that depending on the shape of the car you get a different reading. So the gun gives an erroneous reading that appears randomly wrong. Maybe you find out, as I’ve said here, that the shape is what causes the error, because cars are complex shapes, it’s not possible to correct for the shape error. That is what’s going on here. Because of the way the IEM interacts with the head and ear canal, there is no way to know ahead of time what kind of error it will get when you stick it into a non-accurate ear. So you can’t correct it.

    Brent Butterworth has done a good job addressing the issues and his article is a good read. He also shows measurements compared to his rig which is basically a bare minimum of what is accurate.
    https://www.soundstagesolo.com/inde...-death-of-headphone-measurement-or-its-savior

    He uses this:
    https://www.gras.dk/products/ear-simulator-kit/product/737-43ag

    And the top labs would actually use this:
    https://www.gras.dk/products/head-torso-simulators-kemar

    https://www.gras.dk/products/product/722-43BB
    This is the basic coupler you need to simulate and ear canal. That metal coupler is deceptive. It’s not just a metal tube. It’s dimensions are precise and inside it has a labeyth like chamber that creates resonances that match those or your ear, skull, cheek, etc. It doesn’t create chest resonances which are what he torso simulator adds and that primarily impacts lower frequencies.

    So you can do what you are trying to do, but it’s important to know that the results will have a number of errors that can make even comparing the headphones to each other a problem. Depending on how they react to the non-standard loading. Worse yet, you really can’t use them to judge good or bad because you can’t actually see what is going on when they are loaded in your ear.

    Sadly the price of entry is $5000 and it just goes up from there.
     
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  11. Alex Smith

    Alex Smith New Member
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    Quite dis-encouraging but OK. Then another question coming to my mind. What is the use of iMM-6?
     
  12. Matthew J Poes

    Matthew J Poes Staff Writer
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    I understand Alex. It is discouraging to me as well. I am actively looking for inexpensive DIY solutions and will validate the accuracy of my method. If any of these ideas work out, I'll happily share publicly so you can do the same.

    The iMM-6 is for acoustic measurement with a smart phone or tablet. It can be used with a laptop as well, but a UMM-6 is a better option for that. I have an iMM-6 and I use it when I need a somewhat accurate measurement in a room and I need high portability. Maybe I'm trying to assess the acoustics of an office and I am just running to check out the job before making a bid. Maybe I want to check SPL levels at a concert or event. Something like that. It really isn't designed for measuring headphones, though really there is no reason why it can't be used for that. The problem isn't the mic itself, its the pseudo-ear.
     
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  13. Alex Smith

    Alex Smith New Member
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    Thanks, Matthew, I really appreciate your desire to help me out.
    Could you explain how I can make the graph smooth and re-adjust the axis for 45-105 dB with REW?
     
    #13 Alex Smith, Feb 12, 2019
    Last edited: Feb 12, 2019
  14. John Mulcahy

    John Mulcahy REW Author

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    This button lets you set the graph range. Smoothing is applied via the Graph menu or the associated shortcut keys.
     
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  15. Matthew J Poes

    Matthew J Poes Staff Writer
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    I’m working on a short how to video for you. Just give me a bit to post it.
     
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  16. Matthew J Poes

    Matthew J Poes Staff Writer
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    And here is my hands on video walking you through it!
     
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  17. Alex Smith

    Alex Smith New Member
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    Wow, you've really been so cooperative. I appreciate that so much. :jump:
     
  18. Matthew J Poes

    Matthew J Poes Staff Writer
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    No problem. I won’t promise to make you a video every time you have a question, but we all always try to help out.

    In this case I’ve been working on videos like this and your question was the kind that I suspected would be addressed best by a video. I plan to do more of these five minute videos around REW to help orient people with very specific questions.

    Maybe i should call them 5 minutes with Matt!
     
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  19. Wayne A. Pflughaupt

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    I like that! :)

    Regards,
    Wayne
     
  20. Alex Smith

    Alex Smith New Member
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    You are an angel! :yes:
    Matthew, do you mind if I share the link to this video you kindly made to two Russian audio forums? Most of the users there are good at English and I don't think there will be a problem comprehending the video.
     
  21. Matthew J Poes

    Matthew J Poes Staff Writer
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    Absolutely. Spread the word. We posted it to our front page. If these guys want more tell them to come join our forum. I plan to keep making more of these (and I’m open to topic ideas).
     
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  22. Alex Smith

    Alex Smith New Member
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    Thanks a lot. I'll invite them here.
     

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