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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..

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    $\begingroup$ Maybe there were some special bombs in WWI or II, which had some whistling device. A simple bomb does not "whistle", depending on velocity it will make a hissing noise not more. The rest is hollywood. I have heard the noise of artillery shells and grenades from mortars, it is a rumble resp. hiss. Bombs dropped from airplanes and watched from a mile away are silent, the only thing You hear is the airplanes engine. $\endgroup$
    – Georg
    Commented Jul 10, 2012 at 9:49

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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 http://www.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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    $\begingroup$ +1 Also there's the simple geometry of the hearer's position relative to the path. If the hearer were on the path he or she would not report anything :( Otherwise, as the dot product of the projectile's velocity vector, and the hearer-projectile position vector approach and pass zero, the frequency shift would decline. $\endgroup$
    – Mike Dunlavey
    Commented Jul 11, 2012 at 12:31
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Assume a bomb I dropping straight down at terminal velocity, but not directly overhead. Then, the component of velocity pointing towards you will decrease steadily as the bomb falls ( even though the total velocity is constant). This causes the Doppler shift to decrease in magnitude as the bomb falls.

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My father was a radio operator on B-17 bombers during WWII. As we were watching an old war film together when I was young, we listened to the sound effects of bombs as they neared the point of impact on the ground. I remember my father's comment that the sounds in the movie were wrong, adding "As a bomb moves closer to the ground, the sound frequency increases (get's higher pitched); it doesn't decrease. It's called the Doppler Effect. So, from the perspective of the airplane, the "whistling" effect drops in pitch as the bombs move further from the airplane, but rise in pitch as they near the ground.
Regardless, I don't really care to hear the sounds of dropped bombs from either perspective.

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If you're on the aircraft which is dropping a bomb that whistles, you'll hear the pitch drop as the bomb accelerates downward away from you. Perhaps the classic cartoon sound effect was inspired by the experience of someone who flew on a bomber rather than an observer on the ground.

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In my opinion I think the whistling sounds come from a manufacturing defect or an engineering defect in the shape or dynamic balance of any rapidly moving object through the air. Eddy current caused behind the moving object can set up as standing waves creating a whistling noise, such eddies are used in the form of flow meters to measure the flow of a moving fluid through a pipe. If you notice the design of a simple bullet it has a spritzer nose for supersonic speed and when it slows the boat tail ass means better laminar flow. The creation of any noise at all means loss of energy. There also exists the possibility of psychological impact but since destruction is the primary role the sounds must usually be unwanted artifacts related to construction or design. Anything moving quickly through air will generate turbulence over 30 miles per hour and there seems no way to stop it. But such sounds are usually noise like hissing and not pure tones. Doppler or spin rate may cause frequency changes.

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When a baby cries, the sound of his/her cry becomes lighter. But actually the frequency of the baby's vocal-cord arises. Due to the limitation of our ears (20-20000Hz) we failed to feel the loudness of the baby's cry. Same way, when the bomb closer to ground the frequency of bomb reaches near 20000Hz (with respect to our ears). So we feel the frequency lighter.

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