Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Water glasses experiment theory.

  1. Aug 8, 2011 #1
    I had to come up with a relatively simple physics experiment for an investigation and chose to do a water glass experiment, to make music or at least sound with a wine glass.

    I know that when the vibration of the air inside the glass matches the resonant frequency of the glass it produces sound and that adding water changes the resonance of each glass.

    We were able to record the maximum dB that each water level sounded and also what we could only assume was the fundamental frequency (using the only available equipment an app for an iPad)

    What I'm not sure about is what we can use this information for to help prove the first statement.
    Would doing a calculation to find sound level work?
    SL =10log I/I0
    If it could be used how can we relate the sound level found back to the vibrations of the air and resonant frequency of the glass?

    (Hope this is in the right place, It is a homework assignment)
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Aug 8, 2011 #2


    User Avatar
    Homework Helper

    Are you planning to have the "air column" inside the glass demonstrate resonance or are you going to rub a damp finger around the rim of the glass and have the actual glass resonate.

    The resonating glass will be at a very high frequency.
  4. Aug 15, 2011 #3
    I was planing to run my finger around the top...
    We worked out how to use the equipment available and were able to record the frequency of 6 different glasses with no, 200ml, and for those big enough 300 and 400ml.
    The frequency's obliviously changed when we changed the water level. We were going to relate this to resonance and air columns. Will we be able to do that or is there something else we can relate it to that will work better?
  5. Aug 15, 2011 #4
    The affect of the shape and height of the air in the glass is negligible in this case. The frequency produced will be almost exclusively defined by the shape and size of glass and water. This is because you are adding energy to the glass first, and the coupling between glass and water is very high (because they have similar density and lots of surface area in contact) but the coupling to air is very low and the little sound energy imparted on the air quickly escapes out the top.

    EDIT: assuming reasonably-shaped glasses. if they are very tall and thin, and the mass of the glass and water are relatively low, then the air shape will have more effect.
    Last edited: Aug 15, 2011
  6. Aug 16, 2011 #5
    So I would be safe to assume (hypothesis wise) that an increase in the water level in each glass would decrease the frequency recorded for that glass?
    Though I did find that the frequency only decreased when the glasses were about 3/4's full and anything before that did not affect the frequency readings we were getting. Is there a simple explanation for this or it it just that the equipment we had didn't register the change.
  7. Aug 16, 2011 #6


    User Avatar
    Science Advisor
    Gold Member

    A comment on your premise:
    "when the vibration of the air inside the glass matches the resonant frequency of the glass it produces sound ..."
    Note that by virtue of the vibration of the air there is already "sound". You should be more precise and that should lead you to answer some of your questions. At resonance the amplitude of the sound peaks... so you should show this peak when you vibrate the air externally at different frequencies relative to the resonant frequency of the glass. Can you do that with the equipment at hand?
  8. Aug 16, 2011 #7


    User Avatar
    Homework Helper

    There would be an opportunity to test the part the air is taking here.
    Set a glass "singing" then introduce carbon dioxide. this will eventually displace all the air.
    since Carbon dioxide is more dense than air, there should be a noticeable change to the frequency.
    If there is no change, the frequencies produced have nothing to do with the air in the glass, but the glass itself.

    Perhaps use a strobe light to look at the glass to see if it is changing shape / vibrating. When a trumpet sounds, it is the air column in the instrument that is in resonance, not the metal instrument itself.
  9. Aug 16, 2011 #8
    I'm not 100% sure what I must/should change to be able to show these peaks.

    I am very limited to what my school or I can provide for this experiment (we are only a young small school) so many modifications like the CO2 wouldn't be possible but I will most defiantly research into it as much as I can without actually being able to do the extra part.
    because of these limitations I would be much better off changing what I am discussing to fit the experiment I have done rather then the other way around.

    In my head these two statements "The affect of the shape and height of the air in the glass is negligible in this case." or "the air column in the instrument that is in resonance" kinder contradict each other.... I was initially focusing more on air columns as that's what i believed I should be doing.... but if i follow the first statement I should be focusing on the glass itself?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook