Can't Get Sound Measurement Data on NI 9234

Updated Mar 6, 2018

Reported In

Hardware

  • NI-9234

Issue Details

I am attempting to take a sound measurment on my 9234 but a spectral analysis gives no data. Is my noise floor too high? Is my signal too small? What could be the issue?

Solution

First it is beneficial to estimate the noise floor you are working with in dB(A). This calculation is typically done with an acoustic reference pressure of 20 uPa. 

Using the A-Weighted Power equation and integrating noise density we can get the estimated noise floor for the NI 9234.

Integrating Noise density
A-Weighted Power = ( A weighting power factor for bandwidth - typically .53 ) * (Bandwidth) * ( Noise Level )^2

You can also calculate the noise at full scale range and should get similar results. Subtracting the noise at full scale range from the idle channel noise (found in the 9234 spec sheet) will give you an estimated noise floor. Keep in mind this is relative to environmental noise and is only an estimate. If you are within +/- 10% you are likely at the correct noise floor. 

Calculating from Noise at full scale range:
FS = Voltage Range / Sensitivity
Noise = FS - Idle Channel Noise Spec

Your card likely has the correct noise floor but you are unable to measure a signal due to the amplitude of your signal you are trying to measure. An accuracy calculation will provide you with the context of the natural error in your measurement. If you are measuring signals that are fluctuating within the +/- 5 mV range, you are not taking full advantage of the +/- 5V input range of the card and your signal is being drowned out by the idle channel noise. On these levels you will not get spectral data unless you amplify the signal to better match the input range of the card.

Additional Information

NI offers a wide range of PXI products scoped to this specific issue. This case study gives concrete examples of how the NI PXI system was able to overcome these challenges 

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