Sound and light in the ocean

Lauren Sahl and Joceline Boucher, Corning School of Ocean Studies, Maine Maritime Academy
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This page first made public: May 29, 2013

Summary

Light is our usual method for observing on land, but water quickly absorbs light making it less useful in the ocean. On the other hand, sound in water can travel long distances before it is absorbed to the point where it becomes inaudible. For this reason sound is frequently used to "see" in the ocean, as when scientists map the sea floor.

In this activity students work with sound and light data. In the process they come to appreciate that several factors affect the speed of sound --salinity, temperature and pressure -- and learn how to calculate sound intensity at a distance from the source. By plotting sunlight intensity versus depth in the ocean students learn that light is quickly absorbed in water.

Students also read graphs, make calculations, and interpret both. They view two videos and may listen to underwater sounds (see reference section below for a link to a sound gallery). They read a short article on the changing soundscape in the ocean in modern times. These activities provide an engaging, varied look at sound and light in the ocean.

Context

Audience

This activity is used as a two hour lab exercise in an introductory oceanography course for non-majors.

Skills and concepts that students must have mastered

Prior to this exercise students should have learned about the salinity and temperature structure of the ocean. This activity lets them use that information to come to some surprising conclusions about sound speed and sound and light attenuation.

How the activity is situated in the course

This is a stand-alone lab exercise. With more advanced students it could be assigned as homework.

Goals

Content/concepts goals for this activity

This activity asks students to use their knowledge about temperature and salinity in the ocean, along with provided graphs, to determine the sound speed structure of the ocean. They compare sound speed structure to light intensity structure to discover that light is quickly attenuated while sound can travel great distances. They then examine the frequency dependent attenuation of sound and learn that low frequency sound travels farther than high frequency sound. Finally students are asked to use what they learned in interpreting a news article to improve their scientific literacy.

Higher order thinking skills goals for this activity

  • Learn to extract and apply data from graphs
  • Sharpen problem solving skills
  • Interpret solutions to problems and apply the solutions
  • Learn to assess the importance of several variables in a process

Other skills goals for this activity

A rubric is provided to students to show them what is expected in answering the questions. This is to train them to interpret and answer questions at an appropriately advanced level.

Description and Teaching Materials

The student handout for this activity contains the complete exercise. The PDF file is ready to use and the Word file can be modified by the instructor.

Instructors may want students to have computers, tablets or cell phones available to visit some of the internet links (in this case earbuds or headphones are also recommended since one of the whale fall videos has sound). Alternatively, the instructor may display the sites for the entire class (in that case a projector and speaker are recommended).

The answer key is provided here.

Teaching Notes and Tips

Instructors may want to start the exercise by reviewing the temperature and salinity structure of the deep ocean. Then students may be prompted to volunteer ways in which sound is used to study the ocean. Playing some of the sound clips from the sound gallery (see the reference section below) is fun and draws in the students. It is engaging both to play some sounds and have the students guess at the source, and to let the students see the suite of sounds available and request to hear certain ones. Be sure to play an example of each of the sounds mentioned in the exercise.

Question 1. This question is meant to teach students that sound speed changes with temperature, salinity and pressure. Instructors should have students examine the temperature and salinity profiles. Tell them that as temperature, salinity and pressure increase so does sound speed. Note that we are interested in the change of sound speed as depth increases. So the effect of salinity and pressure is to increase sound speed with increasing depth. Temperature decreases the speed of sound with increasing depth, therefore the delta Ct values the students enter in the table should be negative. There is ample mention of this in the text but this still seems to be a sticking point, so be prepared to assist.

After students have graphed the sound speed profile have them examine it. Use this as a teaching moment to introduce them to the concept that waves bend toward the environment of slower wave speed. The effect on sound is that the sound speed minimum at about 500m is like a lens that focuses the sound (the actual depth of the sound speed minimum varies from place to place). Sound that tries to move up or down is bent back toward the sound speed minimum depth. This means that sounds generated at this depth, known as the SOFAR channel, can travel great distances since spreading attenuation is minimized. This link has more information on this process https://oceanexplorer.noaa.gov/explorations/sound01/background/acoustics/media/sofar.html.

Question 2. This question directs students to examine the data and evaluate the contribution of each variable to sound speed in the surface and bottom layer. Although all three variables affect sound speed, in each layer there is one variable that has a greater impact than the other two.

Question 3. In this question students plot light intensity data for four types of seawater and then speculate on why some of the waters are clearer than others. Urge students to think about both particles that may be suspended in water and dissolved substances that may color the water. Students may not realize that suspended sediment particles are common in coastal waters (since the source is river discharge, shore erosion, and resuspension from the bottom by waves and currents) but rare in the open ocean. This may be a good topic for class discussion.

Question 4. The two whale fall videos are a dynamic way to reinforce the concept that deep sea organisms must use senses other than sight to navigate. Depending on how you choose to pace the exercise students can watch these videos (with earbuds) when they reach this question, or you may show these to the class as a whole. If you do that, you may want to have a class discussion on the question and then have students summarize the discussion in their written answer to this question.

Question 5. Students complete the table doing the same type of calculation four times. The example, coupled with the repetition, should result in even math-phobic students being able to complete the calculations.

Questions 6-8. Students examine the results of their calculations (Question 5) to determine what they mean.

Question 9. In this question students reflect on what they have learned in the exercise by describing the ways in which sound and light are used by organisms and people as tools in the ocean. This should help students connect the problem solving they have done to applications.

Question 10. This question asks students to look at the data and determine how effectively sound and light travel in water. We have found that students need some guidance in how to do this in a quantitative way. We offer a suggestion that they examine the distances over which sound and light reach the same intensity compared to the original signal. Comparing those two distances should make the answer to the question clear. It might be useful to do an example on the board, and to go over how to interpret the results. In the example on the board you may want to show students how to calculate a certain percentage of the light intensity at the surface of the ocean.

Question 11. Some characteristics of sound have biological consequences, and humans have changed the underwater soundscape, not always in ways that are benign to animals. This article discusses interesting changes in submarine sound intensity and bandwidth in historical times, and potential impacts on marine organisms.

Assessment

The rubric (the last page of the exercise) is used to assess the student's work.

References and Resources

Here is the link to the Science Daily article in the exercise. https://www.sciencedaily.com/releases/2012/10/121023123954.htm. Science Daily is a wonderful source of short, easy to read articles about science. We use this resource often to show exciting new developments in science.

This site has many useful pages on submarine sound, so dig down and explore. It provides background information for the instructor. https://dosits.org/science/movement/how-fast-does-sound-travel/. Visit the gallery and choose some sound clips to play for your class. Almost all of the sounds used in the exercise are represented there, including whales, ships, air guns, and sonar https://dosits.org/galleries/audio-gallery/.

This site provides background on light in the ocean, including the ways in which organisms use light for camouflage and to hunt. https://www.whoi.edu/oceanus/feature/shedding-light-on-light-in-the-ocean

The links to two whale fall video clips are embedded in the student handout, and are reproduced here. This video clip shows the change in animal life on a whale carcass (a whale fall) as decomposition occurs https://www.youtube.com/watch?v=vQbGk4sHROg. This video clip shows a whale fall that has been stripped of meat and examines some of the life that feeds on the bones. Play this second clip with the sound on, the scientist's excitement at this discovery is infectious https://www.youtube.com/watch?v=x32PLnBsZsw.

For instructors wishing more detail on sound and light in the ocean: Chapter 5. Light and sound in seawater, in Seawater: It's composition, properties and behavior, 2ed. by an Open University Course Team, is recommended.

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