How do waves following a shockwave catch up to the shock wave?

In summary, the conversation discusses the concept of waves catching up to each other and amplifying, specifically in relation to shock waves and sound waves. It is mentioned that in a free field, the trailing waves do not catch up to the leading shock front, but rather the duration of the disturbance gets longer as the shock wave travels further from the source. It is also noted that in an explosion, the shock wave will be circular, hemispherical, or spherical depending on the situation. The conversation also delves into the mechanics of how the sound pulse from a supersonic craft is received and how it lengthens the sound pulse.
  • #1
Jake DAprile
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TL;DR Summary
Im trying to understand how a wave traveling behind a shock wave can catch up to that wave, such as in a blast.
I was in an argument about a jet engine and I was arguing that since there is a cutoff in terminology what would kill someone approaching a engine is not technically sound, but a shock wave, (I'm probably wrong about this, but that's not the question). That got me wondering how waves can catch up to each other and amplify such as in a blast. I'm studying chemistry, not physics, so forgive me if half of this is wrong.
 
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  • #2
Jake DAprile said:
Summary: I am trying to understand how a wave traveling behind a shock wave can catch up to that wave, such as in a blast.

I'm not sure that they do catch up
and speaking of which, what waves are you referring to ?

Do you have an example of a case where they do ?
 
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  • #3
In a free field, the trailing waves don't catch up to the leading shock front, so the duration of the disturbance gets longer as the shock wave travels further from the source.
 
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  • #4
davenn said:
I'm not sure that they do catch up
and speaking of which, what waves are you referring to ?
They don't "catch up" but they travel outwards with a different Vee shaped wavefront. The angle is determined by the actual Mach number of the object causing the waves. The speeds are different (shockwave and resulting sound wave) but the frequencies (initially) will be the same. You have different speeds and the same frequencies so the wavelengths will be different. This means that the direction of the sound wave has to change from the angle of the shock wave Vee. Higher the Mach number, the sharper the Vee shape.
It's similar to the change of direction due to wave refraction at an interface between substances with different wave speeds (Snell's law etc); you have phase continuity across the transition.
As mentioned above, the pulse will get wider and wider which is due to a dispersion effect, I think.
 
  • #5
Dr. Courtney said:
In a free field, the trailing waves don't catch up to the leading shock front, so the duration of the disturbance gets longer as the shock wave travels further from the source.

sophiecentaur said:
They don't "catch up" but they travel outwards with a different Vee shaped wavefront.

on a plane sound shockwave yes
for an explosion shockwave (that was also mentioned in the OP), no it's going to be circular, hemispherical,
spherical depending on the situationThanks for the backup, guys, I was 99% sure of that :smile:
But I have, on occassion, known to be wrong :wink:

Dave
 
  • #6
davenn said:
on a plane sound shockwave yes
for an explosion shockwave (that was also mentioned in the OP), no it's going to be circular, hemispherical,
spherical depending on the situation
Dr. Courtney said:
In a free field, the trailing waves don't catch up to the leading shock front, so the duration of the disturbance gets longer as the shock wave travels further from the source.
Both comments are very relevant. Clearly something going slower than another 'thing' can't catch it up so the explanation must be that the sound wave is created as energy from a spherical shock wave is transferred into regular sonic waves over a period of time and that, I guess, must mean that the two waves (pulses) exist in the same space for a while (travelling at different speeds). The time taken for the transfer will spread out the length of the sound pulse until there is no more supersonic energy.
I read in a recent PF thread that the sound pulse from a supersonic craft is received at each reception point from contributions along a length of the shock wave. This lengthens the sound pulse.
 

Related to How do waves following a shockwave catch up to the shock wave?

1. How do waves following a shockwave catch up to the shockwave?

The speed at which waves catch up to a shockwave depends on the properties of the medium through which the wave is traveling. In a uniform medium, such as air or water, the speed of sound is constant and the waves will travel at the same speed as the shockwave. However, in a non-uniform medium, the speed of sound can vary and the waves may either catch up to the shockwave or lag behind it.

2. What causes waves to catch up to a shockwave?

Waves catch up to a shockwave because of the sudden change in pressure and density created by the shockwave. This change creates a disturbance in the medium, which propagates as a wave. As the wave travels through the medium, it will eventually catch up to the shockwave.

3. Do all types of waves catch up to a shockwave?

No, not all types of waves will catch up to a shockwave. Only waves that are able to travel through the medium in which the shockwave is traveling will be affected by the shockwave. For example, electromagnetic waves, such as light, do not require a medium to travel and will not be affected by a shockwave in a medium like air or water.

4. Can the speed of waves catching up to a shockwave be controlled?

The speed at which waves catch up to a shockwave cannot be directly controlled. However, the properties of the medium through which the shockwave is traveling can be manipulated to affect the speed of sound and, therefore, the speed at which the waves catch up to the shockwave.

5. What happens when waves catch up to a shockwave?

When waves catch up to a shockwave, they will combine with the shockwave and create a new, larger wave. This phenomenon is known as a constructive interference. The resulting wave will have a higher amplitude and energy than the original shockwave or waves, and will continue to propagate through the medium until it dissipates.

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