Detecting low-frequency spikes from afar

We were monitoring a large rotating device in outdoor conditions. We measured the sound of the device over 1 km from the device (combining environmental and condition monitoring measurements).

Selected sound samples were subjected to spectrogram, FFT, and autocorrelation analysis. As a result, we discovered a pulsating sound in the low frequencies (cf. picture 1).  Two spikes, to be precise: 15,2 Hz and 16,4 Hz. 


These spikes in the FFT analysis were unexpected. They gave us and our client a clear indication that the monitored device was not operating at full capacity. Based on our findings our client was able to take corrective measures and get the device back to full capacity. An automated alert for similar events is easy enough to create.