Hydrophone Spectral Probability Density
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 calculation and calibration of the spectral data is the same as the spectrograms. 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:
A weeks' worth of data accumulates 2016 wav/hyd source files and summarizing upwards of 70GB of data. If the user's request starts or ends part way through a week or is less then a week total, then, accordingly, less data is plotted, and the 'Weekly' wording in the title is dropped (as in the above example). A week is defined as starting Sunday at midnight. For example, if a user searches Saturday the 1st to Monday the 8th, then three plots will be produced. The same logic applies to the daily plot, with file breaks at midnight.
The data is subject to diversion and filtering by the military, as described here. LPF/HPF data is normally fairly obvious as there will be a dramatic cut-off at either the high or low end of the frequencies and the filtered data that is shown will all appear at less than 40 dB. Data product options alter the results as well, see the data product options described below. If a diversion returns 'LPF' or 'HPF' filtered data and the user selects the 'All' data product option, and if the spectral frequencies available do not change, then LPF/HPF and original spectral data maybe combined in the same plot, where one would see the normal (as above) and some data at a much lower PSD. If LPF/HPF or any data has a different number of spectral frequencies, then the plots will break and won't be combined.
Most hydrophones are calibrated as described on the spectral data product page. If calibrated, the vertical axis limits are dB re 1 μPa2 Hz-1, if not calibrated the units are dB re full scale Hz-1. A comment on the calibration will also appear.
Data availability may be limited for low-bandwidth deployments (Cambridge Bay). This data product is produced from hydrophone spectral MAT files, which are pre-processed and stored for fast retrieval. Spectral probability density plots are quick to generate if all the MAT files are pre-processed (~1 minute per week), if not, the spectral data has to be generated on the fly, which could take as long 1/2 day per one week plot. For advanced users, the data that forms the plots, the spectral probability density histogram, the percentiles and linear mean sound pressure level are also available in a MAT file product; please note that this MAT file product is distinct from hydrophone spectral MAT files.
Revision History
- 20150906: Initial release
- 20160704: added more options
Data Product Options
Hydrophone Channel
H1
This option will cause the search to return results for hydrophone channel H1 only. The hydrophone arrays consist of multiple hydrophones connected to a single data acquisition computer, which collects the data into single files that have multiple channels (nominally raw hydrophone array files, although other formats can handle multiple channels). Data products may be produced from these files on a per channel basis and returned as specified.
This is the default option.
Oceans 3.0 API filter: dpo_hydrophoneChannel=H1
File-name mode field
'H1' is added to the file-name when the hydrophone channel option is set to H1, i.e. IOS3HYDARR02_20111211T152404.000Z-spect-H1.pdf.
H2
This option will cause the search to return results for hydrophone channel H2 only.
Oceans 3.0 API filter: dpo_hydrophoneChannel=H2
File-name mode field
'H2' is added to the file-name when the hydrophone channel option is set to H2, i.e. IOS3HYDARR02_20111211T152404.000Z-spect-H2.png.
H3
This option will cause the search to return results for hydrophone channel H3 only.
Oceans 3.0 API filter: dpo_hydrophoneChannel=H3
File-name mode field
'H2' is added to the file-name when the hydrophone channel option is set to H3, i.e. IOS3HYDARR02_20120801T090939.000Z-H3.mp3.
All
This option will cause the search to return results for all available hydrophone channels.
Oceans 3.0 API filter: dpo_hydrophoneChannel=All
File-name mode field
'H1', 'H2', 'H3', etc are added to the file-name.
Hydrophone Data Diversion Mode
Spectral Probability Density Colour Axis Upper Limit ()
Automatic
In this default option, it sets the upper limit of the colour axis to a value that's the nearest 0.1 (unit-less) to the 98th percentile of the empirical probability density. The empirical probably density is generally the most relevant at values less than 0.25. Only in small data sets or extreme frequency / sound level bins does one see values approach the maximum value of 1.0. For this reason, the manual fixed limit options are clusters at the lower values. The spectral probability density plots from Merchant et al. (2013) all had fixed upper limit values of 0.05. The lower limit is always 0.0.
Oceans 3.0 API filter: dpo_spectralProbabilityDensityColourAxisUpperLimit=0
Manual fixed limit settings (0.05, 0.01, 0.012, 0.25 0.5, 1.0)
This option will cause the spectral probability density plot to use a fixed value, as chosen, for the colour axis upper limit.
Oceans 3.0 API filter: dpo_spectralProbabilityDensityColourAxisUpperLimit={0.05,0.01,0.012,0.25,0.5,1.0}
File-name mode field
Spectral probability density plots generated with the manual limits will have a file mode modifier added to their file names of the format: <-CLIMIT><option> where <option> is the value as chosen, but without the '.'.
Spectral Probability Density PSD Range
Automatic
In this default option, it sets the Y-axis range for the Power Spectral Density to the min/max of the data extended to the next 10 dB interval.
Oceans 3.0 API filter: dpo_spectralProbabilityDensityPSDRange=0
Manual fixed limit settings
This option will cause the spectral probability density plot to use a fixed value, as chosen, for the y-axis range. Using a fixed range is useful for comparing multiple plots.
Oceans 3.0 API filter: dpo_spectralProbabilityDensityPSDRange={40_160,20_140,40_140}
File-name mode field
Spectral probability density plots generated with the manual limits will have a file mode modifier added to their file names of the format: <-YLIM><option> where <option> is the value as chosen, with ' to ' replaced by '_'.
File / Plot Breaks
Formats
Hydrophone spectral probability density data products are available in PNG, PDF format for plots, and a MAT file product containing the data used in the plots: the spectral probability density histogram, percentiles and linear mean sound pressure level.
Discussion
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