Hydrophone Spectrogram
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. Spectrogram files (.FFT data files or PNG/PDF image files of the spectrogram) are provided as a summary of the audio recording and for detailed analysis. With spectrogram data, users can determine the sources and nature of sounds they can hear in the audio files: passing ships make hyperbolic curves, whales make chirps and resonances, etc.
Hydrophone spectrogram data is available in FFT data files. Spectrogram image files (PDF/PNG), may also be provided at latter date.
Given the sensitive nature of hydrophone data, the military has authority to high-pass filter the data as required. When filtering occurs, the file-name is appended with '-HPF'. Often the unfiltered data will be passed to us at a later date.
For hydrophones located on low-bandwidth observatories (Cambridge Bay, Brentwood Bay, etc), hydrophone audio files may not be available. In this case, any missing audio data is stored on site to be retrieved yearly, while the observatory will upload the spectrogram files as they are much smaller.
Revision History
- 20130912: Hydrophone spectrogram files initially made publicly available
Format
PNG/PDF (Hydrophone Spectrogram Plot)
This format is a spectrogram plot of 5 minutes of hydrophone/audio data. PDF format is not yet available. Here is an example PNG spectrogram taken from a hydrophone as Cascadia Basin:
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 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. If one were to import the file into Matlab, into a vector called 'data', one can visualize it (i.e. make a spectrogram) quickly with the following commands:
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)');
Please note the above stub of Matlab code is an example only, with hardcoded parameters.
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