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Possible Duplicate:
Why, when one opens 1 car window, does that noise occur?

My knowledge in this area is really out-of-dated and stopped somewhere like ten years ago. So I would like to extend and update it.

I'm driving a car. I have opposite window fully open. I'm speeding to a speed around or slightly over 100 km/h. The air that comes through open window becomes "very loud" with a strange, pitching sound, that can be even interpreted as some kind of explosion.

What is this? Why this happens only at certain (rather high) speeds and only with window fully or nearly fully open? Can it be dangerous (distraction to driver)?

I barely remember that this could have something with sound (air?) pressure or the sound wave diffraction. But, since I had nearly absolutely nothing to do with physics for the last ten years, my information might be very wrong.

Community
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trejder
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    @JohnRennie: I agree, a duplicate. Sorry, for that. I did some search here, but found no results, so I posted a question. Sorry, seems that I searched not quite good. – trejder Jul 05 '12 at 07:24

2 Answers2

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Simple: at higher speeds the kinetic energy of the air skimming the window increases, so the sound waves become larger in magnitude. The amount of air involved is proportional to the size of the window.

The size of the window will also affect the pitch of the note, because the resonant frequency will change.

http://en.wikipedia.org/wiki/Helmholtz_resonance

spraff
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The the car behaves like a musical instruments (e.g trumpet). If the turbulence reach the resonant frequency you hear the huge loud. Changing the size of the open part of the window you have to change the velocity of the car in order to hear the "explosion". It is like you are playing your car :)

Emanuele Luzio
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