Echo Rocks

Principles of Science: Sound Reflection and Transmission, Speed of Sound

The echo rock is a large hollow rock that participants speak into. After a sound is created
into the rock, listen for the echo.

An echo is the reflection of sound waves off of a boundary. Sound waves produced into the rock travel to the far end of the rock, reflect off the inner surface, and return to the experimenter’s ear. The echo rock demonstrates that sound is a wave and can be reflected. It also demonstrates that a reflected sound wave has identical properties as the original wave – it is the same pitch and volume.

It takes time for the sound wave to travel the length of the rock, and then reflect back. Sound, in air, travels at a specific speed depending on temperature. This speed is around 1080 feet per second at room temperature (73 degrees Fahrenheit). This may seem quite fast, but it takes sound almost five seconds to travel one mile! In other words, if you see a rocket take off from four miles away, it will take almost 20 seconds for the sound to reach your ear!