We　report　on　the　fabrication　and　transmission　properties　of　free-standing　single-layer　and　double-layer　THz　bandpass　filters.　These　filters　are　fabricated　on　aluminum　foils　using　femtosecond　laser　micro-machining.　The　aluminum　foils　are　periodically　patterned　with　cross　apertures　with　a　total　area　of　1.75×1.75　cm2,　also　known　as　frequency-selective　surfaces.　Their　terahertz　transmission　properties　were　simulated　using　the　FDTD　method　and　measured　using　a　time-domain　terahertz　spectroscopy　system.　The　simulation　results　agree　with　the　measurements　results　very　well.　The　performance　of　single-layer　bandpass　filters　is　as　good　as　the　commercial　equivalents　on　the　market.　The　double-layer　filters　show　extraordinary　transmission　peaks　with　changing　spacing　between　the　two　layers.　We　show　the　contour　map　of　the　electric　field　distribution　across　the　apertures,　and　ascribe　the　new　transmission　peaks　to　the　interference　and　coupling　of　surface　plasmon　polaritons　between　the　two　layers.　©　2017　Optical　Society　of　America.