How Does the Doppler Effect Influence Sound Frequency at the Great Wall?

In summary, the guy is moving towards the wall with an air horn emitting a constant sound of 500 Hz. The sound will be reflected and travel back to him. He needs to find the frequency of the beat, which is the difference between the frequency of the sound he is hearing and the frequency of the reflected sound.
  • #1
rmorelan
8
0
Hi, I am a little confused about some aspects of sound waves. I was hoping to just check to see if I am on the right track.

I have a problem where a guy is moving towards the great wall of china, with a air horn, emitting a constant frequency sound of 500Hz. The sounds is going to hit the great wall and bounce back, introducing some interferance. The speed of sound is 350m/s. The persons is running towards the wall at 3/ms, and I have to find the frequency of amplitude beats he hears and the frequency of the sound he hears reflected.

ok, so f' = (v/(v +/- Vs)*f

He is moving towards the wall, so Vs is negative? This gives

f' = (350/(347)) * 500 = 504.3Hz

Is the frequency of beats then just the difference, ie (504.3-500)Hz for 4.3Hz beat Hz. Am I making any sense?

thanks!
 
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  • #2
Yes I believe that's right.
 
  • #3
rmorelan said:
Hi, I am a little confused about some aspects of sound waves. I was hoping to just check to see if I am on the right track.

I have a problem where a guy is moving towards the great wall of china, with a air horn, emitting a constant frequency sound of 500Hz. The sounds is going to hit the great wall and bounce back, introducing some interferance. The speed of sound is 350m/s. The persons is running towards the wall at 3/ms, and I have to find the frequency of amplitude beats he hears and the frequency of the sound he hears reflected.

ok, so f' = (v/(v +/- Vs)*f

He is moving towards the wall, so Vs is negative? This gives

f' = (350/(347)) * 500 = 504.3Hz

Is the frequency of beats then just the difference, ie (504.3-500)Hz for 4.3Hz beat Hz. Am I making any sense?

thanks!

No. There is reflection here, so you need to take into account two different Doppler effects. What you found is the frequency of the wave reaching the wall. Now, this wave is reflected and travels back to the guy. You need to do another Doppler effect calculation with this time the guy being the observer so you will need to multiply the frequency you found by another factor (with now the source, which is the wall, being at rest and the observer moving toward the source at 3 m/s).

Once you get the final frequency, you just subtract 500Hz to get the beat frequency, as you had done.

Patrick
 
  • #4
I see, of course :)
 
  • #5
ok, I am still a little confused but things are a bit clearer :)

Actually I made a mistake anyway, the air horn is actually stuck into the ground, so it doesn't move, just the guy running towards the "great wall of china". I should learn to read!

So we have a setup like

(((( horn )))) man -> [ wall ]

So he is moving way from the source, and towards the reflected wave I guess from the wall. So there is two doppler effects? One being:

f' = (350/(353)) * 500 = 495.75Hz ( what the sound behind him sounds like)

and

f' = (350/(347)) * 500 = 504.3Hz (what the reflected sound sounds like )

i am assuming the sound actually reflected is 500 Hz, the frequency of the emitted sounds, as the doppler effect doesn't change the frequency relative to the other standary objects ie the wall, and source are moving.

So the difference would be then 504.3 - 495.8 = 8.6 Hz?

Thanks a lot everyone, this stuff is really messing me up :)

rk
 
  • #6
Looks like you got it just fine.
 

FAQ: How Does the Doppler Effect Influence Sound Frequency at the Great Wall?

What is the Doppler Effect?

The Doppler Effect is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source.

How does the Doppler Effect affect sound?

The Doppler Effect affects sound by causing a change in the perceived frequency of the sound as the source and observer move towards or away from each other.

What causes the Doppler Effect?

The Doppler Effect is caused by the relative motion between the wave source and observer. This can be either the source or observer moving, or both.

What is the difference between the Doppler Effect for sound and for light?

The main difference between the Doppler Effect for sound and light is that sound waves require a medium to travel through, while light waves do not. This means that the Doppler Effect for sound can only occur in a medium, while the Doppler Effect for light can occur in a vacuum.

How is the Doppler Effect used in real life?

The Doppler Effect is used in various real-life applications, such as radar and sonar systems, police speed radars, and medical ultrasound. It is also used in astronomy to measure the relative motion of stars and galaxies.

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