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What can you NOT due with a USB measurement microphone + other tools?

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  • What can you NOT due with a USB measurement microphone + other tools?

    Some recent threads on measurements and laptops got me thinking...what can you not due with a USB microphone, supplemented with other tools? I understand that there are limitations on measuring phase and acoustic offsets directly with a USB mic, but it seems there are (possibly time-consuming) tools that allow the user to overcome this. Thus, is it just that it is easier to use a non-USB (I'm not sure the correct terminology for non-USB mics) or are there things that simply cannot be achieved with a USB mic even if the user is willing to take the extra steps and time necessary?

  • #2
    Originally posted by AEIOU
    What can you not DO ? Well, are you past due? Due to the economy we are cutting back. Due to the Pandemic, we are staying home.
    Yes, I saw the typo as soon as I posted it. Unlike a post, which I can edit, I can't edit the thread title.

    BUT, I'm pretty sure you are one of the forum participants who actually can contribute to the intended question. I have a Umik USB microphone and am willing to change of it is really a significant improvement.

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    • #3
      You give me more credit than due, but I'll get back to you in due time. (Please no hard feelings due to my replies)

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      • #4
        How significant, I do not know. I can only speak from the technology viewpoint.

        Basic problem is the clock that sets start of frame if the in and out converters are not run from the same clock. A USB mic has it's own clock, sound card another. ARTA, HOLM, REW etc all specify a synchronous clock. Just visualizing, the error would increase with frequency, but one would have to dig into MLS and FFT theory a lot deeper. The truth may be "it depends" A sub woofer may be fine, a tweeter may not.

        So, if the clock is not in sync, then phase measurement will be offset by some unknown amount and may drift. I know of no way externally to fix it.

        In the pro sound recording world, they go as far as having a master clock for all the converters so the ambient sounds are in sync with the main sounds.

        I have a USB mic that came with my Anthem AVR. If I get bored, maybe I'll test it against my Berhringer/Focusrite. I would like to do a basic freq cal for it ( their files are encoded proprietary and they won't share it) . It would be handy for quick sweeps and finding room resonances as I would not have to drag out as much "stuff"

        This just brought up a thought. I think my cal file is frequency only. It costs more to get phase. If I remember, it was suggested capsule mics phase does not change much, but this needs to be investigated. THat ay explain why I keep having to deal with larger than expected ripple @ crossover.

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        • #5
          What tvrgeek said. Here's an except from the REW help file:
          • If the input and output are on the same device and so share a common clock longer sweeps will provide higher signal-to-noise ratio (S/N) in the measurements. Long sweeps may be problematic when the input and output are on different devices, such as when using a USB mic, as their sample clock rates will differ. Over a long sweep a significant difference in clock rates could cause severe distortions in the shape of the impulse response and affect the phase response. This can be corrected by using the acoustic timing reference and the Analysis option to Adjust clock with acoustic ref or, if a loopback connection is being used as a timing reference, the Analysis option to Adjust clock with loopback.
          "Everything is nothing without a high sound quality." (Sure Electronics)

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          • #6
            If you have Omnimic, you can test the amount of phase drift on your system by actually watching the phase drift up and down in real time as you test a tweeter.

            I use OmniMic with an old HP laptop (Win7, i3, quad core, 4GB ram) , a Sony CD player, and a 100wpc non-digital KLH amplifier. I generally use track 2 sine sweep, blended mode, 5ms gate, 1/48th smoothing, with the phase curve turned on. As I run the continuous sine sweep, the phase curve will incrementally drift up a few degrees on each sweep, but only at the higher frequencies. The system automatically resets the phase curve back to the starting point after every 7 or 8 sweep cycles. It is a repeating pattern that is cool to watch. Drift, drift, drift, drift, drift, drift, drift, RESET, drift, drift, drift, drift, drift, drift, drift, RESET, etc., etc.,

            This drifting action is only at the higher frequencies, from about 5kHz up. The rest of the phase curve is stable (no drift). Total drift over the 7 or 8 repeating cycles is about 10 degrees at 5kHz and about 20 degrees at 20kHz. So, since the error is repeating, I can eliminate it by always hitting OmniMic's pause button at the 3rd or 4th cycle for each driver. This is no big deal, however, as this small amount of phase error does not seem to affect the accuracy of my models.

            SideTowers: http://techtalk.parts-express.com/fo...corundum-build
            Totally Flat: http://techtalk.parts-express.com/fo...5-totally-flat
            Plumber's Delight: http://techtalk.parts-express.com/fo...notech-winners
            Linehopper: http://techtalk.parts-express.com/sh...Esoteric-build

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            • #7
              Originally posted by 4thtry View Post
              If you have Omnimic, you can test the amount of phase drift on your system by actually watching the phase drift up and down in real time as you test a tweeter.
              ...
              This drifting action is only at the higher frequencies, from about 5kHz up. The rest of the phase curve is stable (no drift). Total drift over the 7 or 8 repeating cycles is about 10 degrees at 5kHz and about 20 degrees at 20kHz.
              ^^^ If I'm understanding this right, it sounds like measurements even with the drifting phase uncorrected should be good enough for data under 5K and crossover work below that should be able to use those measurement results.?

              Should this kind of behavior be pretty consistent between different USB measuring microphones, or will different USB mics have vastly different drift amounts and frequency/beginning (knee point?)?

              EDIT: I suppose even IF ignoring impulse and phase over 5K can otherwise work perfectly fine (which I don't know at all), it'll still be important to have the proper impulse response for the tweeter all the way up to know how the HighPass will tune its FR.
              My first 2way build

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