How Ultrasonic Waves Are Generated
In order to duplicate ultrasonic frequencies, humans have harnessed the electrical properties of materials. When a specially cut piezoelectric quartz crystal is compressed, the crystal becomes electrically charged and an electric current is generated: the greater the pressure, the greater the electric current. If the crystal is suddenly stretched rather than being compressed, the direction of the current will reverse itself. Alternately compressing and stretching the crystal has the effect of producing an alternating current. It follows that by applying an alternating current that matches the natural frequency of the crystal, the crystal can be made to expand and contract with the alternating current. When such a current is applied to the crystal, ultrasonic waves are produced.
Depending on which way the crystal is cut, the waves can be focused along the direction of ultrasound propagation or at right angles to the direction of propagation. Waves that travel along the direction of propagation are called longitudinal waves; as noted above, these waves travel in the direction in which molecules in the surrounding medium move back and forth. Waves that travel at right angles to the propagation direction are called transverse waves; the molecules in the surrounding medium move up and down with respect to the direction that the waves propagate. Ultrasound waves can also propagate as surface waves; in this case, molecules in the surrounding medium experience up-and-down motion as well as expanding and contracting motion.
In most applications, ultrasonic waves are generated by a transducer that includes a piezoelectric crystal that converts electrical energy (electric current) to mechanical energy (sound waves). These sound waves are reflected and return to the transducer as echoes and are converted back to electrical signals by the same transducer or by a separate one. Alternately, one can generate ultrasonic waves by means of magnetostriction (from magneto, meaning magnetic, and strictio, meaning drawing together.) In this case an iron or nickel element is magnetized to change its dimensions, thereby producing ultrasonic waves. Ultrasound may also be produced by a whistle or siren-type generator. In this method, gas or liquid streams are passed through a resonant cavity or reflector with the result that ultrasonic vibrations characteristic of the particular gas or liquid are produced.
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