Re: THE EXPERIMENT
LARRY SULLIVAN ( firstname.lastname@example.org )
Fri, 09 Apr 1999 07:45:58 -0700
LARRY SULLIVAN wrote:
> This is the site that Jerry was refering to. Interesting experiment
> with interesting possibilities.
> THE EXPERIMENT
> A glowing bubble of air cannot be bought anywhere at any price. But
> with an oscilloscope, a moderately precise sound generator, a home
> stereo amplifier and about $100, readers can turn sound into light
> through a process call sonoluminescence. The apparatus is relatively
> simple. A glass spherical flask filled with water serves as the
> resonator--the cavity in which sound is created to trap and drive the
> bubble. Small speakers, called piezoelectric transducers, are cemented
> to the flask and powered by an audio generator and amplifier. Bubbles
> introduced into the water coalesce at the center of the flask and
> produce a dim light visible to the unaided eye in a darkened room.
> The filled flask must be vibrated at its resonant frequency--that is,
> at the sound frequency at which it responds most intensely. The
> resonant frequency equals the speed of sound in water (1,500 meters
> per second) divided by the diameter of the sphere. The glass will
> cause the actual resonance frequency to be about 10 percent higher. We
> used a 100-milliliter Pyrex spherical boiling flask with a diameter of
> 6.5 centimeters. Filled with water, the container resonated at about
> 25 kilohertz. A small necked flask will produce the best results.
> Grease and oil can interfere with the bubble, so the glassware should
> be thoroughly washed with soap and water and rinsed well.
> You will need three ceramic piezoelectric transducers: two to create
> the acoustic wave and one to act as a microphone to monitor the sound
> of the collapsing bubble. We used disks 15 millimeters thick. As a
> courtesy to readers of Scientific American, the three transducers are
> offered as a set for $95 from Channel Industries, Inc.
> Connect fine wire (about 36 gauge) to the piezoelectric ceramics to
> serve as leads (thin wire minimizes the sound loss). The wire is
> soldered to the silver electrodes on the ceramic. Remove the oxide
> layer on the transducers by rubbing them lightly with a pencil eraser.
> Working quickly with a cool soldering iron, place a small dot of
> solder on the silver sides of each piezoelectric transducer. Remove
> six millimeters of insulation from the end of the wire. Tin the copper
> lead (that is, melt some solder on it) and, after briefly heating the
> solder, place it on the solder dot. A wise move is to attach three
> leads to each disk, space equidistantly in the form of a triangle.
> This pattern ensures that each disk rests evenly on the curved surface
> of the flask. The other leads will also act as spares in case the
> first breaks.
> Attach the transducers to the flask with epoxy. The quick-drying,
> five-minute type is recommended because it allows the transducers to
> be broken off the glassware without damage. Use just enough epoxy to
> fill the space between the flask and the transducer. For symmetry,
> place the two drive transducers on opposite sides on the equator of
> the flask and the microphone ceramic on the bottom. The transducers
> are polarized--one side will be identified with a plus sign or a dot.
> Make sure the two drivers are attached to the flask and wired in the
> same way: both should have plus signs toward the flask, or vice versa.
> Solder a short lead to the outside of each transducer. Wire the drive
> transducers in parallel so they will expand and contract at the same
> time. Connect the wires to coaxial cables, which reduces electrical
> cross talk between the components. The microphone wires in particular
> should be short, extending no more than 10 millimeters before being
> connected to coaxial cables. Make the leads long enough so that they
> will not be under tension when connected. Suspend the flask either by
> clamping its neck to a laboratory stand or by hanging it with wires
> tied to the neck. Fasten all cables to the stand to prevent wire
> The piezoelectric speakers act electrically as capacitors. To drive
> them with an audio amplifier (typically a low voltage, low impedance
> source), an inductor must be wired in series with them. The inductance
> is chosen so that it is in electrical resonance with the piezoelectric
> capacitance at about 25 kilohertz--that is, at the same frequency at
> which acoustic resonance occurs. The drivers described here will have
> a capacitance of about two nanofarads, so the inductance required is
> about 20 millihenries. A good trick for adjusting the inductance is to
> use two or more inductors in series. By changing the distance between
> them, the total inductance may be raised or lowered by up to 50
> percent. Two 10 millihenry inductors spaced about five centimeters
> apart would make a reasonable starting point.
> [Driving Circuit]
> To find the correct inductance, you will need to measure the voltage
> and current from the above circuit. Use a two-channel oscilloscope to
> display both quantities simultaneously. Get them in phase (their
> patterns on the oscilloscope should line up) by adjusting the
> inductance. Although the current from a home stereo is low, the
> voltage may give a mild shock, so be sure all exposed connections and
> wiring are insulated, covered with electrical tape or painted over
> with nail polish. The piezoelectric microphone transducer will
> typically produce about one volt; its output may be sent directly to
> the high-impedance input of the oscilloscope.
> A sonoluminescent bubble can be created only in water in which the
> naturally dissolved air has been removed. A simple way to degas water
> is to boil it. Use a 500 to 1,000 milliliter Pyrex Erlenmeyer flask
> with an airtight stopper. Fit a hollow tube about six millimeters in
> diameter and about 10 centimeters long through the stopper and attach
> a short piece of rubber tubing to it. The tubing allows the steam to
> vent and slows the diffusion of air back into the flask.
