Solution
Before we start the noise floor calculation, it is important to obtain below information for ease of calculation.
- Sample rate of the signal
- Sensitivity of the microphone (refer to the specifications of the microphone)
With the above information, we can go to the hardware specification, for example NI-9234 Specifications, with a sample rate around 51.2 kS/s, the idle channel noise is 50 μVrms.
For a microphone with sensitivity = 50 mV/Pa, the channel's acoustic noise level is calculated below from the idle-channel noise (NI 9234 idle-channel noise = 50 µV rms):
Acoustic Noise Level = Idle channel noise / Microphone Sensitivity
Acoustic Noise Level = (50 μV rms) / (50 mV/Pa) = 0.001 Pa rms
Acoustic Noise Level = 20 * log10(0.001 Pa rms / 20 μPa rms*) = 34.0 dB SPL
A-Weighted Noise Level = 34.0 dB SPL - 2.8** dB = 31.2 dBA
For the same microphone (S = 50 mV/Pa), we can also calculate the acoustic noise floor from the integrated noise density (NI 9234 noise density = 310 nV/√Hz at Fs = 51.2 kS/s):
Noise Power = Frequency Bandwidth * ( Spectral Noise Density )²
Noise Power = 25.6 kHz (310 nV/√Hz)² = 2.460n V² rms
Noise Level = √(2.460n V² rms) = 49.6 µV rms
Acoustic Noise Level = (49.6 μV rms) / (50 mV/Pa) = 0.000992 Pa rms
Acoustic Noise Level = 20*log10(0.000992 Pa rms / 20 μPa rms*) = 33.9 dB SPL
A-Weighted Noise Level = 33.9 dB SPL - 2.8** dB = 31.1 dBA
Notes:
* Reference sound pressure level (SPL) is 20 µPa rms.
** For a measurement bandwidth of 0 to 25.6 kHz, A-weighting reduces the SPL by approximately 2.8 dB.