Abstract: An electret sheet is clamped on an apertured support around the aperture so that a sheet portion stretches all across the aperture. A tension-producing ring is advanced against the sheet portion normal to its stretch direction to produce progressively increasing tension therein. Concurrently, a loudspeaker on one side of the sheet emits constant-amplitude sound waves of fixed frequency and directed against the sheet portion to cause it to vibrate at that frequency. When, because of the increasing tension in the sheet portion, it's vibration amplitude rises to a predetermined value at or near resonance, the advance of the mentioned ring is ended, and the sheet portion is adhered to another ring which retains in that portion the tension then existing in it. Such tensioned sheet and adhering ring are then used to make electret diaphragms for electret microphones.