A foil electret microphone is a relatively new type of condenser microphone invented at Bell laboratories in 1962, and often simply called an electret microphone. An electret is a dielectric material that has been permanently electrically charged or polarised. Electret microphones have existed since the 1920s but were considered impractical, but have now become the most common type of all, used in many applications from high-quality public address to built-in microphones in small sound recording devices. Unlike other condenser microphones they require no polarising voltage, but normally contain an integrated preamplifier which does require power (often incorrectly called polarising power). They are frequently phantom powered in sound reinforcement applications.