# Chugging sound when rear windows of vehicle are open
**URL:** https://boards.straightdope.com/t/chugging-sound-when-rear-windows-of-vehicle-are-open/949795
**Category:** Factual Questions
**Created:** [August 30, 2021, 8:23pm UTC](https://boards.straightdope.com/t/chugging-sound-when-rear-windows-of-vehicle-are-open/949795 "2021-08-30T20:23:18Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@PozhanPop](https://boards.straightdope.com/u/PozhanPop)
#### Post date: [August 30, 2021, 8:23pm UTC](https://boards.straightdope.com/t/chugging-sound-when-rear-windows-of-vehicle-are-open/949795/1 "2021-08-30T20:23:18Z")
What causes the chugging sound when we open the rear windows of a vehicle ?
I have noticed it mostly in SUVs.
It has to do with turbulence I know, but chugging pattern of the air rushing past the windows what I am curious to know about.
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### Author:  [@naita](https://boards.straightdope.com/u/naita)
#### Post date: [August 30, 2021, 9:11pm UTC](https://boards.straightdope.com/t/chugging-sound-when-rear-windows-of-vehicle-are-open/949795/2 "2021-08-30T21:11:35Z")
I assume it’s similar to what happens with a flute. You blow across the top and the air in the tube stars vibrating according to the resonance frequency of the volume. Your car has a resonance frequency as well, it’s just a lot lower, so you experience it as “chugging” instead of as a very low base note.
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### Author:  [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)
#### Post date: [August 30, 2021, 9:21pm UTC](https://boards.straightdope.com/t/chugging-sound-when-rear-windows-of-vehicle-are-open/949795/3 "2021-08-30T21:21:41Z")
Opening a window like this turns your car into a giant Helmholtz resonator:
> **[Helmholtz resonance](https://en.wikipedia.org/wiki/Helmholtz_resonance)**
>
> Helmholtz resonance or wind throb is the phenomenon of air resonance in a cavity, such as when one blows across the top of an empty bottle. The name comes from a device created in the 1850s by Hermann von Helmholtz, the Helmholtz resonator, which he used to identify the various frequencies or musical pitches present in music and other complex sounds.
> Helmholtz described in his 1862 book On the Sensations of Tone an apparatus able to pick out specific frequencies from a complex sound. The Helmhol...
> Helmholtz resonance sometimes occurs when a slightly open single car window makes a very loud sound, also called side window buffeting or wind throb.
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### Author:  [@PozhanPop](https://boards.straightdope.com/u/PozhanPop)
#### Post date: [August 30, 2021, 9:59pm UTC](https://boards.straightdope.com/t/chugging-sound-when-rear-windows-of-vehicle-are-open/949795/4 "2021-08-30T21:59:07Z")
Thank you.
I never realized it had to do with the inside of the vehicle.
I had learnt about Helmholtz resonance in engineering physics but never really put two and two together. : )
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### Author:  [@Joey\_P](https://boards.straightdope.com/u/Joey_P)
#### Post date: [August 30, 2021, 10:06pm UTC](https://boards.straightdope.com/t/chugging-sound-when-rear-windows-of-vehicle-are-open/949795/5 "2021-08-30T22:06:04Z")
NASA calls it shape it makes a Strudel Wave.
Start around 2:45 when Adam is talking about taking a model car down to NASA’s water tank.
[![](https://img.youtube.com/vi/snNL5GgOq_c/maxresdefault.jpg "Why "Golf Ball Car" Is Pivotal in MythBusters' History") ](https://www.youtube.com/watch?v=snNL5GgOq_c)
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### Author:  [@naita](https://boards.straightdope.com/u/naita)
#### Post date: [August 31, 2021, 3:18pm UTC](https://boards.straightdope.com/t/chugging-sound-when-rear-windows-of-vehicle-are-open/949795/6 "2021-08-31T15:18:04Z")
A+ reference! Adam puts out so many videos I don’t watch them all, and I’d missed this one!
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### Author:  [@Joey\_P](https://boards.straightdope.com/u/Joey_P)
#### Post date: [August 31, 2021, 4:35pm UTC](https://boards.straightdope.com/t/chugging-sound-when-rear-windows-of-vehicle-are-open/949795/7 "2021-08-31T16:35:56Z")
For a while the only ones I watched were the ones on his Tested channel, with him, in his shop. Typically either doing a build or answering questions. I still don’t watch the ones with the other people (person? is it just Norm?) or the podcasts, but I recently started watching all the ones about the auction for Grant. I didn’t think I’d be interested in them, but they’re very similar to his Q&A videos where he basically just rambles on for 10 minutes about something and goes off on 10 different tangents in the process.
