Ray Garrison Posted April 3, 2006 Share Posted April 3, 2006 I just got a 2006 Jetta. If I roll down the rear right window and drive with the other windows rolled up, I get an *INTENSE* resonance inside the passenger compartment - a woof woof woof woof woof of air pressure that is physically uncomfortable to the point of nausia, and really hurts my ears. My 6 year old son thinks it's way cool, so he keeps rolling down the window. I was reading the KHorn low frequency capability thread, and people are talking about pipe organ installations that can go down to 16Hz, 8 Hz in some cases. Anybody know an easy way to estimate the frequency of the resonance in the Jetta? The beats are too fast to count, but slow enough to feel as individual pulses. I'm thinking if this is what an 8Hz pipe organ would feel like, I'm not going to be paying a visit anytime soon. Any auto engineers out there that can explain the cause? And how Volkswagen could sell a car that does this? Just curious... Quote Link to comment Share on other sites More sharing options...
Speedball Posted April 3, 2006 Share Posted April 3, 2006 Same thing with one of our cars from the past, a Saturn or Acura At about 55 mph with the rear windows down about an inch or two it is unbearable .....not sure if it is the wind or sound as you mention. Quote Link to comment Share on other sites More sharing options...
pauln Posted April 3, 2006 Share Posted April 3, 2006 Lots of cars do that! You are pushing a fairly large cavity with an opening tangent to the wind, so it is like blowing on a soda bottle (like the hillbillys in a jug band - please, no flames from any hillbillys on the forum). The cavity is resonant at a very low frequency - probably 6-15 Hz depending on the car and can be pretty intense. If you open just a front window, I think the 'bow wave' coming off the front of the car or windshield pushes somewhat away from the chassis so the effect if minimal, but the air stream over the back sides of the car are closer and more tangent to the plane of the opening. You might try adhering some damping putty to the insides of the car or fill the back with some fiber fluff?? Imagine if this was a problem with the front window (NASCAR comes to mind). Pauln Quote Link to comment Share on other sites More sharing options...
LarryC Posted April 3, 2006 Share Posted April 3, 2006 Bring back the wing window!! IIRC, these problems didn't exist if one simply cracked open a WW. Here's what you need: Quote Link to comment Share on other sites More sharing options...
mark1101 Posted April 3, 2006 Share Posted April 3, 2006 It sounds like you need some Bose noise canceling head phones. [] Quote Link to comment Share on other sites More sharing options...
ironwoods Posted April 4, 2006 Share Posted April 4, 2006 Ray, can you run the dash or floor vents with fresh outside air, to get the circulation you desire? This may be the reason that "Drivers" are "Wanted"? Quote Link to comment Share on other sites More sharing options...
chops Posted April 4, 2006 Share Posted April 4, 2006 Yeah, this is all old news. Just about any 4-door car or even crew cab truck with 4-doors will do this. It's definately NOT a fault with VW's engineering or design. Quote Link to comment Share on other sites More sharing options...
Speedball Posted April 4, 2006 Share Posted April 4, 2006 Very nice car Ray, they changed the looks of the Jetta considerably.......[] Quote Link to comment Share on other sites More sharing options...
efzauner Posted April 4, 2006 Share Posted April 4, 2006 Some cars like the new Honda Odyssey have a noise cancelling sound system. I wonder if these would help? Er, no, would need one serious subwoofer in the car capable of 10 hz! Quote Link to comment Share on other sites More sharing options...
Tom Mobley Posted April 4, 2006 Share Posted April 4, 2006 my 92 Volvo does that. It's the first 4 door car I've ever owned, absolutely intolerable with a back window down a little. I thought it was me or the car. Glad to see this thread. Quote Link to comment Share on other sites More sharing options...
Ray Garrison Posted April 4, 2006 Author Share Posted April 4, 2006 So, to get back to my original question... is there any way anybody knows of to estimate the frequency? I'm really curious... something like the RS SPL meter that functions as a "What frequency izzat?" meter? Quote Link to comment Share on other sites More sharing options...
chops Posted April 4, 2006 Share Posted April 4, 2006 So, to get back to my original question... is there any way anybody knows of to estimate the frequency? I'm really curious... something like the RS SPL meter that functions as a "What frequency izzat?" meter? Get yourself a dc/ac power converter, a mic and one of these... Quote Link to comment Share on other sites More sharing options...
ben. Posted April 6, 2006 Share Posted April 6, 2006 Try humming a low note along with it. If you find a note that sounds more "right" than any others, it's most likely a few octaves above that resonance in the cabin. Figure out what note it is, then halve its frequency a few times... it could be right. Or mic it up and load it into an audio program on a computer or DAW. Zoom in on the waveform untilyou can see the individual pulses. Then you can just count the peaks between time markers on the display if so equipped. If not, you may be able to cut a 1sec long snippet and view that seperately, or just slow down the playback so you can count the pulses. My Element is the first vehicle I've driven that I like with windows open. There's a rear sunroof and mid-cabin windows that pop open like van windows. With those open & the front passenger window down 4-6 inches, it's really nice on a good day. Quote Link to comment Share on other sites More sharing options...
