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It's the difference in the pressure of the air as it flows around objects. The differing pressure is often caused by a venturi effect... when air is compressed, as in passing through a crack in the wall or even flowing over a branch and then expands as it completes the passage, which causes the different pressures which causes vibrations detected by your ears. In some cases, the object over which the air is passing can bivrate causing sounds as well. The gas law at work here is Bernoulli's Principle, as well as others...
Okay, I see. Thanks! I know about Bernoullis principle (flow rate in=flow rate out right?). But I thought this only would lead to an increased speed (volume/second passing the exit opening compared to the entrance hole because of a smaller area)? But you are quite certain that this also leads to increased pressure/density?
But the branch (or other objects) might also make a sound if it hits a resonnance frequency? Does also the other effect has something to do with resonnance frequencies? Because I noticed that when I blow into the largest opening of a pen cap, you get a whistling sound when the air exits through the smaller hole (this was actually the reason I started thinking about it). But I need to blow at a certain strenght - if I blow too hard or too weak, there is no sound at all. This reminds me about the fundamental and harmonics in Helmholtz oscillators/standing wave experiments (empty bottle), but you can't have standing waves when you have two openings, can you?
Hope you or somebody else are able to provide me with an ever more detailed answer to these questions.
But the branch (or other objects) might also make a sound if it hits a resonnance frequency? Does also the other effect has something to do with resonnance frequencies? Because I noticed that when I blow into the largest opening of a pen cap, you get a whistling sound when the air exits through the smaller hole (this was actually the reason I started thinking about it). But I need to blow at a certain strenght - if I blow too hard or too weak, there is no sound at all. This reminds me about the fundamental and harmonics in Helmholtz oscillators/standing wave experiments (empty bottle), but you can't have standing waves when you have two openings, can you?
Hope you or somebody else are able to provide me with an ever more detailed answer to these questions.
I think that the whistling sound probably comes from a phenomenon called vortex shedding, which occurs on alternate sides of an object in wind.
This can be very destructive when if coincides with the natural frequency of the object e.g. the Tacoma Narrows Bridge. This is why tall thin chimneys often have spirals going up them, so that the vortex shedding is disrupted.
For a small object, the vortex shedding could occur at a frequency that we would hear as whistling.
http://www.princeton.edu/~asmits/Bicycle_web/separation. htmlJust after figure 13 there is a bit of explanation about this topic.
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