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Homework Help Thread ;D

Discussion in 'The Lounge' started by AndThen?, Feb 16, 2009.

  1. AndThen?

    AndThen? pull the trigger.

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    Okay, so I think this thread died in the crash and I could really use it right now.

    Uhmm, here's my question:

    Two small loudspeakers, A and B, are driven by the same amplifier (X) and emit pure sinusoidal waves in phase, The speakers are 3.00m from each other and point P is 4.00m from a line joining the two speakers as shown in the diagram below. Assume the speed of sound is 350m/s.


    A ----2.00m---X--1.00m--B
    ....................|
    ....................|
    .................4.00 m
    ....................|
    ....................|
    ....................P

    Determine the frequencies that
    i) constructive interference occurs at point P, and
    ii) destructive interference occurs at point P.

    This question's due in one and a half hours, so I doubt I'll get a reply, but this is revision stuff and it annoys me that I can't remember how to do it. So yeah, help would be great thankssss...


    Btw, this thread is for anyone who needs homework help, not just me.
    DISCLAIMER: Remember, this is "Homework Help Thread ;D" not "DO MY HOMMWORKZ FOR ME NAO THRED". Thanks for appreciating the difference. ;)
     
    #1 AndThen?, Feb 16, 2009
    Last edited: Feb 16, 2009
  2. Eagles

    Eagles WiiChat Member

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    What actually is the question?

    This just seems like a description of the scenario to me:

    It could be that I haven't done sound for a little while and I'm just not seeing it.

    Edit: Looks like you just added the question in, I'll have a look. :p
     
    #2 Eagles, Feb 16, 2009
    Last edited: Feb 16, 2009
  3. Foxy

    Foxy Super Ninjarator
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    i phuckin hate u ths thred was my idea and i wanted the credit! ffs i baggsed in the thnking thread:mad5:

    ;)

    For maths whip out Lew or the King;)
     
  4. AndThen?

    AndThen? pull the trigger.

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    Physics, Foxy. ;D

    And yeah Pete, just edited it in there... forgot at first.. haha thought no one would notice. xD
     
  5. Foxy

    Foxy Super Ninjarator
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    I saw your diagram and that's all I saw, I jumped to con- ah forget it:frown2:
     
  6. AndThen?

    AndThen? pull the trigger.

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    C'monnn Pete, I'm waiting on you...

    EDIT: I SEE YOU THAR NAPALM
     
  7. Napalmbrain

    Napalmbrain WiiChat Member

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    I can't remember how to do it and I can't be bothered to look.
     
  8. Eagles

    Eagles WiiChat Member

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    First I'd work out the path difference.

    Distance from A to P = Sqrt (2^2 + 4^2) ≈ 4.5 m

    Distance from B to P = Sqrt (1 + 16) ≈ 4.1 m

    These seem like untidy numbers, so could be doing it totally wrong.

    Thus path difference ≈ 0.4 m.

    You'll have constructive interference when the path difference is a multiple of the wavelength or zero.

    So you'll have constructive interference when λ = 0.4 m, 0.2 m, 0.1333 m, 0.1 m etc. [Using your speed of sound.]

    Thus constructive frequencies are: f = v / λ : 875 Hz, 1750 Hz, 2625 Hz, 3500Hz etc.

    I'm not sure if this is right, I haven't done sound for a while. What do you think?

    If this is right, you'll have the destructive interference when the path difference is a multiple of λ/2.
     
    #8 Eagles, Feb 16, 2009
    Last edited: Feb 16, 2009
  9. AndThen?

    AndThen? pull the trigger.

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    Aw, mannn... Anyway, there's plenty of physics to go around! I really don't remember how to do sound-in-a-tube questions, and they always appear in the exams.

    A tube that is open at both ends is placed in a deep tank of water. A tuning fork of frequency 256 Hz is sounded continuously above the tube. The tube is slowly raised out of the water and, at a position of the tube, a maximum loudness of sound is heard.

    The tube is gradually raised from a position of maximum loudness until the next position of maximum loudness is heard. The length of the tube above the water surface is increased by 65.0 cm. Calculate the speed of sound in the tube.


    Care to help? I'd like to know how to do it, not just the answer pl0x. ;)

    EDIT:
    Petey, that looks alright to me, but it says the lowest frequency that the destructive/constructive interference occurs... I know I didn't say that earlier but neither did the quiz.
     
  10. Eagles

    Eagles WiiChat Member

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    Spotted a mistake in my answer post, all fixed up now. The correct equation is: f = v / λ.
    Well that's ok, the lowest frequency refers to the longest wavelength. So just use the path difference as the wavelength. So you'll get 875 Hz as the lowest frequency. Are you getting what I'm doing?

    Want to try ii] yourself?

    Disclaimer: I may be giving you the wrong answers, it happens occasionally. ;)
     
    #10 Eagles, Feb 16, 2009
    Last edited: Feb 16, 2009
  11. AndThen?

    AndThen? pull the trigger.

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    Okee, well I got an answer, I guess we'll wait 'til tomorrow to see if you've led me astray. :p
     
  12. Eagles

    Eagles WiiChat Member

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    So my answer for i] is 875 Hz.

    My answer for ii] is {if you want to check yours} :

    λ = 0.8 m, so f = v / λ = 350/0.8 = 437.5 Hz

    I can't really be assed with your other question tonight, might have a look at it tomorrow.
     
  13. AndThen?

    AndThen? pull the trigger.

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    Yeah, I got the same for ii)... don't worry about the other one, we're most likely going through it all tomorrow anyway. Unless you're keen. Whatever floats your boat.
     
  14. surfinrach90

    surfinrach90 I GOT A JAR OF DIRT.

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    Edit: Problem solved, genius aunty came online and BAM. Sorted =]
     
    #14 surfinrach90, Feb 17, 2009
    Last edited: Feb 17, 2009
  15. alexbel

    alexbel WiiChat Member

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    can someone tell me how to divide decimals
     
  16. Lewi T

    Lewi T WiiChat Member

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    Errm you'd have to give an example to clarify what you mean. Are you dividing a decimal by a decimal, a whole number by a decimal, or a decimal by a whole number?
     
  17. ROB64

    ROB64 40lb box of rape

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    1.0 / 1.0 = 1.0

    ^Dividing decimals.
     
  18. alexbel

    alexbel WiiChat Member

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    nevermind my sister helped me out
     

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two loudspeakers are driven by same amplifier what is the lowest frequency for which constructive interference occurs