Shock Wave Frequency: Observer, Emission & Calculation

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Discussion Overview

The discussion revolves around the frequency of shock waves as detected by an observer, particularly in scenarios where the source may not emit sound waves. Participants explore the implications of shock wave generation when an object moves faster than the speed of elastic waves in a medium, and how to calculate the frequency in such cases.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the frequency of shock waves detected by an observer is infinite, noting that shock waves can occur even without sound emission from the source.
  • Another participant suggests that while the shock wave hits only once, leading to an infinitely small frequency, the detected frequency from the source is finite and subject to Doppler shifting.
  • A different participant discusses the mathematical representation of wave frequency using Fourier transforms, stating that the spectrum of a wave can be analyzed in the frequency domain and mentioning the Heaviside step function as relevant to shock waves.
  • This participant also notes that while the Fourier transform indicates a decrease in magnitude with frequency, it does not drop to zero, emphasizing that real shock waves do not have infinitely fast rise times.

Areas of Agreement / Disagreement

Participants express differing views on the nature of shock wave frequency, with some asserting it is infinitely small while others provide alternative interpretations involving Doppler effects and mathematical modeling. The discussion remains unresolved regarding the exact nature of shock wave frequency.

Contextual Notes

There are limitations in the assumptions made about the nature of shock waves and the definitions of frequency being used. The discussion also highlights the complexity of modeling shock waves mathematically, with references to idealizations that may not hold in practical scenarios.

brasilr9
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Is the frequency of the shock waves detected by the observer infinte?
And, as I know, a shock wave can be produced even if the source didn't emit sound waves. When a object moving in a medium at a speed faster than the speed of medium's elastic wave's speed. In this case how we calculate the wave's frequency?

Thanks for answering my question!
 
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well, the shock wave hits only once so yes, the frequency is infinitely small. On the other hand, the detected frequency from the source is finite, but Doppler shifted to a lower value.
For example, if source is just "beeping", you will still hear the beeps after it will pass you, but the interval between beeps will be bigger.
 
brasilr9 said:
Is the frequency of the shock waves detected by the observer infinte?
And, as I know, a shock wave can be produced even if the source didn't emit sound waves. When a object moving in a medium at a speed faster than the speed of medium's elastic wave's speed. In this case how we calculate the wave's frequency?

Thanks for answering my question!

The only wave that has a single frequency is a sine wave.

Mathematically, the spectrum of a wave is usually defined by its Fourier transform. The Fourier transform takes a functio from the "time domain" to the "frequency domain". The magnitude of the Fourier transform at a specific frequency band can be interpreted as how much of the energy of the wave lies within that band. For a detailed definition, see the wikipedea article

http://en.wikipedia.org/wiki/Continuous_Fourier_transform

The Fourier transform you are probably interested into represent a shock wave is the last entry in the Wikipedia table, the "Heavside step function".

At any frequncy other than 0, the magnitude of the Fourier transform is 1/w - the magnitude decreases with frequency but never drops to zero,.

This is an idealization, real shock waves do not actually have infinitely fast rise-times.
 
thanks for answering my question. :smile:
 

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