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I am looking for anexplanation of this phenomenon:

sorry for bad drawing

I was riding my bike, (not driving it, was sitting on the back seat) with the helmet on. While the glass of the helmet was intact, I could hear the sound of air striking my helmet's outer surface, but when I just opened the glass of the helmet,(just opening implies opening it by a very small measure, like few degrees), all the noise of the air was gone, and then I closed the glass the noise came back.

A Few Details

  1. I was sitting on the back seat, so the wind was striking my helmet on its lateral sides
  2. My helmet is a very simple one--it doesn't bear the golfball-like craters, to counter friction
  3. The noise was coming from all the sides of the helmet, even the back,(or it was just the sides). It was like a 3-d experience,
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In my motorbiking days (now long behind me!) I noticed this. The effect is quite noticable, and it's odd because crash helmet fits snugly against the ears so your hearing should be little affected by whether the visor is up or down. I wonder if the sound is getting via your nose and eustation tube. –  John Rennie Apr 25 '12 at 14:21
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I've experienced this in a different way.. In my apartment, on a windy day one could hear a ghost-like noise with the windows closed. Open 'em a crack and it disappeared. Must be some resonance effect or something. (If you want, I can add this to the question) –  Manishearth Apr 25 '12 at 15:35
    
@John maybe not, see my comment above--that doesn't apply in that case. By the way, "eustachian" is spelled like "moustache" ;-) –  Manishearth Apr 25 '12 at 15:38
    
YEs @Manishearth , feel free to add it, it will be great! :-) –  Tomarinator Apr 25 '12 at 16:15
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I would only speculate why noise "disappears". If there is another, more loud noise source, you can no longer hear a less loud noise source. Additionally, your ears adapt to new noise source, so it does not necessarily sounds stronger than the first one. I think this is why you no longer hear "the sound of air" after you open your helmet. Similar situation happens between day and night. At night less loud noise sources become more disturbing, in absence of more loud noise sources. –  Pygmalion Apr 25 '12 at 16:36
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1 Answer

up vote 1 down vote accepted

You do not hear the wind on the helmet but you do hear something, no? The wind blowing through the opening.

I believe it is two effects:

1) the wind entering the helmet displaces and distorts sounds coming from the helmet to reach your ears as well as creates a sound itself; sound is pressure waves in the air the change in pressure opening the helmet changes the sounds.

2) It is a matter of perception of sound. For the human ear the louder sound masks the softer and the wind through the opening makes louder sounds.

Something similar happens when driving with windows closed or windows open in a car. The sound of the wind coming in masks any sound from the wind on the body of the car.

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