Exploring the data

The audio data files are large in size and long in duration. 10-second samples at 192kHz are 3.5MB in size and and each ACD generates 1.26GB every 6 hours. It is difficult for humans to listen to these files. All of the sounds are scrambled below 7kHz to obscure human voice which makes the sound inherently noisy and disorienting. To spot trends and shifts over long periods of time requires an attentive form of listening that is also very difficult and time consuming.

We have developed an experimental application that allowed us to extract the spectral energy across each 10-second segment and save these aggregated snapshots of spectra alongside one another. With this application, is possible to rapidly ‘scrub’ through the spectral content but still use our ears to pick out areas of interest very quickly. When a spectral snapshot seems interesting or surprising, we can listen to the original sound file that made the spectra.

Another approach we have explored involves concatenating the files into 6-hour chunks and generating spectrograms of that entire period of time, as shown in the following figure [click image to expand]:

Spectrogram spanning six hours of data from 12.00 am to 6.00 am on the 13th June 2018 from ACD3. Towards the righthand side of the image, an increase in spectral activity is noticeable. This correlates to what we would expect around 4.00 am on the morning in question — dawn was 3:56am in Edinburgh that day.

Long-duration spectrograms like this give an instant overview of a period that a researcher may find interesting. Whole days and different ACDs can quickly be compared while trends, anomalies and other features (such as sounds in the ultrasonic band) noticed at a glance. Researchers interested to explore more closely can then home in on these specific files and generate further close-up spectrograms of particular areas of interest. With several of these images lined up researchers can look longitudinally across the season or seasons, laterally across the day and spatially across however many boxes are making recordings.