No doubt, you’ve heard those whistling sounds of bombs in cartoons or documentaries. The whistling sounds were associated when a character used to fall from a hill or cliff. It sounds somewhat like this –
Before the answer, lets understand a nice noisy physics concept called as Doppler effect.
Doppler effect :-
The Doppler effect is the change in frequency or wavelength of a wave as the source and observer move towards (or away from) each other.
It is named after the Austrian physicist Christian Doppler, who described the phenomenon in 1842. The effect also causes sudden change in pitch.
The best example in case of sound waves is a change in pitch of an ambulance/police siren passing by. And the best example in case of light waves is the red shift seen by the astronomers.
As the object(source of sound) keeps approaching towards you, the pitch keeps on increasing & vice versa. Now answering your question,
Why did bombs made whistling sound while they fell ?
The reason was- some bombs were equipped with whistles ! The reason behind attaching whistles on bombs was to weaken the enemy morale & to scare people. It was another way of saying, “We are coming & you are going to die!”
The change in the pitch of sound was due to the Doppler effect. When the bomb is dropped from the airplane, the velocity of bomb goes on increasing due to gravity until it reaches its terminal velocity. As the bomb approaches towards ground, the pitch increases and thus the “kooouuuueeee” sound.
Does it make sense but ?
You are alerting your enemy about the bombardment, aren’t you ?
Well, yes, but it really doesn’t matter because the people don’t know where the bomb will strike plus there’s not enough time to take proper shelter.
Now, the pitch of the sound of bombs decreases in the audio clip I shared with you above & also the ones you hear in cartoons & documentaries.
This is because, that effect is heard with pilot’s perspective.
In the above audio clip, pitch is decreasing ⇒ source moving farther from listener ⇒ therefore pilot’s perspective.
Reverse the audio clip, pitch will increase ⇒ source approaching towards listener ⇒ and this is what the bomb would sound like to a person on the ground.
Now don’t ask me why sound producers decided to use the sound effect heard from a pilot’s perspective.
Ju-87 Stuka dive bomber :-
Beside the whistling bombs, other threatening noises from World War II were the sounds made by the Ju-87 Stuka dive bomber planes !
Stuka were equipped with a siren which were called as Jericho trumpets.
The siren rang when the plane used to dive for bombing. These sounds of siren were louder than the bombs and would panic the enemies & the people(as shown in Dunkirk)
Here’s a video containing the sound of Stuka sirens.
If I had to answer the question in 1 line, that why did the Germans do this, I would say,
To make their enemies shit in their pants !