An injection moulded acoustic filter (10) or filter structure including a foil piece (11) are configured to the foil piece (11) by the sound waves (S1) for filtering collision foil piece (11) as acoustic channels with multiple microcracks (12). Microcrack (13) has a maximum gap width (Ws) and a combined slit length across a surface of the foil piece (11) less than 100 microns, along the surface of at least 10 millimeters of foil pieces (11). One earplug (100) includes acoustic filter (10). A kind of method and mould (20), for manufacturing acoustic filter (10).
