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Measuring flexural vibrations

  1. Jan 18, 2013 #1
    Hi, I'm reproducing a measurement of vibrations and I would like your help with a certain question I have.

    I have a rectangular plate supported by two rods and I want to measure it's fundamental flexural vibration frecuency, among other things. As a base I have a report of such a measurement done in the past by an institution. My inquiry is regarding the surface over which I am placing the plate.

    I am using a microphone to record the sound and a software for fourier analysis. My reasoning is that the vibration on the plate will transmit to the table via the rods, I am thinking the same thing happens on a guitar: the vibration of the strings transmit through the bridge to the front surface of the guitar body, and that's what we hear, the vibration from the body not from the strings themselves. Based on that I know that I will be mostly measuring the vibrations produced from the table (the plate itself it's not big at all, 9x3cm, and hardly makes any sound when striked). But then I thought: Will not the particular vibration modes of the table itself show up on my recordings?

    On the report I read they had placed the plate and the rods over a neoprene sheet over an antivibrating table. I reckon that's what I ought to do as well, but I want to understand if my guitar analogy is valid in this scenario. (And why that doesn't happen on a guitar? If I tune a string on A4 and play it, I will hear A4; the body of the guitar didn't interfere with the sound. Or do we really tune the "guitar" to be A4, not tune just the string itself?)

    Thank you in advance for your kind attention
  2. jcsd
  3. Jan 18, 2013 #2
    This is definitely true to a certain extent, the body of the guitar definitely vibrates and this enhances the sound (compare strumming an unplugged electric guitar and an acoustic guitar) this is because the noise radiates from the vibrating body in the same way your phone sounds louder when it rings on a table, and the same reason your experiment (unless vibration isolated) will be louder when on the table. This is helped in the case of the guitar that a lot of the sound that is produced by the stings enters the body of the guitar via the sound hole.

    You do most definitely tune the sting to a particular note not the guitar. However, everything will have a natural resonant frequency, at this frequency the transmission between the exerted force (incident sound, physical shaking) and the vibration, is most efficient. Things will vibrate and radiate at other frequencies (other than their specific frequency of resonance) but it's 'less efficient'. Now, that's not to say the vibrating body will not interfere with the sound at all, its characteristic resonance will effect the harmonic content (timbre, overtones) of the sounds, but the fundamental note will be the same. Like if you play a C# on a trumpet and a C# on a clarinet the notes are the same pitch due to the fundamental frequency of the sound, but the sounds are quite distinct, the different materials and their properties change the harmonic content of the sound...

    Hope that is, in some way, helpful.
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