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Comment by interestica | Hacker News Reader

> The glass damage must be horrible. Your entire home covered in tiny pieces of glass.

Watch this video at .25x to really see how glass reacts in the pressure wave. You can see the look on the bride's face as the initial vibrations/sound reaches her. Then, you can actually see the window pane bow like a wind sail.

https://youtu.be/PUQ-QvCqDjA

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RT/RUPTLY is well known for stealing other peoples' video and putting their huge gaudy watermark on it. Here is the same video where at least the media logo is on the bottom right corner.

https://www.youtube.com/watch?v=VBV8nnp98vc

Wow, you can really see in the glass reflections, the arrival of the ground compression (?) waves fractions of a second before the air pressure wave hits. And the wife notices it before the husband does.

Is the sound faster than the shockwave or same speed? I think same speed. Might have been a previous explosion that triggered the look before the main one.

I think you're hearing/seeing the difference between the speed of sound through the ground, and through the air.

Sound moves 4-times faster in water than in air, I assume even faster in ground surface, so let’s just assume it arrives instantly via ground. This would mean a delay like in lightning/thunder, which is what one can see in the video (2–3 seconds ~ 0.5–1 km). (This is the restaurant https://goo.gl/maps/td5K1vxUDBN4i1CA8)

But I could equally imagine it’s some lesser explosion before the main blast.

Object moves -> air goes back and forth -> your eardrum moves -> pressure pattern over time registers as sound as a 20-20kHz ish spectrum captured at 0.1s ish timesteps

Supersonic airplanes move past the speed of sound(280-340m/s in air) and generate wavefront at its interfaces with air but the resulting disjoint tsunami wave expands at the speed of sound and heard as single ‘bang’ when it reaches ears or microphones

So there’s no way one could hear the bang through air before visual bang hits

However the speed of sound is about 10x faster on solid mediums such as ground or seawater so could be those

It's actually two "bangs" for a plane. Tum-dum. Source: grew up about 100km away from a military air base in Russia at which at least a couple planes went supersonic almost every day for years.

You can hear several sonic booms in this video, although they're from low-flying planes: https://www.youtube.com/watch?v=jmhU7SEo4gg. The higher the plane, the more distinct and "bassy" the "booms". Here's another example, a bit higher, a bit more distinct: https://www.youtube.com/watch?v=M_KbZON5sW0

> Is the sound faster than the shockwave or same speed? I think same speed.

Shockwaves propagate faster than sound by definition.

The speed of sound depends on the medium, as a few replies have mentioned.

TFA says shock-waves travel much faster than sound's speed in air (343 m/s) whilst pressure-waves (as in the explosion in Beirut at 200+ m/s) travel relatively slower.

I was around during the Enschede, Netherlands fireworks factory explosion [0], just outside the radius where all glass shattered. Saw the glass ripple effect along a line of appartment buildings with full-height windows. A really astounding sight I'll never forget. With the amount of curvature caused by the shockwave it seemed these windows must surely break. But they did not.

[0] https://en.wikipedia.org/wiki/Enschede_fireworks_disaster

Why does the camera lens not shatter in this case?

And in the same vein, what happens to peoples’ eyeglasses?

Does anyone know?

Eyeglass lenses are usually made from plastic or polyurethane variants these days, as they can be much lighter, thinner, and tougher than glass with similar-enough optical properties to cover the needs of most eye conditions.

I consider PC a wonder material yet I went back to 1.7 AR coated glass (higher than that it's a bit overkill unless you really need it IMO). It's crisper, actually cheaper, heat resistant and virtually scratch proof if you clean it properly. It's heavier but I'm not going back to finding pinpricks after bouts of wind and all the extra care. The thing about glass is that it can shatter upon impact, plastic lenses won't but basically get ruined all the same in my experience.

I’ve not seem glass as an option recently. I miss having glass glasses for the exact reasons you describe. Worn plastic is scratched and give nasty artifacts.

The ratio of the thickness of the glass compared to the unsupported span of material makes them relatively much more rigid than a big, floppy pane of glass. A glass window is not only much less rigid, but also maximizes the surface area that pressure from the blast wave can push against.

This and the fact that eyeglasses are usually not firmly attached to a huge chunk of reinforced concrete.

usually.

Also behind a big glass pane is an enclosed room. Air can rush past the glasses.

For camera lenses, they are probably smaller, thicker relative to frontal area, and better protected against shocks and vibrations

Eyeglasses are actually almost always made of dense plastic material these days for better refractive indexes so they shatter differently, also I wonder if physics people consider it to be practically suspended in thin air at the scales of these explosions

Do plastic eyeglasses even shatter? Ever since lenses stopped being made out of glass, the amount of abuse they've been able to take has been through the roof.

I can't imagine a situation where my glasses might shatter before my face would.

Have a look at the vehicle glass in the video.

They're quite small and thick: https://www.personal-view.com/talks/uploads/FileUpload/77/0c...

Windows break because they're thin and extend over large area, which make them flex a lot.

Similarly, you can see in the video how windows of the surrounding cars are largely unaffected.

Oh no. I really hope the bride and groom are OK!

They're fine. And she's an MD who spent the better part of the rest of that day doing what she could for the people around her.

https://www.cbc.ca/radio/asithappens/as-it-happens-friday-ed...

A different couple

That's actually a _different_ wedding photo, happening at exactly the same time: https://www.youtube.com/watch?v=GQssU3dTKUs

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