An injection moulded acoustic filter (10) comprising a foil piece (11) or filter structure having a plurality of micro-slits (12) configured to act as acoustic channels through the foil piece (11) for filtering sound waves (S1) impinging the foil piece (11). The micro-slits (13) have a maximum slit width (Ws) across a surface of the foil piece (11) of less than one hundred micrometres and a combined slit length along the surface of the foil piece (11) of at least ten millimetre. An earplug (100) comprising the acoustic filter (10). A method and mould (20) for manufacturing the acoustic filter (10).
