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midtskogen
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17 Nov 2020 13:44

Watsisname wrote:
Source of the post I plotted its spectrogram

Nicely visualised.  Same thing visible in the spectrogram of the meteor sound.

Now thinking aloud:

The object is decelerating.  Before the sonic transition it leaves a trail of ripples moving slower than the object, so the object is outside the ripples.  As it approaches the sonic transition point, the lower part of these ripples become closer and closer, then they pile up at the transition point and as the object decelerates further inside the ripples they become more and more spaced again.  The packed ripples will reach the witness first, whether they were emitted before or after the transition.  I think that holds regardless of the relative position of the witness, but perhaps how rapid the pitch falls can hint the angle between the meteor and witness, if we know the deceleration rate.

This could probably be effectively visualised in an animation, but that takes a bit of work.

As for the Mythbuster video, maybe there is some initial supersonic expansion, which quickly goes sonic, and what we hear is that expansion approaching sonic speed, reversed.
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18 Nov 2020 03:23

midtskogen wrote:
Source of the post This could probably be effectively visualised in an animation

Always wanted to do something like that, so why not. :) I'll put up three different scenarios for comparison.
For starters let's generate a typical Mach cone. A constant-frequency source passes by at 1.5 times the speed of sound. Theory predicts the Mach cone angle should be about 41.8 degrees, and that looks like what we get:

Image

Intuitively the observer (black star) first sees a sudden burst of sound, which then decreases in frequency. 

Next let's have the source accelerate from rest, so that it breaks the sound barrier as it passes the observer:

Image

This case is dramatically different, since the observer detects sound from the subsonic regime first, so they hear frequencies that first increase, then decrease.

 Finally here's something that's at least qualitatively more similar to a meteor transitioning from supersonic to subsonic. 

Image

This seems closer to the first example, and I think matches your intuitions pretty well.
 
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18 Nov 2020 04:53

Wow.  You beat me to it.   :D
Last edited by midtskogen on 18 Nov 2020 15:05, edited 1 time in total.
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18 Nov 2020 05:21

There is one thing these visualisations tell us, which I didn't realise.  The delay between the sonic transition and the initial sound can not trivially be used to calculate the distance, as indicated by the waves emitted before the transition (red) and the waves emitted after (blue), unless you're almost directly below (which might be unknown).  You are likely to hear the compressed sound of the meteor before it goes subsonic.  That didn't occur to me.  But since "old" fronts fade quickly, it will diminish in intensity quicker, I believe.  The same thing if the meteor impacts before going subsonic.
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18 Nov 2020 15:06

I edited the post above with a visualisation of a more realistic deceleration.  Units on the axes are in km and the animation is in real time.
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18 Nov 2020 17:19

Beautiful! I love how we did this completely independently. :D And yours does clearly show how observers who are not very close will hear the sound from the supersonic flight first, which wasn't obvious.

I also learned something interesting by doing this. I always imagined that if a plane suddenly accelerated through the sound barrier, then the sonic boom would be most powerful for observers directly below the transition point. But not so. An observer directly below will hear the upshift and then downshift of frequencies, with a perhaps substantial increase in sound intensity, but the actual sonic boom propagates more forwards, and the worst of it strikes the ground well ahead of the transition point.  I never knew this. The sonic boom itself also has a very interesting shape when the object is accelerating through the sound barrier.

Another interesting thing to consider is the effect of variation of speed of sound with altitude (or more precisely, with temperature.) It might be very important for meteor entries, as they pass through a very wide range of temperatures.
 
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18 Nov 2020 21:55

Watsisname wrote:
Source of the post It might be very important for meteor entries, as they pass through a very wide range of temperatures.

Yes, the speed of sound is lower at higher altitudes.  And the winds are higher also, which can alter the propagation significantly.  I did not attempt to model that in any way, just assuming constant speed at all altitudes, which gives nice circles.  Otherwise it quickly gets a lot more complex to draw.  Luckily for this simulation, these effects are minimal for the initial waves, as the meteor drops into the temperate and calm troposhere amazingly fast.  And it doesn't affect much the purpose of this visualisation anyway.
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19 Nov 2020 05:32

I made another visualisation where the meteor fragments into three pieces: a few grams, a few kilos and above 100 kilos.  Of course, the smalles fragment wont make much sound, but it's interesting to see the differences in deceleration.
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19 Nov 2020 22:29

midtskogen wrote:
There is a similar sound heard in the Mythbuster cement truck explosion, which I'm not sure how to explain.  The guy referred explicitly to the sound of that explosion saying that it was exactly like that.  The pitch drop in the Mythbuster explosion sounds faster, but in the Swedish videos I suspect that we miss the very first bit as the audio is saturated, so we only hear the slower, continued pitch drop, not so much the more powerful "Mythbuster" sound.

