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Hydrophone Spectral Data

A number of hydrophones are producing audio data with sounds at a wide range of frequencies, having applications in seismology, marine mammal studies, ship noise and more. Hydrophone spectral data (PNG/PDF image files of the spectrogram, FFT data files) are provided as a summary of the audio recording and for detailed analysis. With spectrogram images and data, users can determine the sources and nature of sounds they can hear in the audio data files: passing ships make hyperbolic curves, whales make chirps and resonances, etc. Spectrogram images, particularly the PDF format, are a great way to scan through a vast amount of data quickly, looking for events. Listening to this data would be very time consuming, plus the human ear is not capable of picking up some high or low frequency sounds that may be quite interesting.

Spectrogram image files (PDF/PNG) are available for all hydrophones and hydrophone arrays. For hydrophones located on low-bandwidth observatories (Cambridge Bay, Brentwood Bay, etc), audio data files may not be available. In this case, audio data is stored on site to be retrieved periodically by hand and uploaded to the archiving system. To provide live data, low-bandwidth observatories upload spectrogram FFT files as they are much smaller than the full audio data. FFT files may be used to generate spectrogram images when the full audio data is not available.

Data Diversion

Given the sensitive nature of hydrophone data, the military has authority to completely divert the data and/or filter it as required. When filtering occurs, the file-name is appended with '-HPF' or '-LPF', corresponding to high and low pass filtered data. Often, the military diverted data will be returned at a later date, normally modified from the original data (we will often get the spectral complement to the high or low pass filtered data). See here for more information on the diversion of hydrophone and seismometer data. Data product options are provided to sort out the various types of diverted data (see below). Not all options are available for all devices: for instance, in some cases, we may not offer '-LPF' spectrograms even if there are '-LPF' audio files because such a spectrogram would offer so very little useful data.

Processing Time

Spectrograms may take some time to generate. PNG spectrograms files are normally pre-generated and stored, so their retrieval is quick. However, PDF spectrograms are not pre-generated, but will be computed per each request. Processing speed is nominally about 5 to 20 seconds per 5 minute audio file. The final step of PDF generation, aggregating the individual spectrograms, takes more time for a larger number of files.  Therefore, requesting a day's worth of PDF spectrograms may 20 to 60 minutes. Please be patient, a day's worth of data is roughly 16 GB of WAV files, 500 MB of MP3s, 160 MB of PDFs, 200 MB of PNGs. A single hydrophone can generate terabytes of data in a year. It is a vast amount of data to store and serve to our users.

Revision History

  1. 20130912: Hydrophone spectrogram FFT files initially made publicly available
  2. 20140123: Spectrogram PNG/PDF files made available on all hydrophones
  3. 20140315: Spectrogram images may be produced from FFT files

Data Product Options

Hydrophone Channel

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Hydrophone Data Diversion Mode

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Format

PNG/PDF (Hydrophone Spectrogram Plot)

This format is a spectrogram plot of 5 minutes of hydrophone/audio data. Here is an example PNG spectrogram taken from a hydrophone as Cascadia Basin:

Newer versions may have a human-perceptive colour map, different titles, logos, etc., however the data is the same. The colour scale is fixed to facilitate comparisons between multiple spectrograms. Some spectrograms are calibrated with units of dB re 1 μPa. Non-calibrated spectrogram have a colour scale that is relative to the full range of the source audio file: the extremal values in the audio file are scaled from 0 to 1, so that the dB scale is from -120 dB (0.000001) to 0 dB (1). The newer versions have a fixed relationship between time and horizontal resolution: if less than 5 minutes of data is provided in the audio source file, the spectrogram x-axis will shorter than usual.

PDF spectrograms are appended together into one PDF file with multiple pages. User can quickly scroll through this file looking for events such as whale calls.

FFT (Hydrophone Spectrogram Data File)

The FFT format is an ASCII text file with a single column of data. It is intended for expert users, others may refer to the spectrogram PNG/PDF plots. It will only be offered for devices with low-bandwidth connections, such as the hydrophone currently at Cambridge Bay (deviceID 23155). The file consists of repeating sequences of 512 FFT spectral coefficients, spanning five minutes. The current sampling rate is 256 kHz, with 4 FFTs per second, or 1200 in one file, with a frequency bin spacing of 250 Hz.  Using MATLAB, one can visualize it (i.e. make a spectrogram) quickly with the following commands:

data = dlmRead('myFFTfile.fft', ',');
specData = reshape(data, [512, length(data)/512]);
imagesc((1:size(specData,2))/4, (511:-1:0)*0.250, flipud(specData));
axis xy
xlabel('Time (seconds)');
ylabel('Frequency (kHz)');
cb = colorbar;
ylabel(cb,'(dB re 1 \muPa)');

Please note the above stub of MATLAB code is an example only, with hard-coded parameters. Users can retrieve FFT files and process them with this code using our CodeRunner tool.

Discussion

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