The basics about noise in mics

When we use a microphone, we expect the cleanest possible signal, ideally completely free of annoying noise. Unfortunately, completely free is impossible! To prepare for a good audio result, you should check the microphone's noise specifications. You will find data quantifying the "equivalent noise level”, which describes what goes on at the bottom of the microphone’s dynamic range – basically in the range of low SPLs.

Where does noise come from?

Noise is not something we want to hear from a microphone. However, there is always some portion of noise generated by the microphone itself – self-noise. Primarily this noise originates from a current running in the circuitry (so-called "shot noise" or "Poisson noise"). Also, thermal noise (or "Johnson noise") is an issue; the higher temperature, the higher the noise.

Further, noise is generated due to the presence of air around the microphone. The movement of air molecules causes a bombardment on the diaphragm, which eventually ends up as noise.

The basic nature of the noise is what we call white noise. The name "white noise" comes from comparing audio to light. Just as white light contains the whole spectrum of visible light, white noise contains an equal amount of energy at all audio frequencies.

We call it "noise" because there are no tones in the signal. However, the character of the noise may differ, depending on the distribution of the noise. When we amplify it, we can hear that it sometimes sounds "smooth”, other times it sounds more "crackling”. Sometimes it sounds darker, other times it sounds lighter – all depending on the type of microphone and its design.

What is equivalent noise level?

Noise is expressed by the equivalent noise level.

Self-noise is the signal the microphone produces of itself, even when no sound source is present.

Example: A microphone has an equivalent noise level of 22 dB(A). Now, if the microphone was completely free of noise we could record a sound source that had a SPL of 22 dB(A). If the sensitivity is 20 mV/Pa, the output of that microphpone would be approximately 4 µV. However, there is no sound source to produce the signal; the microphone itself produces the 4-µV signal recorded. The noise is therefore equivalent to that of an external sound source.  

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.

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:

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…).

Preamp noise

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.

Conclusion

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.

References

[1] IEC 60268-4 Sound System Equipment - Microphones.
[2] Recommendation ITU-R BS.468-4: Measurement of audio-frequency noise voltage.