IEPE Mic Sound Level Error

[JDfromRI]

Hi All,

I have recently purchase the Sound and Vibration tool kit for LV8.6 for making (surprise, surprise) sound pressure and vibration measurements on an enclosure that houses a variable speed pump. I use a cDAQ chassis with the 9233 IEPE module and an ACO mic/pre (model 7052SYS). The mic is IEPE, of course, with a specsheet sensitivity of 20mv/Pa.

I am getting started using the S&V example vi named DAQmx Sound Level Meter. When I set up the AI mic channel to the 20mV/Pa sensitivity, I am still seeing large error in my measurement as compared to calibrated handheld sound meter. In specific, the room ambient noise is ~32.9dBA as measured with the Extech handheld meter. The VI and IEPE mic set-up reports this as ~39dBA.  The error when noise is introduce sometimes gets a little closer, but the two do not agree.

What have I overlooked? The IEPE mic is new and has factory cal certs so let's assume this is not the issue.

Thanks for the help!

-JD

[AnalogKid2DigitalMan]

Just a thought-

Does the microphone and the sound meter have the same bandwidth span?

-AK2DM

[JDfromRI]

Good suggestion and thanks for the reply. This could have something to do with the errors.

Meter span is 30-130db across the range 31.5Hz - 8kHz. The IEPE mic is rated for 20-148dbA across the range 3Hz - 20kHz. The meter is more for industrial noise measurements such as factory floor noise. The IEPE mic is more intended actual acoustic measurements. I'm inclined to trust the mic and NI-9233 but, having not used this type of equipment before, I'm trying to gain a better understanding first.

Does this suggest that it could explain a 66-7db difference between 30-40dbA?

[AnalogKid2DigitalMan]

JD:

That indeed could be a potential source of the difference between the readings. To know for sure, you would need to do a spectal analysis fo find the energy content across the frequency bands.

If you have a quiet chamber, you could possibly generate a single tone frequency (at multiple freqeuncies within the range of both meters) and then compare the meter readings.

-AK2DM