Position your ear at the open end of the pipe and clap your hand. Echo of the clap caused by the reflection of sound at the far end of the pipe can be distinctly heard. Our brain can distinguish sounds that impinge on the ear at least 0.2 seconds apart. In our exhibit, sound takes about 0.4 seconds to make a to-and-fro trip. Therefore, the echo or the reflected sound, is heard clearly.
The tube here is thirty metre long. It is placed at a certain height from the ground. One end of the tube has been covered with the same material as that of the tube where as the other end is open.
Clap or say “ha” near the open end. The sound that you hear back is a series of claps or “ha”s one quickly following the other. But if you observe carefully, all of them are not of the same pitch (frequency).
You would first hear a high frequency followed by the low frequencies.
When we clap or say 'ha' we are producing a sound wave. The sound waves travel in air at a speed of ~340 m/s. When any wave encounters a change in the propagating medium, it undergoes reflection, refraction and transmission.
Likewise, the sound waves produced near one end of the tube also get reflected from the walls of the tube and the closed end along with being refracted and transmitted.
These reflected waves travel back to the open end and reach our ears.
If the time difference between the first sound and the next sound is more than 0.1 second, then our brain perceives them as two different sounds which is called an echo. If the time difference is less than 0.1 second, then the first sound would still be in our memory when the second sound strikes our mind. Then the brain perceives it as a prolonged sound. This is reverberation.
If we want to hear an echo, the time difference between the sounds should be more than 0.1 second. If sound travels ~340 metre in 1 second, it can travel 34 metre in 0.1 second. Therefore, if the length of the pipe is more than 17 metre the sound wave would take more than 0.1 second to reach back to the open end. Thus, the required time difference for the echo to be heard can be achieved.
But, the echo heard is a series of sounds of decreasing frequencies. The reason is, the clap or the ‘ha’ sound we make, is actually a mixture of different frequencies.
A high frequency wave will have a short wave length and does not encounter many reflections from the side walls of the pipe. So, it gets reflected from the closed end and reaches back the open end faster than the other low frequencies.
Therefore, the frequencies get separated in the echo.
One can listen to the transmitted sound near the closed end of the tube.