# Waves generated in an explosion

A cracking furnace exploded in a petrochemical industry building near my city somedays ago. The building is more or less far from the city, and even so people could notice the explosion. The windows shook and some of them broke. Thank God, there were no injuries. But the accident made me think a lot of things and I wanted to share them with you because you may solve some of my doubts.

1. what kind of wave was the responsible for breaking the windows? Was it a shockwave? Or a soundwave? Was it travelling at the speed of sound?

2. I noticed the wave when it reached my house because I saw the windows shake, but I didn't feel anything in my body, why?

3. As I've said, there were no injuries, not even people who were working there. But why was that? Because the wave was very strong, that's why the whole city felt it. Also, ears are always the most affected organs and people who were near the explosion were said to be "stunned" after the accident, why?

4. Waves don't carry matter, just energy, so the medium shouldn't move. But the broken glass obvioulsy moved... doesn't that contradict the theory? Obviously theory is never contradicted, but what's the explanation?

5. suppose that there exists such a "strong" wave that can push a person... does it mean that the wave amplitud was "big"? Or was its frequency high? Because, according to Planck's equation, the more frequency, the more energy.

6. Why didn't the wave reflect off the house wall when it hit it and return to its original source?

Many questions here.

1) what you experienced in your remote location was a sound pressure wave, traveling at the speed of sound. a supersonic shock wave would have blown down your building.

2) a window is relatively flexible for its size, behaving like an elastic diaphragm. because of its significant area, it does not require a particularly large amount of pressure difference across it to visibly flex. If the explosion were sufficiently strong, a human would be violently thrown and flexed by it. this is one way explosions kill people.

3) without knowing where the plant employees were situated during the explosion, there's no way to know why they escaped injury. regarding being "stunned", this is a common term people use to describe the experience of being involved in an accident, especially if it involved a very loud noise. Police use "stun grenades" to temporarily disorient armed and threatening people without killing or maiming them.

4) the medium most certainly does move in response to an explosive sound source. see the films of blast damage being done by atomic bomb blasts for a gross example.

5) the strength of an acoustic wave produced by an explosion is measured in terms of the overpressure it develops against objects that the pressure wave strikes. the overpressure is hence a measure of the wave's amplitude. Because the wave front is sharp, a frequency decomposition of it will reveal significant high-frequency components. the sharpness of the front diminishes with distance from the source, so what close by would sound like a "crack" will sound like a "boom" to someone far away. Planck's theorem does not directly apply to acoustic waves.

6) when a wave strikes an object, part of it is reflected and part is absorbed- so some of the sound energy did get reflected back to the site of the explosion.

Any pressure above ambient generated by an explosion (i.e. rapid expansion) causes a supersonic disturbance in the surrounding medium a.k.a. a shockwave. As the shockwave moves away from the source, it weakens by inverse-square law, reflection from obstacles and losses in the medium. Eventually the peak wave pressure drops to nearly ambient and the disturbance slows down to normal sound speed asymptotically. Physical damage requires a significant pressure above ambient, so it was a shockwave.
If you were standing inside the house and nothing was broken, you were shielded.

1. I guess it would be a shockwave. It is not necessary that it was travelling at the speed of sound. Could also be a sonic boom. Blast waves generally travel at a speed higher than the speed of sound.

2. By the time it reached your body, it might have lost some of its energy.

3. Basically, a blast wave is the increased pressure and flow resulting from the deposition of a large amount of energy in a small, very localised volume. So, it wouldn't have caused many injuries except from debris, trees etc. You would feel the wave just like a surge of water. (See P.S.)

4. The glass broke because of the high energy wave. Due to the compressions and rarefactions which took place at an abnormally high frequency, hence resulting in the high energy. (Planck's equation)

For the other questions, please refer this:

https://en.m.wikipedia.org/wiki/Blast_wave

P.S : And, remember, this blast wave is just like another fluid phenomenon. Picturise it like a tsunami. You will automatically find all the answers.

• Even though it depends on many factors like the elasticity of your window, location of your house and many other details, I tried to give a rough picture of what is happening. Commented Jul 1, 2019 at 17:42
• the strength of an acoustic wave produced by an explosion is measured in terms of the overpressure it develops against objects that the pressure wave strikes. the overpressure is hence a measure of the wave's amplitude. Because the wave front is sharp, a frequency decomposition of it will reveal significant high-frequency components. the sharpness of the front diminishes with distance from the source, so what close by would sound like a "crack" will sound like a "boom" to someone far away. Planck's theorem does not directly apply to acoustic waves. Commented Jul 2, 2019 at 3:18