3
$\begingroup$

It's a well known fact that a sonic boom from something such as concord can shatter glass. I'm interested to know why a human is seemingly fine in the presence of the shockwave and yet the glass is completely destroyed?

EDIT: Rephrased: Why is glass the only or one of the only materials that actually completely shatters in the presence of a sonic boom?

$\endgroup$
  • $\begingroup$ The human may look fine on the outside, that doesn't mean there is no damage. Hearing loss, brain damage etc.. are known long term effects of high sound pressure events. "Shell shock" is not just a psychological phenomenon, it is now understood as being caused by physical injury to the brain. As for the glass... unlike humans that can be made as resilient as you like. $\endgroup$ – CuriousOne Jun 1 '16 at 22:17
  • $\begingroup$ Thanks for the reply, however i suppose what i'm trying to get at is why we don't specifically shatter or why glass is the only material that does in the presence of the boom! $\endgroup$ – Oliver Cohen Jun 1 '16 at 22:19
  • 1
    $\begingroup$ I was at Heathrow airport about 10 odd years ago, Concorde came in on it's normal flight path, low over the car park and set off every car alarm around. IT'S LOUD and you can feel it in your upper body. Glass might shatter due to resonance. $\endgroup$ – user108787 Jun 1 '16 at 22:34
  • 1
    $\begingroup$ We do "shatter", but initially we shatter on the inside, which is what leads to these internal injuries. The human skin is a very strong "bag" that keeps the internal organs contained, if you like. Make a similar bag around glass and it would behave the same: break on the inside but be contained. That's essentially what shatter resistant glass is. That ordinary glass is prone to cracking is related to the way it is being produced, which leaves strong internal tension behind (sometimes on purpose). Remove that tension by heat and chemical treatment and you get much tougher glass. $\endgroup$ – CuriousOne Jun 1 '16 at 22:39
  • $\begingroup$ Sorry, I wrongly attributed glass shattering to resonances, I was half asleep, I meant overpressure. As well as Floris' answer, this might help: en.m.wikipedia.org/wiki/Sonic_boom $\endgroup$ – user108787 Jun 1 '16 at 23:15
4
$\begingroup$

The effect you noticed is a function of strength and size.

When you have a large window, and a sonic boom comes along, a relatively small pressure difference can set up very large tensile forces in the surface of the glass (especially if the shock wave cannot easily "go around the back" of the glass). When there is a bending stress, all you need is a small (surface) crack to act as a stress concentrator and you will get crack initiation: and once the crack is initiated, it will propagate as long as the stress is maintained*).

This very simplistic answer shows there are several differences between windows and humans:

  1. size: the human is small relative to the wavelength of the boom: this means there will be less opportunity to build up stress, as pressure is "all around"
  2. elasticity: when you are hit with a shock wave, your lungs will collapse very slightly (your chest compresses) but that doesn't hurt anything; by contrast, the glass tries to resist the motion and builds up stress
  3. compressibility: you are mostly made of water, which is highly incompressible. A sonic wave will hardly compress anything made of water - but it can deflect glass when there is just air behind it.

The notable exception to all the above is the ear: there is an enclosed air pocket behind your ear drums, and the ear drum will move violently back as the shock wave hits. This can do permanent damage to the structure of the ear, including rupture of the ear drum.

When people go scuba diving, they can be subjected to pressures that is many times greater than atmospheric pressure - yet they survive. It's not the pressure that kills, but the pressure difference. Humans "equalize" quickly - ears and sinuses are possible exceptions. Incidentally, when divers try to surface quickly, they have to make sure to breathe out continuously or they will rupture their lungs (as a pressure difference would build up between lungs and surroundings).

None of the above is meant to imply that it is not possible to create a shock wave that can damage human tissue; but the combination of compliance, size and toughness means that a glass pane will shatter rather easily compared to more diffuse damage in human tissue (at sufficiently high pressure levels).


*) Specifically, the fact that the glass will first bend and store elastic energy means that once the critical stress is reached (which is a function of surface condition and possible built-in stresses) a lot of energy is available / released quickly, resulting in acceleration and branching of the cracks and (frequently) complete disintegration. For some glass, this is a deliberate effect: during manufacturing the glass is tempered in such a way that internal tension is built up and the outer surface is under compression. This raises the threshold before cracks will initiate (for that, the surface has to be in tension); but once it reaches that threshold, the crack, propagating into the center of the glass, encounters material that is already in tension. This makes the crack accelerate and bifurcate. The result is that the glass breaks "completely". You are less likely to be hurt by many small pieces of glass than a single big one, because the small pieces will have less momentum.

$\endgroup$
  • $\begingroup$ Blast-associated shock wave-induced traumatic brain injury is being widely researched these days. The conventional wisdom that tissue other than e.g. the middle ear and maybe the lungs is not significantly affected by shock waves has long given away to a scientific inquiry in how they are being affected, after all. Curiously, the people who designed the daisy cutter style ordinance were using the devastating effects of shock waves on the human body at the same time as the rest of the military community was still denying them. I guess the anatomy of the US warfighter must have been different. $\endgroup$ – CuriousOne Jun 1 '16 at 23:12
  • $\begingroup$ @CuriousOne in that case, the military are really going to have to do their homework, if they want to use noise as the basis for non lethal weapons: en.m.wikipedia.org/wiki/Sonic_weapon. I think they used sonic weapons on Somali pirates, or tried to. $\endgroup$ – user108787 Jun 1 '16 at 23:22
  • $\begingroup$ If you look at medical laboratory experiments, they are being performed, among other things with rats, which are clearly much smaller than humans. Nevertheless one can see the same type of injuries as in humans, i.e. living on a smaller scale (at least within an order of magnitude) is not enough to prevent these injuries. $\endgroup$ – CuriousOne Jun 1 '16 at 23:23
  • 2
    $\begingroup$ @CuriousOne. No, I will stick to "Gunsmoke" westerns on YouTube, the doc digs out the slug, tells the cowboy he will be fine in a day or two and nobody suffers from PTSD. $\endgroup$ – user108787 Jun 1 '16 at 23:48
  • 1
    $\begingroup$ Did you read my last paragraph (before the footnote) in the edited answer? How is that not saying the same thing? I am just saying "glass breaks more easily, and here is why". Which is the question that was asked. $\endgroup$ – Floris Jun 2 '16 at 0:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.