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Klaas Robers wrote:If you look at the holders, in most of them the crystal is mechanically held flat. That implies that the crystal cannot vibrate in thickness or bending mode. That is understandable, because in that mode the crystal is giving sound, that is it emits power to the air. That decreases the resonance sharpness, which is the only task of a crystal for frequency control of a transmitter or receiver. So crystals made for that aim will not give sound into a liquid or a solid (glass).
So the crystals that you opened are not usefull for making ultrasonic sound in water / oil. Then the ceramic transducer is a better thing. Those transducers are used as under water microphones for sonar. They emit sound and receive it again.
Klaas Robers wrote:The modern high frequency crystals vibrate in a "shift-mode". That is, if you lay the crystal flat on the table, that the top plane and the bottom plane move left-right-left with respect to each other. This gives only visible and feelable movement at the very thin left and right edges. The larger old crystals have much lower resonance frequencies, because the frequency is directly coupled to the dimensions of the crystal.
At resonance there is an accoustic movement / standing wave in the crystal. In most cases the length / thickness is 1/2 wavelength of the accoustic wave in the crystal. Accoustic waves travel in crystal, but also in glass, with a speed of about 5 km/sec. So a crystal of 10 MHz carries a accoustic wave with a wave length of 10 000 (m/sec)/ 10 000 000 (MHz) = 1/1000 meter. You will see that the crystal has a thichness of 1/2 mm, i.e. the size of a half wavelenght. If you have a micrometer, measure the thickness of your 4.4 MHz crystal.! Then you can calculate the exact speed of sound in the quartz.
Low frequency crystals are large. A 100 kHz crystal has (in this case) a length of 5 cm. Yes, it is as simple as that.
Klaas Robers wrote:If the sound wave runs in the thickness of the crystal, the size doesn't matter. A 4 MHz crystal had always the same thickness, if the speed of sound is the same in both crystals. For quartz that will be the case I believe.
I made a small drawing if the vibrating crystal. As far as I remember, this is the AT-cut. The red and the blue lines indicate both shapes. The arrows show the directions of movement of the upper and the lower surface. You might see that all the action is in the horizontal direction and there is hardly any activity in the vertical direction.
This implies too that if you make the crystal wider, nothing changes. Then the thickness is the "mean" thickness. Now that we can make crystals more precize, we can also make them smaller.
Harry Dalek wrote:Now i don't know what to make of this on Quartz crystals ! new to me ,i would of thought we would have flying saucers by now if this were true ...
Andrew Davie wrote:
You're all setup to perform that experiment, aren't you? A decent pair of digital scales would be cheap.
I'd be very curious to see if you can nullify gravity, Harry!
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