# Homework Help: Interfence with reflected sound

1. Oct 5, 2008

Hey guys, was hoping you could help me with a sound question. When sound is emitted from a speaker and it strikes a wall, it rebounds. Since both waves are in phase, a standing wave is created. Are nodes located at wavelength/2, 3wavelength/4? If not, where is there location?

2. Oct 5, 2008

### Rake-MC

Re: Sound

Well please attempt this question, I'll give you some pointers. Both waves have the same wavelength, and you said they are in phase.

So if they were travelling the same direction then the antinodes would become 2A and the antinodes would stay as 0A right? Where A is standard amplitude, and the x nodal positions would not change. ie. nodes at 0 and wavelength/2
Does travelling in opposite direction change this?

3. Oct 5, 2008

Re: Sound

Well if thats the case for them travelling in the same direction, they should change shouldnt they? Does this mean that anti-nodes are now located at 0 and wavelength/2?

4. Oct 5, 2008

### Rake-MC

Re: Sound

Well that depends on if you are measuring your 0 as from the start of a wave from the original pattern or a distance from the wall on which they are reflected.

If you keep it as the start of the original wave, then it should be identical.
Nodes at 0, wavelength/2

5. Oct 5, 2008

Re: Sound

Perhaps it may be easier if i post the question that has sparked my confusion:

A science student connects a dynamic loudspeaker to a low voltage AC power pack in an Australian school. The loudspeaker sends sound waves directly towards a concrete wall 3.0 metres away. Take 340 ms-1 as the speed of sound. Wavelength = 6.8m

How many softspots are there located between the wall and speaker? Explain.

Okay, a soft spot is formed due to the presence of a node. When sound is emitted from the speaker it is reflected from the wall and intefers with the wave emitted from the speaker. This produces a standing wave.

That is as far as i can get, i do not understand how to find how many nodes there would be. My understand of a node is when the path difference of two sound waves in phase is wavelength/2, 3wavelength/2, etc. If thats the case then the first soft spot should occure 3.4m from the wall, therefor no soft spots formed. However the answer states that there should be 1 soft spot???