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Homework Help: Wave interference?

  1. Feb 21, 2015 #1
    1. The problem statement, all variables and given/known data
    A student enters Best Buy prepared to buy a pair of speakers. Before he does so, he conducts an experiment with them. He places the speakers 4.0 m apart and connects a signal generator to both speakers that produces a single and consistent tone. (constant wavelength and frequency) He then walks to a point that is 2.0 m from one speaker and2.3 m from the other. At that point he notices a quiet “spot”. If the speed of the sound in the room is known to be 350 m/s, calculate the possible frequencies being played by the speakers.

    2. Relevant equations

    PD = (n-1/2) λ

    3. The attempt at a solution
    f = v/(lambda) = 350/6(2n + 1) = 58.33/(2n+1) .... n belongs to integer
    Sorry i have no idea what I'm doing nor do i understand the question...
    Could someone please direct me to a couple of videos/helpful links that would actually help me understand what's going on here? This is for my online physics course that I'm taking, and it's hard to actually follow along through a webpage.
  2. jcsd
  3. Feb 21, 2015 #2


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    Make drawing of the set up showing the distances.

    Do you know what they mean by "a quiet spot" ?
  4. Feb 21, 2015 #3
    I'll be honest this course is osmething I haven't touched for a while as I've been busy with my actual school. I'm rusty on this subject.

    Pardon my ignorance where necessary, but I believe a quiet spot indicated that the area had a destructive interference?

    Do you know of any reliable youtube channels that I can rely on?
  5. Feb 21, 2015 #4


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    That's correct.

    Try making the drawing and work out what must happen to get destructive interference at that point. Remember that both speakers are connected to the same signal generator so both speakers produce waves with the same wavelength.

    Try this youtube clip...

  6. Feb 22, 2015 #5
    hi; i just wanted to thank you for the help you have given me.

    That video was helpful. But I am stuck here, as i know the extra distance is .3m, and i know that they are out of phase and because of this the quiet spot is created. What i dont know is the wavelength nor the frequency. I understand that they are the same because of the signal generator, but where do I go from here? How can i find(and confirm) what the potential frequencies are?
  7. Feb 22, 2015 #6
    Okay; Just to ask. Since i know it is .3m apart, can i use submultiples of this to formulate information for the wavelength?
    E.g, .1 m ->

    frequency = 350/.1 ->3500 Hz
  8. Feb 22, 2015 #7


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    But you do know the product of them.

    By what fraction of a wavelength can the waves be out of phase, in order to create a quiet spot?
  9. Feb 22, 2015 #8
    1/2 of a wavelength ? So they can be out of phase.

    I ended up getting 583.33 Hz as a potential frequency? I used the speed of sound in the room and divided it by .6 as that is the wavelength
  10. Feb 22, 2015 #9


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    That is one possibility; are there any others?
  11. Feb 22, 2015 #10
    I believe that any multiple of .5 will create a deconstructive interference?(.5, 1.5, 2.5).

    Okay! I think I got it now. THank you so much for your help!
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