Show students how to secure the string to a chosen object so that it can hang freely, feeding the free ends of the string through the loop to hold it tightly if necessary.
Then place the ends of each free string onto the flap of skin that covers your ear canal (the tragus), and use the same fingers to push the flap closed. (Do not push the string into your ear.) Your ear should be blocked and the string pushed against your ear flap and the bone underneath it.
(Alternatively, to keep string out of ears, wrap the free ends three turns around each index finger, then push the finger against the flap of the ear.)
Lean over and swing the object so that it bangs against a table or chair, which starts it vibrating. You only hear sounds coming up the string, and not through the air.
Students can anecdotally share their observations at the end, or use a worksheet (see attached below).
Questions to prompt students with experimentation:
Did all materials make a sound? If not, what were those objects made of, or what was their shape?
Compare two similar objects made from different materials (e.g. metal and wood or metal and plastic).
Try making the ringing sound, then touching the object or string to stop the ring.
After students have successfully heard some sounds through the string, ask them to compare with the sounds through the air. Swing and bang an object, but do not put the strings in your ears.
What is happening?
As you bang the object it vibrates. The vibrations travel up the string to the bones in your ear, where you hear them as a sound that has passed only through solids. Different materials transmit vibrations differently and so the sound changes.
To hear sound through another solid, lay your ear flat on a desk. Knock on the other end of the desk with your knuckles.
Animals hearing through different materials
Just as the sound is quite different through the string compared to through the air, animals that hear sounds through water or the ground hear sounds quite differently from us.
Sound travels faster and further through liquids (e.g. the ocean) and solids (e.g. the ground), than the air. But more energy is needed to transmit sounds through liquids and solids, so very quiet sounds do not transmit well.
Animals that hear sounds through the water:
Fish and marine mammals such as whales, dolphins. Lower frequencies travel well under water, and a long way.
Animals that hear sounds through the ground:
Snakes lack external ears and internal eardrums. They hear through their jaws (two jaws - can hear in stereo), the sound going directly to the cochlea. They bury themselves in sand to make their hearing more precise.
Elephants detect these seismic waves with the skin of their feet and trunk. Communicate danger from miles away.
Blind mole rat knocks its head against the walls of its tunnels to signal to its neighbors.
Termites in danger will bang their heads on the ground, which spreads like a chain reaction through the colony.
Kangaroo rats drum their foot on the ground with danger.
Frogs and spiders also hear through the ground.