IEPE Mic on 4431 USB DAC Not Working

[SOET_Prof]

We're recording 3 ICP (NI calls this IEPE) accelerometers and 1 ICP microphone using LabView and an NI 44331 DAQ.  The mic is on channel 1 and the accels are on channels 2-4.  We are using the system to record dynamic data on a truck on the road - lots of noise and vibrations. 

The accel data looks fine, but the mic data is dominated by some kind of noise.  We calibrated the mic using an  acoustic calibrator and the cal signal looks fine.  Recording voices in the office also works fine.  Road data on the mic channel is junk.  Accel data is fine.

The obvious answer is an overloaded mic.  However, the bad data doesn't look like an overloaded ICP signal.  The other choice is an overloaded DAQ, but we've got it set for 122 dB.  Hard to believe we're overloading the front end.

We've checked the cables and replaced one, even though it was probably good.

The VI is dirt simple.  We're wondering if there is some wierd quirk in how LabView treats IEPE sensors.  The manual says that the channels are individually programmable, but I'm beginning to wonder if there is a problem somewhere.

We've got a customer waiting on the data.  We'd be most obliged for any suggestions.  Thanks folks.

[AndyCandy]

Hi SOET Prof,

Thanks for presenting the issue you're seeing.  There doesn't seem to be a straightforward solution, as I'm not aware that LabVIEW interacts differently with IEPE devices.  My guess is that the issue is stemming from the hardware.  Here's an article that outlines using long cables, which may be helpful depending on the length of yours--http://www.ni.com/tutorial/7274/en/.  It would also be helpful to test the various sensors in different channels, test other 4431 modules if they exist, and test the microphone under less extreme conditions, to verify it is functioning properly.