I pulled this quote from the "mic damage" thread as it is a good introduction into the second part of my question which is typical distortion levels of the 6mm measurement microphones when measuring subwoofers (ie 10-100Hz).ricci said:Very high volume close mic LF driver measurements are going to be rough on the mic and it will exhibit high distortion if you are up over 120db, which really isn't even driving the sub very hard at all when the mic is so close to the cone. It may affect the accuracy of the measurements adversely especially any distortions. I'd limit things to under 120db indicated by the mic. (Look for a test that Herb kindly did for me on the ECM8000 capsule). Some of the better Earthworks or AO Pacific mics would probably be fine, but not these.
I've been able to find very little information on this subject but am trying to get a sense of typical distortions in this scenario. Especially when used in a near-field mic arrangement where the levels tend to be quite high.
Here is the results that Herb provided Ricci. Very useful information!
Also ARTA has an application note (http://www.fesb.hr/~mateljan/arta/AppNotes/AP5_MikroMeasChamber-Rev03Eng.pdf) which includes this chart in the appendix:anechoic said:I just ran a quick test comparing the distortion of my one of my reference mics (ACO Pacific 7052, rated to 144 dB) to an ECM8000 at 1 kHz.
At 115 dB, my ref mic gets 0.13% distortion, the ECM8000 reads 0.54%
At 120 dB, my ref mic gets 0.22% distortion, the ECM8000 reads 0.81%
At 125 dB, my ref mic gets 0.46% distortion, the ECM8000 reads 1.37%
Tests at 30 Hz:
At 115 dB, my ref mic gets 1.8% distortion, the ECM8000 reads 3.1%
At 120 dB, my ref mic gets 2.95% distortion, the ECM8000 reads 5.0%
At 125 dB, my ref mic gets 4.2% distortion, the ECM8000 reads 7.4%
FWIW, this is a different ECM8000 than the one used for the 1 kHz test.
Note the measurements are at 300Hz which is inbetween the data points Herb measured. The WM60 is presumably the precursor to the current crop of 6mm Panasonic capsules in mics from Behringer, Dayton etc.
Since my subwoofers usually show a significant increase below 30Hz I wonder about the distortion at 10Hz and 20Hz? Does distortion typcially increase significantly below 30Hz?
And I wonder if the native 6mm capsules inherently have much lower distortion than shown by the Behringer. I have an IBF Acoustic microphone that appears to have significantly less distortion than my Behringer for example. This raises the question if the Dayton mic may be lower distortion than the Behringer?
I also have a Nakamichi CM-300 mic rated at 3% at 140dB. Presumably rated at 1kHz (they don't say). It is a 12mm microphone. I've noticed it has significantly less distortion than my Behringer but more than my IBF Acoustic mic.
So, does anyone have data providing more information on this subject? Typical distortion levels of other mics relative to SPL and Frequency? Distortion patterns at 10 and 20Hz? Other comments?