14
$\begingroup$

When you're driving and you open 1 car window, say the front one, there comes a horrible noise, but when you open another window just the slightest bit, this noise goes away (I'm sure most people know what I'm talking about then I mention this 'noise').

  1. Why does this noise occur?
  2. Why does it go away when another window is slightly opened?

(Not sure about the tag).

Improve this question
$\endgroup$

2 Answers 2

Reset to default
18
$\begingroup$

The car is behaving like a closed pipe, so you get a resonance set up. There's a Wikipedia article here, but for once the Wikipedia article isn't that great, so there's another better article here. I imagine you (like most of us) will at some point have discovered you can make a sound by blowing across the top of an opened bottle, and it's the same thing happening in your car with the open window acting like the opening in the bottle. Since your car is much bigger than a bottle the resonance frequency is uncomfortably low.

When you open a second window you get an air current flowing through the car and this destroys the resonance.

Improve this answer
$\endgroup$
6
  • 2
    $\begingroup$ Finally I know. Someone published a paper, do scientists get paid for publishing papers? $\endgroup$
    – ODP
    May 22, 2012 at 15:33
  • 7
    $\begingroup$ @OllyPrice - no we just do it for the fame,glory and women $\endgroup$
    – Martin Beckett
    May 22, 2012 at 15:42
  • $\begingroup$ Perfectly good reasons if you ask me $\endgroup$
    – ODP
    May 22, 2012 at 15:43
  • 1
    $\begingroup$ "You don't have permission..." - link broken? $\endgroup$
    – Floris
    Aug 4, 2015 at 3:22
  • 3
    $\begingroup$ @Floris: gone beyond the ability of Google to relocate it. Oh well, that's the Internet for you. $\endgroup$
    – John Rennie
    Aug 4, 2015 at 5:11
2
$\begingroup$

In another post of a supposedly identical question today, which was closed while I was writing the following answer, it was claimed that the frequency did not change if the opening of the window was changed. And Helmholtz resonators change frequency with the area of the opening. Therefore I post my answer here:

The excitation of air oscillations in a moving car interior due to the open window is probably due to a vortex shedding phenomenon at the edge of the open window which creates vortices and thus air oscillations with a frequency proportional to the speed of the car. This phenomenon causes the whining or howling that is emitted from objects (e.g. ropes) in strong wind. These oscillation can also couple to the resonances of standing air waves in the car interior which would enhance the effect.

Improve this answer
$\endgroup$
9
  • $\begingroup$ I was also about to contribute a similar answer related to the Helmholtz resonance being driven by coupled vortex shedding and I believe this is the key to why the frequency does not change much with the area of the window. Given $f_s = \frac{S \ U}{D}$, $S = 0.2$, $U = 24.5 \ m/s$, $D = 4.9 \ m$, $f_s \simeq 1 \ Hz$, (which seems a bit low) but would explain the noticeably periodic 'woofing' sound of the window. $\endgroup$
    – Halyn Betchkal
    Feb 12, 2018 at 21:03
  • $\begingroup$ @D.Betchkal - Thank you for your helpful and reassuring comment. I think this is a good estimate, because the characteristic linear dimension $D$ might not necessarily be the total car length. I have experienced the phenomenon myself and think it has a higher frequency than 1Hz but still mostly infrasound. Maybe you can also vote to open the original question: physics.stackexchange.com/questions/385962/… $\endgroup$
    – freecharly
    Feb 12, 2018 at 22:06
  • $\begingroup$ that takes 3000 rep right? I'm a few thousand shy. $\endgroup$
    – Halyn Betchkal
    Feb 12, 2018 at 22:16
  • 1
    $\begingroup$ The frequency of the Helmholtz resonator scales with $\sqrt{V/L_{eff}}$, where the end correction for the finite aperture is $\frac{L}{L+0.3D}$ and the value $D$ is approximately $2h$ where $h$ is the size of the crack of the window (when $h \ll w$). It might be instructive to calculate the expected shift in frequency, and ask whether a casual observer could detect that difference. $\endgroup$
    – Floris
    Feb 12, 2018 at 22:21
  • 1
    $\begingroup$ Doesn't frequency scale with $\sqrt{\frac{1}{V \ L_{eff}}}$? Depending on the value of $L$ (which seems quite mysterious in the case of a car's window) $\Delta f$ is definitely large enough to detect. For $L = 0.1 \ m$, $\Delta f = 359 \ Hz$. Even for an unreasonably large $L=1 \ m$, $\Delta f = 72 Hz$, which is much greater than the Just Noticeable Difference (JND) for the human ear. Treating the car as a simple Helmholtz resonator doesn't appear to be consistent with the observation. $\endgroup$
    – Halyn Betchkal
    Feb 13, 2018 at 1:27

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.