by W.A. Steer PhD
by W.A. Steer PhD
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This document assumes you are familiar with my main sonoluminescence experiment page.
Questions and answers given are lightly-edited extracts of email correspondence I've had over the years.
I have tried to categorise the questions according to aspects of the setup. In the coming weeks I hope to add more questions, and a whole section on bubble-behaviour and achieving sonoluminescence for the first time.
I hope you find this useful.
A.
The source of the transducers is in the post-script at the very bottom of my webpage:
The transducer set are available from:
Channel Industries
839 Ward Drive
Santa Barbara
California
Tel: [US] (805) 967 0171
Fax: [US] (805) 683 3420
I never did manage to obtain a part-number from them, but if you explain you want the sonoluminescence transducer set (which is 3 transducers) as used in the Scientific American article (or mention my webpage), they should know what you mean. The last I heard, the transducer set cost about $100.
[If anyone finds out any specifications or order code for the set, please let me know - I'll post it here.]
Q. I have some piezo transducers I scrounged, which were apparently used in an industrial cleaner of some variety. They have two elements, 6mm thick and 35mm in diameter. It was easy to determine the polarity from the way the transducer was wired. I am unsure of the composition of these elements, but they were used in a very similar application so I reckon they should work. My question is, are they too large? Obviously their resonant frequency will be lower than the ones you used. However, you say that less than 1 watt of power needs to be applied, so not running them at their resonant frequency should be ok?
A. The transducers you have may work - the electrical resonance of the transducers is matched by the inductor (since your transducers may have a different capacitance, you may need to compensate with a slightly different inductance), and the acoustic resonance shouldn't be much affected. The larger size (and weight?) of the transducers may (in theory) acoustically load the flask and adversely affect the sharpness of the acoustic resonance of the flask. Try it and see!
Q. For our report we need to know how a transducer works because we have never heard about it before.
A.
The transducers are made of a piezo-electric ceramic material.
Piezo-electric materials change their physical size very slightly (perhaps up to ±0.1%) in
response to an applied electric field. Conversely, if mechanically
strained, they develop an electric voltage across their faces.
Quartz is a common example of a crystal exhibiting a piezo electric
effect, used as a very stable electromechanical resonator in clocks,
watches, and general electronics. Large crystals of quartz are used in some older style gas-lighters - you
can generate a spark several millimetres long from the force of your hand
and a simple lever arrangement!
Synthetic piezo-ceramic materials are used to make various transducers such as that used in the sonoluminescence experiment.
For more information, see the Wikipedia article at
http://en.wikipedia.org/wiki/Piezoelectricity
Q. Did you choose the glue you used for any particular reason? Would any rigid-curing glue that bonds to glass well, work?
A. There's nothing special about the glue. Obviously you need something rigid/brittle rather than rubbery.
Q. Why did you put three dots of solder on the surface of your piezos? Some sort of balancing symmetry because you had to at least put one dot of solder on for the wire connection??
A.There was a far more practical reason: the silvering on piezos is usually quite fragile, and a sharp tug on the wire is likely to remove the wire and, locally, the silver. By having 3 wires, we got three chances! In practice we were careful enough and never had to use the spares...
A. Of all the 100ml round-bottomed flasks pictured in the catologue, the "distillation flask" seemed to have the most spherical bulb. Some other flasks seemed to widen out more where the neck joins on... also the distillation flasks were cheaper than some others because they didn't have a "Quickfit" neck. That's about all there was to it.
Q. Your flask was soda-glass. Will pyrex work? Is there anything special about the soda glass?
A. "Soda-glass" is the more technical term for common glass. There's nothing special about the glass. Pyrex glass might be slightly harder (stiffer), but works okay as far as I know.
Q. We have not yet received our 100ml flask so we are going to try a 250ml flask we have handy while we wait.
A. Good luck. Do bear in mind that with a 250ml flask the acoustic resonance will be more like 18kHz (rather than 25kHz for 100ml), so you'll need a slightly larger inductance.
Q. I didn't realize the frequency would be as low as 18kHz. Could you tell me how to calculate the resonant frequency? I have looked, but can't seem to find a formula.
A. To calculate the resonance from first principles you'd look up the speed of sound in water (about 1500m/s from memory), work out the diameter of the flask from the volume V = 4/3 × pi × r^3 (in practice a 100ml flask is approx. 65mm diameter) then knowing that you want a standing wave (I think that means the radius needs to be half a wavelength).
For a quick conversion, calculate that the resonance frequency scales inversely with the cube-root of the flask capacity, i.e. to convert 100ml (25kHz) to 250ml, use f = 25kHz × cuberoot(100/250) = 18.4kHz
R. Boy, am I glad that you told me that the larger flasks frequency would be so low. I set up to test the inductor, and yes, it would have to be larger. Not only that, but 18kHz is painfully audible.
For a 1000ml flask, the resonant frequency will be around 11kHz.
A. I built my own amplifier based around an HY60 module (30W mono) from ILP, designed for 4- or 8ohm loads. As explained on my web page, you need an output swing around 30V. An amplifier designed for slightly higher-impedance loads (eg 16 or 32 ohm) at 30W should also be ok - or even more desireable since the voltage swing will be greater. The output *currents* we require are substantially *less* than for putting 30W into 8ohms.
I did appear to cause some damage to the module during the sonoluminescence work, resulting in some minor corruption of the waveform. This was not really an issue for continuing sono, but would affect audio quality if used for its proper use! The damage may or may not have been caused by experiments with other transformers or 'funny' loads, or by driving the output into saturation (which in conjunction with the inductive/capacitive load may not be too clever). In short, you probably need an amplifier which is either robust, cheap enough to be 'disposable', or homebrew so you can easily replace any output transistor should they become damaged!
A.
Here's the best info I have on the homebrew oscillator. I apologise that it's not up to my usual
standard of writing and detail, but it should be enough to get you started if you have some
electronics experience.
http://www.techmind.org/sl/osc/
A. If you have the budget, a digitally-synthesised signal generator should be the way to go. I haven't personally used a digital generator for sonoluminescence, although I have used them more recently for other applications. The Fluke/Philips PM5136 or PM5138A are very easy-to-use and I expect will work well here.
Q. You are pulsing the piezos with sine waves. Why not triangle or square?
A. A sine wave is a pure frequency. Triangle or square waves can be anaylsed (using Fourier Analysis) as comprising the sum of a sine wave and a series of harmonic frequencies. Harmonics will cause additional nodes in the flask besides the prime one at the centre. Also it'll be very much more difficult to drive non-sine waves anyway because you can't use resonant tank circuits...
to be continued...
©2007William Andrew Steer