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Increase in UMIK-1/UMIK-2 2nd harmonic distortion at increased SPL levels? 3 months 2 weeks ago #55845

  • mtg90
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A number of moths back I noticed some odd behavior from my UMIK-1. I was taking some distortion readings of a speaker using REW and noticed that the 2nd harmonic distortion increased or decreased depending on the microphone placement (nearer or further from the speaker) while the output level of the speaker remained constant.  I thought this was odd but had forgotten about it until more recently as I was starting a large series of comparison tests or various compression drivers and waveguides and realized it was doing the same thing.  Thinking my mic must have been damaged at some point (my mic stand has been knocked over a few times over the years) I purchased another UMIK-1 from Cross Spectrum Labs as well as a UMIK-2 here.

To my disappointment both new UMIK's also show the same exact issue, increasing 2nd harmonic distortion levels as the measured SPL increases (it reaches ~1% at 105dB, ~2% at 114dB). No issue with other the other harmonics. I tried playing with the gain setting on the mics with no change in the distortion behavior.  I have done the same tests with my Dayton UMM-6's and they do not seem to exhibit this behavior of increasing 2nd harmonic distortion at higher SPL levels at the mic.

So I am wondering if anyone knows what's causing this issue?  I have a feeling it's an issue with the hardware of the UMIK's themselves either the mic capsule or the preamp.

Here are some measurements I have taken comparing one of the UMIK-1's to my UMM-6:

Using the Celstion CDX1-1731 in a waveguide (No crossover, only an L-pad to drop noise level hiss from the amp) and taking measurements at 40" - 20" - 10" - 5" (roughly 1m, 50cm, 25cm, 12.5cm). Note the speaker output level remains constant only the microphone position is changing.

SPL/response measurements from the two mics at these four positions:


UMIK-1 2nd harmonic distortion overlay.
Note the rise in distortion as SPL increases at the closer distances. The microphone hits 1% distortion at just over 105dB:


Now the UMM-6 2nd harmonic distortion overlay.
Only a very minor increase seen at the closer distances (high SPL) between 1-3khz, not anywhere near as bad as the UMIK-1.  Note the distortion is lower then the UMIK-1 even at the furthest distance (90-95dB level) which means the UMIK distortion readings are inaccurate/elevated even at that level:


 

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Last edit: by mtg90. Reason: typo

Increase in UMIK-1/UMIK-2 2nd harmonic distortion at increased SPL levels? 3 months 2 weeks ago #55847

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 Note the speaker output level remains constant only the microphone position is changing.
 

I'm no expert but surely the second harmonic levels may vary with position depending on the cause of the distortion and its radiation pattern. This doesn't explain the different results for your other microphone though. I take it they were positioned identically for the comparison?

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Increase in UMIK-1/UMIK-2 2nd harmonic distortion at increased SPL levels? 3 months 2 weeks ago #55851

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They can vary a little with regards to the mic placement but that's certainly not the issue I'm experiencing here.  But yes the two mics were positioned identically for the four distances.

There are a few other measurements posted here:
www.avsforum.com/threads/the-great-waveg...age-12#post-60626209

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