Sound travels to the far end of the tube and bounces back to make an echo. As it travels, the sound bounces off the sides of the tube.
If you clap into the tube, the echo that returns sounds like a drawn out whine that begins with a high pitch and ends with a lower pitch. Higher-pitched sounds take a more direct path than lower-pitched sounds do, so they get back to you sooner and stronger—causing the strange high-to-low “pinging” you hear when you clap into the tube.
For your ear to hear a reflected sound as a distinct echo, the delay between the initial sound and the arrival of the reflected sound must be greater than about a tenth of a second. Since sound travels at roughly 340 meters (1,100 feet) per second, a sound-reflecting surface has to be at least 17 meters (56 feet) away to produce an echo.
Challenge: Our Echo Tube is about 60 meters (200 feet) long. Can you use the speed of sound (340 meters per second) to estimate the expected time delay between a sound and its echo? Photo © 2013 Bruce Damonte, all rights reserved. (click to image to enlarge)
An Echo Tube doesn’t need to be straight to work—sound travels by bouncing between the walls of the tube, be it straight or curly. (click image to enlarge)