some acustics

ChrisWerner · Post by **ChrisWerner** » Tue Aug 22, 2006 10:18 pm

I have a question in my mind and I can´t find an answer, or at least no real explaining answer.

A sonic vibration or oscillation, let´s say a sine, hits a wall and gets reflected.
It hits the wall with 0 degree.

Does the sine turn its phase or not.
Does it come back with a 180 degree turned phase?

[ This Message was edited by: ChrisWerner on 2006-08-22 23:19 ]

Gordon Gekko · Post by **Gordon Gekko** » Wed Aug 23, 2006 1:12 am

i'd say yes, like a ball hitting a wall would do, or a wave in a pool. but hey, my physics manuals have been lost for a long time

Counterparts · Post by **Counterparts** » Wed Aug 23, 2006 2:01 am

I not sure if you've quite visualised the problem correctly.

I think that you're thinking of a sine wave travelling towards the wall, going up and down nicely (like all good sine waves do).

However, what's really happening is the air is being compressed and expanded (forming the wave) in the direction that the sound is travelling in. That waveform will be reflected off the wall, the 'phase' not being changed at all; the wave carries on as it was before, with loss of energy depending on the wall's surface.

Immanuel · Post by **Immanuel** » Wed Aug 23, 2006 12:16 pm

I believe it goes something like this:

Code: Select all

        __           __
       /          W/  
      /          /A    
    /          / L         /
 __/        __/  L      __/

It does not go up and down, but think of the wave as high and low preasure points. And all here is caught in time over a distance. What you have to do is to mirror the stuff that passes thru the wall on my little thing above.

Code: Select all

    __  __      __ 
   /  /      /   W
  /   /     /   /A
/   /     /   / L
/__/    __/__/  L

and then sum it up, because there is only one airpreasure at a certain point at a certain time ...

Code: Select all


Sorry - I have to change to numbers here. The low _ will have value 0 the high _ will have value 4.

1-1-1½-2-2½-3-3-3-2½-2-1½-1-1-1-1½-2-2½-3-3-3-...

but "one sample" later you will have:

[code]
   __    __    __
  /    /    /   W
/    /    /    A
/    /    /    /L 
  __/  __/  __/ L

wich translates into

0-0-0-0-0-0-0-0-0-0-0-0-0

and you can make more drawings, if you want to. If you do so, you will se, that this sine wave with this size of room will be able to reach both 0 and 4, when you sum things up. These drawings are however rather square ... and not really sine.

I don't know, if this answers your question.

Post by **hubird** » Wed Aug 23, 2006 12:41 pm

jeez!...
good typing

ReD_MuZe · Post by **ReD_MuZe** » Wed Aug 23, 2006 12:50 pm

the phase by hitting the wall is reversed.
as it is reflected to the oposite direction

astroman · Post by **astroman** » Wed Aug 23, 2006 3:01 pm

c'mon - it's about a tone, not a laser...

(I mean the setup, not your answer, Red)

[ This Message was edited by: astroman on 2006-08-23 16:02 ]

ChrisWerner · Post by **ChrisWerner** » Wed Aug 23, 2006 9:39 pm

Thank you very much for your answers, the correct answer is that it does not change the phase.

But we have an on going little war at school about this one, our lecturer didn´t gave us a real reason, yet. I will keep you in touch.

_________________
Music starts where any language ends 
<a href="http://www.spring-of-sound.de">Spring-Of-Sound.de</a>

[ This Message was edited by: ChrisWerner on 2006-08-24 00:17 ]

ChrisWerner · Post by **ChrisWerner** » Wed Aug 23, 2006 9:42 pm

I think / thought like immanuel draw us so nicely.

_________________
Music starts where any language ends 
<a href="http://www.spring-of-sound.de">Spring-Of-Sound.de</a>

[ This Message was edited by: ChrisWerner on 2006-08-23 22:43 ]