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  • Using Omnimic in ARTA

    UPDATED, See last comment from me...
    If you have Omnimic and want to use it in ARTA you can and it appears to be accurate but you must do a few things to properly calibrate it.

    Using mic sensitivity provided by Omnimic:
    1. Open the ####.omm calibration file that you downloaded with your mic, find the line that says "sens=-xx.xx" at the very top and write down the sens number. It will be negative, mine is -7.732 db. Make sure you enter all 4 numbers, don't round this down!
    2. Go to -->this web page<-- and enter the sens number into the box that says "Sensitivity dB re 1 V/Pa ", press the "calculate" button and write down the Transfer Factor value it spits out.
    3. Open ARTA->Setup->Audio Devices and set preamp gain for your mic channel to "0.135", enter the "Transfer Factor" for your mic into the box where it says "sensitivity (mv/PA)", (Mine is 410.582) and hit OK.
    4. Now, open the ####.omm file and remove the first line that says "Sens Factor =-X.XXXXdB, SERNO: XXXXXXX" from the file and save is as #####.mic where you can find it easily from ARTA. It will probably work OK with the line in there but better safe than sorry.
    5. Under Setup-->FR compensation load the .mic file you just saved and click "use frequency response compensation".
    6. Last run a sweep with both Omni and ARTA and double check the calibration is correct. Check more than just one frequency to be sure and make sure you are using the correct scale.


    Alternate Calibration:
    As an alternate you can use a speaker and the mic and the "estimate mic sensitivity" procedure in ARTA calibration. Both give reasonable results. I set up a test speaker at 1Khz (using LIMP->Setup->Generator set to 1000 Hz for the tone). Adjust volume till SPL reading is above 80 and constant under Omnimic, switch to ARTA, enter the DB value from Omnimic for SPL and press the "estimate mic sensitivity" button making sure the tone is still running. With mic channel gain set to 1 I get 159.4 over several measurements repeated and the SPL peak meter in both programs agrees within .1 db or so. This is essentially what a mic calibrator does so it should be quite accurate, at least as accurate as the original Omnimic.

    For best results the test tone should be 1000 Hz set to exactly 94.0 db and the mic should be as close to the speaker as is reasonable for the calibration since that's the SPL and frequency at which most microphones are calibrated. A mic calibrator uses an acoustic coupler (a sleeve) around the mic to remove external noise and couple the mic to the transducer. Anything you can do to duplicate this will probably help accuracy. This entire procedure assumes that the Omnimic is properly calibrated to begin with so any error in the Omnimic sensitivity will be magnified in ARTA. For this reason it's not a good idea to use the ARTA calibration to calibrate other devices.

    Checking your calibration(s):
    If you save a gated .FRD of your test sweep (gated to reduce LF drift) in Omni, rename it from .FRD to .MIC and load it as a mic compensation file in ARTA you can see just the difference minus microphone calibration. It should be very flat. Mine is very close using either procedure.

    Feedback welcome
    Attached Files
    Last edited by siggma; 12-19-2012, 02:54 PM.
    Tom

    NEW CNC Site http://cnc.trbailey.net/
    My Projects BLOG http://speakers.trbailey.net/

  • #2
    Re: Using Omnimic in ARTA

    How lame is this, replying to my own post but I really want to be able to take measurements using the Omnimic in ARTA.

    After reading a bit more about microphones I think the correct way to use the Omnimic calibration in ARTA is to just convert the db voltage in the .omm calibration file that came with the mic to mv/pa (Web Page Here) and use it with a gain value of 1.

    While the sensitivity of the mic reported in the calibration file undoubtedly includes the internal mic gain, unless the guys at Dayton Audio want to post a correct microphone gain value (they may not know what it is...??) I'm shooting in the dark tinkering with the gain value in ARTA. I think I'll just have to live with a little bit of error or just use the estimate from the ARTA calibration page. As for using the Omnimic renamed to .mic as a frequency compensation it "should" work just the way it is. However, other mic calibration files I've see set the phase field to 0. I suspect ARTA doesn't use the phase data anyway but it's probably a good idea to zero out the last field just to be sure. This also assumes the values in the calibration are db SPL values and not db volts or mv/pa volts. Hopefully they are SPL values. Note below that phase does not change regardless of frequency compensation file.

    Maybe one of the Dayton Audio techs can verify the meaning of the .omm file fields, specifically is the second field value DBSPL, MV/PA or DBV?
    ... answering my own question they can't be dbv or they would all be negative values with high precision.