DrWho Posted April 6, 2006 Share Posted April 6, 2006 So, to get back to my original question... is there any way anybody knows of to estimate the frequency? I'm really curious... something like the RS SPL meter that functions as a "What frequency izzat?" meter? A portable RTA is what you're talking about and they usually run around $500 and don't usually measure below 20Hz (typically they measure the same frequencies as a 31-band graphic EQ....) That said, you probably don't want to go that route [] A cheaper alternative would be to use free RTA software...if you've got a laptop your laptop mic might be able to pick up which frequency it is...if not, then you will need to get a better mic (again, $$$...$50 to be exact) I like the idea of trying to find an upper-harmonic and trying to calculate back...though it's probably not too accurate...it might possibly be done with a CD with a bunch of 1Hz increment test tones. Get the car resonating and then start playing the CD and see if you can't find a tone that keeps a constant volume. Btw, what are the dimensions of your cabin? I'll see if I can't calculate it out....your car is simply behaving like a helmholtz radiator. No amount of damping inside the cabin is going to make a difference either - it is entirely a function of the air trapped inside. You should be able to get rid of the resonance if you open more than one window at once...like keep both rear windows open (and the energy should cancel each other out). Quote Link to comment Share on other sites More sharing options...
WMcD Posted April 6, 2006 Share Posted April 6, 2006 Very interesting. I think it might be a whistle effect rather than a Helholtz resonator (there is not trapped mass but we do have a cavity). I was reading some book long ago, It might have been, The Invention Which Changed the World. That ambitious title was a bit far fetched. The theory went that radar used diode detectors (like crystal radio) that led to transistors, that led to computers and chips. Ergo, radar engineers invented LSI. By these standards, the crystal radio invented LSI too.. In any event, there is a story ( in the book) about the cavity magnitron invented by the British and necessary to microwave (radar) generation. A sample was shipped to the USA under great security so that the Yanks could pick up on production before England was invaded or factories shut down.. One of the persons looking at the diagram was Enrico Fermi. He says to his fellow great minds, "Oh, I see it works like a whistle." And the rest of of the great minds say, "Oh, but how does a whistle work?" Well I'm not sure either. But basically there is a fixed blade in an airflow. The airflow has two unstable conditions in flowing to one side or the other side of the blade. So it flips back and forth. I can only think that your car's open window is the blade and the airflow is vehicle motion. I'm note quite sure how the cavity in the car effects the frequency. You might be able to Google based on that theory. Gil Quote Link to comment Share on other sites More sharing options...
oldtimer Posted April 7, 2006 Share Posted April 7, 2006 Sympathetic and para-sympathetic vibrations? Is it worse at some speeds compared to others? Quote Link to comment Share on other sites More sharing options...
Ray Garrison Posted April 7, 2006 Author Share Posted April 7, 2006 Gil - cool thought. I guess I was having trouble figuring out how I was getting a resonating beat inside the car - thinking about it like a very big, very low frequency whistle with the C panel acting as the fipple and the passenger compartment the resonant chamber makes a lot more sense. Thanks! Oldtimer - it gets really bad at about 60 - 70 Ft/Sec. I haven't had the courage to accelerate past that point as my eardrums feel like they're going to be ripped out of my head if it gets any worser. Quote Link to comment Share on other sites More sharing options...
WMcD Posted April 7, 2006 Share Posted April 7, 2006 About measuring. Some hand held multimeters have a frequency read out. You could use the line level output of an RS meter to feed a multimeter input. I've been playing with the ability of laptops to record. My present one ony has a mike input; no internal mike, and no line level input. I've not had time to mess with an outboard audio card. But a little junk mike from a voice recognition program worked okay for tests on voice. You might have a laptop with an internal mike, or you can lash up something. I've fooled a bit with the Audacity free ware for recording. In playback there is a very cool real time display (not a fft) with a cursor wiping along the waveform recorded. It might be possible to expand the scale and estimate the period of the recorded signal. Finally, Windows media player has a crude RTA in the "bars" display. You'd have to find a way to calibrate it. Audacity will put down a pure tone. Gil . Quote Link to comment Share on other sites More sharing options...
Paladin Posted April 12, 2006 Share Posted April 12, 2006 My car does this too... I'm not sure about the frequencies and such...but a good way to cancel it away is to open the front and rear windows the same amount. I always drop my driver's side rear window with my front window (I suppose this only applies if you have power windows...but I'm assuming that power accessories in a new VW is pretty much a given...). I suppose that doesn't help if you son keeps putting it down on you though Quote Link to comment Share on other sites More sharing options...
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