That's why I always lower the windows in pairs. Back windows together, front windows together. No air flow problems.

 

Apparently this is a very common problem:

http://cartalk.com/board/showflat.ph...22&an=0&page=2

Intriguing... must be a way of estimating the resonance frequency based on the volume of the cabin, size of the window opening and the speed of the wind...

Wind deflectors should help prevent this, but putting permanent deflectors on the rear windows I would think would look both ugly and add unnecessary drag when the windows are closed. Wonder if there's a way to design wind deflectors that automatically fold out when a window is rolled down and retract when a window is rolled up...

 

At least for the HiHy I found a very simple way to kill the "helicopter" resonance. If you have the passenger-side back window open and your head is pounding from the helicopter thumping, simply open the passenger-side front window about 2 inches. *Poof*, helicopter goes away. I still notice a slight residual pressure on my ears, but it's constant and not very noticeable.

Similarly, if you have the driver-side back window open and Mr. Chopper is beating its drums, open the driver-side front window about 2 inches. Again, *poof*. Problem solved.

 

This phenomena is called Helmholtz Resonance

http://en.wikipedia.org/wiki/Helmholtz_resonator

 

Hmm.. very interesting. Thx. The velocity in the formula is the speed of sound, which is about 340 m/s.

So, let's plug in some numbers! With my trusty tape measure I get:

A (area of the window opening) = 24" x 17" = 408 in^2
V (volume of cabin) ~ 57.5" x 33" x 108" = 204930 in^3 (ignore volume over dashboard and leg-room pockets)
L (length of the window outlet) ~ 3.5" (i.e. roughly the thickness of the door)

f = (Vs/2*pi)*sqrt(A/(V*L))
= (340 m/s)/(2*pi)*sqrt(408/(3.5*184437))*(1in/2.54cm)(100cm/m)
~ 51 Hz

Pretty low frequency... which is expected given how uncomfortable that pounding chopper sound is...

However, offhand this number still doesn't seem low enough. This number indicates that we should hear roughly 51 "chopper beats" every second. I remember a lot fewer than that... Time to take a little cruise in the HiHy with a window down...

Yeah... I know, my spouse is giving me that same look, same shaking head and that same sigh...

 

Just drove today with the sunroof open and boy was it deafening. My 2000 ML had the same issue. My 2003 ML they programmed in a fix. When auto opening, the sunroof stopped just a few inches short of full open. I did the same and it worked the highlander.

 

The FEH does this too -- full open when using one touch open is still about 1.5 to 2 inches shy of full open. It's been too hot for windows down here so I haven't played with what exact windows positions resulted in the 'chopper noise in the FEH.