Why does NI-9234 with IEPE microphone report 39dBA to 41dBA as minimum noise level in a semi anechoic room?

[Mohammed.Ashraf]

Dears

I hope my message finds you well , 

We are facing a similar problem as the posted in this thread : http://forums.ni.com/t5/Dynamic-Signal-Acquisition/Why-does-NI-9233-with-IEPE-microphone-report-39dB...

Re: Why does NI-9233 with IEPE microphone report 39dBA as minimum noise level?

We are using the PCB Free Field IEPE microphone 378B02 (this is an assembly of 377B02 microphone and 426E01 preamp) 

The datasheet of the preamp : http://www.pcb.com/contentstore/docs/PCB_Corporate/Vibration/products/Manuals/426E01.pdf

The maximum allowable IEPE current from the 9234 is 2mA, and it's compliance IEPE voltage is 19VDC as you know 

but the 426E01 preamp has the following specs for excitation : 

Excitation Voltage 20 to 32 VDC 
Constant Current Excitation 2 to 20 mA

Is it sufficient to switch to an external current source and give better results for minimum noise level ? since we read something close to the 39dBA ? will this be equivalent to pregain the signal ?

actually we are in the acceptance phase and bought a number of 9234 with a cDAQ ethernet chassis , and we can't switch to other option (e.g. PXI)

I'm Really in a big need to your reply , Thanks alot for your time 

Best Regards

[dsb@NI]

External signal-conditioning electronics seem like the right option, but I don't think that we should expect the gain to scale with the constant current IEPE. Instead, I recommend that you investigate a signal conditioning option that provides pregain. PCB lists several compatible options on their website (http://www.pcb.com/TM_Electronics.asp) that provide required excitation and selectable gain.

[dsb@NI]

External signal-conditioning electronics seem like the right option, but I don't think that we should expect the gain to scale with the constant current IEPE. Instead, I recommend that you investigate a signal conditioning option that provides pregain. PCB lists several compatible options on their website (http://www.pcb.com/TM_Electronics.asp) that provide required excitation and selectable gain.

[Mohammed.Ashraf]

Is there any cheaper solution ? Since their 16 channels Signal conditioner with gain selection is about 7,000 USD price list 

Thanks a lot for your time 

[dsb@NI]

If you need to insert the signal conditioning, try working with a vendor such as PCB to see if they can recommend a low-cost solution.

We started this discussion with the assumption that we wanted to be able to measure the noise floor of a quiet semi-anechoic room. What are the expected sound pressure levels during the measurement of the DUT? Are they >20 dB higher than 40 dBA such that we have confidence that the system noise floor is not significantly contributing to sound power measurement? Can 'quiet' be measured with just a single channel (thus requiring only one channel of signal conditioning with pregain)?