A mechanical acoustic filter is formed of boards made by using erosion etching. The filter may be used in electric condenser microphones. The filter may be made of one or a plurality of boards each being made by erosion etching according to different requirements. The boards can be printed circuit boards, ply boards, or of other materials. Every etched board has corresponding paths, which after the boards are assembled, form an acoustic filtering path that not only can extend the length of the path to filter out high frequency noise more efficiently, but also can reduce the entire volume of the microphone assembly.