NI-9234 Noise Floor Calculation

Updated Dec 31, 2024

Reported In

Hardware

  • Sound and Vibration Device
  • NI-9234
  • PCI-4462

Issue Details

When we verified the noise level in the soundproof room, we found that the value measured using B&K microphones and NI-9234 was around 32 dB. There was no way to measure below 30 dB at a sampling rate of 51200 samples per second (S/s). However, when we used a sound level meter to test in the soundproof room, the actual result was around 24.5 dB. Why can't NI-9234 measure noise levels below 30 dB?

Solution

According to NI-9234 Specifications, the idle channel noise is 50 μVrms at 51.2kS/s.

Assuming the microphone sensitivity is 50mV/Pa, the channel noise is calculated as below:
AI noise (in Pa) = Idle channel noise / Microphone Sensitivity
AI noise (in Pa) = (√2*50 μVrms) / (50mV/Pa) = 0.001414 Pa
AI noise (in dB SPL) = 20*log10(0.001414/0.00002) = 36.988 db SPL

Assuming the microphone's inherent noise does not exceed this value, the noise floor reading measured by NI-9234 would be 36.988 dB SPL. Note: this is just a theoretical value and is used for your reference only.

To reduce noise floor, please use a microphone with higher sensitivity or other NI Sound and Vibration modules with smaller idle channel noise, such as NI-446x series modules.

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