Two measures, why?
When we measure the noise of a microphone – and present the result in the spec sheet – we use two different methods as stated in the standards (IEC 60268-4 Microphones, Clause 16.2 Method of measurement).
The two methods are A-weighted, RMS and ITU-weighted, Quasi-peak.
The numbers we get from each of these methods are different. We will explore what the difference means.
Weighting
We use two different frequency-weighting curves: A-weighting and ITU-weighting (formerly also known as the CCIR-weighting). These curves are much alike, introduced to compensate for the frequency response of the ear at low levels. The weighting-curves reduce the signal at low frequencies. However, above 1 kHz, the signal is boosted, especially in the ITU-curve methodology.
RMS or Peak
When measuring the level of a signal, we often use RMS, which means Root Mean Square. This measure expresses a kind of average value based on the energy contained within the signal. The peak value, however, is the portion of the waveform (on either side, plus or minus) that is the farthest away from the 0-line.
Here is the waveform of a signal. The peak value and the RMS value of that signal are both indicated in the diagram.
The peak-level therefore always is higher than the RMS-level (unless the signal is a pure square-wave).
When the measurement instead of "peak" is described as "quasi-peak", it means that the bandwidth is not infinite but limited to the standard audio range from 20 Hz to 20 kHz.
The ITU-weighted quasi-peak level is typically 11-13 dB higher compared to the RMS level. However, some lesser quality condenser microphones suffer from what is called "popcorn-noise" (sounds like the popping of corn). When this occurs, the difference is larger as the peak-measure better expresses the single "popping"-impulses.
Some manufacturers only publish the A-weighted RMS detected self-noise.
The spectrum of the noise
As mentioned, noise is broadband but is shaped differently depending on the microphone type, brand, model, etc. Below is the noise spectrum of a d:screet™ CORE 4061 Omni Mic, Loud SPL and a competing product in the market.