Setting different channels gains on stereo ARU to maximize detections

Question

When trying to detect particular animal vocalizations, is it a good idea to use stereo autonomous recording unit (ARU) and set different channels gains?

I would think that having a channel with a low gain prevents saturation when animals vocalize very close to the ARU and having the other channel with a high gain maximizes the detection distance (given ambient noise is low enough). Therefore, whenever the animal is very far or very near, there will always be at least one channel on which the signal is good enough to be detected, thus maximizing the detections.

For example, on SMmini, user can choose between 6/12/18/24 dB channel gain. I would use the lowest (6dB) and the highest (24dB) since the 18dB difference is still a very small difference when compared to the microphone dynamic range of about 80dB, hence ensuring that both microphones still have about 60dB common dynamic range.

Any thoughts/ideas/experiences on this topic?

Answer 1

When setting up the system gain (IOW, effective sensitivity) I orient myself on the background noise. Analog gain (i.e. gain before ADC) should be set such that background noise is not dominant but still present. This gives you maximal sensitivity for signal detection.

In case you are in a quiet environment and you need amplitude information for classification and expect clipped sound due to close-by sound sources, then you would decrease gain setting, loosing the capability of detecting faint signals.

Using different gain settings in a stereo configuration could make sense do have both requirements, faint detection and amplitude based classification.

If you do spectral based classification you should further consider the FFT processing gain (noise distributed over the bandwidth), e.g. a broadband noise RMS value of 10 bit gives you after a 1024 point FFT a spectral RMS noise level of 1 bit.

In summary, I set my gain such that during analysis I still 'see' the ambient noise and yes it makes sense to use different gain settings on different channels.