Echo Tubes

Principle of Science: Wave Reflection, Sound Frequency

Clap into the tube and listen to the sound when it returns. The returning sound does not sound like the original clap, but rather like a long drawn out whine beginning with a high frequency and ending with a lower one. Notice also that it takes less than a second for the sound to return.

Clapping into the tube creates a disturbance in the air: the sound wave. This disturbance radiates out in all directions, like the surface of a sphere, at the speed of sound in air (about 1080 feet per second). Not all parts of this sound wave will reach the ear at the end of the tube at the same time. The part traveling along the center of the tube will reach the ear first.

Since the sound wave spreads out in all directions, parts of it are reflected off the walls of the tube. The more bounces the wave makes, the longer is the path over which it travels in getting to the ear and the longer it takes to get to the ear. The pulses are close together at the beginning of the sound but gradually become spread out. When the pulses are close together you hear a high frequency (pitch); when they are far apart you hear a low frequency.