1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Fundamental note vs fundamental frequency of string

  1. Aug 8, 2014 #1
    1. The problem statement, all variables and given/known data
    A guitr player changes the frequncy of the note produced by a guitar string by pressing his fingers along the string. The fundamental frequency of the string is 264hz. What are the frquncies of the fundamental note if the player plucked the string at 1/4 of the way from one end?


    2. Relevant equations



    3. The attempt at a solution
    my ans is fundamental freuqncy of note = fundamental frequncy of string which is 264hz. but the ans is 352hz. why is it so?
    what's the diffrence between fundamental freuqncy of note and fundamental frequncy of string actually?
     
  2. jcsd
  3. Aug 8, 2014 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    I think the problem has a typo - they do not mean "plucked".
    If the string is pressed 1/4 of the way from the end - what frequency does it play?
     
  4. Aug 9, 2014 #3

    rude man

    User Avatar
    Homework Helper
    Gold Member

    Two answers are required. One is the answer you were given. The other is > 352 Hz.

    Assume no change in string tension when the string is pressed or plucked.
     
  5. Aug 9, 2014 #4
    what's the diffrence between fundamental freuqncy of note and fundamental frequncy of string actually?
     
  6. Aug 9, 2014 #5
    If the string is pressed 1/4 of the way from the end , the frequency that's being played is 264 x 2 =528hz (first overtone occur)
     
  7. Aug 9, 2014 #6

    NascentOxygen

    User Avatar

    Staff: Mentor

    Explain your working.
     
  8. Aug 9, 2014 #7
    when the string is plucked at 1/4 of its end , first overtone( 2nd resonsnace ) occur. when the string is plucked at 1/2 of its end , fundamnetal node produced. (1st resonance)
     
  9. Aug 9, 2014 #8

    rude man

    User Avatar
    Homework Helper
    Gold Member

    You now have two separate strings, effectively.

    Unfortunately, neither one gives 528 Hz.

    What formula relates frequency to length, tension and linear mass density?
    Then, realize that when you push on a string the only thing that changes is length. And as I said there are now effectively two strings so possibly two separate frequencies.
     
  10. Aug 9, 2014 #9

    AlephZero

    User Avatar
    Science Advisor
    Homework Helper

    If you pluck at the 1/4 point, you excite both the first and second resonances (and also higher ones).

    If you pluck at the 1/4 and also touch the string at the mid point, you prevent the vibration of the fundamental frequency and the sound is mainly the 2nd resonance.

    But that has nothing to do with the OP's question.
     
  11. Aug 9, 2014 #10

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    No - I mean: what is the physical effect on the string?

    A standing wave happens when waves reflect off the ends of the string, come back, and interfere with each other. You should have this in yur notes.

    When you press part of the string to a surface, your finger prevents the vibration reaching some of the string. The waves reflect off your finger instead of reflecting off the far end of the string.

    The effect is to have made the string shorter.

    The question is making a distinction between the fundamental for the whole string and the fundamental for the note that is being played by pressing on the string while plucking.
     
  12. Aug 9, 2014 #11

    olivermsun

    User Avatar
    Science Advisor

    It's a guitar. When you "stop" the string 1/4 from the end, you don't pluck it behind the (finger) stop.

    Edit: Although I suppose you could be playing a note very high on the fingerboard, in which case a physicist might interpret 1/4 from the end as being 1/4 from the bridge instead of the nut. I don't think a guitar player would ever interpret it that way, though.
     
    Last edited: Aug 9, 2014
  13. Aug 9, 2014 #12

    olivermsun

    User Avatar
    Science Advisor

    The "plucking" question is more interesting than the "stopping" question… but then the original question ought to ask what are the fundamental frequencies produced.
     
  14. Aug 9, 2014 #13

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    It's a different way of describing it - i.e. in terms of the modes of vibration of the overall string.
    It is important to distinguish between the effect of plucking and pressing+plucking.

    I want to emphasize some of your points that may get lost on a quick read through:
    - In a guitar string, there is significant enough damping for overtones to quickly vanish ... so you have to press then pluck.
    - Press first and it is possible to excite tones which are not modes of the whole string - though it is possible to describe them as a sum of those modes.
    - any wave on the string can be represented as a sum of modes. When described like this, pressing on the fret-board is, mathematically, selecting available modes.

    However:
    Could it be more direct to consider that pressing on the fret-board changes the effective length of the string? Then you can treat the string as playing the fundamental for the effective length?
     
  15. Aug 9, 2014 #14

    AlephZero

    User Avatar
    Science Advisor
    Homework Helper

    That's not true. The way you excite individual harmonics is by touching the string at a nodal point (1/2, 1/3, 1/4, etc, along the length) when you pluck, not by pressing the string onto the fingerboard. You can then release the "touching" finger and the harmonic will continue almost as long as a normal note.

    Here's a demo. (Skip to about 1:30 if you want to cut to the chase). I think somecelxis was getting confused about exciting harmonics in this way.

    https://www.youtube.com/watch?v=mZ6HKrFFM_4

    You can to represent the shape of a string that is partly vibrating and partlly static as a Fourier series, but the you can't represent the vibration at a different frequency in that way. (Well, not without getting way ahead of the level of the OP's question).

    When you press the string against the fingerboard, you change the boundary conditions of the vibration problem, and it is much easier to
     
  16. Aug 10, 2014 #15
    thanks for telling me that When you press part of the string to a surface, your finger prevents the vibration reaching some of the string. The waves reflect off your finger instead of reflecting off the far end of the string.
    if the question ask for 2nd resonance of string (first overtone) . isn't that euqla to 528hz?
     
  17. Aug 10, 2014 #16
    by saying that The waves reflect off your finger instead of reflecting off the far end of the string.


    do you mean the frequncy of sound played is only affected by the length of string from the finger plucked to the right node? this is not affected by the length of left node to the hand plucked?
     
  18. Aug 10, 2014 #17

    NascentOxygen

    User Avatar

    Staff: Mentor

    No. No. No. The string has now changed. By holding it down firmly it at some point along its length you have created a new string. This new string is shorter than the old string, so it has a new fundamental frequency of vibration (and, in turn, it follows that it has also a new overtone).
     
  19. Aug 10, 2014 #18

    olivermsun

    User Avatar
    Science Advisor

    The fundamental frequency corresponds to the mode with only 2 nodes — one at each end of the vibrating string. Thus the fundamental frequency is only affected by the distance between the nodes, which you have altered by pushing the string down behind a fret with your left hand. Now you have a shorter string which has its left end at the fret.

    Overtones correspond to modes with extra nodes in the middle—they will have frequencies which are multiples of the (new) fundamental. How the string was plucked tends to change which overtones get excited, but it doesn't change the list of overtone frequencies.
     
  20. Aug 10, 2014 #19
    my book give wehn the string is plucked at l / (2n) , n##f_0## produced , which n##f_0## = first ,second overtone and etc , n= 2,3,4.....
    this is making me confused. what's the diffrence between the note in photo and what you've said earlier?
     

    Attached Files:

    Last edited: Aug 10, 2014
  21. Aug 10, 2014 #20

    olivermsun

    User Avatar
    Science Advisor

    All the overtones produced are multiples of ##f_0##, the fundamental frequency. At the moment of release (after plucking), the string contains a mix of all the overtones (in varying amounts).
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Fundamental note vs fundamental frequency of string
Loading...