In Vino Veritas - a Wineglass Oscillator

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After I had finished a tuning fork oscillator, my brother challenged me to make an oscillator using a wineglass.
( https://www.instructables.com/id/Tuning-Fork-Osci... )

He thought it would be more difficult to use a wineglass than a tuning fork as the frequency determining element. It is.

Everyone knows the sound a (wine) glass makes when you gently tap it, usually it sounds like a fast decaying "ping". Some, more expensive glasses can keep "singing" when you rub a wet finger over the edge. The sound this produces is caused by the glass quickly vibrating in a special way. The round shape of the glass changes into an ellips, back into a circle and then into an ellips but rotated by 90 degrees, and so on. The air vibrates with the glass and a tone is the result.

You can even find serious research on the vibrations of wineglasses, just Google for : "a study of wineglass acoustics" and see the pdf below. (I admit I didn't read it all)

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Step 1: Making the Wineglass Vibrate

When I build the tuning fork oscillator, making it vibrate was easy, you just have an electromagnet repeatedly attract it. But with glass magnetism not an option. I could have made a contraption with a mechanical wet finger, constantly rubbing the glass. But mechanical solutions aren't really my strong suit.
Then I thought of attaching a piezo element (as you can find in "musical" picture cards), but I didn't like the idea of anything touching the glass. And it would change the natural frequency of the wineglass too.

It is possible to make a wineglass vibrate with soundwaves. I guess everyone has seen movie clips of wineglasses being shattered with powerful soundwaves. I didn't need sound that powerful, I thought... So I chose an ordinary loudspeaker to produce the soundwaves that makes the glass vibrate.

Step 2: Detecting the Vibrations

An oscillator needs a closed loop, so I had to register the vibrations, amplify them and feed them back (with the correct phase) via the loudspeaker to the wineglass. How to detect those vibrations. Well that proved to be the most difficult part.

On TV I have seen guys working for "three letter organisations" listening to the vibrations of window panes that in turn were vibrating because of voices in the room behind it, with what is called laser-microphones. I thought it wouldn't be all that difficult to make such a device myself as the glass I am listening to is just a few millimeters away as is the laser.

I was wrong. Those laser microphones use the interference of the original laser light and the reflected light to detect the vibrations of the window panes. I can't think of any way I could make a device to do that. Maybe someone else here does, please tell me in the comments below.

Using a microphone to listen to the wineglass doesn't work either, the sound coming from the loudspeaker will be stronger and the system will oscillate, but not with the frequency of the wineglass, you possibly know the squeal when someone turns the amplifier up too much and that sound returns via a microphone.

With the tuning fork oscillator I used an optical interrupter to detect the vibrations of the tines. That worked well, could I repeat that with something made of glass?

Glass bends light, maybe that could be used. So I tried with leds of different colours shining through the wineglass in different ways and detect any changes with a photo transistor. It didn't work. Then I tried a laser light beam reflecting off the glass and trying to detect any vibrations in it. That didn't work either.

What did work was skimming the laser beam across the glass in such a way that the wineglass would block most of the light, the light that reaches the photo transistor is modulated with the vibrations of the wineglass. The trouble with this setup is that it is extremely sensitive to the smallest of movements of the laser, the glass and the detector. But it is the way I made it work.

Step 3: Green Lasers Are Dangerous

First I used a green laser as I know that green laser light is made with an IR laser and a nonlinear crystal that doubles the frequency of the IR light to green light. But that process isn't perfect so some IR light still comes out of it. With the cheap green lasers (e.g. mine) there is no IR filter to block it. And my photo transistor is sensitive to IR light. But in the end I changed to a red laser when I saw that there was *a lot* of IR coming out of the laser and as your eyes do not react to it, that can be dangerous. Luckily my photo transistor reacts just as well to red light as to IR.

Step 4: The Right Frequency

By tapping the glass and recording it on the oscilloscope I saw (at least) two frequencies pop up. One appeared to be about 100 Hz, which is very low and the other around 800 Hz. That one looked like the frequency I was looking for. I didn't want that 100 Hz so I made a high-pass-filter to block it (and at the same time block low frequency noise such as the 50 Hz hum of mains). I used the Filter Wizard by Analog Devices to calculate the correct values of the parts, they not only make outstanding electronic parts, they are also very helpful with their use.
( https://www.analog.com/designtools/en/filterwizard/ ) Later I realized that the 100 Hz may have been produced by the entire glass shaking on it stem because of my tapping it.

