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Physics/strange question re: sound


QUESTION: This is a very off the wall question I know. I don't really expect you to know the answer, but thought I'd try... Since I have owned my new car, over the last 18 months, when driving it I hear a very slight crackle noise coming from the plastic on the interior of the passenger door, near the door handle and it drives me nuts when it does it; temperature doesn't seem to matter. I've sprayed silicone on every spot that may be causing the sound but it persists. But this is the very strange thing...
when I'm in the driver's seat and  lean my head towards the middle of the car, towards the sound, it stops or at least I don't hear it. Then when I move my head back, I hear it again. It's like a paradox. So I wonder if it's a physics type thing, which I know nothing about. Is there any scientific reason that could explain not hearing a sound the closer you get to it?? And vice versa? Sounds crazy I know, but that's what's happening and I find it more than baffling. Thanks for any feedback.

sound focusing
sound focusing  
ANSWER: Is there any curvature at all to the door on that side of the car, or the passenger window?  What you could have is a sound that doesn't originate there, but is reflected in a focusing way on you when you're in the driver's seat.  That would explain why you can't hear it when you lean over.  It doesn't have to be a parabolic dish to focus sound, just a simple curve would do it.  So now you might want to think, what between you and the door (given the shape of most cars) could be emitting the sound that you hear?

See my badly photoshopped picture of a car.  The source, depending on geometry, could be the engine, some random rattle in the transmission, something on the floor of the passenger side, even the tires on the drive's side.

---------- FOLLOW-UP ----------

QUESTION: Now that's interesting, fascinating really and never occurred to me. The door does have some curve, but not a lot. But it definitely sounds like a very faint plastic crackle, so I doubt it's engine related, who knows. But I'm for sure going to check the floor on that side and anything else that may be reflecting back to me. Thanks!

Well, if that doesn't solve it and you can't hear it coming from the driver's side when you sit on the passenger side (does having a passenger stop the sound?) and the car is running under the same conditions, then you have one of those perniciously sneaky car sounds that's hard to determine.  Sometimes weird resonant vibrations form in cars that cause sounds which are hard to pinpoint without an array of transducers...can't help with that from here.


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Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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