Background, I picked up a UMM-6 from Parts Express and set up REW on my MacBook (running latest OS X 10.9.4). My goal is to use REW to more logically set the filters for my Velodyne SMS-1 and get a nice flat response from 15-80hz with my Seaton SubMs HP+ Master and Slave setup.
I used the calibration file from Dayton - deleting the sensitivity line, and level calibrating the mic as recommended by AustinJerry in his step-by-step guide. All seems fine so far.
Comparing the readings to what the SMS-1 is graphing lines up pretty well (although there is a lot of smoothing going on with the SMS-1 graphing). But there is one large discrepancy.
Below 20hz the SMS shows very substantial output from the subs all the way to 15hz, which is the graphical limit of the device. I am using the included Velodyne mic -- which my research tells me is a Behringer ECM8000. The REW/UMM-6 measurement shows a huge nosedive just below 20 hz, and the a bounce around 16hz or so. It's a shape I see in other graphs posted where the scale goes down past 20hz.
My SubM's are in SW2 mode which should provide solid extension to 15hz in room. Both the master and slave are in the rear corners of the room which Mark Seaton recommended as a good place for them (and for other limiting factors -- it's really where they have to be).
While having lower resolution, I think the SMS-1 is perhaps providing more accuracy below 20hz. I see on the UMM-6 datasheet that the FR on the mic only extends to 18hz (as I side note - the UMIK-1 shows FR down to 20hz).
Question: am I running into a measurement limitation of these USB mics that the Velodyne mic does not suffer from?
I am at work right now -- but I can post a REW graph and screen shot of what the SMS-1 is showing for reference.
Thanks in advance for any help.