Step 5: Closing the Loop

Now tapping the wineglass gave me some nice pictures on the oscilloscope, so it was time to test with a loudspeaker. It worked instantly, the wineglass started to resonate with a frequency of 807 Hz. From there it was simple, I amplified the signal coming from the (now filtered) photo transistor and fed it to the loudspeaker.

Step 6: Conclusion

Conclusion, it is possible to make an oscillator with a wineglass instead of an RC, LC, crystal or any other "normally used frequency determining device, but it isn't easy. At least it isn't easy the way I did it. The positioning of the laser, the wineglass and the photo transistor is extremely critical, it isn't just a millimeter forward or backwards, it is less than that, as I said to my brother, the phase of the moon influences the positioning too much.

Maybe someone knows of better, less critical ways to detect the vibrations of a wineglass (and no, a microphone does NOT work) Please tell me in the comments below.

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14 Discussions

This may perhaps give you an idea. I design microscopes and teach optics, and I am always looking for nice demonstrations. One useful source is a book by Lewis Wright published by Macmillan in 1892 and titled 'Light: a course in experimental optics, chiefly with the lantern.' The lantern in question was often equipped with a polarizer and an analyzer to reveal birefringence. One of the demonstrations ( on page 285) is a demonstration of 'Effects of Sonorous Vibration' on a strip of glass. It uses the fact that glass shows strain birefringence when it is vibrated. I have not tried it yet, but I summarize it as follows:
1. Cut a strip of glass 5 to 6 feet long, two inches wide and about 1/4 inch thick.
2. Render the edges safe by means of a stone ( modern diamond sharpening plate would work well).
3. Grip the glass at the exact centre in wooden jaws in a vice. I am guessing that the jaws should not be more than an inch wide.
4. Shine a powerful light through the glass, near to the vice, projecting an image of the strip on a screen and place a polarizer and analyzer in the beam on either side of the strip in the crossed orientation, with the directions of polarization at 45 degrees to the long axis of the strip.
5. Draw a 'wet flannel cloth' down the strip from the free end above the light beam to produce a 'shrill but wonderfully clear musical note'. Other kinds of cloth 'have too much bite, and tend to drag the vice about'.
6. During this musical note the glass becomes bright between the crossed polarizers, showing the strain induced by the vibration. The brightness varied sinusoidally at a high frequency, as could be demonstrated by projecting the beam on a rotating mirror and observing the dotted line of light.

If you can send light through your wineglass from a source of polarized light and cover your photodiode with an analyzer, rotating the analyzer to block the signal when the loudspeaker is off, you may be able to pick up the vibrations in the wineglass without needing exact alignment. Hope this works! I tried to make a version once with a piezo transducer to vibrate the glass, but could not get the right frequency.

7 replies

Polarized light, another thing I hadn't considered. I do have some polarizing lenses (somewhere) so I might try that too. I'm not a scientist but I have seen those pretty colours in plastics and glass when you put some stress on it, and these oscillations in the wineglass are stresses. Do you think it would help if the sensor has a colour filter in front of it? Or will it be changes in the intensity of the light, not changes in colour?
About the instructions from that book, I have no idea how one could cut a two meter long strip of glass just 5 cm wide without breaking it. And what is meant by "analyzer"?

Dear WilcoL,
My advice would be not to put any colour filter over the sensor: as the strain increases the glass will go from totally dark between crossed polarizers to grey and then to white before reaching the colours, starting with red. For maximum sensitivity you want your sensor to pick up all the wavelengths in the grey and white light.
Concerning the instructions in the book, I would not recommend that anyone who is not a pro glass cutter should attempt to cut such a long strip. Special tools and protective clothing would be needed. The sequence of components historically was light source, linear polarizer, glass specimen, second linear polarizer in the 'crossed' orientation so no light gets through, then the eye of the observer. 'Analyzer' was used to mean the second polarizer, because it could be used to analyze the light coming through the specimen. For example, if you saw colour, you could test whether it was due to strain birefringence by turning the analyzer through 90 degrees, which would cause the colour to change to the complimentary colour ( red to green, blue to yellow etc).
I suggest that you do not try to repeat the hazardous experiments of 1892. The simplest experiment would be to add a bright light source of large area to your existing kit, with crossed polarizers on either side of the wineglass and just look visually to see if the wineglass gets bright when you make it vibrate with the speaker. You might even be able to use your laptop screen as the light source: the LCD emits linearly polarized light with the direction of polarization at 45 degrees to the vertical. Then you need find only one of your polarizing sunglass lenses....
By the way: a scientist is defined by doing experiments to satisfy his or her curiosity, not by whether he is paid to do this professionally, So you are one, and a highly ingenious one too! Your apparatus could lead to a useful device for testing glass under acoustic stresses.

