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B Resonant frequency in acoustics

  1. May 7, 2016 #1
    Hi,

    A question popped into my mind which I've never thought of in 20 years of making music: when you hold a tuning fork against a simple resonator (a guitar body, a table, any object with some volume really), the resonator amplifies the tuning fork's sound and you hear the tuning pitch louder.

    I've read that this works because of the way the tuning fork is designed: the oscillating tines move horizontally and transmit vertical oscillation to the handle, which isn't dampened when holding it. The vibrations are then transmitted to the resonator by physical contact.

    My question is: why is the resonator amplifying the tuning fork's frequency rather than resonating at its own natural resonant frequency (cf. singing a note close to a guitar at a specific pitch will make the guitar resonate and amplify that pitch)?

    Thanks a lot for your help.
     
  2. jcsd
  3. May 7, 2016 #2

    Simon Bridge

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    A guitar is designed to function as a musical instrument ... so it would be silly if it's box was constructed to suppress a musical note.
    So the simple answer is that the box was constructed that way.

    But you will notice that the sound you get from the box+fork is different from the sound of the fork by itself, or the fork and other objects.
    This is the result of the resonances ... so the box does favourably amplify particular frequencies.
    The specific resonances in a box can be quite complicated ... consider how a singing voice often sounds better in a shower stall... why does a shower stall not suppress non-resonant sounds?
    http://www.indiana.edu/~emusic/acoustics/resonance.htm
    http://www.ncbi.nlm.nih.gov/pubmed/12051448
     
  4. May 7, 2016 #3

    FactChecker

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    The guitar has a large surface area to move air. The tuning fork will not move much air. Even though the guitar may dampen out the tuning fork frequency, it still moves air as it does that. I would be very surprised if the guitar was designed to do much to resonate with or to dampen out the frequencies of interest. That would lead to a very uneven volume at different frequencies of a song.
     
  5. May 8, 2016 #4
    Thank you very much for your very helpful replies.
     
  6. May 11, 2016 #5

    CWatters

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    A struck tuning fork or plucked string can only contain a certain amount of energy, so presumably bringing one near a resonator shortens the duration of the note as well as making it louder?
     
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