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Microphone sensor sensitivity and noise floor using NI-9233

Hi, I'm new to LabVIEW and I've inherited a program that produces 1/n octave band sound pressure level plots from a microphone connected to the NI-9233.
My setup is B&K mic, B&K Nexus Conditioning Amp, NI-9233, LabVIEW on a Windows XP machine

The program itself works well. However, when I try to measure very low level sound fields, the sound pressure level output is higher than for a similar setup using a legacy B&K analyser. After much playing about, I believe it is to do with the sensor sensitivity value obtained during calibration of the microphone. If I alter the settings on the Nexus amplifier I can get the sensor sensitivity higher and the noise floor lower. The problem is that if I change the amplifier to get the LabVIEW program to output a similar noise floor to that obtained on the legacy analyser, then the amplifier clips when I put the mic in high sound fields (i.e. greater than the calibration level of 94dB).

The legacy analyser has the ability to alter the input attenuation separately from the calibration settings, so I only need to calibrate the analyser at the beginning of the day and then by adjusting the input attenuation I can measure both high and low sound fields.
Is there any way of achieving something similar in LabVIEW?
I've had a look at the 'Scaling and Calibration' information in the Sound and Vibration Toolkit but can't see anything relevant, although I could be missing something being a newbie!

Thanks for any help,
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Hi Susan, 


I understand that you have a service request with us here in the office with one of our engineers regarding this (if I've confused this with another person then please let me know). We will continue to use that to communicate with you as we work on this.


I am posting to bump this back up to the top to see if any members of the community have anything they'd like to suggest. 


If you have any other questions or problems then let us know.


Kind Regards,


Jason W.
Applications Engineer
National Instruments UK & Ireland
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The Nexus has a RS232 interface. So you can set the appropriate amplification with LabVIEW. The technical documentation of the Nexus interface is more than 100pages .....

For the LabVIEW side you need the VISA driver installed to communicate via RS232. Search the NI driver network maybe there is already a B&K Nexus driver ....




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The USB-9233 has a 24-bit analog-digital converter (one 1 bit is lost for bipolar; theoretically this means about 138 dB), but the datasheet defines a range of about 102 dB .

The input range is 5 V.


As the range is 102 dB the finest voltage is about 39 µV (microVolt).

Now, 39 µV (microVolt) is about 7,94E-04 Pa. This value, to the base level of 20e-6 Pa for 0 dB, defines 32 dB (linear).


For low noise measurements, about 32 dB is the effective noise floor of the 9233.    If you use the Nexus signal conditioning to increase the gain, you can lower this but you run the risk of clipping if you don't get it right and have higher signals.

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