Strike the set of suspended steel pipes of varying lengths. Each of them produces sound of a particular pitch. The pitch produced depends on the length of the pipe – the longer the pipe the lower the pitch. The sound is produced because the pipes vibrate when struck.
The model here has seven metal pipes of same material with same inner and outer radii, but different lengths.
The pipes are suspended from a rigid support. When the pipes are hit by a wooden rod one after the other, we can notice that the sound generated by each pipe is different.
Why should it be so?
When a pipe is hit by the wooden rod, the air molecules inside the pipe vibrates causing alternate compressions and rarefactions. That means there will be regions with more air molecules and high pressure (compression) and regions with lesser air molecules and low pressure (rarefaction) compared to the atmospheric pressure. This is how the disturbance is propagated through the pipe.
At the open end, the sound wave gets reflected and standing waves are formed inside the pipe.
The wavelength of the standing waves is directly proportional to the length of the pipe.
Speed of sound in air is about 330 m/s. Speed is actually the product of wavelength and frequency. Here the wavelength increases with length of the pipes. Therefore, the frequency decreases correspondingly.
The shorter pipe produces a sound of higher frequency and longer one produces a lower frequency sound.