Electret condenser microphones for acoustic measurement.

In recent years, condenser microphones which are polarized with an electret film (herein referred to as "electret microphones") are increasingly used instead of condenser micro phoneswhich need an external voltage source for polarizing (herein referred to as "con densermicrophones") as acoustic measurement microphones, especially for sound level meters. This is because electret characteristics have been remarkably improved, due to great progress in electret source materials and manufacturing technology. In this paper, first we will present an outline of electret materials and some evaluation methods of electret characteristics.