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Spectral Probability Density (SPD) plots are useful for assessing the quality and performance of hydrophone audio data, as well as looking for signals such as marine mammals, ships etc. The plot was first described by Merchant et. al. (2013), see reference here. The plots are a 3D histogram with audio frequency on a logarithmic scale on the horizontal axis and power spectral density on the vertical axis. The body of the plot is then a heat map of the relative number of data points occurring at each intersection of frequency and power spectral density; this is labelled as the "Empirical Spectral Probability Density", the units of which are relative and a function of the horizontal and vertical bin sizes (and the relative occurrence of the data). By default, a weeks' worth of data is plotted per plot, although daily and no break options are available. This provides a summary of up to 2016 individual five minute spectrograms. The The calculation and calibration of the spectral data is the same as the spectrograms. The , see Hydrophone Spectral Data for more information. The distribution of the data is used to investigate the nominal frequency response of the hydrophone, including background noise, while outlier data points could be interesting transient signals such as whale, ships, etc. The data limits and bin sizes are determined so that they are consistent from plot to plot for ease of comparison. Line plots are overlain to indicate the 1, 5, 50, 95, 99th percentiles in the distribution for each frequency bin, plus the linear mean sound pressure level (known as SPLlin in Merchant's paper, also note that lines are not available if there's less than 6 hours of data). Information on the processing done, calibration and data availability is noted below the plot, while the data product options are indicated in the title above the plot, along with the usual information on location and device (the data time range maybe dropped from the title if it gets too long, it's redundant with the filename). See the example below:
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