Book Review

Ringing in the New Year

Happy 2016 all you Oceanbiters! Starting this year, we, the bloggers, have decided to change things up a little bit. Each month we will be devoting one week to a particular ocean-related theme. Every post during that week will cover a different aspect of that theme. We’ve got a bunch of cool ideas to get the ball rolling, but we’re always looking for suggestions. If you’ve got a topic you want to hear more about, let us know via Facebook or Twitter! To start things off, we will be “Ringing in the New Year” with a series of posts about sound in the ocean.

Long time readers may have noticed how often acoustics comes up in the study of the ocean. This is no coincidence and there are a myriad of reasons why. The most basic, however, has to do with the physics of sound.

Sound is a kind of wave that moves energy through a medium (ex. air, water, solids, etc). In particular, sound propagates as a longitudinal, or compression, wave. This means that sound gets from one place to another by vibrating molecules in the direction it is traveling. A slinky makes for a great illustration. Imagine that you stretch the slinky between your hands. If you move one end back and forth, the coils expand and contract along the axis of the slinky. If you instead move one end of the slinky up and down, the whole thing wiggles like a snake. A longitudinal wave is analogous to the first situation.

Figure 1: An illustration of longitudinal, or compression waves. The sphere in the center expands, compressing the molecules near its surface. Those molecules, in turn, press on the molecules nearest them. The process repeats itself as the wave expands outwards. (Animation courtesy of Thierrey Dugnolle via Wikipedia).

Check out the animation in figure 1 for another example: The circle in the center expands, compressing the particles that touch its surface. The resulting change in pressure pushes on the next set of particles. The process repeats itself as the wave expands outward. This is kind of like how your stereo works. The speaker vibrates, pushing on the air nearby. The sound wave expands and eventually makes it to your ear.

Since sound is a compression wave, the speed at which it propagates depends on the density of the material it is traveling through. Generally, the denser the medium, the faster sound can travel. Consider the difference between air and seawater; at sea level, air has a density of ~1.2 kg/m3 while seawater is ~1025 kg/m3. Likewise, the speed of sound in air is 343 m/s. In seawater, sound travels at about 1500 m/s.

The difference between the density of air and water is quite stark. More subtle changes in the density of seawater cause an incredible phenomenon known as the SOund Fixing And Ranging, or SOFAR, channel (which is definitely a name an engineer came up with). The SOFAR channel is a layer of seawater where the sound speed is at its lowest level, as shown in figure 2. Sounds can get “trapped” in the channel and propagate thousands of miles before fading away. This happens because the sound waves will get refracted, or bent, back toward the middle of the channel when they hit regions of higher sound speed (for a more detailed description of how this works check out the resources at DOSITS and NOAA Ocean Explorer).

Figure 2: An example of a sound speed profile from a patch of ocean north of Hawaii. The y-axis is depth and the x-axis is the sound speed. Note that the sound speed is faster at the surface and at the bottom. The minimum occurs at about 750 m deep. That is where the SOFAR channel is this particular part of the ocean. (Figure courtesy of Nicoguaro via Wikipedia)

The SOFAR channel is a pervasive feature in the world’s oceans. By simply placing a hydrophone (or underwater microphone) in it, oceanographers can study all kinds of interesting things. Marine biologists can study whales by listening to their calls from thousands of miles away. Physical oceanographers use it to measure the temperature of huge amounts of deep ocean water. The U.S. Navy has even used the SOFAR channel to monitor Soviet submarines during the Cold War.

The use of acoustics in oceanography is not, however, limited to the SOFAR channel. Scientists have also developed acoustic systems to map the sea floor, track large groups of fish, and probe the Earth’s structure beneath the bottom of the ocean. These methods are used for everything from primary research to ecosystem monitoring and beyond.

For the rest of the week, you’ll get an in depth look at some of the wonderful results – and unforeseen consequences – from using sound in the ocean. Tomorrow, Austen will cover an article discussing the use of autonomous robots for monitoring fish populations. Wednesday is a double header: Megan will look at whale earwax and how that affects their perception of sound. Gordon will discuss a paper on how fish can find the sounds they are interested in amid all the subsurface noise. On Thursday, Valeska will describe a study of how fish use sound at multiple stages of their life cycle and how those processes are affected by ocean acidification. Sarah will wrap up the theme week on Friday with a story about how seismic surveys may be affecting sea turtle behavior and distribution.

We hope you enjoy this new endeavor of ours! We’ve certainly had fun putting it together. And, as always, keep checking back after the theme week for all the latest in the oceanographic literature. Happy New Year and many happy returns!

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