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### Author:  [@Telemark](https://boards.straightdope.com/u/Telemark)
#### Post date: [August 31, 2021, 4:56pm UTC](https://boards.straightdope.com/t/chugging-sound-when-rear-windows-of-vehicle-are-open/949795/8 "2021-08-31T16:56:03Z")
Here are a couple of previous threads on the topic:
> [@Does this really annoying sound happen in your car, and what causes it, and is there a fix?](https://boards.straightdope.com/t/does-this-really-annoying-sound-happen-in-your-car-and-what-causes-it-and-is-there-a-fix/755466):
>
> When I have either or both back windows open more than a small crack, and at speeds between about 30 and 60 mph, a loud low frequency flutter develops. It is very intense, almost painful, and it’s something in the neighborhood of maybe 150 hertz. It seems not to be audible to passengers in the back seat, since they are invariably the ones who start it by opening their windows, but they show no real awareness of it. I thought it was something peculiar to my Mazda 3, but my sister says the exac…
> [@Modern car design: windows up when driving?](https://boards.straightdope.com/t/modern-car-design-windows-up-when-driving/774304):
>
> What is it about the design of more modern cars that driving with the windows (particularly rear ones) down creates an unpleasant booming noise? Sure, aircon being standard now means we shouldn’t need the windows down, but sometimes I just quite like an open window as I’m cruising along back country roads. My first car/s sans aircon were fine with the windows down, and also had that little triangular pop-out window in the front seats to ensure a stream of wind in your face on the warmer days. …
> [@Ear pressure in car](https://boards.straightdope.com/t/ear-pressure-in-car/662238/2):
>
> Just a guess, but air pressure could be building from air being forced into the car, but not having a ready exit. Both windows open would allow both entry and exit for the air, lowering the effective air pressure.
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### Author:  [@Napier](https://boards.straightdope.com/u/Napier)
#### Post date: [August 31, 2021, 7:02pm UTC](https://boards.straightdope.com/t/chugging-sound-when-rear-windows-of-vehicle-are-open/949795/9 "2021-08-31T19:02:52Z")
It’s a nice example of infrasound, that being sound of such low frequency that we don’t hear it as much as sense it. Your stereo speakers would need to work well down to, oh, 5 or 10 or so Hz to be able to reproduce it.
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### Author:  [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)
#### Post date: [August 31, 2021, 7:14pm UTC](https://boards.straightdope.com/t/chugging-sound-when-rear-windows-of-vehicle-are-open/949795/10 "2021-08-31T19:14:06Z")
A rotary woofer can do the job.
> **[Rotary woofer](https://en.wikipedia.org/wiki/Rotary_woofer)**
>
> Pages for logged out editors learn more
>
> A rotary woofer is a subwoofer-style loudspeaker which reproduces very low frequency content by using a conventional speaker voice coil's motion to change the pitch of an impeller rotating at a constant speed. The pitch of the fan blades is controlled by the audio signal presented to the voice coil, and is able to swing both positive and negative, with respect to a zero pitch spinning blade position. Since the audio amplifier only changes the pitc...
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### Author:  [@garygnu](https://boards.straightdope.com/u/garygnu)
#### Post date: [August 31, 2021, 7:47pm UTC](https://boards.straightdope.com/t/chugging-sound-when-rear-windows-of-vehicle-are-open/949795/11 "2021-08-31T19:47:16Z")
There’s also the diesel-powered Mythwoofer the Mythbusters built.
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### Author:  [@california\_jobcase](https://boards.straightdope.com/u/california_jobcase)
#### Post date: [August 31, 2021, 8:09pm UTC](https://boards.straightdope.com/t/chugging-sound-when-rear-windows-of-vehicle-are-open/949795/12 "2021-08-31T20:09:06Z")
Many cars used to have a feature that prevented the booming from happening. They had vents in the rear that let air circulate through the trunk and the cabin to the outside, or through the hatch back or doors. Vehicles I had that were so equipped were a Sentra sedan (holes in the package shelf and hidden trunk vents built into the tail lights), a 1988 Ford Festiva (vents through the hatch), and a 2009 Ranger (through the doors- vent outlets on door edge outside of weather strip, but inside lip of door). A Celica I had also had the vents but I don’t remember if they were through the tail lights or the hatch. There were others I don’t recall, I’m sure- I’ve owned over twenty cars over the years.
This isn’t a difficult feature to incorporate into the design of a car. Curiously, the most expensive car I ever bought (2018 Impala) doesn’t have it. Yes, it booms like crazy.
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### Author:  [@PozhanPop](https://boards.straightdope.com/u/PozhanPop)
#### Post date: [August 31, 2021, 8:50pm UTC](https://boards.straightdope.com/t/chugging-sound-when-rear-windows-of-vehicle-are-open/949795/13 "2021-08-31T20:50:48Z")
Does the shape and size of the car affect the booming ?
Does the large open cargo area behind the rear seat have an effect ?
My vehicles still does it spectacularly with a full passenger load.
I don’t remember this happening with a couple of small cars that I’d owned.
A little Suzuki Alto and a Nissan Micra.
I first noticed it in my Nissan Xterra and then in almost all SUVs I’ve been in.
