# Sound travelling without any disturbances?

1. Jun 20, 2014

As we all know, we are always surrounded by a "sea" of sound(Of cars,air,farts etc . Lol)
How can our voice be transmitted by the air without getting disturbed by other sounds?

Sound is a longitudinal wave. So imagine another wave travelling perpendicular to our wave. Then at the point of intersection, there will be movement of air in both forward-backward and right-left. So the sound wave should be disturbed here. Am I wrong?

2. Jun 20, 2014

### SteamKing

Staff Emeritus
It's not clear by what you mean when you ask, "How can our voice be transmitted by the air without getting disturbed by other sounds?"

Our voices can be drowned out by louder noises. (Ever try to carry on a conversation behind a roaring jet engine?)

If you are asking how we can 'distinguish' a voice from other noises in the background, that's something the brain does for us.

3. Jun 20, 2014

I don't think you understood what I meant. See this image of two sound waves travelling in perpendicular directions to each other

What will happen in the circled area?

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4. Jun 20, 2014

### ZapperZ

Staff Emeritus
There's nothing special there. It is still a superposition of both waves, even when they are perpendicular to each other.

Have you never seen waves in ripple tanks before?

http://www.555electronics.com.au/scanripple2.jpg [Broken]

Look carefully. As some point, the wave fronts from each of the two sources are moving perpendicular to each other.

Zz.

Last edited by a moderator: May 6, 2017
5. Jun 20, 2014

### olivermsun

Sound waves, just like waves in ripple tanks and waves in strings, do in fact interfere with one another. However, at commonly encountered amplitudes the effect is very very tiny and can be ignored.

6. Jun 20, 2014

I see. Thanks everyone!

7. Jun 20, 2014

### AlephZero

The fact that the equations describing the behavior of sound waves in air, over the range of amplitudes and frequencies that humans can hear without being damaged by them, are linear (in most situations) is "something special". But using linearized approximate models in physics is so common that it's easy to forget how "special" they are.

Also there is probably an anthropogenic principle here - if "sound waves" behaved in a nonlinear way so they were no use for communication, humans and other animals would be using something different to communicate!

8. Jun 20, 2014

### ZapperZ

Staff Emeritus
How is this relevant to the topic of this thread?

The "specialness", or lack of it, that I referred to was not in the range of frequencies, but rather in the DIRECTION of propagation!

Zz.

9. Jun 20, 2014

### olivermsun

I think the relevance is that the (non)linearity ("specialness") of the waves is a function of both amplitude and wavelength (i.e., frequency).

10. Jun 20, 2014

### ZapperZ

Staff Emeritus

Zz.

11. Jun 20, 2014

### olivermsun

The OP asked why voices aren't disturbed by other ambient sounds. One good answer is that sound waves are fairly linear at the amplitudes and frequencies that we typically encounter. The corollary is that waves which cross perpendicularly won't affect each other even though it seems like they should.

If the nonlinearity is significant, plane acoustic waves crossing perpendicularly does seem to be an interesting special case.

BTW, why did you assume that I didn't read the OP?

12. Jun 20, 2014

### ZapperZ

Staff Emeritus
Because it appears as if you didn't read the explanation that was given in Msg. #3 that clarified what the OP was looking for! He clearly wanted to know what happened when two perpendicular wavefronts meet!

My reply was that there's NOTHING SPECIAL about this when compared to other angles, since the principle of superposition works the same way. I even showed a picture where superposition/interference occurred at many different angles.

Somehow, my use of the term "special" got picked up and now we're talking about linear regime of the frequency and sound range of human ears, which are NOT what I was referring to! I know there is a common tendency of PF threads going off on a tangent, but this is a bit silly!

Zz.

13. Jun 20, 2014

### AlephZero

Indeed. See the animations at the bottom of this page http://lie.math.brocku.ca/~sanco/solitons/gkdv_interactions.php for the sort of things that can happen when two strongly nonlinear waves interact with each other.

The green plots are the two waves without interaction (note, for the nonlinear waves the velocity is amplitude dependent, not constant) and the red plot shows the result of the interaction. This can even generate a "tail" moving in the opposite direction to the incoming waves.

14. Jun 20, 2014

### olivermsun

By definition, the principle of superposition works for linear waves. For nonlinear waves the principle no longer works.

Because of what I said above, it's crucial to understand that the waves are, indeed, in the linear regime.

If the waves are not in the linear regime, then much more interesting things can happen (as AlephZero explains in the post above).

15. Jun 20, 2014

### ZapperZ

Staff Emeritus
I give up. Somehow the OP's elementary question and his level of understanding never entered into your consideration when answering such questions.

Zz.