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Hydrophone Spectral Probability Density

Spectral Probability Density 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 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). Normally, a weeks' worth of data is plotted. This provides a summary of up to 2016 individual five minute 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 99% upper limit, 1% lower limit and mean for each frequency bin (only if there is 50% data coverage in the time range of the plot). 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. See the example below (computed from test data where we don't have sufficient data to add the line plots):

A full weeks' worth of data requires collecting 2016 wav/hyd source files and summarizing upwards of 70GB of data. If the user's request ends part way through a week or is less then a week, then less data is plotted, and the 'Weekly' wording in the title is dropped (as in the above example). The plot file-name is contains the time range of the raw data, while time range shown in the plot itself indicates the time range over which data was searched for (in the example above, the search time range is only 1.5 hours). The data is subject to diversion and filtering by the military, as described here. 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 plot. Such a result will normally be obvious as there will be a dramatic cut-off at either the high or low end of the frequencies. 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. For an example of a non-calibrated plot with 'HPF' filtered data, see below (example is also from limited test data, and you can observed that the high-pass filter doesn't appear to have a very good roll-off at low frequencies, but is quite sharp at the expected 3.6 kHz). 

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. 

Revision History

1. 20150906: Initial release

Data Product Options

Formats

Hydrophone spectral probability density data products are available as plots only, in PNG or PDF format.

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