One of the more famous videos coming out of Beirut in the past couple of days is this clip of a new bride participating in a glamour video shoot just as the blast wave arrives from the massive explosion at the port:
A second or so before the blast wave hits, there is some other sound which gives a clue to her and others that something bad is about to happen. It would seem that this sound is associated with the explosion, but what is it? How is this sound arriving before the shock wave, which is presumably propagating at supersonic speed?
The sound almost seems like it might people yelling. Perhaps someone with a better vantage point who could see the blast coming before it actually hit.
The sound arrives the same time as the shock wave – watch the woman’s dress. The dust takes a little longer, but I’d expect it to be slowed a bit because it’s heavier than air molecules.
A blast wave has a leading supersonic shock wave. Only high explosives (including ANFO) produce a blast wave, perhaps this explosion did not. In any event, I think technically if it’s not supersonic, it’s not called a blast wave.
Nope - watch the video again, but with the volume turned up. The main blast wave hits at about 0:15 and blows her dress back, and it’s accompanied by an awful racket, yes. But there’s a noise before that, right around 0:13; it’s enough to cause the bride to grab her dress, someone in the background (the groom?) to turn toward the camera (or more probably toward the source of the noise), and someone else to exclaim “allahu akbar”. It’s that early noise I’m wondering about, and I think Pleonast has got it: the airborne blast wave is supersonic (relative to the speed of sound in air, ~763 MPH), but earthquake waves can propagate really fast. So maybe the ground in that area started shaking before the main blast wave arrived, cracking a few windows and rattling some wind chimes before the blast wave blew the entire neighborhood apart?
100% this. We were destroying Saddam’s arsenal of crud in Iraq. The shot was every day except Friday around 4:00 - 4:15 pm so the locals were used to it. Contract ammount was 125,000 lb. gross wt. each day. We averaged about 200,000 lb. with an explosive net weight of around 40,000 lb. (We used lots of donor explosive). From our vantage point, 4.5 miles away, we’d see the visual explosion, mushroom clouds (4 separate pits), next the ground shock, and finally the air blast.
It could be the sound of tremors in the earth. But it could also be the sound of splintering building facades farther north, closer to the port area, a moment before the airborne shock wave reached these people in downtown Beirut.
The guy in the background appears to look up to the sky right after the noise starts at 0:13. Is it possible that the sound from the blast traveling above the buildings (straight line from the blast site) got to their location two seconds before the sound traveling at ground level (which had to propagate through who knows what maze of streets? The sound traveling just above the rooftops would randomly reflect down to street level so would sound like it was coming from above, and moving left to right (as the camera was facing at 0:13)?
That bride was hauling ass in that bulky dress - I can’t believe how far she got towards shelter by the time the camera panned back to her. I guess if you grow up in Beirut you learn how to take cover…
I found the spot where the video was taken on Google Earth (search for Purrl Jewelry in Beirut, that was the shop in the background of the video). It is about 1300m from the site of the explosion. Area between is pretty solidly built up, no streets run directly in the direction between the explosion and the video site. Pretty much every building along the direct line between the explosion and the video site is oriented at a 45 degree angle to the path, so let’s say that the ground level path length is sqtr(2) time the direct distance so about 1830m. 530m longer than the direct route, 1.5 seconds @ 343 m/s.
I’m sticking to my hypothesis - the roaring/rushing sound before the bang is the sound from the blast propagating over the rooftops and reflecting off obstructions down to the ground, starting about 1.5 seconds before the main sound of the blast arrives along ground level.
The “main sound of the blast” is a supersonic shock wave - and as the video shows, it had enough intensity to shatter doors and windows. The “sound from the blast propagating over the rooftops and reflecting off obstructions down to the ground” would also be a supersonic shock wave, with similar destructive power. It clearly does not have that kind of power; it’s just acoustic waves, which - if they were traveling through the air from the blast site - would arrive long after the (supersonic) blast wave.
Spherical shock waves from explosions decrease quickly in strength with distance from the explosion center, rapidly leveling out to Mach 1.0, or the speed of sound.
If we consider two portions of the shock wave - one traveling above the tops of buildings and one traveling along the ground between buildings - it still seems to me that the portion above the buildings has a more direct path from the explosion to the video site than the portion taking the circuitous path between buildings. The initial noises could be the effect of the shock wave passing along the tops of the buildings, generating noise from turbulence as well as from damaging what it is hitting - a bunch of scattered noises which coalesce into the general roar heard on the video. The shock wave itself is not sensed at ground level, the only thing you can hear are such secondary effects.
The portion of the shock wave along the ground, in addition to taking a more circuitous route, would slow down faster than the overhead portion of the wave as it loses energy interacting with the buildings it passes.
I think you are all forgetting something there - there were actually TWO explosions, a first one then the much, much larger one.
What makes heads turn is the first explosion and its effects.
THEN comes the big boom.
It’s not so obvious on the bridal shoot video because we can’t see past the buildings but if you watch footage of the actual blast site burning then exploding you can see the sequence.
Not seeing/hearing the “first explosion”. Here’s a collection of videos, with one of them starting ~30 seconds before the really big bang:
Please understand that a shock wave is, by definition, a supersonic disturbance propagating through a medium - supersonic meaning “traveling faster than the speed of sound.”. If the blast wave leaves ground zero headed south, and buildings behind the shock wave are making basic impact sounds as they collapse, those sounds will never catch up to the shock wave.
Possibly the expanding Wilson cloud had propagated high enough to be seen from his vantage point at that moment?
In the end, I’d defer to smithsb’s actual experience:
