Sound frequency of dropping bomb

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..

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

Two things

1. German WWII Stuka bombers had wing-mounted sirens to strike terror into their targets. Since they were driven by air pressure, the pitch would probably drop as the bomber came out of its dive and returned to level flight (slowing down).

2. Tone painting is a very standard music composition technique. Bombs drop from high altitude to low, so the corresponding pitch is high to low.

Answer 5

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.