Why do objects whistle when falling? Like here.

[luxun2]

This is my first post but just as it says in the title I'm curious about the physics behind falling objects. Specifically if you could examine this video which despite the title is not actually graphic and school me on why we heard what we did, Thanks.

http://www.youtube.com/watch?v=0ABGIJwiGBc

 

[DennisN]

Do you know what sound is? I don't mean "something we can hear", I mean, do you know how sound is produced and transmitted?

 

[davenn]

have a think about what would happen to the shape of the bullet after its impact/ricochet
and how that shape may cause a whistling sound


Dave

 

[luxun2]

Alrighty, well sound in my mind are the vibrations that propagate through air, water, and solids. If the bullet was shot then I would assume when it hit the target it became deformed like so(http://www.dreamstime.com/stock-image-deformed-flattened-out-bullets-image28978371) and therefore less aerodynamic. I believe either that the bullet struck the iron so that the bullet vibrated at its resonate frequency or the bullet is now displacing a lot of air and it is merely the wind that we are hearing along with the Doppler effect.

 

[davenn]

you don't have to get as complicated as resonant freq or Doppler effects

look at that deformed bullet 5th image along ... see all the ragged edges/bits curled out
The air is literally going to create a whistling sound as it moves through the gaps

similar to when you blow air through your slightly opened lips to whistle or blow air across an open top of a bottle etc

I really don't think its any more complicated than that

cheers
Dave

 

[luxun2]

I do understand that we hear the whistling of air because of the edges but don't we hear sound come from the bullet vibrating itself too or is that negligent? I know its hard to tell it just being a video but how loud (dbs) do you imagine that the whistling itself to be, or can I calculate it someway? Thanks for you help.

 

[DennisN]

Alrighty, well sound in my mind are the vibrations that propagate through air, water, and solids.

Yes, excellent.

...along with the Doppler effect.

Yes, I heard the Doppler too.

...but don't we hear sound come from the bullet vibrating itself too or is that negligent? I know its hard to tell it just being a video but how loud (dbs) do you imagine that the whistling itself to be, or can I calculate it someway? Thanks for you help.

I actually don't know (about the bolding part). But I will do something an experimentalist might had done - I will run a frequency analysis of the sound . I'll post it here when I'm done. Oh, the wonders of internet...

 

[DennisN]

Ok, I'm done. I downloaded the clip, run a noise reduction on it and cut out the part where the bullet can be heard going through the Doppler shift. This part was approx. 0,75 seconds long. Here's the result of the analysis (dB on the y-axis, frequency on the x-axis):

Now, regretfully I am a little too tired to think about this thing , but the frequency analysis shows basically frequency peaks which start at about 3200 Hz and end at about 800-900 Hz. I'm not sure what the 250 Hz peak is at the moment (i.e. if it is from the bullet). I can not give an approximation at the moment how loud the sound actually was (it depends on e.g. the microphone and recording level, and I'm also too tired to think clearly about it at the moment ). Never mind, something for further discussion. I'll hopefully drop in here tomorrow.

 

[256bits]

I do understand that we hear the whistling of air because of the edges but don't we hear sound come from the bullet vibrating itself too or is that negligent? I know its hard to tell it just being a video but how loud (dbs) do you imagine that the whistling itself to be, or can I calculate it someway? Thanks for you help.

The edges would contribute to the all the frequencies heard, but have you also ever heard the sound of a bullet ricochet off a rock in the cowboy western movies. Being deformed, the bullet will now tumble in a chaotic fashion ( using chaotic here to mean you would have a difficult time determining how the bullet will be spinning ). The tumbling will produce a sound at a certain frequency and then add in all the other sounds from the jagged edges.

 

[luxun2]

Nice answers thanks for the help all.