Radio antennas can be tuned by changing their length. For example on a portable radio antenna of changing length.
Guitar strings can also be tuned to a different frequency by changing their length. For example by decreasing the length of the string by our finger pressing on a fret.
Why does length change the frequency?
Antennas and strings behave similarly. Inside the antenna is traveling a radio (light), (electromagnetic) wave at nearly the speed of light, actually the speed light travels inside this material. Inside the guitar or piano string is traveling a mechanical wave at the speed of sound in the material the string is made of.
When the wave reaches the end of the antenna or the string there is impedance mismatching and it reflects back. When it reaches the other end it reflects back again and so forth. There is oscillation.
The frequency of oscillation clearly depends on the length of the waveguide and the speed of travel of the wave.
The natural frequency of a radio antenna can be for example 100MHz or 100 million vibrations or second as light travels very fast inside the antenna.
An acoustic wave travels slowly hence a frequency of the order of 400Hz.
Further reading:
The Feynman Lectures in Physics - Feynman, Leighton, Sands
Foundations of Wireless - Scroggie
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