# Homework Help: Points of destructive interference of sound waves?

1. Feb 14, 2015

### Maged Saeed

1. The problem statement, all variables and given/known data
Two speakers (S1,S2), emitting sound waves of frequency 340Hz and separated by a distance of 3 m, are driven by the same oscillator. A listener starts walking from point A to S2 Along the line that joins A and S2> How many points of destructive interference will be observe? Speed of sound in are is 340 m/s

2. Relevant equations
w=v/f

3. The attempt at a solution
Although, it seems quite easy,I've tried to solve it many times with no reasonable result,
Wavelength will be 1 meter. And I don't know how to do after that.

the right answer is 4 but how it comes??

2. Feb 14, 2015

### Staff: Mentor

What is the condition for destructive interference?
This condition has one parameter that will vary as you go towards S1. What is its initial and final value?

3. Feb 14, 2015

### Maged Saeed

The condition says :
for a destructive interference, (delta L/wavelength)=n.0.5
But how can I apply it here?

The initial value of delta L equals to 1 and the final value equals 1.

4. Feb 14, 2015

### Staff: Mentor

I agree with the initial value, but not with the final one. What are the two distances to the speakers at the final position?

5. Feb 14, 2015

### Maged Saeed

Sorry, I meant 3
The vertical distance between the two sources.

6. Feb 14, 2015

### Staff: Mentor

Right, that allows to find how many times you get destructive interference. But the answer is not 4.

7. Feb 14, 2015

### Maged Saeed

I think that it is 3
Am I right??

8. Feb 14, 2015

### Staff: Mentor

Why 3?

9. Feb 15, 2015

### Maged Saeed

Oh , sorry
delta L will be (3-1) =2
So the numer is two.
I think I'm right now .

10. Feb 15, 2015

Looks right.