Doppler Effect
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This Wiki entry has been taken from the OpenAL Dopper Tutorial (http://www.devmaster.net/articles/openal-tutorials/lesson7.php) by Jesse Maurais, and modified to fit the Wiki.
The Doppler effect can be a very tricky concept for some people, but it is a logical process, and kind of interesting when you get right down to it. To begin understanding the Doppler effect we first must start to understand what a "sound" really is. Basically a sound is your minds interpretation of a compression wave that is traveling through the air. Whenever the air becomes disturbed it starts a wave which compresses the air particles around it. This wave travels outward from it's point of origin. Consider the following diagram.
In this diagram (on the left) the big red "S" stands for the sources position, and the big red "L" stands for (you guessed it), the Listener's position. Both source and Listener are not moving. The source is emitting compression waves outward, which are represented in this diagram by the blue circles. The Listener is experiencing the sound exactly as it is being made in this diagram. The Doppler effect is not actually present in this example since there is no motion; the Doppler effect only describes the warping of sound due to motion.
What you should try to do is picture this diagram animated. When the source emits a wave (the circles) it will look as though it is growing away from it's point of origin, which is the sources position. A good example of a similar effect is the ripples in a pond. When you throw a pebble into a calm body of water it will emit waves which constantly move away from the point of impact. Believe it or not this occurs from the exact same physical properties. But what does this have to do with the Doppler effect? Check out the next diagram.
Wow, what's going on here? The source is now in motion, indicated by the little red arrow. In fact the source is now moving towards the Listener with an implied velocity. Notice particularly that the waves (circles) are being displaced inside each other. The displacement follows the approximate path of the source which emits them. This is the key to the Doppler effect. Essentially what has happened is that the source has emitted a wave at different points in it's path of travel. The waves it emits do not move with it, but continue on their own path of travel from the point they were emitted.
So how does this effect the perceived sound by the Listener? Well, notice too in the last diagram that the waves (circles) that are between the source and the Listener are kind of compressed together. This will cause the sound waves to run together, which in turn causes the perceived sound seem like it's faster. What we are talking about here is frequency. The distances between the waves effects the frequency of the sound. When the source that emits the sound is in motion, it causes a change in frequency. You may notice too that distance between the waves varies at different points in space. For example, on the opposite side of the moving source (anywhere along the previous path of travel) the distances are actually wider, so the frequency will be lower (the distance and frequency have an inverse relationship). What this implies is that the frequency perceived by the Listener is relative to where the Listener is standing.
The motion of the Listener can also affect the frequency. This one is a little harder to picture though. If the source is still, and the Listener is moving toward the source, then the perceived frequency by the Listener will be warped in the same exact manner that we described for the moving source.
If you still have trouble picturing this, consider the following two diagrams:
These two diagrams will represent the sound in the form of a sine wave. Look at the first one. Think of the peaks as the instance of the wave. The very top point of the wave will be the same as the instance of the blue circle in the previous set of diagrams. The valleys will be like the spaces in between the blue circles. The second diagram represents a compressed wave. When you compare the two you will notice an obvious difference. The second diagram simply has more wave occurrences in the same amount of space. Other ways of saying this are that they occur more often, with a greater regularity, or with a greater frequency.
For anyone who is interested in some added information: The velocity of the waves is the speed of sound. If the velocity of the source is greater than that of the wave, then the source is breaking the sound barrier.
