# Rank the volume of the three points with the weakest first

1. May 19, 2015

### Nina stena

Hey all

1.
Two speakers emit sound in phase with the frequency of 13.2 kHz, the sound from the loudspeakers directed at points A , B and C. Rank the volume of the three points with the weakest first.
speed of sound 340m/s

2.

3.
I think I need to use Pythagoras says to calculate the
path difference from H1 TO A. This is what i have done but I am sure it's wrong.

60^2+220^2=52000 root square of (52000)= (20) root square (130). I don't really know what I am doing my self.

2. May 19, 2015

### haruspex

You wrote H1 to A, but in fact calculated H1 to B, right? That's fine. You mentioned a path difference, so you need another path length. What two path lengths do you need to compare to find what happens at B?

3. May 19, 2015

### Nina stena

Ohh do you mean i calculated the path to B?Beacuse iam not sure if i calculated the right thing.

Oh you mean i calculated the path to B insted of A? And my idea was to use Pythagorean theorem to calculate the path from H1 to A,B and C and then calculate the path from H2 to A,B,C, then compare everything and see what point in A,B,C are the lowest. But is that even a the right method?

4. May 19, 2015

### Staff: Mentor

For each point, you need to know the phase difference for the two speakers sending the sound to the point. To calculate the phase difference, you need the two distances (from each speaker to a point), and you need the wavelength of the sound. Does that help?

Can you write the 6 distances (two for each point)? And what is the wavelength of that frequency in air?

5. May 20, 2015

### Nina stena

It helps a bit, but i dont really know how to write it down and get an answer. I have done pretty good job with the othere quesions on this old exam but this question are impossible.

6. May 20, 2015

### haruspex

Let's start with point B. Can you work out the distances from H1 to B and from H2 to B?

7. May 20, 2015

### Nina stena

H1-B:
When I sketch , I get a triangle , then I can use the Pythagorean theorem . So i get 60^2+220^2=52000 root square of (52000)= (20) root square (130)

But if i calculate the first max with the grating equation, will i get the answer?

8. May 20, 2015

### haruspex

Not yet. You need H2-B as well.

9. May 20, 2015

### Nina stena

H2-B: 60+40=100, 100^2+220^2=58400 root square of 58000= (20) root square of (146)= 214 is that even right?

Here is a better picture:

10. May 20, 2015

### haruspex

How do you get 60+40?

11. May 20, 2015

### Nina stena

Hmm the length from the middle to B? or is't 60+30? I really dont know iam trying my best.

12. May 20, 2015

### haruspex

We want the distance from H2 to B. Draw a straight line connecting them on your diagram.

13. May 20, 2015

### Nina stena

Can i just take then the answer from H1-A (-) the distance 220?

Sorry for all the trouble but iam really trying. I have not been in school for the past 2 month , ( i got diabetes) So i have missed much.

14. May 20, 2015

### haruspex

I need to label some more points in your diagram for reference.
Call the point half way between H1 and H2 the point O. This is 30cm from each of H1 and H2, right.
What shape do the four points O, H2, B, A make?

15. May 20, 2015

### Nina stena

rectangle?

16. May 20, 2015

### haruspex

Right. How long is the side H2-B?

17. May 20, 2015

### Nina stena

Sorry i lost my connection, H2 and H1 have the same distence? Beacuse H1 and H2 have the same hight (220) and the base of the tringel are 30 cm?

18. May 20, 2015

### haruspex

You have.the rectangle O-H2-B-A. According to the diagram, what are its dimensions?

19. May 20, 2015

### Nina stena

Oh is H2 to B 220cm??

20. May 20, 2015

### haruspex

Yes.
So what is the difference in length between H1-B and H2-B?