# Can a sound wave be both standing and traveling?

1. Jul 26, 2015

### rwooduk

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

"An ultrasonic field will be a combination of standing and travelling 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.

2. Jul 26, 2015

### Staff: Mentor

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.

3. Jul 26, 2015

### CWatters

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

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

4. Jul 26, 2015

### rwooduk

Thanks. That helps a little (if I can visualise them as two seperate waves as two waves moving in opposite directions with unequal amplitude) but is it possible to somehow separate the componants 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.

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.

5. Jul 26, 2015

### Staff: Mentor

That is just a matter of definition. It can be useful in some setups.

6. Jul 27, 2015

### rwooduk

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

7. Jul 27, 2015

### tech99

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.

8. Jul 28, 2015

### rwooduk

I didn't know that, thanks!

9. Jul 29, 2015

### Finny

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)

10. Jul 31, 2015

### rwooduk

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? wouldnt that be zero impedance? or were you just directing me to the standing wave gif below it?

Thanks again!

11. Jul 31, 2015

### Finny

12. Jul 31, 2015

### Philip Wood

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 travelling towards the wall.