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I was going to ask a question similar to: Why, when one opens 1 car window, does that noise occur? but saw that it was already asked and answered very helpfully. However, there are a couple other things about it which still confuse me, which were not answered by the answer to that question. I understand how the noise is made, but why is it made only by the back windows, and only when the window is partly down and not when it's all the way down?

If it is made like the way you blow across a bottle opening to make sound, as I understand by the way it is explained in that other answer, then I don't see why it wouldn't just the same with a front window, or with a window that's all the way down.

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Ester Montague
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1 Answers1

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Why the rear window?

The sound occurs when wind is blown across a lip:

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At the arrow, the wind is blowing like the air that a flutist blows across the mouthpiece - so it sets up a resonance.

Now for a resonance to work, the cavity must have a sufficiently high "Q" - quality factor. The air must reflect from most surfaces, and only have a limited chance to escape, right where the blowing wind is acting. If the hole in the window is too big, the velocity of the air escaping is too small to significantly affect the direction of the wind blowing over the edge, and so it does not build up the resonance. Thus, the window must be open "just enough" to provide the energy to set up the resonance, but not so much that the Q of the car goes down. I described this in more detail in an earlier answer about whistles - that might be helpful background.

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Floris
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