Chugging sound when rear windows of vehicle are open

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.

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.

Opening a window like this turns your car into a giant Helmholtz resonator:

Helmholtz resonance sometimes occurs when a slightly open single car window makes a very loud sound, also called side window buffeting or wind throb.

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. : )

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.

A+ reference! Adam puts out so many videos I don’t watch them all, and I’d missed this one!

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.

Here are a couple of previous threads on the topic:

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.

A rotary woofer can do the job.

There’s also the diesel-powered Mythwoofer the Mythbusters built.

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.

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.

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.

And it will make you deaf very quickly. Road trip science: Why the car's open rear window hurts your ears | Ottawa Citizen

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.

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?

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.

Air is highly compressible. What on Earth do you mean by “you couldn’t close the door”?

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.