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A Resonant Frequency

  1. Nov 4, 2016 #1
    I hit a glass and ran the sound though a FFT and I found the tone is 775.195 Hz. I then tried to play the tone out of my computer and this glass did not break.
    This is the equation I am using:
    Amplitude * Math.Sin(Math.PI * Frequency * n2 / 44100D)

    I set the Amplitude to 3 and the Frequency to 775.195 but the glass did not break.
    why?
     
  2. jcsd
  3. Nov 4, 2016 #2

    Charles Link

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    Did the glass resonate at all from the tone you played?
     
  4. Nov 4, 2016 #3
    No,
    I even put a spoon inside the glass and it did not shake at all.
    Also, I ran the sound in to another fft from Audacity and that one said the tone was 815 hz, but I think my fft works a lot better than that one, because I have tested mines a lot.
     
  5. Nov 4, 2016 #4
    this is the FFT
     

    Attached Files:

  6. Nov 4, 2016 #5

    Charles Link

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    Your tone generator might have had an incorrect frequency. It might take a tremendously loud volume to break the glass, but it should resonate with even a low volume if you match the frequency properly. If you have any experience with playing musical instruments, you should be able to tell whether the tone from the computer tone generator matches the tone of the glass. It appears the two are not a match.
     
  7. Nov 4, 2016 #6
    this one says 814Hz but that did not work. Also I set the Amplitude to 5 and it plays really loud. I have to cover my ears. but the pin inside the glass cup does not move
     

    Attached Files:

  8. Nov 4, 2016 #7
    this is the fft of the sound I am playing is say 814 is that not the same as the glass ?
     

    Attached Files:

  9. Nov 4, 2016 #8

    Charles Link

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    When playing a guitar or violin, if the strings are in tune, you can play a low (e.g. on the D string on a violin with finger pressed about one inch up the fingerboard), and the high E string resonates. You should be able to get the glass to resonate. I don't know that the resonant frequency of a drinking glass is quite as precise as the string of a musical instrument (it might be a broader resonance=i.e. a wider frequency spread), but it should resonate if you match the tone. Even if the tone is an octave or two off (a factor of two or four in frequency), it should still make it resonate, just as a low E causes the high E to resonate.
     
  10. Nov 4, 2016 #9

    NascentOxygen

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    Can you get your generator to slowly step up in frequency, starting at 750 Hz and going up in increments of ½ Hz every 2 or 3 seconds? Keep the volume low and see whether there's a point where the sound around the glass magnifies.

    I'd sit the glass base on something hard like glass. Ordinary glass is not as lossless as is true crystal glass, but I think you should discern a resonance.

    Have you looked for youtube clips on this experiment?
     
  11. Nov 5, 2016 #10
    ok I tried playing different frequencies between 770hz-775hz and 810hz- 820hz and the glass did not move. I set Amplitude to 0.5. I played all frequencies for longer than one sec. and nothing... the glass did not move...
    idk what I am doing wrong.
    has anyone got this to work ?
    oh the glass I am using is not a wine glass. it is a glass cup... idk if that make a difference or not
    here is a pic of the glass
     

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  12. Nov 5, 2016 #11

    NascentOxygen

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    So you did not notice that the sound from the glass seemed a bit louder around a particular frequency? How are you coupling the sound to the tumbler? Do you notice a prolonged ringing after you tap the wall with a metal spoon handle?

    If you can generate a tone close to the tumbler's resonance, then tap the lip, a sharp ear should discern a beat note between the two sources.

    You are never going to get the sound to break the glass; at best the circular lip will deform and wobble a bit.
     
  13. Nov 5, 2016 #12
    So you did not notice that the sound from the glass seemed a bit louder around a particular frequency?
    I do not understand what you mean be this question, can you explain ?
    How are you coupling the sound to the tumbler?
    again, i am not understand what you are asking.

    Do you notice a prolonged ringing after you tap the wall with a metal spoon handle?
    Yes, I do...
    If you can generate a tone close to the tumbler's resonance, then tap the lip, a sharp ear should discern a beat note between the two sources.
    it sound the same to me...

    You are never going to get the sound to break the glass; at best the circular lip will deform and wobble a bit.
    whyt not ?
    Reference https://www.physicsforums.com/threads/resonant-frequency.892172/#post-5612615
     
  14. Nov 5, 2016 #13
    Your problem is excessive damping and the reason is the shape of your glass. Wine glasses have thin walls, a wonderful resonating bell shape and a thin stem attachment that prevents the resonance from being coupled to the supporting structure. When you tap a wine glass it doesn't just make a pitched sound it actually rings for a noticeably long time. That is to say the ringing is very poorly damped. A heavier walled nearer to cylindrical tumbler with a broad base touching the counter may make a distinguishable frequency , but it is short lived and doesn't survive much longer than the off resonant frequencies so you don't get a pure tone. A "tink" rather than a ring. This is why you never see a musician playing water glasses use anything but bell shaped stem ware right down to the bass brandy snifter. Tumbler shapes are too highly damped to be made to resonate with any reasonable input. Sound, even amplified sound from a speaker, is too weak to cause resonance unless the resonator has a nice high q.

    Get a wine glass, wet your finger and rub it around the edge. If you can make the glass sing it will work for you experiment. If you can't it won't. You will also see the resonant quality (q) in your captured Fourier transform. The wine glass will ring for a much longer time and the transform will resolve to a much purer tone. If you measure while running your finger around it will be a beautifully pure tone.

    As for putting a spoon in the glass, it is important that the item be light. Try a straw. Also, no matter what anyone says, you can shatter a wine glass this way. Please be careful. Start at lower amplitudes, and wear safety glasses. Better yet, stay well back.

    If you have any sort of a high speed camera, the first resonant mode vibration of a wine glass looks very impressive achieving amplitudes without breaking that are a significant fraction of the diameter of the glass. You won't believe glass can bend that far.
     
  15. Nov 5, 2016 #14

    NascentOxygen

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    I think you are placing the tumbler near the speaker on the side of your screen? That won't direct sufficient energy into the glass walls to achieve much, but with care it should still demonstrate resonance.

    Glass is very strong, and springy (despite what you may think). To stand any chance of breaking glass using an audio tone it needs the lowest loss drinking vessel, these are made of expensive quartz crystal. It needs to be thin-walled and flexible and, as mike.albert99 indicates, shaped like a bell so it rings and keeps ringing, allowing it to store and magnify the weak audio stimulus.

    You'll get some ideas here: http://www.meyersound.com/news/2005/myth_glass/ [Broken]
     
    Last edited by a moderator: May 8, 2017
  16. Nov 5, 2016 #15
    I am not sure what a tumbler is...
    but I have the glass cup in front of my laptop if that is what you mean
     
  17. Nov 5, 2016 #16
    That which we usually call a glass is a tumbler: tall, glass, relatively straight sides, kind of heavy duty every day stuff, no handle. Glass is not specific. We generally think of tumblers when we say a glass, but really we call almost anything a glass. A glass of wine (wine stem) a glass of champagne (flute) a glass of beer (beer mug) etc. I wanted to be more specific. When you pull them out of your kitchen cabinet you call it a glass, but when you buy it a Target, the box says "tumbler" (or stange, or highball, or pilsner, or old fashioned, or cocktail, or ... well, it depends on what shape of "glass" you're buying)
     
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