A method for manufacturing a resonator is disclosed in which a sleeve insert is placed into a fixture within a blow molding apparatus. The sleeve insert has a wall with a first plurality of apertures in the wall at a first axial distance and a second plurality of apertures in the wall at a second axial distance. A parison is slid over the sleeve insert; the mold is clamped over the parison causing the parison to press into the sleeve insert at three locations: near the ends of the sleeve insert and at a location between the pluralities of apertures; and air is blown into the sleeve insert, via a blow pin, to expand the parison into the walls of the mold to form cavities proximate the first and second pluralities of aperatures. After cooling, the mold opens to release the newly formed resonator.