    I tried the compensation with phase values intact and setting all phase numbers to 0. No phase difference shows up but the calibration clearly does change the frequency response by the presumed SPL listed in the calibration. To test if it's correct I'll have to put my thinking cap on and get back to you. If the calibration values specified are in SPL it will produce a correct frequency response compared to other microphones (properly calibrated of course). I have a Dayton Audio EMM6 mic I can use to compare which I'll do later and post the results.

    In the mean time, if the noted sensitivity is -7.*** which is impossibly high for any microphone, it has to include a preamp gain. Assuming the mic itself has a sensitivity of about 10 mv/pa (-40 db volts) which is typical for measuring microphones, the internal preamp gain is about 40 or so. That seems a little high to me but it's not outside the normal preamp gain range. I'll try comparing it with my EMM6 and see what I come up with. But, following logic, if the reported sensitivity includes preamp gain the correct gain value in ARTA is "1", which means "no increase" since gain is a multiplier. I'll have to investigate why I'm getting different SPL values between the two programs. The most likely reason is the output volume between the two programs is not consistent combined with the different stimulus each program uses. They may not have the same overall power curve.

    Thumbs:
    First is Sound Card Loopback impulse response gated at 10 ms. Note scale is 1 db per vertical division
    Second is first sweep as overlay and second sweep with mic calibration enabled using same gated response, same scale.
    Third is separate sweep with compensation enabled and phase field changed to 0 in calibration file, same scale.
    Attached Files
    Tom

    NEW CNC Site http://cnc.trbailey.net/
    My Projects BLOG http://speakers.trbailey.net/

    Comment


    • #3
      Re: Using Omnimic in ARTA

      Even more lame but I'm making more progress and thought I'd share it with anyone interested.

      The Omnimic has a gain control available under it's software setting that I didn't see before. The omni software sets it between 5 (lowest slider position) and 94 (highest slider position). The mic is overloaded (clipping the input) at about 92 db SPL with the sensitivity slider at 100% under the windows recording properties for the Omnimic. I also noticed it has two separate output levels, one on LEFT channel and one on RIGHT channel that are approximately 10 db apart with the largest output being on LEFT channel. I tried a new calibration by setting the gain to max under it's software control, applying a 90.2 db 1Khz signal with a speaker very close to the mic and estimating its output. Theoretically as long as I'm using the LEFT channel for measurements and I set the gain to max before taking any readings my measurements should be correct. I'll post if I find out differently, otherwise I think I've found a way to use the Omni under both ARTA and STEPS since both use the same calibration. STEPS seems to be a better way to take measurements for crossover design as I can sweep it at 1/6th octave then apply 1/3 octave smoothing and get a much smoother FR to work with under PCD. I still don't understand all the ins and outs of STEPS but that's a separate thread.
      Tom

      NEW CNC Site http://cnc.trbailey.net/
      My Projects BLOG http://speakers.trbailey.net/

      Comment


      • #4
        Re: Using Omnimic in ARTA

        Hi,
        I also own the Dayton Omnimic V2 and would like to use it in ARTA.
        In conclusion, if I understand correctly (due to my poor english...), I just have to calibrate the microphone in ARTA following 6 steps mentioned in your first post.
        It's right?

        Thank you.
        Best regards.

        Comment


        • #5
          Re: Using Omnimic in ARTA

          Originally posted by d888sp4 View Post
          Hi,
          I also own the Dayton Omnimic V2 and would like to use it in ARTA.
          In conclusion, if I understand correctly (due to my poor english...), I just have to calibrate the microphone in ARTA following 6 steps mentioned in your first post.
          It's right?

          Thank you.
          Best regards.
          Well, after fiddling with it for a while if you aren't concerned with absolute SPL you can just set the amp gain to 1, use the LEFT channel input and load the calibration file that came with it. Set the sensitivity to 7 or so and it works OK. You'll have to copy the .emm file and rename it to .cal. It works fine but does not display SPL correctly. To calibrate it for SPL you'll need a mic calibrator. I found a used on on eBay for $50 that works fine. To calibrate set the gain to 1, attach the calibrator and let it calculate the sensitivity. I don't recall what it came up with but I think its close to 8.

          Hope this helps
          Tom

          NEW CNC Site http://cnc.trbailey.net/
          My Projects BLOG http://speakers.trbailey.net/

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