- This simulator by T. Walsh shows the Doppler effect with adjustable speeds for both the source and the observer in one-dimensional motion. It displays the observer’s perceived wavelength and frequency. Watch what happens when the relative velocity exceeds the speed of sound!
https://www.ophysics.com/w11.html
This pair of animations by B. Surendranath are best used complementarily.
- The first shows the Doppler effect with a fixed observer, but a variable frequency and speed for the moving source.
https://www.surendranath.org/GPA/Waves/Doppler/Doppler.html
- While the second compares the wavelength as measured from the two points.
https://www.surendranath.org/GPA/Waves/Doppler/DopplerB.html
- D. Russell has a page with a series of animations breaking down different cases of the Doppler effect, to compare motion below and above the speed of sound