Working on students’ ideas

Students in Brian’s science class brought a number of important ideas about sound stemming from their personal experiences with music and sound. Many of the students’ ideas provided fruitful foundations for developing further understanding of sound. For example, many students participate in band at school and understood how their instruments vibrated to create sounds. Other students were familiar with seeing speakers vibrate and feeling vibrations pounding from loud car speakers and speakers on stage at a concert.

While some of the students’ ideas were important foundations for understanding sound, some of the students’ ideas were in need of close analysis in order to determine if these ideas impede understanding or if they are helpful foundations. Two lines of thinking became important for Brian to follow during this unit of instruction. First, some students envisioned sound as a physical entity traveling through the air analogous to how molecules of a smell can diffuse through the air. Second, many students envisioned sound using a wavy line – a transverse wave – as seen in graphical representations in media. These two lines of student thinking provided the focus for Brian’s instruction in much of this unit.

Line of students’ thinking 1. Sound traveling through air.

We often speak of sound traveling from an instrument to a listener, and music is often depicted as notes flying through the air in cartoons and other images. A few of Brian’s students also represented sound as musical notes traveling through the air. These students expressed ideas that sounds are separate physical entities from the air and that these sounds travel directly to the ear of the listener. Often students showed these notes floating through the air on wavy lines like a musical staff. It is easy to understand how students might have developed these mental models of sound, but the challenge for Brian was to figure out how to help students reshape their musical note models of sound into compressional wave models of sound.

Brian first noticed students representing sound as musical notes during the introductory lesson where students drew diagrams of sounds created by musical instruments. As Brian visited each small group of students he used spoken questions to invite students to elaborate on their ideas about sound. From responses to these questions, Brian learned that some students held tightly to an idea that sounds pour out of the bell of the saxophone or stream out of a singer’s mouth in a direct line towards the listener’s ear.  After seeing how students imagined sound, Brian decided to provide a number of representations of sound to help students reshape their models (see continuum below). First, Brian had students read and talk about vibrations of musical instruments bumping into air particles. For many students, this was enough of a support to help them make sense of sound as a compression of air particles. But, some students needed additional support. For one group in particular, Brian needed to work one-on-one through the use of questions to stretch and shape a student’s ideas about sound as a musical note traveling through air.

Later, Brian pressed students about the idea of sound traveling directly to the ear of a listener by having students carefully listen to sounds and talk about where they could hear the sound. After some “just in time” instruction about the nature of sound, Brian challenged students to think not just about what sound was doing, but also asked them to think about where sound was traveling. When students revised their models, Brian was pleased to see that students were beginning to think about sound traveling in three dimensions. He decided to follow this line of thinking in subsequent investigations to help students think about sound across both time and space.

Line of students’ thinking 2. Sound represented as a transverse wave.

Some students showed prior knowledge of sound as “a sound wave” in their initial drawings and conversations, but instead of drawing vibrations or compressions, students drew transverse waves similar to those shown in textbooks. Brian had concerns about the students’ use of the transverse wave representation because it does not represent how sound energy travels through compressions and rarefactions of air particles. The transverse wave representation does not help to explain how sound actually travels through air, and it does not help students think in terms of air particles. However, the transverse wave representation is important because it is a common way of showing properties like amplitude and wavelength so Brian did not want to discard this idea entirely. Instead, Brian worked to help students relate compressions and rarefactions in compressional waves with the crests and troughs seen in transverse waves.

During the first two days of instruction, Brian heard students use the phrase “sound wave” to describe the movement of sound through the air. He also saw some students draw a wavy line to represent this “wave” while other students drew lines that looked like ripples or vibrations coming off of musical instruments or tuning forks. Brian built on these ideas by asking questions to help students elaborate their existing ideas about “sound waves.” Later Brian helped students incorporate science ideas about waves with their own wave ideas.

One of the most powerful moments for student thinking was the use of a Rubens’ Tube apparatus to show the connection between waves of energy pulsing through a gas and transverse wave representations. A Rubens’ Tube is a metal tube sitting horizontally with tiny holes drilled in a line along the top. One end of the Rubens’ Tube is connected to a small tank of propane like those used when camping, and the other end of the Tube is connected to a speaker. When a sound is played through the speaker, the wave of energy compresses the propane gas inside of the tube and propane escapes out of the tiny holes on top. If the gas coming out of the holes is ignited, the flames will be taller where the gas is compressed and shorter where the gas particles are less dense. The resulting image helps students “see” both the compressions/rarefactions of a gas carrying a sound wave and the relationship between compressions and a transverse wave.

  

 

For many students, the Rubens’ Tube demonstration provided an opportunity to make sense of the relationship between compressions of air and the transverse wave image frequently seen in textbooks. Some students continued to hold on to an idea about sound traveling as a wavy line through the air, so Brian also used two strategies during his conversations with small groups. First, Brian provided Slinky toys as manipulatives to help students act out the movement of a sound wave through air. Students often would move the Slinky side-to-side to create the wave instead of pushing one end of the Slinky to create a compression. This action gave Brian an insight into student thinking and a quick opportunity to suggest an alternative. A second strategy – the use of a “backpocket question” – served as a rapid assessment and another way of opening up a conversation about waves. Brian drew a compression and a wavy line on a card and pulled the card out of his backpocket each time he visited a small group asking which one better illustrated the movement of sound through air. The backpocket questions and the Slinky toy manipulative were important for helping to shape students’ ideas about the nature of sound waves throughout the unit of instruction.

 

Tool Systems to Support Progress toward Expert-Like Teaching by Early Career Science Educators 2008-2013 Discovery K-12 Grant National Science Foundation No. DRL-0822016