The whistling of mortars/rockets/bombs

[MRT144]

So I was thinking about this the other day and how in movies and on TV bombs always whistle. but the sound of them is always from high pitched to low pitch which doesnt make any sense if you consider the doppler effect. so is there more at play than this?

[KrazyHorse]

a) Rockets typically travel faster than the speed of sound. People on the receiving end probably don't hear anything before they hit

b) Given that whatever whistling actually occurs is due to the air rushing over the surfaces of the projectile and that the frquency at source is therefore also a function of projectile velocity any discussion of the doppler effect is premature if you don't understand these other effects first

[KrazyHorse]

Also, by the time gravity bombs reach target they should generally be travelling at a relatively constant velocity (terminal velocity), so if anything the frequency of the sound reaching target should be constant.

[Cort Haus]

a) Rockets typically travel faster than the speed of sound. People on the receiving end probably don't hear anything before they hit

I was taught by a history teacher who had a V2 land nearby him in WW2. He claimed he heard the whistle after the explosion.

[Flip McWho]

You can hardly add suspense if you can't hear the bombs approaching.

[MRT144]

Originally posted by KrazyHorse
a) Rockets typically travel faster than the speed of sound. People on the receiving end probably don't hear anything before they hit

b) Given that whatever whistling actually occurs is due to the air rushing over the surfaces of the projectile and that the frquency at source is therefore also a function of projectile velocity any discussion of the doppler effect is premature if you don't understand these other effects first

okay, i shouldnt have inclued rockets. i meant any projectile that is using gravity solely. so bombs or mortars descending.

also i realize that the whistling is a function of that. the frequency should either be constant or increasing.

so continue... teach me.

[KrazyHorse]

I don't know what effect increasing velocity has on the source sound frequency. My initial guess would be that it increases it, which, coupled with the doppler effect would make an accelerating approaching object (such as a mortar shell) rise in pitch, but again, I am by no means certain. My fluid dynamics background is weak, to say the least (I know the Navier-Stokes equations and not much else)

[notyoueither]

I have heard that mortar shells don't make much noise at all. Haven't had one fired at me, so I can't testify.

[MRT144]

so would it be reasonable to say that this is a movie effect and not actually real or as honest as showed in the movies?

thanks for trying though KH, I appreciate it.

[KrazyHorse]

I honestly can't say for certain.

Naively it seems wrong. Be careful to note that this is only for those at the wrong end of a bomb, say. Those dropping itsee the bomb accelerate away from them, giving a plausible (though by no means airtight) reason to assume that the pitch trends downward.

[notyoueither]

I hate to break it to you, but cars don't explode on impact (well, most of them anyway) and explosives in movies have all sorts of effects added to them.

Oh, and the good guy doesn't always win.

[KrazyHorse]

However, note that if the bomb travels directly overhead and then strikes somewhere past you then for the last few instants it will be getting further away from you, not closer. In that instance it might seem reasonable to say that the pitch would trend downward.

[MRT144]

i thought about that for a little bit. the perspective of where the observer of the sound is. I think it's interesting to watch a movie and see how the effect is universally used for bombs whether the person is the bomber or bombee.

If it was a bombee and missed it would probably experience a pitch rise coupled with a pitch dip at the very end as it moved away from the person.

regardless this would be a fun experiment to conduct.

[MRT144]

Originally posted by notyoueither
I hate to break it to you, but cars don't explode on impact (well, most of them anyway) and explosives in movies have all sorts of effects added to them.

Oh, and the good guy doesn't always win.

I know this, I did foley work in school. Im actually asking this for future reference if i ever do this for someone as a favor.

[KrazyHorse]

If it was a bombee and missed it would probably experience a pitch rise coupled with a pitch dip

No. There is no reason to expect a pitch rise.