- #1

Nikitin

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Hi guys, I have problems with a high-school assignment:

http://img843.imageshack.us/img843/3748/74698735.png

An experimenter tries to determine the speed of sound in air. He tries to lower and elevate the water in the tube at the same time as he hits on the fork, until he hears an increase of the tone. He marks the spot with A. He repeats the same thing and finds a new spot. He marks it "B". The fork has a movement frequency f=440hz.

1) Determine why the air in the tube sends out sound of the same frequency as f?

2) Explain that the distance AB is half the wavelength of the sound-wave in the air.

3) Calculate the speed of sound in air.

v=f*[lambda]

1) Because the fork vibrates at 440 hz, thus it must send out 440 complete sound waves per second down the tube. Correct?

2) OK, here is the tricky question. I know that the water reflects the sound waves, and at point A & B the reflected sound waves and the incoming soundwaves interfere, thus increasing the amplitude and increasing the tone of the new wave (new wave = incoming wave + reflected wave).

I am finding it extremely hard to prove that distance AB = 0.5*[lambda]

3) No problem. v=f*2*(AB) This part is pretty easy.

Any help is highly appreciated..

## Homework Statement

http://img843.imageshack.us/img843/3748/74698735.png

An experimenter tries to determine the speed of sound in air. He tries to lower and elevate the water in the tube at the same time as he hits on the fork, until he hears an increase of the tone. He marks the spot with A. He repeats the same thing and finds a new spot. He marks it "B". The fork has a movement frequency f=440hz.

1) Determine why the air in the tube sends out sound of the same frequency as f?

2) Explain that the distance AB is half the wavelength of the sound-wave in the air.

3) Calculate the speed of sound in air.

## Homework Equations

v=f*[lambda]

## The Attempt at a Solution

1) Because the fork vibrates at 440 hz, thus it must send out 440 complete sound waves per second down the tube. Correct?

2) OK, here is the tricky question. I know that the water reflects the sound waves, and at point A & B the reflected sound waves and the incoming soundwaves interfere, thus increasing the amplitude and increasing the tone of the new wave (new wave = incoming wave + reflected wave).

I am finding it extremely hard to prove that distance AB = 0.5*[lambda]

3) No problem. v=f*2*(AB) This part is pretty easy.

Any help is highly appreciated..

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