I've always imagined the pitch changes of whizzing bullets as doppler effects, but perhaps there are other factors as well.  For bullets it could certainly be a change in rotation speed, but a spinning rock of this size surely doesn't have a spin frequency matching the sound frequency.  Still, perhaps high sound frequencies can be produced by lower rotation periods due to funny air displacements around the rock.

I listened to a number of Chelyabinsk videos trying to find the same sound, but nothing like it.  Chelyabinsk came in quite shallow, though, and didn't enter the troposhere in such insane speed as in Sweden (probably well over 10 km/s at 15 km altitude!).  The pressure from that point and below must have been enormous.  It will be interesting to see what kind of rock could survive that.  But iron is unlikely.  10 tonnes of iron at 70 degree angle would just punch through the atmosphere and hit the ground supersonic.

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This is seriously great stuff!  Will you venture out to look for fragments, Mid?
About Chelyabinsk if I remember correctly, wasn't our attention fixed on a different (predicted) asteroid that was forecast to make a close pass to Earth and meanwhile this asteroid "snuck up" on us from sun-side?  Interesting coincidence (unrelated?) that it came on the exact same day as the one that was well predicted to be a close pass.
 
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19 Nov 2020 22:32

Watsisname wrote:
Beautiful! I love how we did this completely independently. :D And yours does clearly show how observers who are not very close will hear the sound from the supersonic flight first, which wasn't obvious.

I also learned something interesting by doing this. I always imagined that if a plane suddenly accelerated through the sound barrier, then the sonic boom would be most powerful for observers directly below the transition point. But not so. An observer directly below will hear the upshift and then downshift of frequencies, with a perhaps substantial increase in sound intensity, but the actual sonic boom propagates more forwards, and the worst of it strikes the ground well ahead of the transition point.  I never knew this. The sonic boom itself also has a very interesting shape when the object is accelerating through the sound barrier.

Another interesting thing to consider is the effect of variation of speed of sound with altitude (or more precisely, with temperature.) It might be very important for meteor entries, as they pass through a very wide range of temperatures.

Having grown up with the Concorde flying right over my head, I can verify that you are 100% correct.  Heard the sound well before I saw the Concorde.
 
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20 Nov 2020 01:38

A-L-E-X wrote:
Source of the post Will you venture out to look for fragments, Mid?

No, only from my desk.  There are people out in the field.  It's hard to find the time, besides there are quarantine restrictions in effect.

A-L-E-X wrote:
Source of the post About Chelyabinsk if I remember correctly, wasn't our attention fixed on a different (predicted) asteroid that was forecast to make a close pass to Earth and meanwhile this asteroid "snuck up" on us from sun-side?

It didn't "snuck up" because focus was on the close pass.  We wouldn't have detected it anyway.  The other close pass was unrelated.

A-L-E-X wrote:
Source of the post Having grown up with the Concorde flying right over my head, I can verify that you are 100% correct.  Heard the sound well before I saw the Concorde.

If you heard the sound before it passed right over you, it simply means that it was still subsonic.  I think Wats point was that the greatest sonic boom will not be heard right below the transition point, but later, well ahead of it.
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20 Nov 2020 01:56

How long after take off did the Concorde take to become supersonic?  The booms were pretty unmistakable.

About the asteroid, I heard that no one knew about it because it approached us from the "blind side" (the sun side) and so it was in the sun's glare and that's why it went undetected until it made impact.  It was amazing that the world was focused on one asteroid and a completely different one made impact.
 
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20 Nov 2020 04:54

A-L-E-X wrote:
Source of the post How long after take off did the Concorde take to become supersonic? 

I don't think it was allowed to do that over land?

A-L-E-X wrote:
Source of the post About the asteroid, I heard that no one knew about it because it approached us from the "blind side" (the sun side) and so it was in the sun's glare and that's why it went undetected until it made impact.


Yes, it was very difficult to spot before because of that.  But they are still hard to detect.
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20 Nov 2020 09:34

midtskogen wrote:
A-L-E-X wrote:
Source of the post How long after take off did the Concorde take to become supersonic? 

I don't think it was allowed to do that over land?

A-L-E-X wrote:
Source of the post About the asteroid, I heard that no one knew about it because it approached us from the "blind side" (the sun side) and so it was in the sun's glare and that's why it went undetected until it made impact.


Yes, it was very difficult to spot before because of that.  But they are still hard to detect.

Hmm not sure about the Concorde, perhaps that is correct, but I'd have to see the flight path.  There is about 100 miles of land east of JFK airport on Long Island.  But the FAA may have put those regulations in, many around here have complained about high airplane noise!
The asteroid spotting difficulty makes me wonder....how many years (decades?) are we from spotting and stopping an asteroid that would be big enough to rise to an ELE?  Of course such a thing statistically probably only happens once in 100 million years but being prepared is still important.  If we had a Shoemaker-Levy type collision like Jupiter had, I dont want to think about the amount of damage that could cause.

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