Why Can't My C-Series Module Measure Low dB SPL Even Though My Microphone Can?

Issue Details

When trying to make a measurement at high frequencies and low dB SPL (Sound Pressure Level) with my C series module, the measurement is much higher than expected, even though my microphone is sensitive enough to measure at those ranges. 

Solution

When using a microphone with a C-Series module with a microphone, the inherent noise and sensitivity of the microphone needs to be greater than the idle channel noise of the module.

In order to determine this, you need to multiply the inherent noise of the microphone by the sensitivity. Depending on the specifications of your microphone, you may need to convert the units to get this value. This can then be compared with the idle channel noise of the device, and if it is less than the idle channel noise the sensor is not a good fit.

Example:

The inherent noise of a microphone is 20 db SPL and the sensitivity is 50 mV/Pa.

To convert db SPL to Pa use the following equation:

              dB SPL = 20log10(P1/P0)

Where P1 is the sound pressure level (in pascals) that you’re measuring, and P0 is the reference sound pressure level of 20 micro pascals (i.e., 0.00002 pascals).

In this example, the resulting Pa is 0.0002 When multiplied by the sensitivity (50 mV/Pa) you get 10.0 uVrms.

The NI-9250 has an idle channel noise of 8.7 uVrms at 51.2 kS/s and therefore would be a good fit for this application. Other modules, such as the NI-9232 (idle channel noise=171 uVrms at 51.2 kS/s) and NI-9234 (idle channel noise=50 uVrms at 51.2 kS/s) would not be good fits.