why when you’re driving and lower the back windows but not the front it makes that debilitating helicopter noise?
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The interior of the car has a certain volume of air inside. The relatively uniform shape of the interior (cube) goes the air a simple path to go any direction to reach a surface.
Air reacts to sudden pressure application by transferring the force as a wave. Once the wave reaches a wall, it bounces back toward the original source. The speed of the wave has a certain characteristic speed. By calculating the distance the wave need to travel in order to make a trip from the open window, to the walls, and back, and by knowing the speed of the sound wave(the speed of sound), you can find that certain numbers keep coming out of the calculator. This becomes the resonant frequency of the car. Also, you can get multiples of the fundamental frequency, because the phase of the wave as it arrives back at the source happens to line-up for a certain frequency, while all other frequencies align imperfectly, or even totally opposite which would result in silence.
The air rushing by the window is turbulent, and applies forces against the air inside the car at lots of random frequencies, but sometimes it hits ‘just right’ and starts a resonant wave that self-reinforces with each application of the turbulence at the window. This is why the ‘helicopter’ sound tends to build-up to some maximum amount and then stay like that until you break the cycle somehow, by changing the window position, slowing down, opening another window, etc.
It’s literally the same as a whistle, flute, or noisy cola bottle whistle, only much larger size and therefore, lower frequency since those properties are so related.
Ok, that ended up being unintelligible to most 5 year olds I have ever known, but it’s kind of hard to dumb it down much further without ruining it completely.
I guess, it boils down to: the size of the space inside the car, and the application of repetitive force from the turbulent air outside, crates the hum that you hear. It’s an efficient resonant-cavity. Not all cars can achieve the effect thanks to the shapes inside, and their specific sizes, but when it works, it usually works really well!
Try this: if you have access to a shower stall that’s flat on all four sides(square or rectangle) with hard walls(tile), hum loudly inside there varying the pitch as far up and down as you can. You will hear that certain tones are much louder than others, all thanks to the waves interacting with each other and your voice/neck. Certain other tones will also be very muted(anti-resonance).
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