Is This Destructive Interference and Ambulance Speed Calculation Correct?

[tornzaer]

Hey! I got two questions. Hoping someone can please help me out.

1) I'm doing a physics assignment and there's a question about constructive/destructive interference of sound.

One wave is square and its on top of and the other wave is a triangle and its on the bottom. The square is bigger than the triangle. I'm thinking its a destructive interference since they are both on a different plane. Therefore, when the waves meet, I have the square with a missing portion of the shape of the triangle.

Something like this: http://img503.imageshack.us/img503/6374/physicsinterferencefu3.png

Can someone help me with this. This is what I have but my friends have something else.


2) An ambulance is moving away from you and its siren is making a sound with a frequency of 457 Hz. It the original frequency was 620 Hz and the temperature is 25 C, how fast is the ambulance going?


Someone please help. I'm desperate.

 

[Doc Al]

1) Your answer looks good to me. When the two waves overlap they will look as you drew it.

2) This is a Doppler effect problem. Look it up!

 

[tornzaer]

Thanks for the reply.

For the second question, I knew it was a Doppler Effect problem. I did the calculations too. However, my final answer for Vo turns out to be something unbelievable, as in something couple of times the speed of sound. I'm thinking I made a mistake. Could you please double check for me?

Thank you very much. Truly appreciated.

 

[Doc Al]

You're right, those numbers aren't particularly believable. (Could be a typo.)

 

[tornzaer]

Just out of interest, is the answer you got 124 m/s? Because that's what I got.

 

[Doc Al]

Just out of interest, is the answer you got 124 m/s? Because that's what I got.

I thought you got an answer that was several times the speed of sound? Show what you did.

 

[tornzaer]

Well I did it again and I got something along the lines of 124 m/s.

f2 = f1(vs/(vs+vo))

f2/f1 = vs/(vs+vo)

f2(vs+vo) = vsf1

vo = vsf1/f2 - vs

So when I plug in the digits, I get 123... for vo.

To prove it, you just just plug in vo and the other values into f2 = f1(vs/(vs+vo)) and f2 should come out to 457, just like in the question.

 

[Doc Al]

You are correct. (I messed up my calculation earlier! D'oh!)