On January 15th, we deployed Europa off Puako, Hawai’i to begin her journey towards Mexico. On March 13th, we turned Europa around to start her journey back to Hawaii. Approximate temps are updated periodically.
Europa's acoustic payload is recording and archiving audio continuously. We plan to analyze the full data-set after the mission; however, since something could happen to Europa during the mission, we are periodically requesting short sound files back to shore via satellite. Our Ocean Sonics icListen 200kHz digital hydrophone allows us to set five “triggers” (or Epochs) that detect whale frequencies and help filter out frequencies that aren’t whales, such as the sub's self-noise.
When the Epoch detects a whale we can then request another file as a text-file, called Fast Fourier Transforms (FFTs), a complex set of algorithms that samples a signal over a period of time and divides it into its frequency components.
The FFTs can be downloaded and displayed as a graphical representation of the audio our hydrophone is capturing (spectrogram). Below is what a FTT spectrogram looks like when played back on “Lucy,” icListen’s software program.
Once we confirm something of interest from the FFT, we can request an actual mp3 audio. The spectrogram below (zoomed in to the 0-10kHz range) is an mp3 audio file that was uploaded into Audacity (a free, open source audio software). Baleen whales’ frequency range is from 20Hz to 22kHz. The humpback whales' song is generally thought to be from 80Hz to 4kHz. Human hearing is from 20Hz to 20kHz.
We have outlined three specific (and different) events:
RED CIRCLE: This shows a range of the humpbacks’ vocalizations. The slanted blue lines that stack vertically are harmonics. Their color represents the decibel level (loudness). As you can see, they fade as the frequency gets higher.
GREEN CIRCLE: This is the sub’s rudder noise. The three lines are the primary harmonics (325Hz, 650Hz, and 975Hz) that you'll hear from the rudder, but you can actually hear a few more above it when there's no additional noise. They also continue at 325Hz intervals until they fade out completely. The glider’s “self noise” also falls within this range (300-1,500Hz), and includes wing stops, springs and surface noise from the float pushing through the water.
BLUE CIRCLE: This is a great example of a long low-frequency “groan” that the humpbacks make throughout their song. In this example, it lasts for about three seconds and increases from 450Hz to 530Hz.
Take a listen to the matching MP3 file and try to distinguish specific noises with the events taking place on the spectrogram.
The majority of the higher decibel (dB) levels are located in the < 1kHz range. Above, we've zoomed in to show this in more detail.
Again, we have outlined three specific (and different) events:
GREEN CIRCLE: This points out the three primary harmonics of the rudder noise (as described above).
BLUE CIRCLES: A better look at the long low-frequency “groan” that the humpback whale makes throughout their song.
GREEN BOX: This shows some of the "self noise" described above. As you can see, everything under 250Hz is pretty noisy, creating a constant background noise in this frequency range. Luckily, we can set triggers to filter most of those sounds out and pick up on the humpback whales’ vocalizations above the “self noise.”