Here we have a set of PVC pipes of different lengths. Place your ear at one end of a pipe - one at a time. The sound that is heard through each of these is distinctly different.
Any place, even a seemingly quiet one, has stray sound waves of different frequencies. Only, the sound is too faint to be heard. However, when a pipe is held over the ear, it reinforces sound of a particular frequency, depending upon its length. Shorter the pipe, higher is the frequency of the sound reinforced.
In this model, there are seven pipes of different lengths that are horizontally mounted parallel to one another on a rigid support.
If you place your ear at one end of a pipe, you can hear a sound of a particular frequency. As you move from one pipe to the other, the frequency of the sound also changes.
Why should you hear this kind of a sound when you place your ear at one end of the pipe ? And why should it be different for different lengths of the pipe ?
Sound wave is characterized by frequency & wavelength. Sound travels in air at constant speed which is about 330 m/s. The speed of sound waves is given by the product of its frequency and wavelength.
The background noise is a mixture of many sounds of different frequencies and of very low intensity. Therefore human ears cannot hear any of them as such.
When the same background noise passes through the pipe, there will be a particular sound whose wave length matches with the length of the pipe so that the wave gets reflected at the open ends forming a standing wave inside the pipe.
Only this sound gets reinforced to a large extent. Therefore one can hear only particular sound when they place their ears near one of the ends of the pipe.
As mentioned before, the wavelength of the sound that gets reinforced inside a pipe is directly proportional to length of the pipe.
A pipe of greater length picks up a sound of longer wavelength or lower frequency and a pipe of shorter length picks up a sound of shorter wavelength or higher frequency. It is for this reason, one can hear different sounds from different pipes.