Introduction to Ocean Acoustics: Guided Problem Solving

Joceline Boucher, Corning School of Ocean Studies, Maine Maritime Academy, joceline.boucher@mma.edu

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Initial Publication Date: June 25, 2014 | Reviewed: July 21, 2015 (see revision history: 2 events)

Summary

Students learn about sound propagation in the ocean through instructor-guided problem solving. The activity promotes a conceptual understanding of the physical and chemical factors that influence ocean acoustics, with applications to how whales communicate over long distances. The activity serves as a way to introduce the topic of ocean sound.

Oceanography | College Introductory

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Context

Audience

This activity is suitable for an undergraduate introductory oceanography course.

Skills and concepts that students must have mastered

Familiarity with ionic compounds and the polarity of the water molecule is needed. Knowledge of the definition of density is also useful, as is the ability to use units in solving problems.

How the activity is situated in the course

This could be a stand-alone activity on ocean acoustics followed by a short lecture, or it could introduce a section on sound pollution in the oceans.

Goals

Content/concepts goals for this activity

Conceptually understand sound waves and their propagation in the ocean
Learn how ocean conditions (temperature, salinity, and pressure) influence sound propagation
Apply knowledge of ocean conditions and sound to understand acoustic communication at sea.

Higher order thinking skills goals for this activity

Use knowledge of physical properties to predict acoustic properties.
Interpret and use graphical information.

Other skills goals for this activity

Practice with:

presenting information graphically
numerical problem solving
reading log-log plots and interpreting exponential notation

Description and Teaching Materials

Worksheet and answer key. Students will need a calculator and pencil with eraser. A ruler may be helpful.

Ocean Acoustics Worksheet (Microsoft Word 517kB Jun24 14)

Ocean Acoustics Worksheet KEY (Microsoft Word 2007 (.docx) 559kB Jun24 14)

Teaching Notes and Tips

This activity fits a 1 h 15 minute class period, or a 50 minute class period if Part D Questions 2-6 are done as homework.

To begin the activity, I project the animation of "sound waves in gas" from this site: http://web.archive.org/web/20161120065855/http://www.educationscotland.gov.uk/resources/s/sound/amplitude.asp . Students might also view the site on their own electronic devices.

The atomic displacement in Part A, which is easiest to see in the still image, will likely challenge students. Asking about it requires students to think about the molecular scale movement associated with sound waves. An alternative would be to ask a more general question about compressional waves.

Some students will draw a blank on the compressibility question (Part B, Question 1). Instructor prompts such as "what could you do to the box to measure compressibility?", or "think of the box as the 'before' part of a 'before and after'" image, may be needed.

The thought experiment of dissolving salt in water could be difficult for students who haven't taken chemistry. Instructors can help by sketching the ionic structure of a salt crystal (e.g. NaCl) and the molecular structure of a water molecule and indicating charges on the respective component ions (in the crystal) and charge centers (of the water molecule). Most students will know that opposite charges attract.

Inspect the graphs at the beginning of Part C as soon as students complete as a quick check that they have answered Part B correctly. Students with wrong answers in Part B will reach incorrect conclusions in Part C.

For Part D, Question 1, students may need prompting to find the sound speed and distance information; remind them that the information is on the ocean section plot just above the question. They might also need help translating "circum-Pacific transect at 5^o N". If Part D is to be done for homework, be sure to verify that students know how to interpret exponential notation and log-log plots.

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Assessment

Students are tested on the contents of this activity.

References and Resources

Sound attenuation calculator (see how attenuation varies with pressure, salinity, temperature, and pH): http://resource.npl.co.uk/acoustics/techguides/seaabsorption/

Audio gallery of sounds (play whale calls along with the activity!): http://oceanexplorer.noaa.gov/gallery/sound/sound.html

Short overview of sound properties and SOFAR channel: http://oceanexplorer.noaa.gov/explorations/sound01/background/acoustics/acoustics.html

More comprehensive overview of sound in the oceans (peer and teacher reviewed): http://www.dosits.org/

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