Electrostriction is a behavior of all dielectrics in which the material suffers mechanical deformation, or changes in shape, under the application of an electric field. Class II and Class III ceramic dielectric are made using ferroelectric materials that display greater effects of electrostrictive movement. You will recognize these ceramic types as X7R, X5R, Z5U, and Y5V.
When the mechanical deformation occurs, the result can be a sound emission such as an audible hum (i.e., “singing”).
Several capacitors mounted closely together on the board can amplify the sound to the point that it is noticeable.
See also Piezoelectric Effect.
Wednesday, September 16th, 2015