Diaphragms have different uses they allow movement between two surfaces. Invariably thin discs attached on inner and outer diameter.
They allow vibration and are key to creating sounds waves in telephone or speaker devices.
A diaphragm seal is a flexible membrane that seals and isolates an enclosure. It’s a regulator. It can controls light passage, as well as gases. They can be made of most rubbers depending on the application.
Evident in detection industries. Home stereo speakers. Car turbos.