What is a spectrogram?.......A Spectogram is a visual representation of an acoustic signal. The horizontal and vertical components of the spectrogram represents frequency and time respectively, while intensity of different signals is indicated by color.

What is a hydrophone?.......Hydrophones record pressure changes underwater as a function of time. Acoustic signals created from earthquakes, ships, whales, and underwater volcanic eruptions cause pressure changes that can be recorded by hydrophones. The result is a data set that preserves underwater events through time.

This illustration shows the schematics of a hydrophone array. Three cables connecting a hydrophone and a glass float are anchored to the ocean floor. The phones are positioned approximately 2 km apart in the sound channel, and a single cable connects all three hydrophones in the triad to shore.

What is the sound channel?.....The sound channel is a depth range in the ocean where sound waves can become trapped and travel long distances with little attenuation. Because sound waves travel fastest under high pressure and warm temperature, a velocity profile is created resulting in a sound channel with little wave attenuation at approximately ~900 m below sea-level.

Figure 2 Howard 1998
Howard 1998

The spectrograms shown here are from different hydrophones positioned at various locations throughout the Indian Ocean. Phones H01W are located off the coast of Australia, and H08N and H08S are to the north and south of Diego Garcia Island. Phones H04 will be recording data in the spring of 2003.