Science of Sound in the Sea
Sound Measurement
How are sounds viewed and analyzed?
Sounds are often displayed as a picture to help understand them. Musicians and scientists use many of the same methods to look at sound. Throughout this web site sounds are represented by pictures.
The most common graphic presentation of a sound is as a waveform. A waveform shows sound pressure in a graph as it changes with time. Sound pressure is positive when the medium through which the sound is traveling is being compressed by the sound wave. Sound pressure is negative when the medium is being expanded by the sound wave. Sound pressure is zero when the medium is neither being compressed nor expanded by the sound wave and is at its equilibrium pressure. Just because the sound pressure is zero does not mean that the equilibrium pressure in the medium is zero. The equilibrium pressure deep in the sea is always high due to the weight of the overlying water, for example. This is a waveform for a single frequency of 250 Hz generated on a computer. It looks just like a simple wave.
Click either choice below to hear the computer-generated sound:
Below is the waveform of a sound made by a fish, the Atlantic Croaker. There are two tracks shown for this sound because it is in stereo(the right and left tracks are identical in this case).
Click either choice below to hear the fish:
The sound from the Atlantic Croaker is much more complex than the computer generated single frequency sound. Most natural sounds are quite complex.
Scientists and musicians often need to know the frequencies present in a sound. Most natural sounds have many different frequencies present. The illustration below shows a series of sounds at different frequencies. Listen to the sound (47K):
To look at all the frequencies present, the sound is converted to a frequency spectrum. The image below is a graph of the frequency spectrum for the sound above (the one with a series of distinct frequencies in a row). The graph is made by plotting the amount of energy in sound at each frequency.
Each frequency shows up nicely as a sharp peak in this example.
Look at the frequency spectrum below of the full Atlantic Croaker sound.
Click either choice below to hear an Atlantic Croaker :
There are many peaks, broad regions with some energy and lots of little peaks. Like most natural sounds, it is a complex sound. It is also hard to tell which frequencies go with which part of the sound.
The frequency spectrum above shows all the frequencies over the whole length of the sound. An animated frequency spectrum shows just the frequencies present at each moment in time. You can see what frequencies are associated with each part of a sound. The animated spectra are presented as QuickTime movies. Click anywhere on the movie below to see an animated spectrum of the "tones" sound.
You can see how each tone has only one peak. As the sound moves from one tone to the next, there is some overlap due to the analysis method. You can see this in the animated spectrum if you look closely or use the QuickTime controls to step through the animation one frame at a time.
The animated spectrum below (136K) is of the same Atlantic Croaker. Notice that there is energy at many frequencies in each part of the sound. The amount of energy specific frequencies changes for the different sounds made by the Atlantic Croaker.
The higher frequencies are from the first part of the sound and the strong peaks in the lower frequencies are from the second part of the sound. We can easily hear the different frequencies and where they occur in the Atlantic Croaker sound. However, there are many sounds for which the human ear cannot sort out the frequencies. The frequency spectrum allows us to "see" the frequencies.
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