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### Author:  [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)
#### Post date: [September 1, 2021, 10:18am UTC](https://boards.straightdope.com/t/chugging-sound-when-rear-windows-of-vehicle-are-open/949795/14 "2021-09-01T10:18:25Z")
> [@PozhanPop](#):
>
> Does the shape and size of the car affect the booming ?
> Does the large open cargo area behind the rear seat have an effect ?
The shape of the car can modify the airflow across the open window, making the sound louder or quieter. This is analogous to blowing across the top of an empty beer bottle and changing the angle from which you blow, which affects the resulting sound volume.
The frequency of the booming is primarily related to the surface area of the open window and the volume contained inside the vehicle: a larger window opening increases the frequency, and a larger cabin volume decreases the frequency. So if you’re dealing with an SUV, the volume of the rear cargo area works together with the volume of the passenger seating area, effectively forming one big interior volume and making the booming frequency lower than it would be if the cargo area were sealed off with a rigid wall.
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### Author:  [@Unintentionally\_Blank](https://boards.straightdope.com/u/Unintentionally_Blank)
#### Post date: [September 1, 2021, 12:10pm UTC](https://boards.straightdope.com/t/chugging-sound-when-rear-windows-of-vehicle-are-open/949795/15 "2021-09-01T12:10:06Z")
And it will make you deaf very quickly. [Road trip science: Why the car's open rear window hurts your ears | Ottawa Citizen](https://ottawacitizen.com/news/local-news/road-trip-science-why-the-cars-open-rear-window-hurts-your-ears)
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### Author:  [@jjakucyk](https://boards.straightdope.com/u/jjakucyk)
#### Post date: [September 1, 2021, 1:26pm UTC](https://boards.straightdope.com/t/chugging-sound-when-rear-windows-of-vehicle-are-open/949795/16 "2021-09-01T13:26:46Z")
> [@california\_jobcase](#):
>
> Many cars used to have a feature that prevented the booming from happening. They had vents in the rear that let air circulate through the trunk and the cabin to the outside, or through the hatch back or doors.
Is that the purpose of those vents or just a side benefit? My 2014 Mazda 3 has vents under the rear window that go into the trunk, but they’re really too small to do much about booming. My understanding is that they’re to allow airflow into the trunk partly as a means of conditioning it but also to provide a path for air from the climate control system to get out when it’s not in recirculate mode.
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### Author:  [@california\_jobcase](https://boards.straightdope.com/u/california_jobcase)
#### Post date: [September 1, 2021, 3:08pm UTC](https://boards.straightdope.com/t/chugging-sound-when-rear-windows-of-vehicle-are-open/949795/17 "2021-09-01T15:08:02Z")
I think they were designed for better flow-through ventilation from the windows. Air conditioning used to be an option, and these were supposed to help. Have you determined if there are vents to the outside in the trunk of your car?
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### Author:  [@jjakucyk](https://boards.straightdope.com/u/jjakucyk)
#### Post date: [September 1, 2021, 3:35pm UTC](https://boards.straightdope.com/t/chugging-sound-when-rear-windows-of-vehicle-are-open/949795/18 "2021-09-01T15:35:54Z")
> [@california\_jobcase](#):
>
> Have you determined if there are vents to the outside in the trunk of your car?
No but they’re usually hidden behind the bumper, and with all the carpeting, hidden compartments, spare tire well, etc., they could be anywhere. In some cars there’s just gaps around the taillights or the trunk gasket itself.
I’ve also heard they’re pressure reliefs for when you close the doors. Older VW bugs would need a window to be cracked open otherwise you couldn’t close the door. This mitigates that. Having vents only between the cab and trunk wouldn’t work, they need a path to the exterior.
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### Author:  [@naita](https://boards.straightdope.com/u/naita)
#### Post date: [September 1, 2021, 7:05pm UTC](https://boards.straightdope.com/t/chugging-sound-when-rear-windows-of-vehicle-are-open/949795/19 "2021-09-01T19:05:13Z")
> [@jjakucyk](#):
>
> I’ve also heard they’re pressure reliefs for when you close the doors. Older VW bugs would need a window to be cracked open otherwise you couldn’t close the door. This mitigates that.
Air is highly compressible. What on Earth do you mean by “you couldn’t close the door”?
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### Author:  [@jjakucyk](https://boards.straightdope.com/u/jjakucyk)
#### Post date: [September 1, 2021, 7:47pm UTC](https://boards.straightdope.com/t/chugging-sound-when-rear-windows-of-vehicle-are-open/949795/20 "2021-09-01T19:47:09Z")
When closing a car door you need to give it some oomph to get it to latch, and the large area of the door when closing against an airtight cabin would cause it to bounce off the bubble of air in the cabin. Yes air is compressible, but the act of closing the door causes a spike in pressure that tries to get out around the same door, pushing it out. The harder you slam the door the harder the air bubble pushes back. If you could slowly creep the door closed then it would be fine, but in most cars then it’s not fully latched.
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