Sound is a longitudinal wave: a vibrating source makes compressions (particles squashed together) and rarefactions (particles spread out) that travel through a medium, with each particle only vibrating to and fro in place. Frequency sets the pitch and amplitude sets the loudness. Without particles, as in a vacuum, there is nothing to carry the wave.
In space, an explosion is completely silent. Yet on Earth you can hear a thunderclap from kilometres away. What is sound actually made of?
You have heard sounds all your life. This lesson looks at what is happening in the air between a sound and your ear.
Make your prediction first. Help opens once you've locked it in.
Use the sliders or the + and − buttons. Follow the orange particle. What changes about the waves, and what changes about the particles?
Make a high, quiet sound: pitch 4 or more, loudness 2 or less. Wait for it to reach the listener.
Use the board. This ticks when you get there.
Now a low, loud sound: pitch 2 or less, loudness 4 or more.
Use the board. This ticks when you get there.
In your book: draw a row of dots for air before a sound arrives, then again with a compression and a rarefaction marked. Add one sentence saying what each particle does.
Match the change to the effect on the sound.
Choose Vacuum from the menu. Watch the listener.
Use the board. This ticks when you get there.
Now choose Steel. Wait for the sound to arrive.
Use the board. This ticks when you get there.
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
Students describe sound as a longitudinal wave that needs a medium, use compressions and rarefactions to explain how energy is transferred, and link frequency to pitch and amplitude to loudness (AC9S9U04, node 2, a hinge node). About 25 minutes.
Open the teacher guide: answers, solutions, slides and a worksheet.
A concept tutorial for introducing or revising the idea. Set the width with ?width=minimum, typical (default) or wider. It follows How does heat get through? and comes before the charge lessons.
The hinge: the wave model first appears here. Students who picture particles flowing from speaker to ear cannot go on to light as a wave or to compare the wave and particle models later. The prediction targets it, and the orange particle on the board is the answer.
Board notes: the animation is slowed down, the speeds in the three media are not to scale (the numbers shown are the real ones), and the particles are drawn in one row of 7 for clarity. Real sound waves have far more particles per wavelength.
That the wave travels but the particles don't. When a student says air rushes to the ear, ask them to point to the orange particle and say where it is a second later.
Stretch: A sound has a frequency of 200 vibrations a second. How many compressions pass you in a second? (200.) If the speed is 340 m/s, how long is each wave? (340 ÷ 200 = 1.7 m.)
Support: Give a drawn row of dots with a pencil mark on one dot. Ask students to draw the row at three moments as the wave passes, keeping the marked dot in place.
Next: The charge lessons (the particle model of electricity), then the electromagnetic spectrum, where the wave model has to work without a medium.
In class: stretch a slinky across a table and push and pull one end, then shake it sideways; use a tuning fork on water; a bell in a vacuum jar if available. A yidaki or other instrument from a local First Nations community, with the community's guidance, is a strong real example.