> Fill the flask halfway with distilled water. Slowly heat the water and
> keep it at a rolling boil for 15 minutes. Then remove the flask from
> the heat, clamp the rubber tubing to prevent air from entering and
> allow the flask to cool (refrigeration will speed things up). After
> cooling, the flask will be under a strong vacuum, and the water will
> be well degassed. Keep sealed until ready to use, as the liquid will
> reabsorb air in a few hours when the container is opened.
> Carefully pour the degassed water into the resonator flask, letting it
> run down the wall. Doing so will introduce a little air, but that
> actually brings the amount in the water to about one fifth the
> atmospheric concentration, which is the correct level for
> sonoluminescence. The water will slowly regas but will remain useable
> for several hours. Fill the flask with water up to the bottom of the
> neck so that the fluid level makes the mass of water approximately
> For a 100-milliliter spherical boiling flask, tune the audio generator
> to the approximate resonance frequency of 25 kilohertz. Set the
> oscilloscope to display simultaneously the output voltage of the
> amplifier and the current through the drivers. Turn the volume control
> so that the amplifier output voltage reads about one volt peak to peak
> and adjust the inductance so that the current is in phase with the
> voltage. Monitor the current, because it can exceed the limit for the
> coil, causing it to overheat. Also, periodically check that the
> frequency is set to the resonance peak, as it make change with the
> water level and temperature.
> Now, on the oscilloscope, display the piezoelectric microphone output.
> As you vary the generator frequency, you will notice a broad peak in
> the microphone signal, about one to two kilohertz wide. This peak
> corresponds to the electrical resonance between the inductor and the
> capacitance of the drivers. The acoustical resonance shows up as a
> much sharper peak in the microphone signal, less that 100 hertz wide,
> and as a slight dip in the current.
> An easy way to find the resonance for the first time is to examine the
> bubbles in the flask. For best viewing, position a bright lamp just
> behind the flask, because small bubbles scatter light much more
> efficiently in the forward direction. A dark background improves
> visibility. With an eyedropper, extract a small amount of water. While
> looking into the backlit flask, squirt the eyedropper at the surface
> hard enough to create about 10 to 30 bubbles. Adjust the generator to
> find the frequency at which the bubbles move toward the center and
> eventually coalesce into one. When everything is tuned correctly, it
> is usually possible to create a bubble just by poking a wire at the
> When you have a bubble in the center of the flask, slowly increase the
> amplitude. The bubble will be stable at first, and then it will
> "dance" over a few millimeters. Still greater amplitude will cause the
> bubble to stabilize again and shrink, becoming almost invisible,
> before growing again. Above a certain sound intensity, the bubble will
> disintegrate. Best light emission is obtained just below this upper
> amplitude limit.
> Small ripples should be visible on the oscilloscope trace from the
> microphone. This signal is high- frequency sound emitted by the bubble
> as it collapses with each cycle. Watching the ripples is an easier way
> to monitor the status of the bubble than is looking at the bubble
> itself. An electrical high-pass filter can be used to attenuate the
> driving sound, making the ripples on the oscilloscope more apparent.
> To view the glow emitted by a bubble, turn off the room lights and let
> your eyes adjust to the darkness. You should see a blue dot, somewhat
> like a star in the night sky, near the center of the flask. The bubble
> may be made brighter and more stable by fine-tuning the frequency and
> amplitude of the driving sound. If the glowing of the bubble is moving
> in the flask or varying in brightness over a few seconds, the water
> contains too much air. Try substituting freshly degassed water.
> If the bubbles do not move at all or if they move toward the side of
> the flask, you are probably at the wrong frequency. Set the generator
> frequency at 24 or 26 kilohertz, readjust the inductance so the
> current and voltage are in phase and try again. Changing the water
> level or the way the flask is hung may improve the acoustics. As a
> last resort, carefully remove the transducers with a razor blade and
> try a different flask.
> Having produced sonoluminescence, you can now explore many questions
> about the phenomenon. For example, how do magnetic and electrical
> fields affect the light emission? How will various substances
> dissolved in the water change the behavior of the bubble? What new
> ways are there for probing the transduction of sound into light? With
> this home setup, you are ready to research the frontiers of science.
> From Scientific American, Feb. 1995, pp. 96-98
> Problems I've encountered!
> During the building of the apparatus, the article above calls for the
> use of 36awg wire to be soldered to the piezoelectric transducers.
> This is not as easy as it sounds. An inherent part of a good
> piezoelectric transducer is that it is an excellent conductor. Well,
> that goes for heat as well as for electricity. So, the solder is cool
> almost immediately after touching the transducer, and no amount of
> heat will re-melt that solder once its cooled without damaging the
> transducer. A better solution is to solder the leads for the coaxial
> cable directly to the transducer plates, which are both easier to
> solder to the transducers, plus they will provide a considerably
> better connection. Also, set up the entire wiring scheme on a
> breadboard before connecting all the pieces with solder. This will
> help you get the correct wiring. A wiring diagram will be coming soon!
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