Soundwave Mapping

My students are greeted as they enter the classroom by a rather annoying humming noise that emanates from the rear. Upon further inspection, they discover that there are two speakers placed at each end of the room. Each one of these speakers is giving off the same tone and each is set to the same volume.

by Michael P. Kennedy

published online October 16, 2003

Courtesy of the Subaru National Science Teaching Awards
Created by:

Michael P. Kennedy
Neugua Valley High School
Naperville, Illinois
2003 Midwest Regional High School Winner

After they settle in, I tell my students that they are to walk about the room and make a map of places where the sound seems the loudest and places where the sound seems the softest. Students bustle about and quickly find these strange places everywhere. Some students find places so loud that it is uncomfortable to stay there very long and others find places that are so soft that the sound can barely be heard. All the while my students are taking notes about the volume that they experience at each of these locations.

Once the students have mapped the room, we begin to discuss constructive and destructive interference. I ask them what happens when two waves meet. Quickly we are discussing waves that meet to become louder since wave crests will match up and waves that become softer as wave crests meet wave troughs. Students become excited as they realize that they have experienced the wave phenomena first hand.

After the concepts have been established, I begin to ask about what we would need if we were to design a theatre, entertainment system, or a car stereo system so that we did not have such variations in the volume. I ask what improvements we could make to our classroom so that the room would not have such "live" and "dead" spots. We begin to systematically make changes to isolate those variables that will affect our maps.

The students then use a web-based simulation of two wave sources to check their results. Quickly students discover that the places that they found as being constructive or destructive are usually close, but do not quite fit. After some questioning, they discover that the walls are causing the sound to reflect and therefore extra waves are present in the room after reflecting. Reflection, after all, is a topic that is soon to come!

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