MadSci Network: Physics

Re: Tempreature of liquid and sound frequency!

Date: Mon Mar 26 12:51:26 2001
Posted By: Todd Jamison, Staff, Image Science, Observera, Inc.
Area of science: Physics
ID: 984889943.Ph

Good question, Winnie.  Although, I must say, I'm not entirely sure of the 
answer.  I can tell you that the easiest way to find out is to run a small 
science experiment yourself!

Here's my guess:  The ringing you hear is due to the resonant frequency of 
the glass - not the water.  I love to do this myself, entertaining my 
children by making different notes with different shapes of glasses and 
different amounts of water.  I believe that the water "dampens" the ability 
of the glass to resonate below the level of the water.  So the pitch goes 
higher as you add water, because the amount of glass above the water gets 
smaller.  If it is truly just the dampening effect of the water that changes 
the pitch, I would have to say my guess is that the pitch will not change 
with the temperature of the water.  

(As an aside, take a look at the water surface as you perform the "ringing 
trick".  Do you see little round circles in the water?  These are "standing 
waves" that are caused by the vibration of the glass pushing on the surface 
of the water.  )

On the other hand, the pitch MAY vary with the temperature of the GLASS!  As 
you probably know, glass will contract or expand with temperature, just like 
many other types of materials.  IF the glass contracts or expands much, I 
would guess that its pitch would have to change.  Glass is, however, very 
stable compared to, for instance, most metals.  So it might be that the 
glass does not change enough to vary the pitch over the range of 
temperatures that you would want to encounter.  

IF, however, the pitch does vary with glass temperature, then you might 
postulate that hotter water would conduct some heat into the glass, thus 
raising the glass' temperature and hence the pitch of the sound.  Does this 
sound plausible? 
Can you think of an experiment (or several) that would allow you to confirm 
or reject these hypotheses?  I'd be curious to hear your answer and the 
results of your experiments.  Let me know how things go at   

Good luck and good science, 

Todd Jamison
Chief Scientist, Observera, Inc.

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