I tried with polarized light in the setup on my "optical table" (haha) but have failed. I used a white 10 Watt led mounted in a plastic tube with a polarizing lens at the other end. Shine that light on and through the wineglass. On the other side I first tried to see any effect with and without sqeezing the wineglass. With the polarizers aligned and disaligned, I didn't see any changes. Then I tried with a photo transistor but unfortunately also without any results. See the pictures. Should I try in some other way? Or do i need an other wineglass (with some build in stress)?

Dear WilcoL,
I am not sure why it is not working, but it may be that squeezing the glass does not produce enough strain. As you know, acoustic stress can make a glass vibrate so much that it breaks and I am sure you were not squeezing hard enough to shatter the glass. You might try the polarized light experiment again, but making the glass sing by the moist fingertip trick. There is a YouTube video called 'Wineglass resonance in slow motion' which shows strong sound from a speaker being delivered at the resonance frequency. The width of the wineglass oscillates by several percent without the glass breaking, so the glass waggles visibly like it is made of soft rubber, but you can only see this if you have a fast camera and view the scene in slow motion.
A tube of black paper and turning the room lights off may help to see faint signals with your apparatus . Were you able to get a good 'extinction' ( i.e. total darkness) at two rotational positions of the analyzer? Your polarizers look like the correct linear polarizers that you need, but I should mention, in case you don't know, that there are circular polarizers that will not give an extinction unless you orient both of them with the polarizing sides facing each other. [Circular polarizers are often sold for photography. They consist of a polarizer with a birefringent plate cemented to one side ( i.e. they are not symmetrical like ordinary linear polarizers).] But I think you were probably using the correct kind.
I have to go away for a week now, so I will not be able to try anything in my home lab, but I will do so when I get back.

It wasn't the squeezing, I even managed to break a glass doing so (no I'm not hurt). It was that my filters are circular polarizers.
I didn't even know that existed, and I must say I have problems visualizing circular polarized light. Linear polarization is easy but circular? Do the waves (I assume we use the wave representation, not the particle) rotate? Like the first is vertical, the next rotated a few degrees, etc.?

My son came up with a good solution, 3D-glasses! One of his 3D glasses had to give up the polarizers and they work great, even somewhat better than the "official" 52 mm lenses. When you have them at 90 degrees almost no light comes through.

And it does work, when I squeeze the wineglass I see the amount of light increasing. I did manage to see the resonance of the wineglass, picked up with a photo transistor on my oscilloscope. I couldn't get it to work as an oscillator, but that doesn't matter, I have seen it working!

