Everyone has seen cartoons of bombs being dropped, accompanied by a whistling so

Question

Everyone has seen cartoons of bombs being dropped, accompanied by a whistling sound as they drop. This sound gets lower in frequency as the bomb nears the ground.

I've been lucky enough to not be near falling bombs, but I assume this sound is based on reality.

Why does the frequency drop? Or does it only drop when it is fallling at an oblique angle away from you, and is produced by doppler shift?

I would have thought that most bombs would fall pretty much straight down (after decelerating horizontally), and therefore they would always be coming slightly closer to me (if I'm on the ground), and thus the frequency should increase..

Explanation / Answer

I can't claim any experimental experience in this area (fortunately :-) but I thought it was interesting enough to be worth a bit of Googling. The results suggest there is a difference between shells and bombs.

There is an extensive collection of eye witness accounts of WW2 at bbc.co.uk/history/ww2peopleswar/categories/, and searching this suggests that falling bombs make little if any sound. I couldn't find any of the eye witness accounts that mentioned a whistling sound.

However if you Google for stories from, for example, the current troubles in Syria there are lots of reports of the whistling sounds shells make. Chapter 5 of The Art of Noises describes the stereotypical whistling sound falling in tone, and as this dates from the years before Hollywood it's presumably relatively uncontaminated. The author attributes this to fact that the shell velocity is highest immediately after firing and falls during flight due to air resistance.

It's probably relevant that shells are generally fired at greater than the speed of sound so you wouldn't hear them approaching. You'd only hear them after they passed you, and of course the sound of those shells would be red shifted.

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