precludes electric motors. Crystal earpieces are sometimes used in old or low power radios. High-intensity focused ultrasound for localized heating or creating a localized cavitation can be achieved, for example, in patient's body or in an industrial chemical process. Refreshable braille display. A small crystal is expanded by applying a current that moves a lever to raise individual braille cells. Piezoelectric actuator. A single crystal or a number of crystals are expanded by applying a voltage for moving and controlling a mechanism or system. Piezoelectric actuators are used for fine servo positioning in hard disc drives. Frequency standard The piezoelectrical properties of quartz are useful as a standard of frequency. Quartz clocks employ a crystal oscillator made from a quartz crystal that uses a combination of both direct and converse piezoelectricity to generate a regularly timed series of electrical pulses that is used to mark time. The quartz crystal (like any elastic material) has a precisely defined natural frequency (caused by its shape and size) at which it prefers to oscillate, and this is used to stabilize the frequency of a periodic voltage applied to the crystal. The same principle is used in some radio transmitters and receivers, and in computers where it creates a clock pulse. Both of these usually use a frequency multiplier to reach gigahertz ranges. Piezoelectric motors Types of piezoelectric motor include: The Ultrasonic motor used for auto-focus in reflex cameras Inchworm motors for linear motion Rectangular four-quadrant motors with high power density (2.5 W/cm3) and speed ranging from 10 nm/s to 800 mm/s. Stepping piezo motor, using stick-slip effect. Aside from the stepping stick-slip motor, all these motors work on the same principle. Driven by dual orthogonal vibration modes with a phase difference of 90°, the contact point between two surfaces vibrates in an elliptical path, producing a frictional force between the surfaces. Usually, one surface is fixed, causing the other to move. In most piezoelectric motors, the piezoelectric crystal is excited by a sine wave signal at the resonant frequency of the motor. Using the resonance effect, a much lower voltage can be used to produce a high vibration amplitude. A stick-slip motor works using the inertia of a mass and the friction of a clamp. Such motors can be very small. Some are used for camera sensor displacement, thus allowing an anti-shake function. Reduction of vibrations and noise Different teams of researchers have