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  Monitoring the global ocean through underwater acoustics
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  Acoustic Monitoring Program: Underwater Acoustics Tutorial
  The following is a very brief overview of the basic principles of underwater acoustics.
The tutorial is presented in twelve topics beginning with "What is Sound?".
  1. What is Sound?  
  A propagating sound wave consists of alternating compressions and rarefactions which are detected by a receiver as changes in pressure. Structures in our ears, and also most man-made receptors, are sensitive to these changes in sound pressure (Richardson et al.1995, Gordon and Moscrop 1996).

wave componentsThe basic components of a sound wave are amplitude, wavelength, and frequency:

The amplitude of a sound wave is proportional to the maximum distance a vibrating particle is displaced from rest. Small variations in amplitude produce weak or quiet sounds, while large variations produce strong or loud sounds. The wavelength of a wave is the distance between two successive compressions or the distance the wave travels in one cycle of vibration. The frequency of a sound wave is the rate of oscillation or vibration of the wave particles (i.e. the rate amplitude cycles from high to low to high, etc.). Frequency is measured in cycles/sec or Hertz (Hz). To the human ear, an increase in frequency is perceived as a higher pitched sound, while an increase in amplitude is perceived as a louder sound. Below are examples of sound waves that vary in frequency and amplitude.

same frequency graph
These two waves have the same frequency but different amplitudes.
  different frequency graph
These two waves have the same amplitude but different frequencies.

Note that increasing the frequency of a sound in equal steps will lead to perceived increases in pitch that seem to grow smaller with each step. For example, click on the sound frequencies below, and you'll see a more noticeable difference between 200 Hz and 225 Hz than 400 Hz and 425 Hz.

200Hz 225Hz 250Hz 275Hz 300Hz 325Hz 350Hz 375Hz 400Hz 425Hz 450Hz 475Hz

Entire Series

Humans generally hear sound waves whose frequencies are between 20 and 20,000 Hz. Below 20 Hz, sounds are referred to as infrasonic, and above 20,000 Hz as ultrasonic.

infrasonic (about 20 Hz) < human hearing < ultrasonic (about 20,000 Hz)

frequency rangesWe know a great deal about human hearing, but what about the hearing of large whales?  Currently, we do not have detailed audiograms for the larger, baleen whales (note: we do have information on the hearing of smaller porpoises and dolphins from research with captive animals).  Instead, we assume whales can hear the range of sounds they produce.  The figure above compares human vocalizations with the sounds baleen whales are known to produce.

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