Grandma Moon Speaks: The Doppler Effect

Topics Covered:

  • The Doppler Effect

I don’t care if anybody gets it. I’m going as the Doppler effect. If I have to, I can demonstrate. NYEEEROOOOM – Sheldon, The Big Bang Theory

The Doppler Effect is the change in frequency of a wave for an observer moving relative to its source.

It is not exclusive to sound waves, but to all kinds of waves, even for ducks:

Probably the most well known triumph of the Doppler Effect is its use in the discovery of the fact that the universe is expanding. How does it work?

Imagine you are waiting at a bus stop. Let’s say there’s a drummer standing down the road from you, hitting a constant beat. You count in sync with it, i.e., both your count and the beat are in step.

The drummer, while still hitting the same constant beat, approaches you. Beat one takes a certain time to reach you. Beat two will reach you faster because he has walked closer to you. Each subsequent beat will reach you faster as he gets closer and closer. Since the beats get to you faster and faster, the frequency of the beat increases. If you’re still counting the beats, you are no longer in sync with the actual beat – you are counting faster!

Imagine a person listening to the same drummer directly opposite you, on the other side. To that person, the frequency of the beats will get slower and slower because each beat is coming from further away.

This is why police sirens sound shriller (at a higher pitch) as they approach and appear to lose pitch as they disappear into the distance. Sometimes this low pitch is lower than the actual pitch of the sirens! In the case of sirens, the Doppler Effect actually helps their practical utility, and they are designed keeping this effect in mind.

When it comes to sound waves, there are three kinds of ‘motions’:

  • Motion of the Source
  • Motion of the Observer
  • Motion of the medium in which sound travels

Each of this introduces the Doppler Effect and affects the frequency of the wave at different points. It’s study is mathematically complicated.

The Doppler Effect, when applied properly, gives rise to interesting affects in acoustics. One practical example is the Leslie Speaker. At the very least, the effect can be used to loop for a siren in a movie to make it sound more realistic!

Takeaways:

  • The Doppler Effect is the change in frequency of a wave for an observer moving relative to its source.

Links for further study:

Next: Audio Bit Depth of the Human Ear
Previous: Reverberation and Echo

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