Dear WilkoL,
I am delighted that you got it to work! I will now try to explain circularly polarized light, though this message may be too long for the Instructables format. Light is exactly the same as radio, except the wavelength is a million times smaller. If you think of a WWII jeep, it has a whip antenna which is vertical. The transmitter also has a vertical antenna, which emits radio waves, consisting of a spreading electric field which oscillates from upwards to zero to downwards: i.e. the force is a vector quantity which varies sinusoidally in strength but always has a vertical direction. An electron in the jeep antenna gets driven upwards and downwards by the field, because it is charged and the field exerts a force on it. If the antenna is horizontal the electron cannot move along it and so no current flows and the receiver does not pick up the signal. The radio wave is said to be linearly polarized in the vertical direction. Natural light, e.g. from the sun, contains a mixture of waves polarized in all different directions. Linear polarizers allow electrons to travel in only one direction and convert their energy into heat, so the only light waves that get through are the ones with the field vector at right angles to that direction : it just goes through as if the electrons were not there. In the nineteenth century it was discovered that an array of very fine gold wires placed in parallel acts as a polarizer, and some fancy linear polarizers act in that way, being made of conductive nanoneedles running in parallel in glass.Their output is polarized at right angles to the direction of the needles. So much for linearly polarised light. The most important thing is that in the path of linearly polarized light
an electron experiences a force which oscillates in strength from positive to zero to negative, to zero, to positive again but always up and down along the same direction, called the direction of polarization.
Many materials such as liquids, unstrained glasses and even some crystals let light waves travel through at one velocity, which is always less than the speed of light in a vacuum. However, most of the classes of mineral crystals and many strained materials have a direction to them, rather like the grain in wood. If the e field of a light wave is parallel to the 'grain' the light is slowed down relative to the speed of a wave with the electric field at right angles to the grain. Such a material has two velocities for light and is called birefringent. This is where everything gets complicated. Imagine you are an electron sitting at the exit face of a birefringent sheet, with the 'grain' at 45 degrees to the direction of polarization of the input light. Suppose the polarization is originally North-South. You will experience one wave oscillating NE to SW and another one SE to NW. If these waves arrive in phase with each other the result will be a comfortable force on the electron just like the force exerted by the original wave, and it will be always NS. But if one wave is slowed down relative to the other, by 1/4 cycle because of the effect of the 'grain', the hapless electron will suffer a force which does not vary in strength at all, but rotates continuously. You have to draw the vector arrows on paper to see why this is so. This is called 'circularly polarized light' and comes in two types, with the field vector rotating either clockwise or anticlockwise when you look along the wave in the direction of travel. If the thickness of the sheet of birefringent material is varied, you get other results, where the electronic field strength varies in such a way that the head of the vector arrow describing the electric field travels in an ellipse. A circular polarizer consists of a sheet of linear polarizing material cemented to a birefringent plastic sheet of just the right type to induce the quarter-wave phase difference described above. As you know, ordinary polarizers are useful for photography since if the polarizer is oriented appropriately is can cut out the reflection from a wet road or a window. An ordinary linear polarizer might improve the photo, but it would transmit linearly polarized light into the camera which might not be reflected properly in an SLR or might affect metering. Circularly polarizing filters are preferred because the light they send into the camera has no trace of the direction of polarization of the light that came through the linear polarizing layer. I suspect that all the circular polarizing filters that are now sold have the same handedness, but if you can get some 3D spectacles of the type that have circular pol lenses they will be left- and right- handed, with the amazing property of being crossed whatever the rotational position, provided you have the birefringent layers facing each other. Also, with these, your wineglass will bright up whatever the direction of the strain and this is used in commercial inspection equipment for detecting strain.
Try to get hold of a little book by Shurcliffe and Ballard if you are getting interested in this branch of optics.

Thanks for a very thorough explanation. I still have difficulty getting my head around circular polarization but I will get there. At the moment I think that my visualization of the light wave wasn't all that bad, with one wave coming in say vertical, the next rotated just a little and so on. Only the process of rotation won't be in discrete steps but a constant rotation. That would also make more sense of those circular polarized antennae you sometimes see.

And yes, I bought those 52 mm lenses for my Canon AE1 SLR camera (somewhere in the late 70's or early 80's) It made the sky look much better :-)

While reading your story here I remembered that we (actually my son) have some crystals here with this double refractive index. Calcite. We looked through it to some text and then held one "glass" of the 3D glasses above it. When rotated the picture jumped from on to the other position, we had never done that before, it was nice to see. I guess that what comes out of this calcite crystal is not circular polarized though. It is two "beams" of light, both with another (linear) polarization.

First I'll read and reread your reply a few times, together with Wikipedia. And I might buy that book by Shurcliffe and Ballard as well. Never too old to learn.

Some years ago for one of my kids science experiments we constructed an experimental apparatus to detect the resonant frequency of a wine glass. We used a signal generator-audio amplifier-speaker for glass excitation. The detection employed an old phonograph stylus needle. The stylus piezoelectric output was preamplified and connected to a frequency meter and oscilloscope (for signal amplitude). When the signal generator reached the resonant frequency, the detection signal reached a sharp peak. It was very sensitive to exact frequencies and amplitudes, but does make contact with the glass. This contact is VERY slight, and we concluded negligible to the distortion of the glass resonance.

3 replies

I know I made life more difficult by demanding that nothing was to touch the glass but this is a good idea. One could glue a very small (but strong) magnet to the glass and try to detect the movement with a coil or hall-sensor. As you can see in the movie clip the resonant frequency does not change all that much when you pour in water, so a small magnet will not do that either (I think).

No need to glue a anything to the glass. Just touch the needle of the phono cartridge lightly to the glass, then connect the phono cartridge to a phono pre-amp, as usual. The output of the pre-amp will be about 0.5V peak-to-peak. Plenty strong to work with at that point.

No doubt you are right, but I'm not (yet) prepared to disassemble my record player. Call me sentimental but I still love my old vinyl records. In the meantime I already have glued a small neodymium magnet to the glass, only to find my hall sensors missing. I will buy some and try later. ...or maybe I will mount a coil next to it...

Hi,

101 Spy gadgets for the evil genius by Brad Graham & Kathy McGowan has a chapter on laser spy devices where a laser beam is reflected of a window and the returned beam is amplified...it is available on kindle.

Regards,

Once again, I am completely impressed!