SOUND IN THE OCEAN
Grade Level: Secondary
Unifying Concepts & Processes: Constancy, change, and measurements
1. Understand how changes in temperature and pressure (due to going deeper in the ocean) each affect the speed of sound.
What is the SOFAR Channel? It's a layer of water deep in the ocean (near Bermuda it's around 1000 m deep) where the speed of sound is at a minimum. Sound waves can get caught in this channel and travel hundreds of kilometers. Scientists think that humpback whales may dive down to this channel and "sing" to communicate with other humpback whales many kilometers away.
The SOFAR channel is formed by the interplay between changes in ocean temperature and pressure with increasing depth. Temperature and pressure are the two main factors that determine the speed of sound in the ocean.
Let´s take a closer look at the SOFAR channel and how it works.
1. First introduce the concept of a sound channel by performing Hands-on Activity #1 below.
2. Now have the students use OceanExplorer data to explore what parameters affect the speed of sound and to answer the following three sets of questions.
2a. Open OceanExplorer and use the autofilter button to select data from a single year (e.g., 1990).
Question Set 2.1: Effects of temperature on the speed of sound
How does temperature vary with depth? Does it change at the same rate the whole way down? If different students have extracted and plotted data from different years or cruises, ask them to describe and discuss the differences among their temperature profiles.
Next, we'll look at how sound speed varies with temperature (taking pressure out of the equation).
With the data set you acquired above, press the "Sound Speed Depth Profile" button on the "Speed_of_Sound" toolbar. Choose the second option: Including temperature, disregarding pressure. You should get a sound speed profile that resembles Figure 2. How similar is this profile of sound speed vs depth (where pressure was disregarded) to the previous profile of temperature vs. depth? Do you think temperature is a major factor on the speed of sound in water?
Question Set 2.2: Effects of Pressure on the speed of sound
With the same data set as above, press the "Sound Speed Depth Profile" button on the "Speed_of_Sound" toolbar. This time choose the third option: "Including Pressure, disregarding Temperature." You should get a sound speed profile that resembles
Question Set 3.2: Effects of both Temperature & Pressure on the speed of sound
Near the surface, where the water temperature changes quickly (Figures 1 and 2), temperature has a large effect on the speed of sound. Thus the top of the speed-of-sound profile tends to follow the temperature profile, and the speed of sound decreases as temperature decreases. At depth, the temperature becomes more constant, but pressure is still changing (Figure 3). Thus the bottom part of the speed-of-sound profile tends to follow the pressure profile. Since the speed of sound varies with pressure, the speed of sound increases as pressure increases with increasing depth. Thus there is a mid-water sound speed minimum, visible in Figure 4. At what depth is this sound speed minimum found? Does the depth of this layer change from year to year or from season to season? Why?
So, now we've found a region where the speed of sound is at a minimum.
3. SOFAR Channel: Sound waves bend towards a region of minimum sound velocity due to refraction. Thus a sound channel forms where the speed of sound is at a minimum in the ocean. This is the "SOFAR" channel. To demonstrate this concept, have the students perform Hands-on Activity #2 below.
4. Uses of the SOFAR Channel by Humans and Animals: After having gone through this discussion, what ways can you think of to use an underwater sound channel? Have the students list possible ways of using the SOFAR channel. Ways that have been used in the past by humans and have been suggested to be used by animals include:
SOSUS array: US Navy Sound Surveillance System – array of hydrophones (underwater microphones) used by the Navy for deep ocean surveillance during the cold war. The SOSUS array has since been used for seismic monitoring, marine mammal monitoring, and for the ATOC project. For more information on the SOSUS array, see http://newport.pmel.noaa.gov/geophysics/sosus_system.html#GENERAL
Tracking of vessels in distress – Before GPS (Global Positioning System) the SOFAR channel was used for locating ships and aircraft in distress as well as for tracking floats for the study of ocean currents.
Hands-on-Activity #1: The garden hose as a sound channel
Bring in a few garden hoses. Assign several students to each hose and have them stretch it as far across the room as space permits. Have the student on one end put his/her ear to the hose, and have the other student whisper into the hose. Tell the students to whisper softly so the sound is not too loud. Then have the students switch who is whispering and listening and pass the message back. See how softly they can whisper before the message is inaudible. Make sure each student is able to both listen and whisper.
Now ask the students to put down the hoses and perform the same task by just whispering across the room. Ask the students which method was most likely to get their message across just by whispering. Also have them hypothesize why this is true, and how humans could use this concept in their daily lives (expanding from the garden hose example, of course).
Have a pair of students fill a large bucket with water (fresh water is fine). Ask for volunteers (some of whom are willing to get their heads wet). Ask one student to put his/her head (at least up to both ears) in the bucket of water. Then ask another student to say something to the first student (e.g. "You water-wallowing walrus!"). The student with his/her head in the bucket tries to understand what is being said.
Have the students come up with ideas why the student with their head in the bucket wasn't able to hear the other student. (The speed of sound in air is slower than the speed of sound in water. Since the sound-speed difference is so large, the sound waves bounce off the boundary between the media).
Other Web-based Resources concerning the SOFAR Channel:
|[Home] [SOFAR Fig 1] [SOFAR Fig 2] [SOFAR Fig 3] [SOFAR Fig 4]|