Direct answer
An ePTFE acoustic membrane is a microporous component placed over a microphone or speaker opening to help block water and particles while allowing sound to pass.
How it works
The membrane contains interconnected microscopic pores. Airborne sound can excite air through the pore network, while surface energy, pore size and membrane construction help resist liquid-water entry. The final acoustic result also depends on active area, adhesive opening and the device sound channel.
What engineers should specify
Provide the microphone or speaker type, frequency range, acceptable insertion loss, target water and dust protection, available active area, housing geometry and assembly process. A membrane should be evaluated in the actual device structure, not from membrane data alone.