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Practical Project Help : The Resonance of different pieces of metal.

  1. Mar 1, 2005 #1
    Hello forum.

    I'm in abit of a mess. My high school physics class is in the process or doing individual investigations into an aspect of physics we are slightly familar with. I had planned out an experiment to do with sound, but last minute i was told another person was doing something similar to i had to scrap that work, and my teacher reccomended investigating resonance of metal strips.

    We hadn't covered this is class yet, so i firstly i am still not that much of an expert on the subject. My teacher helped me set up some apparatus; i have a signal generator connected to a vibration generator, in which i can screw on different lengths, widths, and shapes, of metal, and cause them vibrate and find their resonant frequencies.

    My problem is, at this stage, i honestly dont really know what the heck i'm doing, like, what i'm investigating, what i should be doing regarding my metal strips. Different shapes have different resonant frequencies, so what. I did some digging around and found that i could like, get a large oval shape of metal and then attach smaller shapes of metal and compare something to this that or the other.
    This is where i'm stuck. Can anyone point me in the direction of what i can be investigating with this type of apparatus? I feel like i'm nagging the Teacher too much which is why i thought i'd come here. I would have got this sorted sooner if i had of known i'd never get to do my sound investigation : /

    Thank to anyone who can help.

    -Joe
     
  2. jcsd
  3. Mar 1, 2005 #2
    You might investigate vibration modes (harmonic modes) of various shapes. Concentrate on 3 or 4 basic shapes. For example, a rod, a circular plate and so on. Should make an interesting project.
     
  4. Mar 2, 2005 #3

    Ouabache

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    After you attach each piece (strips of metal) to your vibration generator. As you slowly vary the intensity of the generator, you will find a setting (frequency) where the strip vibrates a lot. :surprised

    If you increase the intensity past that point, the vibrations will settle down. The setting of the vibration generator, where you observed the most active vibrations, corresponds to the fundamental or resonant frequency of that strip.

    If you run through a whole range of frequencies, you may also find additional settings where the metal vibrates a lot, but not as strong as the resonant setting. These correspond to harmonic frequencies (or modes as kevinalm mentioned).

    To visualize resonance for different shapes, this site has some nice demos http://www.gmi.edu/~drussell/Demos.html
    go down to "Vibrational Modes of Continuous Systems" (the ones for rectangular and circular membranes may be useful). :cool:

    If you were to vibrate a long cylindrical piece of metal (wire), you will find the very same phenomenon occurs acoustically. If you play a stringed instrument, you may be familiar with resonance and harmonics you can hear, along each string. (Though I realize you cannot do the experiment with sound).
     
  5. Mar 2, 2005 #4

    FredGarvin

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    Since a lot of people want to know if what they are doing has any real world applications, I'll fill you in a bit on this one. What you are doing is inducing a vibration to find the natural frerquencies (resonant frequencies) of a given geometry. The natural frequencies of cantilever beams (what you are doing) are very well documented. Anyways, in my line of work (jet engines) we do something very similar to turbine disks, compressor disks, fan disks, stators and vanes prior to running in an a research environment. Granted it is a slightly different procedure and technique, but the information gained is very similar.

    We induce on every blade, vane, etc... an impulse to let the part vibrate. The item will vibrate at all of it's natural frequencies. By using a certain piece of equipment we can measure all of the natural frequencies that the part has (assuming we have imparted enough energy into the part). You will be doing it one frequency at a time as you sweep up in your frequency sweep on your beams and plates. You will find at what frequencies the part will resonate at. These frequencies are very important in our line of work for quite a few reasons. From these frequencies we can tell if the range that we run in will induce these vibrations into the part and thus cause things like increased fatigue, noise, and safety issues.

    So hopefully you can see the real world application of what you are starting to delve into here. You might want to also look under "modal analysis" for some information.
     
  6. Mar 2, 2005 #5
    Thats a pretty cool real world application. What do you do with jet engines? Design them? This also has an application with insturments too. Have any of you ever wondered why certain instruments are made from some materials and not others? Its because of the resonance of the metal. Also the reason why if you dent a brass instrument, it will sound a little differently then before you dented it.

    You might also want to look at how thickness of the metal affects the resonace of it. If it creates too many variables to keep track of, then I would suggest you avoid it. However if you can do it, adding thickness would be an intriguing twist on your project. If you do well, sounds like an "A" worthy project to me.
     
  7. Mar 3, 2005 #6
    Thanks for all this info so far it is very usefull.

    hehe, yes, sorry, i am aware of how resonence is an important consideration in the design of aircraft, bridges, cars, etc, i was just being slightly sarcastic in regards to my project because i was unclear what i would be showing if i gather all this data on these different shapes' harmonic modes.
    What i'm getting at is, for example, someone else in the class is investigating the effect that different temperatures have on the bounce of squash balls, so this data can be used to say "blah blah if i was designing a squash ball i would blah blah". How can i apply something like this to my experiment?

    i cant really alter the thickness of my shapes, as i've been limited to what the school has, so i've got one big sheet of aluminium to play with.

    And no, i dont play stringed instruments, more of a Syntheszier player myself [i guess you could call it a very fancy signal generator :biggrin: ]
     
  8. Mar 3, 2005 #7

    FredGarvin

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    There are a lot of references out there that have the theory of resonant frequencies of beams. My suggestion would be to look up equations for the various geometries that you can replicate. Then, based on that, change certain variables and observe the changes in natural frequencies.
     
  9. Mar 6, 2005 #8
    that sounds like a very good direction to go in. I've as yet had no luck finding a good resource for what you mentioned via google, hopefully there will be something in my school library.
    Im assuming these equations will be mighty complicated though? They'll have to take into account the length, width thickness, and material right?
     
    Last edited: Mar 6, 2005
  10. Mar 6, 2005 #9
    Hello Acidvoodoo,

    I would start by examining what effect the length of the metal strip has.
    What happens with the resonance frequency, does it become higher or lower if I make the strips longer? Make a diagram with y-axis: resonance frequency and x-axis: length of metal strip.

    Then go on further by investigating a circle shaped metal plate. What happens if you change its radius? What happens if you take oval shapes.

    Then take square shapes. How does the resonance frequency change if you make the square greater? What happens if you take rectangular shapes?
     
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