strange behavior of piezoceramics

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I am working on a project concerned with sonoluminescence,in which we
have to create standing ultrasonic waves in water inside a 250cc
flask. Testing the piezoceramics I encountered with two problems:
1. The amplitude of the waves, which have been propagated to the water
(detected by a microphone), differs when I change the pole of the
ceramic that contacts with the flask, i.e. when (-) pole is connected
to the flask, the amplitude is so much lower than the connection of
(+) pole of the ceramic to the flask.
2. The resonance frequency differs for the connection of (-) and (+)
poles to the flask as described above, e.g. for (-) pole it resonates
at around 21200Hz, but for (+) pole it happens at about 21500Hz.
Please let me know if this behavior is natural or not.
Also please explain what I can do for solving the problem.


Re: strange behavior of piezoceramics


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Yes, I would expect that behaviour to be normal, piezoceramic elements
are not usually physically bidirectional, at least on all the ones
I've ever worked with.

Why is it a problem for you? Wouldn't you just mount the piezoceramic
the way you want it, and then characterize the system?

Dave.


Re: strange behavior of piezoceramics


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Dear Dave
As I have asked the ceramics must behave symmetrically. The project
 which I work I am concerned about if it will perturb the symmetric
 acoustic pressure inside the fluide.Please explain more if you have
 experienced such a matter. http://techmind.org/sl/index.html may be
helpful for realize the electronic circuit.
 Sincerely yours
 A. A. Ahangary


Re: strange behavior of piezoceramics


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If it's a problem. why can't you just characterise and hand match the
two ceramics?

Like I said, physical asymmetry in piezoceramic material is normal in
my experience, especially given that you are mounting it on a surface.

Why does such a small change in the resonant frequency bother you
though? are you using them at or near their resonant frequency?, if
so, why?

Dave.


Re: strange behavior of piezoceramics



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Interesting website, interesting phenomenon. Firstly, if you can swap the
direction of the transducers around, then you must have just clamped them to
the flask, rather than gluing them. Under those conditions, the resonance
behaviour can be influenced by the influence of the attachment device,
whatever that is, and by possible transient separation of the transducer
from the flask - which you can generally detect as a faint 'singing' at a
high frequency (subharmonic of the driving frequency, just like you hear
when you have your teeth ultrasonically cleaned).  Either of these will
cause unpredictable effects.

Secondly, you seem to be confusing the transducer resonance and the system
resonance. In the article, two resonances are mentioned - is this the same
as you're finding? Transducers of the size they mention will have a
self-resonant frequency *way* higher than 20+ KHz. The system resonance is
due to a mass of fluid confined by the glass walls, the transducer
contribution is relatively minor.

It's an unfortunate setup in that if you glue the transducers on and there's
a problem, then you have to chuck it all out. But doing it as they say seems
to me to give you the best chance of success. IME, ultrasonics isn't a field
that responds well to ad-hoc setups.



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