WSRS Technical note - Mic Preamplifier Noise and why does it matter

This technical note describes how the mic preamplifier noise of your recorder can affect the quality of your recordings. Preamplifier noise tends to be an issue at the budget end of the equipment scale, where recordists need to match their equipment and microphone choices well to get the best results in quiet environments.

All signal processing systems have two natural volume limits.  At the low end there is the noise of the system itself, and in any competently designed recorder it is the internally generated noise of microphone preamplifier that sets this limit. At the high end is the maximum input level the system will accept without distorting the signal. In a linear (for our purposes uncompressed) system the difference between these two extremes is called the dynamic range of the system.

Four systems concern you as a recordist before you have your signal recorded. The first system is what you are recording. The volume of a bird depends on its natural volume level at the bill, and when recording this is modified by how far away you are from the bird. The noise level is the level of unwanted noises in the environment - wind noise, traffic noise, other signing individuals (if they are not wanted - in a soundscape recording for instance they are often part of the wanted signal).

The next system in the chain is the microphone. This adds its own noise to the signal, and often has a maximum sound level it can handle. In most wildlife situations the latter is never approached., but the lower limit can be problematic.

Now the signal gets into your recorder, and the next system of concern are the microphone preamplifiers of the recorder. These too have a maximum signal level and a minimum signal level they can handle. If the noise level of the microphone preamp is lower than the electrical noise produced by the microphone itself, the microphone noise dominates and there is little to gain in engineering the preamp noise much lower.  Unfortunately, cheaper microphones are often less sensitive as well as being noisier, and paradoxically this combination means the mic preamplifier oftne needs to have a lower noise level dealing with a cheaper microphone of poor sensitivity.

Finally the signal reaches your recording medium, which in the case of digital media tends to have clearly define maximum and minimum signal levels it can cope with. These levels are fixed for any given recording medium, and it is the job of the record level control to adjust the incoming signal so that it lies within the dynamic range of the recording medium.

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