Sound and light in the ocean
In this activity students read graphs, make calculations, and interpret both to see how sound is a more effective tool than light when we want to "see" in the ocean.
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 would have learned about the salinity and density 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.
Content/concepts goals for this activity
This activity asks students to use what they know 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 see that light is quickly attenuated while sound can travel great distances.
Then the frequency dependent attenuation of sound is examined and students learn that low frequency sound travels better than high frequency sound.
Finally students are asked to use what they learned in interpreting a news article.
Higher order thinking skills goals for this activity
Students read values off of graphs, make calculations based on those values, then plot calculated values on a graph.
Other skills goals for this activity
A rubric is provided for the student 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 exercise contains the complete exercise. Students may want to have computers available as one part of the lab asks them to use Google.
Teaching Notes and Tips
Question 1. 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 C-temperature values in the table should be negative.
After students have graphed the sound speed profile have them examine it. Use this as a teaching moment to reinforce 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 lense that focuses the sound. 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 can travel great distances since spreading attenuation is minimized.
Question 4. Instructors can use this question to have a discussion with students on the difference between linear decay of a signal and exponential decay.
Question 5. Tying whales into the exercise will "hook" some students into remembering the content. Since this exercise has no field or lab component having students Google a whale fall adds a bit of excitement.
Question 6. Instructors may want to do the calculations for one of the rows in the table.
The rubric (the last page of the exercise) is used to assess the student's work.
References and Resources