Piezoelectric materials are materials that generate electrical voltage in response to applied mechanical stress (direct piezoelectric effect) and that change shape when electrical voltage is applied (converse piezoelectric effect). In addition to crystals such as quartz, certain types of ceramic materials also have these properties and are called piezoelectric ceramics. When alternating current is applied to a piezoelectric material, the material vibrates, and consequently, such materials are used in piezoelectric buzzers and ultrasonic generator oscillators. Also, piezoelectric receivers in mobile phones.