SOUND YOU CANNOT HEAR BUT SEE

We know, vibrations produce sound. When a bell is stroked; it vibrates; creates air pressure waves; reaches our ear; vibrates air drum; and we perceive the sound. Every object vibrates with its own frequency (no. of vibrations per second). Our ear is designed to hear the frequency range 20-20000 cycles per second. If the sound frequency is above 20000 cycles, it is called ultrasound. Some animal can hear ultrasound but we can not. People have found many uses for the ultrasound. First, how to produce such a high frequency sound.
Here the quartz crystal comes to our help. It is 'silicon oxide'. Positive and negative charges are evenly distributed in it. So it is electrically neutral. But if we press that crystal, the charges are displaced; positive charges move to one side; negative charges move to another side; electrical field is created; a voltage develops. If we connect the two sides of quartz externally, current (electrons) will flow. Hence, using mechanical force, electrical current is produced. This is called 'Piezo effect'.
"Reverse piezo effect" is also possible. That is , by giving suitable voltage to the crystal, we can compress the silicon oxide crystal. Suppose, we give oscillatory electric voltage, the crystal will vibrate according to the frequency of voltage. That is we have produced mechanical vibrations using electricity. And if the frequency is above 20 k cycles, the crystal will produce ultrasonic waves or sound. Now we have a source of ultrasonic sound. We will see one of the uses.

SONOGRAM
Say, we want to check the health of the baby in the mother's womb. The ultrasonic waves are sent to the womb. The waves pass easily through body fluids. But stopped and reflected (echoed)by body organs. The echoed waves are received by the machine. The received waves are analyzed by the software in the machine. The software interprets the 'time of travel' and 'strength' of the echo-waves and output a fine picture of the organs called sonogram. Heart is also analyzed using 'echo' method. So we, 'see' the ultrasound and not hear it.
Ultrasound is just a pressure wave in the fluid(air, liquid). It is not a harmful like x-ray. So it is safe for the unborn child.

FOOT NOTE: Gas lighter also works using quartz crystal. We press it, crystal get pressed. Electrical voltage is produced. When the current flow to the outer metal body, an electric spark is emitted. It lights up the gas. It is micro lightening. No battery is required.
Quartz crystal -another wonder material.
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