Can a sound wave be both standing and traveling?

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Discussion Overview

The discussion revolves around the concept of sound waves being both standing and traveling simultaneously. Participants explore the implications of this idea, particularly in the context of ultrasonic fields and transmission lines, while seeking to visualize and understand the nature of these wave types in combination.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants express confusion about how sound waves can be both standing and traveling, noting that traditional understanding suggests they are distinct types of waves.
  • One participant suggests that a combination of standing and traveling waves can be visualized as two waves moving in opposite directions with unequal amplitudes.
  • Another participant mentions that the total power output might be the sum of the powers in the traveling and standing waves, implying a relationship between the two.
  • There is a discussion about whether it is possible to separate the components of a combined wave into standing and traveling waves, with some suggesting it is a matter of definition.
  • Examples are provided, such as transmission lines with mismatched loads, where a mixture of standing and traveling waves is common.
  • Participants discuss the implications of tuning waves to be standing and the effects of detuning them, raising questions about the nature of wave behavior in different contexts.
  • One participant draws parallels between ultrasonic and sound waves, suggesting that they behave similarly in terms of resonance and dissipation.
  • There is mention of constructive interference and the role of reflections in creating standing waves, with a focus on the conditions under which these phenomena occur.

Areas of Agreement / Disagreement

Participants generally express uncertainty about the nature of combining standing and traveling waves, with multiple competing views on how they interact and whether they can be distinctly separated. The discussion remains unresolved regarding the visualization and implications of this combination.

Contextual Notes

Some participants note limitations in their understanding and the definitions used in the discussion, particularly regarding the conditions for standing and traveling waves and the implications of power distribution between them.

rwooduk
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Apologies if this is a stupid question, I have a reference that says:

"An ultrasonic field will be a combination of standing and traveling waves. An increase in the traveling wave will decrease the proportion of a standing wave and vica versa".

I'm trying to visualise this, I understand the two separately but having trouble with both. I thought the wave was either standing or traveling. How can it be both?

Any help visualising this would really be appreciated.
 
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You can take the sum of of a standing and a traveling wave. Which is the same as the sum of two waves moving in opposite directions with unequal amplitude.
 
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Not really my field but it doesn't say that there is just one wave. It says "combination of standing and traveling waves" (plural). eg a mixture of standing and traveling waves.

It's not unreasonable to imagine that the total power output might be the sum of the power in the traveling and standing waves so that reducing the power in one allows more power in the other... or something like that.
 
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mfb said:
You can take the sum of of a standing and a traveling wave. Which is the same as the sum of two waves moving in opposite directions with unequal amplitude.

Thanks. That helps a little (if I can visualise them as two separate waves as two waves moving in opposite directions with unequal amplitude) but is it possible to somehow separate the components of a combined single wave into standing and traveling? And why do books not say anything about this?

The paper I'm reading imposes conditions for a standing wave, and then does the same for a traveling wave, the effects of which are then analysed to see which has the greatest effect. Therefore in the combined standing and traveling wave system it suggests you can see which type of wave contributes most.

CWatters said:
Not really my field but it doesn't say that there is just one wave. It says "combination of standing and traveling waves" (plural). eg a mixture of standing and traveling waves.

It's not unreasonable to imagine that the total power output might be the sum of the power in the traveling and standing waves so that reducing the power in one allows more power in the other... or something like that.

Yes, it's the mixture I'm struggling with. How is this possible when you usually tune a wave to be standing, if you detune it slightly it is no longer standing and is traveling.

Thanks for the replies.
 
rwooduk said:
but is it possible to somehow separate the components of a combined single wave into standing and traveling?
That is just a matter of definition. It can be useful in some setups.
 
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mfb said:
That is just a matter of definition. It can be useful in some setups.

Ok, thanks. I will just consider it a defined event for the purposes of calculation etc and try not to get drawn into how to see it.

Thanks again
 
rwooduk said:
Ok, thanks. I will just consider it a defined event for the purposes of calculation etc and try not to get drawn into how to see it.

Thanks again
A mixture of standing waves and traveling waves is very common and is found, for instance, in the case of a transmission line with a mismatched load.
 
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tech99 said:
A mixture of standing waves and traveling waves is very common and is found, for instance, in the case of a transmission line with a mismatched load.
I didn't know that, thanks!
 
rwooduk said:
How is this possible when you usually tune a wave to be standing, if you detune it slightly it is no longer standing and is traveling.

yes, that's the idea.

Ultrasonic and sound waves are pretty much the same...longitudinal waves...so think of any wind or string music instrument for example...the resonant frequency produces the desired tones, or notes, while the non resonant waves' travel' and are quickly dissipated.

And in the case of transmission lines, you want the opposite situation ...no reflections...so that higher frequencies, say rf, are entirely traveling waves...no resonances to sap additional power.

rwooduk said:
try not to get drawn into how to see it.

No reason to avoid drawing your own picture...
Here is an illustration of a traveling wave...like a water wave moving along...and just picture steady state peaks and valleys in addition to what is shown...
https://en.wikipedia.org/wiki/Transmission_line#Input_impedance_of_lossless_transmission_line

Oh, and constructive interference illustrated here:
https://en.wikipedia.org/wiki/Interference_(wave_propagation)
 
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  • #10
Finny said:
yes, that's the idea.

Ultrasonic and sound waves are pretty much the same...longitudinal waves...so think of any wind or string music instrument for example...the resonant frequency produces the desired tones, or notes, while the non resonant waves' travel' and are quickly dissipated.

And in the case of transmission lines, you want the opposite situation ...no reflections...so that higher frequencies, say rf, are entirely traveling waves...no resonances to sap additional power.
No reason to avoid drawing your own picture...
Here is an illustration of a traveling wave...like a water wave moving along...and just picture steady state peaks and valleys in addition to what is shown...
https://en.wikipedia.org/wiki/Transmission_line#Input_impedance_of_lossless_transmission_line

Oh, and constructive interference illustrated here:
https://en.wikipedia.org/wiki/Interference_(wave_propagation)

Thanks for the detailed reply, the instrument analogy is very good.

From your link to Input impedance of lossless transmission line, how can there be impedance if there is lossless transmission? wouldn't that be zero impedance? or were you just directing me to the standing wave gif below it?

Thanks again!
 
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  • #12
Consider when a standing wave arises by the interference of a wave and its (normal) reflection from a wall. If the reflection is not 100%, then you have a standing wave superimposed on a progressive wave traveling towards the wall.
 
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