Measurement Mic Tests
When I was restarting the great waveguide shootout in early 2021 I noticed odd behavior in the distortion measurements of the compression drivers. Specifically I wasn't seeing the amount variation in the 2nd order harmonic distortion from the different compression drivers or horns that I was expecting. Upon further investigating I came to the realization that the UMIK-1 itself was distorting at the SPL levels being used and masking the real 2nd order harmonic from the drivers being tested.
Thinking my UMIK-1 had been damaged I picked up another from CSL along with the new at the time UMIK-2 from MiniDSP however both showed similar issues when tested in the same way (rising self distortion above ~90dB) which leads me to believe the capsule itself is distorting . Finally I caved and purchased an Earthworks M23 and audio interface which thankfully is distortion free well beyond the levels where it becomes an issue on the miniDSP mics.
This got me curious to test/measure all my measurement microphones to look for this distortion issue and have a look at the frequency response accuracy between the mics at the same time.
Thus the following is a look at the relative distortion performance and quick frequency response comparison above 1kHz.
I may add low frequency testing of distortion and response accuracy <1000hz in the future.
List of Mics being tested:
2x Dayton UMM-6
2x Behringer ECM8000
2x MiniDSP UMIK-1 (older micro USB version and newer USB-C version)
A2000's ACM 192
Tests performed using a Celestion CDX1-1731 in a waveguide at fixed output level, the compression driver has only an L-Pad to lower hiss from the amplifier and simple passive filter to lift the top end so the response shape is a little more flat then normal.
XLR mics are used with a Focusrite 2i2 Gen 3 interface with the mic gain adjusted up slightly so that REW gave ~135dB of headroom using the M23.
Measurements performed with each mic at a distance of 10cm, 20cm, 40cm and 80cm using a standard mic stand a clip holder so some hash will be seen in the top octaves. I normally use a boom/pole to extend the mic out away from the stand for speaker measurements which greatly reduces those reflections off the stand but my primary focus for these tests was the distortion performance of the mics for which a little bit of hash does not matter much. The frequency response comparisons will therefor be smoothed to better show the general response trend.
A quick look at the SPL level/frequency response at the four distances for all 10 mics tested, those with calibration files have them loaded:
Most the mics are relatively close up to a few ~3khz (UMIK-2 is outlier) they diverge beyond that to a spread of ~5dB above 10khz.
No Smoothing used, 4.5ms gate.