6.5 The Doppler Effect

Critical Questions:

• Why does the sound of an ambulance or car engine seem to change as the vehicle passes by?
• Why do we hear a loud bang when a supersonic jet flies by?
• What does all this have to do with the Big Bang?

I’d like you to do me a favour. This is especially important if you’re in a public place, surrounded by lots of strangers.

I’d like you to imitate, out loud and at a significant volume, the sound of a racecar going by.

The noise you made probably sounded something like, “weeeeeeeeEEEEEOOOOOOOooooo”. It’s a fairly common sound effect, one most people can perform even if they’ve never attended a car race. But why does it sound like that? Why does the car’s engine go from a higher pitch to a lower one as it passes us by?

This is the Doppler effect, and it’s fairly easy to explain. Imagine someone standing in the middle of a room and humming a single note. This person is a wave source, and they are creating ripples of sound that spread out in all directions. If the person isn’t moving, then anyone in any direction will receive those ripples at the same frequency, meaning that they will hear the same pitch.

Now imagine a moving source of sound – say, a car. As the car moves, it creates sound waves that spread out in all directions, just like the humming person. But the difference between the car and the person is that after creating one single ripple of sound, the car moves forwards – it moves closer to one part of that first ripple.

After it makes its second ripple then, two things have happened: in front of the car, the ripples are bunched together. And behind the car, the ripples are spread apart, because the car moved away from that part of the sound wave.

If you’re having trouble visualizing this, it would look something like the image below – with the exception that those rings of sound should constantly be spreading outwards.

Any person standing in front of the car receives more of those soundwave-ripples per second than normal – the frequency of the waves is higher, which means the sound has a higher pitch. A person standing behind the car receives fewer waves per second (because they’re spread out), which results in a lower-pitched sound.

You can hear this same effect anytime an ambulance passes by, or a police car with its siren going, or even a normal car if it’s moving quickly enough. As the sound approaches, it gets louder (simply because you’re closer to the source), and has a higher pitch. Then, as it passes, the sound suddenly dips down, and as it drives away you hear a lower pitch, plus a decreasing volume as the engine gets farther and farther away.

Hence, “weeeeeeeeEEEEEOOOOOOOooooo”.

Now, sound is an interesting type of wave here, because it goes much faster than racecars and ambulances – specifically, it travels at around 1200 km/h[1. This number will change depending on air temperature, pressure, and so on.]. So what happens when the source of the wave catches up to the wave it’s creating?

You might be surprised to know that you’ve probably seen this happen, in person. Any time you’ve been in a motorboat, yacht, or luxury steamer, you’ve probably looked backwards and watched the two lines of waves that trail off behind the boat in a big ‘V’ pattern. Maybe you’ve even been so lucky as to see a wakeboarder do a sweet jump off one of those permanent waves.

What’s happening here is that as the new ripples are being created, the wave source (the boat) is keeping up or even passing the old ones. This results in each new wave being created right on top of the old one, and a trail of extra-large waves forms behind the wave source.

In the case of a boat, you get two lines (because the surface of the water is two-dimensional. But if an airplane moves faster than the sound it’s creating, the pattern is a cone.
A speed faster than the speed of sound is called “supersonic”, and it usually requires a special jet. And that cone of extra-intense sound – which we call a ‘shockwave’ – does two remarkable things. First, it results in the “sonic boom” that you’ve heard if you’ve ever been to an airshow. And second, because sound is just compression waves moving through the air, the shockwave can actually knock dissolved water particles out of the atmosphere, resulting in a conical cloud around the airplane, like the one in the photo below:

Ok, so by now you’re thinking that maybe I told you about the somewhat yawn-worthy Doppler effect in order to tell you about the reasonably-cool sonic boom thing, right?

Well it turns out that I had an even better reason. You see, this phenomenon became incredibly important to science – and, really, to the entire planet – when a certain astronomer named Edwin Hubble looked into the night sky and found that the frequencies of light coming from distant galaxies seemed to be Doppler-shifted in a way that suggested those galaxies were moving away from us.

This seemed rather strange: everything out in space was running away from us, in all directions, everywhere. Did we forget to shower this morning? Is there an enormous t-rex standing directly behind us that we haven’t seen yet?

After surreptitiously sniffing his own armpits, Hubble decided that if everything was moving away from us, at some point it all must have started in the same place. This was the beginning of the theory of the Big Bang, and as time went on and more evidence was discovered, this bizarre idea began to seem more and more likely.

But we’ll have to leave that behind until we get to Chapter 11, which is all about outer space and the universe.

In the next chapter, we’ll delve into the mysterious world of light.

Big Ideas:

• When a wave source (sound or otherwise) is moving, it creates different frequencies of waves in front of and behind it. This is called the Doppler Effect.
• In front of a moving wave source, the waves are bunched together and thus have a higher frequency. Behind the source, the waves have a lower frequency.
• If a wave source is travelling faster than the waves it creates, a shockwave will result.
• The Doppler Effect, observed in light waves emitted by distant galaxies, was the first indication that the universe had started with the Big Bang.

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Next: 7.1 – Introduction to Light and Optics

Previous: 6.4 – Resonance