Doppler Effect
The Doppler effect was first described in 1842 by Austrian Physicist Christian Doppler.
When sound is emitted from an object, it will have a particular wavelength and frequency.
The wavelength will seem to change if the object is moving towards the observer or away from an observer.
Sound is a physical phenomenon by tiny, fast movements called vibrations that transfer energy from one point to another.
These vibrations are transferred to the air medium, and propagated in all directions in the form of waves.
All sources of sound wave have some part that vibrates.
Sound that is below the range of human hearing is called infrasound.
It is the range of frequencies detected by seismographs in detecting earthquakes.
These waves can cover long distance with little obstacle.
Animals like bats and dolphins generate ultrasonic waves and use the reflections of these waves to find their way.
Hence they can fly about avoiding the obstacles.
This effect where there is an apparent change in frequency and wavelength of a sound wave due to the relative motion between the source of the sound and the observer is called the Doppler effect.
The Doppler effect is observed whenever the source of waves is moving with respect to an observer.
The wavelength will increase and the frequency will decrease if the source of wave is moving away from the observer.
If the source of the wave is moving towards the observer, then the wavelength will decrease and the frequency increase.
Let us assume a source sends out a sound wave that moves out at a speed of sound.
Consider two stationary observers (1) and (2) located at either side of a source at rest.
Each observer hears the same frequency produced by the stationary source.