Doppler shift with 2 trombones

In summary: That's just my opinion though. In summary, the question is asking for the maximum speed at which Paul can approach Mary, while playing the same type of trombone, and still cancel out her trombone noise. Using the formula for perceived frequency, with a known maximum frequency of 500Hz and Mary's playing frequency of 400Hz, we can solve for the maximum speed of 68 m/s. The answer of 85 m/s would make sense if the question was asking for the maximum speed at which Paul can move away from Mary and still hear her trombone noise at 400Hz.
  • #1
Gauss M.D.
153
1

Homework Statement



Mary is blowing her trombone at 400hz.

Paul is approaching her at speed v, blowing the same type of trombone. These trombones has a maximum frequency of 500hz. What is the maximum speed Paul can approach Mary, and still cancel out her trombone noise?

Homework Equations



f = f0/(1-vs/v)

Speed of sound = 340m/s

The Attempt at a Solution



Max freq 500, so:

500 = 400/(1-v/340)

Solving for v, I get 68 m/s. The answer is 85 m/s. What's going on?
 
Physics news on Phys.org
  • #2
Gauss M.D. said:

Homework Statement



Mary is blowing her trombone at 400hz.

Paul is approaching her at speed v, blowing the same type of trombone. These trombones has a maximum frequency of 500hz. What is the maximum speed Paul can approach Mary, and still cancel out her trombone noise?

Homework Equations



f = f0/(1-vs/v)

Speed of sound = 340m/s

The Attempt at a Solution



Max freq 500, so:

500 = 400/(1-v/340)

Solving for v, I get 68 m/s. The answer is 85 m/s. What's going on?

I don't think your equation is correct. Why are you saying the velocity v is equal to the speed of sound in the air?
 
Last edited:
  • #3
The perceived frequency is equal to the real frequency divided by (1-v(speed of approaching source)/v(speed of sound)). Is that the wrong formula?
 
  • #4
Gauss M.D. said:
The perceived frequency is equal to the real frequency divided by (1-v(speed of approaching source)/v(speed of sound)). Is that the wrong formula?

No that's not what I'm saying. This is indeed 100% correct after dividing through by v :

##f = \frac{f_0}{1 - \frac{v_s}{v}}##

I meant why are you using ##v = 340 m/s## in your calculation, it doesn't make any sense.

Paul is approaching her at speed ##v##, blowing the same type of trombone.
 
  • #5
Pauls speed divided by the speed of sound... What's wrong with dividing by 340?
 
  • #6
I agree with your calculations and answer of 68 m/s. 85 m/s is the answer if the question is how fast do you have to move away from a source of 500/s to hear it as 400/s.
Zondrina, you seem to be confused by the fact that the formula quoted uses vs for the speed of the source and v for the speed of sound, whereas in the question v stands for the speed of the source.
 
  • Like
Likes 1 person
  • #7
Gauss M.D. said:
Pauls speed divided by the speed of sound... What's wrong with dividing by 340?

So Paul is coming at Mary at mach 1? I think you mean Paul comes at Mary with a speed of ##v_s## right :)?

As for your question though, I think you have worded it incorrectly, left information out or the answer is a typo.

If Paul is going away from Mary, then the answer of 85 m/s makes sense. ##f## is the apparent frequency and you said Mary plays the trombone at 400Hz. Hence ##f = 400 Hz## and so we can also say ##f_0 = 500 Hz##. Then you get :

400 = 500/(1 - (v/340))
v = 85 m/s

EDIT : I'm not confused harup, I just believe he didn't put any effort into typing out the question properly.
 
Last edited:
  • #8
Zondrina said:
EDIT : I'm not confused harup, I just believe he didn't put any effort into typing out the question properly.
Seems to me Gauss MD stated the question as given to him/her, and stated a formula in a reasonably standard form (though it is better to state what all the variables represent when quoting a formula). There should be no expectation that the two agree on allocation of symbols to entities.
 
  • #9
haruspex said:
Seems to me Gauss MD stated the question as given to him/her, and stated a formula in a reasonably standard form (though it is better to state what all the variables represent when quoting a formula). There should be no expectation that the two agree on allocation of symbols to entities.

Yes I agree I suppose, I'm just picky about notation IMO. I think information can get lost in translation through a problem if there isn't any consistency.
 

1. What is Doppler shift with 2 trombones?

Doppler shift with 2 trombones is a phenomenon in which the pitch of a sound appears to change when it is heard from a moving source or observer. In this case, it refers to the change in pitch that occurs when two trombones are played simultaneously and one of them is moving towards or away from the listener.

2. How does Doppler shift with 2 trombones occur?

Doppler shift with 2 trombones occurs because of the relative motion between the source of the sound (the trombones) and the observer (the listener). As the moving trombone gets closer to the listener, the sound waves are compressed, resulting in a higher perceived pitch. When the trombone moves away, the sound waves are stretched, resulting in a lower perceived pitch.

3. Is Doppler shift with 2 trombones the same as the Doppler effect?

Yes, Doppler shift with 2 trombones is a specific example of the more general Doppler effect, in which the perceived frequency of a sound changes due to relative motion between the source and observer.

4. How does the speed of the trombones affect Doppler shift?

The speed of the trombones directly affects the amount of Doppler shift that is perceived. The faster the trombones are moving, the greater the change in pitch will be. This is because the frequency of the sound waves is directly proportional to the speed of the source.

5. Can Doppler shift with 2 trombones be heard in everyday life?

Yes, Doppler shift with 2 trombones can be heard in everyday life, particularly in situations where there are multiple moving sound sources, such as a marching band or a passing emergency vehicle with multiple sirens. However, it may not always be noticeable, as the amount of Doppler shift depends on the relative speed and distance between the source and observer.

Similar threads

Blood flow velocity via Doppler effect
    • Introductory Physics Homework Help
Replies
1
Views
1K
Doppler Shift and Astronomy
    • Introductory Physics Homework Help
Replies
1
Views
2K
Doppler Effect: Velocity of a train
    • Introductory Physics Homework Help
Replies
4
Views
1K
Doppler Shift/ Doppler effect HELP
    • Introductory Physics Homework Help
Replies
5
Views
3K
Doppler effect train frequency question
    • Introductory Physics Homework Help
Replies
4
Views
1K
Bouncing ball and Doppler effect
    • Introductory Physics Homework Help
Replies
5
Views
2K
Calculating frequencies with the Doppler Effect
    • Introductory Physics Homework Help
Replies
1
Views
2K
Doppler Shift: Communication from a Space Probe
    • Introductory Physics Homework Help
Replies
8
Views
2K
Doppler effect w/ percentage frequency shift?
    • Introductory Physics Homework Help
Replies
1
Views
3K
Doppler Shift Special Relativity Question
    • Introductory Physics Homework Help
Replies
8
Views
2K

Hot Threads

Recent Insights

Back
Top