A microphone’s signal-to-noise ratio
The signal-to-noise ratio of a microphone is defined with reference to a SPL of 94 dB (the same as 1 Pascal). It expresses the interval from 94 dB SPL to the level of the self-noise, RMS, A-weighted.
Note: The signal-to-noise ratio must not be confused with the dynamic range, which is always much higher (from max SPL to self-noise).
Can we hear any signal buried in the noise?
Below is an example where white noise is mixed with a 1 kHz tone. The unweighted level (RMS) of the noise is exactly the same as the RMS-level of the 1 kHz sinewave.
When looking at the waveform, it is impossible to see the sinewave. However, we can hear it if we play it back. Further, we can reveal the sinewave by making a frequency analysis of the signal.
The frequency measurement is performed by applying FFT-analysis. Four different settings are applied:
1 (red upper curve) FS: 48 kHz FFT size: 1024 (bandwidth: 46.88 Hz)
2 FS: 48 kHz FFT size: 4096 (bandwidth: 11.72 Hz)
3 FS: 48 kHz FFT size: 16834 (bandwidth: 2.93 Hz)
4 (green lower curve) FS: 48 kHz FFT size: 65536 (bandwidth: 0.73 Hz)
Sometimes we can extract signals even when recorded below the level of the self-noise. (But this is not a technique you should aim for…).
Using a low-noise microphone does not help you if the preamp is noisy. Low-sensitivity microphones may especially need a lot of amplification. In these cases, it often is the noise of the preamp that is responsible for the resulting noise level.
The self-noise of dynamic microphones is seldom specified because the preamp usually determines the noise level.
Microphone self-noise is unwanted but its presence is due to the laws of physics. It is up to the manufacturer to minimize microphone self-noise and make it as smooth and inaudible as possible. It is up to the user to select the right microphone – and the right preamp – that fits the application.
 IEC 60268-4 Sound System Equipment - Microphones.
 Recommendation ITU-R BS.468-4: Measurement of audio-frequency noise voltage.